Protection against malaria by variants in red blood cell (RBC) genes

INTRODUCTION — The evolution of human red blood cell (RBC) genes is intertwined with the history of human malaria infection.

Over the course of many millennia, new RBC gene variants that reduce the risk or severity of malaria infection have been selected by evolutionary pressure. The geographical distribution of protective variants is heterogenous across malaria endemic regions. Outside of endemic areas, some protective variants are clinically silent in the heterozygous state but cause potentially serious disease in the homozygous or compound heterozygous state.

This topic discusses the relevant RBC genes and their mechanisms of protection against malaria.

Details of malaria pathogenesis and the role of immune response genes are presented in separate topic reviews:

●Pathogenesis – (See "Pathogenesis of malaria".)

●Immune response – (See "An overview of the innate immune system".)

EVOLUTIONARY SELECTION PRESSURE FROM MALARIA

Parasite-human interactions

●Origins and discovery – Exposure of human populations to malaria is estimated to have arisen approximately 10,000 years ago with the emergence of agriculture [1-4]. The sources appear to have been crossovers of a Plasmodium falciparum-like parasite from gorillas and of a P. vivax-like parasite from chimpanzees [5,6]. Most of the protective RBC genetic variations in human populations, including the sickle cell variant, have been selected during the last 3000 to 11,000 years [1,7-9].

The influence of inherited factors on malaria risk was initially observed in a study of malaria in North America in 1887, which noted the apparent resistance to malaria of those of African descent, who "better than any other resist (malaria's) action" [10].

Subsequently, the "malaria hypothesis" or "Haldane hypothesis" was published in the 1940s, followed by accumulating evidence for many genetic changes providing protection from P. falciparum malaria [11,12]. The "malaria hypothesis" is thought to have been based on the evolving descriptions of the global distribution of beta thalassemia; as described by Haldane in a 1949 publication: "The corpuscles of the anaemic heterozygotes are smaller than normal, and more resistant to hypotonic solutions. It is at least conceivable that they are also more resistant to attacks by the sporozoa which cause malaria, a disease prevalent in Italy, Sicily, and Greece, where the gene is frequent" [11].

●Selection pressure – In theory, all of the human malaria species (P. falciparum, ovale, vivax, malariae, and knowlesi) can cause selective pressure on the human population; evolutionary selection pressure has been most studied for P. falciparum infection [13,14]. (See "Pathogenesis of malaria", section on 'Genetic factors'.)

Gene frequencies >1 percent imply positive selective pressure [13,15]. Besides genetic background, protection comes from prior malaria infections, environmental factors, as well as immune status; it is also modulated by determinants of parasite virulence.

•A study from Sri Lanka of the relative importance of genetic and acquired factors on the burden of malaria found that 20 percent of the variability in disease intensity was explained by differences in patient susceptibility to clinical disease [16]. Approximately one-half of the susceptibility differences were attributable to host genes, and one-half were due to age, sex, occupation, and local preventive measures or transmission patterns.

•A similar analysis in Africa demonstrated 25 percent of the variation in hospital admission for malaria was explained by the additive effect of multiple protective host genetic variants [17]. Only 2 percent of this variation was due to the sickle cell trait; this suggests that other, largely undefined genetic resistance traits play an important role.

•A study from 2020 suggested approximately one-third of the variability in the risk of severe and complicated malaria is explained by additive host genetic effects [18].

Categories of protective mechanisms

Parasite life cycle disruption — Plasmodium falciparum malaria, the deadliest form of malaria, has a life cycle that includes a sexual cycle in the insect vector (a female Anopheles mosquito) and a human cycle that includes a liver stage and an erythrocytic (RBC, bloodstream) stage (figure 1). Genetic resistance is much better defined for the RBC stage. (See 'RBC gene alteration' below.)

Mechanisms of protection can occur at the stage of parasite invasion, growth, sequestration (which prevents splenic clearance and contributes to severe disease), and clearance. Some genetic variants can be protective at more than one stage. As an example, alterations in RBC surface proteins and cytoskeletal proteins generally affect parasite entry into the RBC, although they may also play a role in sequestration. Hemoglobin and enzyme alterations generally affect parasite growth within the RBCs, sequestration, and/or clearance of infected RBCs from the circulation. Together these variants often result in decreased parasite biomass and reduce severe malaria.

The following summarizes stages of RBC infection and protective genetic changes:

●Merozoite entry into the cell – Malarial invasion of the RBC is a complex, multistep process involving initial nonspecific attachment of the merozoite to the RBC, reorientation of the merozoite so its apical region opposes the RBC membrane, and then zipper-type introjection of the merozoite into the RBC [19,20].

•P. falciparum uses multiple receptors – The falciparum parasite has multiple redundant pathways, with different parasite ligands and host RBC receptors for different steps of the process of invasion. These include [2,21]:

-Glycophorin A of the MNS blood group system. (See 'MNS blood group system (glycophorins A and B)' below.)

-Glycophorin C, part of the Gerbich blood group system, binds P. falciparum erythrocyte binding antigen (EBA)-140, an invasion protein with Duffy-binding like domains (DBLs) [22]. (See 'Gerbich blood group (glycophorin C)' below.)

-Many more functional EBA proteins have been found in P. falciparum merozoites, including erythrocyte-binding antigen-175 (EBA-175); erythrocyte-binding antigen-181 (EBA-181); erythrocyte-binding ligand-1 (EBL-1) [23].

-ABO, Knops, and Ok blood groups also affect parasite entry, but the association of human genetic variants with inhibition of invasion through the respective pathways is less well understood [24]. (See 'ABO blood group system' below and 'Knops blood group (complement receptor 1)' below and 'Ok blood group antigen (basigin)' below.)

-Parasite ligands at the apical region establish specific and high binding affinity interactions mediated by ligands from the erythrocyte-binding-like (EBL) or the reticulocyte-binding homologous protein (PfRh) families, which are able to deform the RBC membrane [24].

•P. vivax uses the Duffy antigen – Invasion of RBCs by vivax malaria merozoites is almost completely dependent on the Duffy blood group antigen [25,26]. (See 'Duffy blood group system' below.)

●Intracellular parasite growth – Parasite growth inside the RBC requires a favorable intracellular environment. Variants that reduce parasite growth include altered hemoglobins (Hbs) including Hb S, Hb C, and Hb E; thalassemias; membrane disorders such as Southeast Asian ovalocytosis and some forms of hereditary elliptocytosis; and enzyme deficiencies such as glucose-6-phosphate dehydrogenase (G6PD) deficiency; and others [27]. (See 'Hemoglobinopathies' below and 'RBC cytoskeletal changes' below and 'RBC enzyme deficiencies' below.)

●Other aspects of infection – Other aspects of infection that might be affected include:

•RBC lysis at the end of parasite maturation, leading to release of merozoites into the bloodstream

•Immune response and phagocytosis of parasite-infected red cells [28-31]

•Cytoadherence of infected erythrocytes to endothelial cells, uninfected RBCs, platelets, or antigen-presenting cells

The association of HLA class I allotypes with protection from malaria suggests genetic traits conferring resistance also exist during the hepatic stage of infection [32,33].

RBC gene alteration — Parasitization with P. falciparum induces a loss of RBC deformability [34,35]. This effect is more prominent in more mature forms (trophozoites and schizonts) and can induce microvascular occlusion (contributing to the ischemic complications of severe falciparum malaria) as well as the clearance of parasitized cells by the reticuloendothelial system.

Alterations in various RBC genes can be protective against malaria, including genes that encode [36]:

●Cell surface proteins, many of which are blood group antigens

●Cytoskeletal proteins

●Hemoglobin

●Enzymes

RBC SURFACE PROTEINS — RBC surface proteins that affect the risk of developing malaria likely do so by preventing entry of the parasite into the RBC. The best described are the Duffy blood group determinants and the glycophorins; there is also increasing evidence for the role of other blood group systems such as ABO, Knops, and Ok.

Duffy blood group system

●ACKR1 gene and Duffy antigens – The Duffy blood group system includes five antigens, two of which are codominant alleles (Fy^a and Fy^b) that differ by a change at nucleotide 306: guanine in Fy^a and adenine in Fy^b [37].

Both Fy^a and Fy^b reside in a glycoprotein of approximately 40 kD known as the Duffy antigen receptor for chemokines (DARC, CD234), which acts as a receptor for certain proinflammatory cytokines (eg, IL-8, monocyte chemotactic protein-1, RANTES) [38,39]. The gene that encodes the receptor is ACKR1 (previously called FY). (See "Red blood cell antigens and antibodies", section on 'Duffy blood group system'.)

●Duffy-null phenotype – Duffy antigen negativity corresponds to a null genotype in which neither the Fy^a nor Fy^b antigen is expressed, also designated as Fy(a-b-). Most Duffy-negative people of African descent have a silent Fy^b allele with a single nucleotide substitution that impairs promoter activity in erythroid cells [40].

The proportion of the population with the Fy(a-b-) phenotype reaches 100 percent in much of Sub-Saharan Africa. It seems that its selective advantage (resistance to invasion by the merozoites of P. vivax) has driven this phenotype nearly to fixation. In Gambian individuals, for example, 100 percent are Duffy negative [41]. This trait is also seen in many other African populations [42,43].

The ACKR1 variant that causes the Duffy-null phenotype is also associated with a lower baseline neutrophil count. (See "Gene test interpretation: ACKR1 (Duffy blood group gene)" and "Overview of neutropenia in children and adolescents", section on 'Normal variants' and "Approach to the adult with unexplained neutropenia", section on 'Definitions and normal values'.)

●P. vivax resistance – The Duffy-null phenotype confers protection against P. vivax infection, although infection can still occur. A 2022 systematic review and meta-analysis of studies from African countries determined that Duffy-null individuals had a reduced risk of P. vivax infection (odds ratio [OR] 0.46, 95% CI 0.26-0.82 [44]. Another study comparing the Fy^a versus Fy^b phenotype found that Fy^a individuals had a reduced risk of P. vivax infection relative to Fy^b individuals [45].

The role of the Duffy blood group system in P. vivax infection has been evaluated in several ways:

•Human volunteers – In a 1958 study in which human volunteers were exposed to mosquitoes infected with P. vivax (before in vitro malaria cultures were available), Duffy-negative individuals were resistant to infection, while controls developed malaria 11 to 15 days after exposure [46].

•Epidemiologic studies – Epidemiologic studies have shown that Duffy-null individuals are protected from P. vivax infection, including soldiers in Vietnam and in children in Papua New Guinea [43,47].

•Laboratory studies – Duffy-null RBCs were first shown to be resistant to invasion by the merozoites of the simian parasite Plasmodium knowlesi, which is immunologically related to P. vivax [48,49]. Proteins from the two species share sequence homology, and a homologous cysteine-rich domain binds to RBCs [25]. Subsequent studies using blocking antibodies against Duffy antigens (Fy6) or ligands (IL-8; CXCL1, previously called melanoma growth stimulatory activity) demonstrated reduced parasite invasion of RBCs [50-52].

•Platelet factor 4 (PF4) – PF4 released during platelet activation has been implicated in the killing of parasites within RBCs. PF-4 appears to bind to infected RBCs via the Duffy antigen. These in vitro phenomena suggest somewhat paradoxically that Duffy negative individuals would be at a disadvantage when infected with P. falciparum malaria due to lack of PF4 binding [53]. The role of this pathway of parasite killing in patients with malaria remains to be determined.

P. vivax Duffy-binding protein (PvDBP) – The RBC binding domain in PvDBP is highly variable [54]. The binding domains within the PvDBP lie within a conserved N- terminal cysteine-rich region of 330 amino acids [55,56]. Recombinant proteins have been developed that stimulate blocking antibodies and can prevent RBC invasion [57].

●Lessons from vaccine development – Vaccination with PvDBP was used to identify an antibody with neutralizing activity against all tested strains of P. vivax [58]. Other groups have identified regions of PvDBP that may be useful for vaccine development [59-62]. Duffy binding domains also show sequence conservation with the RBC binding domain of P. falciparum EBA-175, suggesting the possibility of a functionally significant structure that could serve as a vaccine candidate [21,26]. (See 'Categories of protective mechanisms' above.)

Malaria vaccine development is discussed separately. (See "Malaria: Epidemiology, prevention, and control", section on 'Vaccination'.)

MNS blood group system (glycophorins A and B) — The glycophorins act as receptors for several malarial ligands [23]. Glycophorins A and B are responsible for MNS blood group antigens. (See "Red blood cell antigens and antibodies", section on 'MNS blood group system'.)

Engagement of glycophorins may change RBC deformability to permit parasite invasion of the RBC [63]. There are 15 O-glycosidically linked oligosaccharides on each molecule of glycophorins A, B, and C. The capacity of malarial parasites to invade cells with modified O-linked saccharides is reduced [64-70].

●Glycophorin A – Increasing evidence suggests that some glycophorin A variants are protective against P. falciparum malaria.

●Glycophorin B – There is also experimental evidence that S-s-U- RBCs, deficient in glycophorin B, are relatively resistant to malarial invasion. The blood group S-s-U- is found in 2 to 5 percent of people from Africa, suggesting malaria may have provided a selective force for this polymorphism [71]. There is also wide variation in the expression level of glycophorin B, which may be associated with susceptibility to malaria [72]. The parasite ligand for glycophorin B has been identified as the P. falciparum erythrocyte binding ligand-1 (EBL-1) [73].

In a study that evaluated invasion of two different malarial strains, the efficiency of RBC invasion was 20 percent of control in those lacking glycophorin A and 50 percent of control in those lacking glycophorin B [74].

Complex polymorphisms affecting the glycophorins, such as the Dantu+ (NE type) blood group phenotype, are associated with reduced susceptibility to severe malaria and reduced parasite growth in RBCs.

●Dantu blood group and DUP4 structural variant – A novel malaria resistance locus was reported close to the cluster of genes encoding glycophorins that provided 33 percent protection against severe malaria (OR 0.67, 95% CI 0.60-0.76) [75]. Genome sequencing demonstrated complex copy number variants (CNVs) at this locus [76]. Eight deletions and eight duplications were found, as well as 11 single nucleotide variants in the glycophorin region. The combined allele frequency of glycophorin CNVs in African populations was 11 percent compared with 1.1 percent in non-African populations. One of the imputed CNVs, DUP4, is associated with decreased risk of severe malaria (OR 0.60, 95% CI 0.50-0.72), reducing the risk of both cerebral malaria and severe malarial anemia. The DUP4 structural variant encodes a fusion protein in which the extracellular domain of glycophorin B is joined to the transmembrane and intracellular domains of glycophorin A, creating the Dantu+ (NE type) blood group phenotype. Parasite growth is impaired in Dantu+ RBCs, consistent with the genetic epidemiology that the DUP4 variant encoding the Dantu+ NE blood group antigen is associated with protection from malaria [76,77]. The Dantu blood group may limit parasite invasion by increasing RBC membrane tension [78].

Gerbich blood group (glycophorin C) — Glycophorin C and D carry the Gerbich (Ge) blood group system. Glycophorin C is a receptor for P. falciparum erythrocyte-binding antigen 140 (EBA-140, BAEBL), a protein with Duffy-binding-like domains (DBLs) [79]. (See "Red blood cell antigens and antibodies", section on 'Gerbich blood group system'.)

●Epidemiologic evidence

•Individuals who are negative for glycophorin C appear to have a lower risk of malaria. In a survey of 266 individuals from Melanesia, the prevalence of P. falciparum and/or P. vivax infection was 6 percent in Gerbich-negative individuals, versus 19 percent in Gerbich-positive individuals; Plasmodium malariae infection was not affected (8 percent in both groups) [80]. However, other studies have not found differences in either P. falciparum or P. vivax infection in individuals with glycophorin C deficiency [81].

•Deletion of exon 3 in the glycophorin C gene is seen at a frequency of 47 percent in coastal Papua New Guinea, where malaria is hyperendemic [79].

●Mechanistic evidence

•A protein complex containing protein 4.1, glycophorin C, and p55 appears to be important for P. falciparum invasion and development [82-84]. The EBA-140/BAEBL allele containing threonine rather than lysine at position 121, which creates the amino acid sequence VSTK rather than VSKK, can bind to glycophorin C [22].

•The 4.1 protein is also involved in the binding to P. falciparum erythrocyte membrane protein-1 (PfEMP1), a protein involved in adhesion of malaria-infected RBC to the vascular endothelium [85,86]. This may contribute to the mechanism of protection in hereditary elliptocytosis. (See 'Hereditary elliptocytosis (HE)' below.)

ABO blood group system — The ABO blood group system is defined by carbohydrates (sugars) that are added on top of the carbohydrate backbone known as the "H" antigen. The glycosyltransferase enzymes responsible for adding these sugars are encoded by the ABO gene. (See "Red blood cell antigens and antibodies", section on 'ABO blood group system'.)

●Epidemiologic evidence – Two studies, including a large study of 9000 children, have shown that blood group O confers protection against malaria (OR 1.2, 95% CI 1.09-1.32) [27,87,88]. Two additional studies have suggested that group A red cells are associated with severe malaria [89,90]. These observations are consistent with the low frequency of blood group A in many endemic malarial areas.

●Mechanistic evidence – Possible mechanisms for the protective effect of group O blood type include the modulation of rosetting of uninfected RBCs by infected RBCs as well as the adherence of infected RBCs to host receptors on monocytes, platelets, and endothelium [88]. This effect has been exhibited by some P. falciparum strains in vitro [91,92].

Clinical evidence from Thailand and East Africa has supported the role for the influence of rosetting by ABO blood group type. In these studies the frequency of rosetting parasites was less in blood isolated from Group O patients than from patients with blood groups A, B, and AB [93-95]. The association of blood group O with protection and with reduced rosetting has also been confirmed in a large case control study of malaria [96].

ABO blood group polymorphisms also modulate sialic acid recognition by some pathogen receptors and may alter the rate of phagocytosis of infected RBCs [97]; it remains unclear if these phenomena are mechanisms of protection in malaria [98].

It has been suggested that during exchange transfusion for resistant P. falciparum, it may be appropriate to consider the ABO blood group of the transfused cells in treatment planning [99]. (See "Treatment of severe malaria", section on 'Exchange transfusion'.)

Knops blood group (complement receptor 1) — The Knops blood group system is controlled by the CR1 gene, which encodes complement receptor 1 (CR1), also called CD35 [100]. Knops antigens include McCoy (McC) and Swain-Langley (Sl). (See "Red blood cell antigens and antibodies", section on 'Knops blood group system'.)

●Epidemiologic evidence

•Children with certain Knops genotypes were less likely to have cerebral malaria (OR 0.17, 95% CI 0.04-0.72) than children with Sl(1/1) [101].

•A study from Ghana showed that the McC^b/b genotype was associated with a reduced risk of severe malaria (OR for severe malaria relative to uncomplicated malaria 0.12, 95% CI 0.02-0.64, p = 0.013); heterozygosity (McC^a/b) was associated with increased susceptibility to severe malaria (OR for severe malaria relative to uncomplicated malaria 2.31, 95% CI 1.03-5.20) [102].

•A case control study from Kenya found that the Sl2 polymorphism was associated with reduced odds of cerebral malaria and death, while the McC^b polymorphism was associated with increased odds of cerebral malaria [103]. The protective association of Sl2 was greatest in children with normal alpha globin.

These complex associations may explain some previous studies that did not show an association of CR1 alleles with protection from malaria [104].

●Mechanistic evidence

•CR1 is the sialic acid-independent receptor used by P. falciparum to invade RBCs [105]. CR1 may also be a receptor during invasion by P. vivax merozoites [106]. The P. falciparum adhesin PfRh4 binds to CR1 at the complement control protein modules 1 to 3 (CCP1-3); this is the region of CR1 that binds to complement components C4b and C3b [107]. (See "Complement pathways".)

•CR1 has been implicated in rosetting of uninfected RBCs to P. falciparum-infected cells, a phenomenon associated with severe malaria [108].

Ok blood group antigen (basigin) — Ok blood group antigens are not obviously polymorphic in malaria endemic regions.

However, the Ok blood group antigen basigin has been identified as a receptor for PfRh5, a parasite ligand that is essential for blood stage growth. RBC invasion was inhibited by soluble basigin or by basigin knockdown, and invasion could be completely blocked using low concentrations of anti-basigin antibodies across all laboratory strains [109]. Furthermore, Ok(a-) RBCs, which express a basigin variant that has a weaker binding affinity for PfRh5, had reduced invasion efficiencies.

PfRh5 forms a complex with the P. falciparum proteins CyRPA and Ripr when interacting with basigin [110]; PfRh5-basigin interaction triggers remodeling of the RBC cytoskeleton to allow merozoite invasion [111]. Binding of PFRh5 to basigin may be responsible for the species specificity of PfRH5 binding and provide a molecular basis for the restriction of P. falciparum to its human host among other primates [112].

These findings have provoked renewed interest in further explorations of PfRh5 as a vaccine candidate antigen. Recombinant PfRh5 can induce antibodies in humans that block invasion of RBCs by merozoites [113]. A recombinant chimeric antibody (Ab-1) against basigin inhibited the PfRH5-basigin interaction and blocked erythrocyte invasion across multiple parasite strains [114].

Membrane channels — Preclinical studies have suggested that variants affecting membrane channels might be protective, although further study is needed. As examples:

●Calcium pump ATP2B4 – A genome-wide association study showed that polymorphisms that map to the major RBC calcium pump (ATP2B4) were associated with protection from severe malaria [27,115]. Erythroid cells with a deletion of the ATP2B4 enhancer have abnormally high intracellular calcium levels [116].

●Ferroportin – Ferroportin is an iron transport channel expressed on many cell types including developing and mature RBCs. Ferroportin knockout mice show increased susceptibility to malaria. A common polymorphism (FPN Q248H) renders the protein resistant to downregulation by the iron-regulatory hormone hepcidin and reduces intracellular iron accumulation, hemolysis and anemia, although there are conflicting reports on whether it reduces the risk of malaria or the degree of parasitemia [117,118]. (See "Regulation of iron balance", section on 'Ferroportin'.)

●Piezo1 – Parasite growth is reduced in dehydrated cells [119]. Gain-of-function mutations in PIEZO1, which encodes a mechanically activated ion channel, cause hereditary xerocytosis (HX), which is characterized by RBC dehydration and mild hemolysis. Mice with a germline gain-of-function PIEZO1 mutation are protected from cerebral malaria with the rodent parasite Plasmodium berghei [120]. The same study evaluated a population from Africa and found a gain-of-function PIEZO1 allele, E756del, in one-third of the participants. RBCs from individuals carrying this allele are dehydrated and display reduced Plasmodium infection in vitro. (See "Hereditary stomatocytosis (HSt) and hereditary xerocytosis (HX)", section on 'Control of RBC solute and water content'.)

RBC CYTOSKELETAL CHANGES — Cytoskeletal changes may affect RBC invasion and/or parasite growth. One possible mechanism by which parasite growth is impaired in protein 4.1-deficient cells is accumulation of a protein that regulates cytoadherence. Other possibilities include involvement of host cytoskeletal proteins in the generation of the parasite membrane or the insertion of parasite proteins into the RBC surface [121].

Southeast Asian ovalocytosis (SAO) — SAO is a hereditary condition that results from a specific 27 base pair deletion in the SLC4A1 gene, which encodes band 3. Band 3 is an integral membrane protein that functions as a chloride-bicarbonate exchanger and provides structural cohesion between the RBC membrane and the underlying spectrin-based cytoskeleton. The SAO variant causes increased RBC rigidity. (See "Southeast Asian ovalocytosis (SAO)", section on 'Characteristics of RBCs'.)

It is unclear if other Band 3 variants are associated with protection from malaria.

●Epidemiologic evidence

•Epidemiologic evidence suggests that SAO confers resistance to high levels of parasitemia with P. falciparum, P. vivax, and P. malariae [122,123]. It may also protect against cerebral malaria in patients infected with P. falciparum.

•In the Madang area of Papua New Guinea, heterozygous SAO was present in 15 percent of all inhabitants, 9 percent of those who had uncomplicated P. falciparum malaria, and in none of those with potentially life-threatening cerebral malaria [124]. However, the prevalence of SAO did not differ between children with and without acute P. falciparum malaria, suggesting that ovalocytosis protects from dying of malaria but not from acquiring malaria [125].

•SAO is present in approximately 30 percent of the Melanesian population of Papua New Guinea and other aboriginal populations of Southeast Asia.

●Mechanistic evidence

•In vitro, SAO RBCs are highly resistant to invasion by P. falciparum merozoites [126-128]. They are also resistant to invasion by P. knowlesi, a plasmodium similar to P. vivax [127].

•A similar resistance to P. knowlesi invasion can be induced in control RBCs by monoclonal antibodies directed against band 3, the protein that is abnormal in SAO [129].

•Why merozoites are unable to invade the SAO cell is not well understood. Inhibition of invasion of two types of plasmodia suggests involvement of a later stage of invasion than receptor recognition. In vitro studies suggest that it is decreased deformability, not the ovalocytic shape per se, that is of primary importance in protecting against malaria [128]. The degree of impaired invasion is closely related to the degree of reduced deformability [130].

•Malaria infection also induces the generation of ovalocytes. In one series from Thailand, individuals infected with malaria had a higher percent of ovalocytes than uninfected controls (6.3 percent with P. falciparum, 8.3 percent with P. vivax, and 0.6 percent in uninfected controls) [131]. Infected ovalocytes contained significantly fewer parasites than infected discocytes, suggesting that the development of ovalocytes could represent a host response to parasite multiplication within the circulation.

Hereditary elliptocytosis (HE) — HE is cytoskeletal disorder caused by pathogenic variants in the genes for protein 4.1 (EPB41), alpha spectrin (SPTA1), beta spectrin (SPTB), and glycophorin C (GYPC). (See "Hereditary elliptocytosis and related disorders", section on 'Gene variants' and 'Gerbich blood group (glycophorin C)' above.)

●Epidemiologic evidence

•The prevalence of HE is 2.5 to 5 per 10,000 in the United States but may be >1 to 10 percent in areas in which malaria is endemic [132-134]. (See "Hereditary elliptocytosis and related disorders".)

●Mechanistic evidence

•HE RBCs demonstrate resistance to invasion by both P. knowlesi and P. falciparum. The resistance may involve diminished invasion, poor intraerythrocytic growth, or diminished cytoadherence of infected RBCs [135]. In particular, a normal protein 4.1/glycophorin C/p55 complex appears to be important for parasite invasion and development [83]. (See "Red blood cell membrane: Structure and dynamics".)

•Less common structural variants that increase the content of spectrin dimers also exhibit impaired parasite growth [136]. This pattern is distinct from that seen in hereditary spherocytosis (HS), in which parasite growth is diminished in proportion to the reduction in spectrin content [136]. (See 'Hereditary spherocytosis (HS; uncertain role)' below.)

•A phosphorylated form of protein 4.1 appears during P. falciparum growth that is not seen in infected RBCs [137]. The possible role of this phosphoprotein in parasite growth and survival is uncertain.

Hereditary spherocytosis (HS; uncertain role) — HS is caused by variants in one of five genes for proteins that link the RBC cytoskeleton to the overlying lipid bilayer. Its role in protection against malaria is uncertain. (See "Hereditary spherocytosis".)

●Epidemiologic evidence – HS has not been reported at increased frequency in regions of the world with increased malaria.

●Mechanistic evidence

•In an in vitro study, P. falciparum growth in HS erythrocytes showed normal growth for two days, followed by impaired growth, the onset of which was more rapid in cells with greater degrees of spectrin deficiency [136].

•In ankyrin/spectrin deficient mice, spectrin appeared to be necessary for the invasion and/or growth of malarial parasites [121]. Heterozygous ankyrin-1 null mice have wild-type levels of ankyrin-1 and spectrin with normal red cell survival but are nevertheless protected from malaria. Resistance is not due to reduced invasion; the exact mechanism of protection is unclear [138].

HEMOGLOBINOPATHIES

Hemoglobinopathies overview — Several case-control and cohort studies have explored the role of specific hemoglobinopathies in resistance to malaria.

A 2012 meta-analysis of case-control studies showed a decreased risk (lower odds ratio [OR]) of severe P. falciparum malaria in individuals with the following hemoglobins (Hbs) [139]:

●Hb AS – OR 0.09, 95% CI 0.06-0.12

●Hb CC – OR 0.27, 95% CI 0.11-0.63

●Hb AC – OR 0.83, 95% CI 0.67-0.96

●Homozygous alpha thalassemia – OR 0.63, 95% CI 0.48-0.83

●Heterozygous alpha thalassemia – OR 0.83, 95% CI 0.74-0.92

Only Hb AS showed protection from uncomplicated malaria (incidence rate ratio [IRR] 0.69, 95% CI 0.61-0.79), and none of the hemoglobinopathies were consistently protective against asymptomatic parasitemia [139].

Sickle cell variant — The sickle cell variant is a single point mutation in the gene that encodes beta globin (HBB). It is thought to have arisen as a founder mutation in Africa thousands of years ago. (See 'Parasite-human interactions' above.)

The point mutation creates an amino acid substitution (valine instead of the normal glutamic acid) at amino acid 7, referred to as p.glu7val; a previous numbering system referred to this as p.glu6val. (See "Pathophysiology of sickle cell disease", section on 'Sickle hemoglobin'.)

A connection between the epidemiology of the sickle cell variant and the distribution of malaria was observed decades ago, and the suggestion that variant conferred a selection advantage was made as early as the 1940s [140,141].

Sickle cell trait (heterozygosity for the variant) is protective without causing significant morbidity. In contrast, sickle cell disease (homozygosity, with Hb SS; or compound heterozygosity with another beta globin variant such as Hb SC or Hb S-beta thalassemia) causes significant morbidity. (See 'Sickle cell trait' below and 'Sickle cell disease' below.)

Sickle cell trait

Epidemiological evidence

●Severe or fatal infection – Case-control studies have demonstrated that sickle cell trait is 90 percent protective against severe and complicated malaria (cerebral malaria and severe anemia) and 60 percent protective against clinical malaria leading to hospital admission [32,142].

A cohort study has shown that sickle trait confers 60 percent protection against mortality between 2 and 16 months of age in an area of high transmission [14]. There is some evidence for protection against mild clinical malaria, but this generally manifests as reduced parasite densities rather than protection against parasitemia per se [143-145]. Another study showed that sickle cell trait protected individuals with severe P. falciparum malaria against hyperparasitemia [146].

Reducing severe infections may also reduce other complications of malaria:

•Bacterial superinfection – Malaria infection strongly predisposes to bacteremia, often with non-Salmonella typhi species. In malaria endemic areas, Hb AS may protect against bacteremia (OR 0.36, 95% CI 0.2-0.65) [147].

•Iron deficiency and malnutrition – Hb AS may also confer protection from iron deficiency anemia. Using Mendelian randomization, Hb AS was shown to be associated with a 30 percent reduction in iron deficiency among children living in malaria-endemic countries in Africa, but not among individuals living in malaria-free areas [148].

Those with Hb AS are also protected from neonatal conditions (perhaps indirectly; OR 0.79, 95% CI 0.67-0.93) and malnutrition (OR 0.67, 95% CI 0.55-0.83) [149].

●Mild infection and epistasis – The most comprehensive longitudinal studies of the role of sickle cell trait in protection from malaria have shown protection from mild disease but negative epistasis (or interaction) between the malaria-protective effects of alpha thalassemia and sickle cell trait. The protection afforded by each condition inherited alone was lost when the two conditions were inherited together, so that the incidence of both uncomplicated and severe P. falciparum malaria was close to baseline in children with both Hb AS and homozygosity for alpha⁺ thalassemia [150]. (See 'Thalassemia' below.)

●In utero transmission – Hb AS does not protect against placental malaria across a range of parasitologic, clinical, and histologic outcomes [151].

Mechanisms — The mechanisms by which sickle cell trait protects against P. falciparum are not fully understood and may be both innate and acquired [152,153]. Several mechanisms, not mutually exclusive, have been proposed to explain impaired parasite growth or death in deoxygenated sickle trait RBCs or removal of parasites growing in sickle trait RBCs from the circulation [154-163].

Some parasite genotypes are found more frequently in people with Hb AS than people with Hb AA, suggesting some parasite traits confer a selective advantage [164]. The Hb S-associated alleles include nonsynonymous variants in the acyl-CoA synthetase family member PfACS8 on chromosome 2 and two other regions of the parasite genome [164]. Understanding the biological mechanisms that confer advantages for some parasite genotypes may help clarify the mechanisms by which Hb AS trait is protective against malaria.

Two principal factors are increased sickling and impaired growth of the parasite during vascular sequestration.

●Increased sickling – The first line of defense is an acceleration of sickling rates in parasitized sickle cell trait RBCs that facilitates their removal from the circulation. Studies using cultured parasitized sickle cell trait RBCs at physiologic levels of deoxygenation showed that cells infected with ring forms had accelerated sickling compared with noninfected cells [165,166].

●Impaired parasite growth – P. falciparum growth is impaired in deoxygenated sickle trait cells [167-169]. P. falciparum grows normally in infected sickle trait cells exposed to 17 percent oxygen, but reducing the oxygen content to 3 percent results in parasite death within a few days due to formation of sickle hemoglobin [170,171].

This effect may be more biologically significant in reducing morbidity and mortality from malaria than a reduction in cytoadherence of infected red cells to the endothelium.

The following changes may contribute to impaired parasite growth in sickle cell trait red cells:

•Loss of potassium – Sickle RBCs have two abnormal mechanisms that facilitate potassium loss. One is the increased activity of the K-Cl cotransporter, due in part to absence of the normal inhibition of the transporter by low oxygen tension, along with retention of its ability to be activated by low pH; both of which occur at sites of stagnant circulation. The second mechanism is increased activity of the calcium-dependent (Gardos) potassium channel, due to a transient increase in cell calcium during sickling [172-174]. (See "Pathophysiology of sickle cell disease", section on 'Effects on the RBC'.)

•Cellular dehydration – The osmotic loss of water that follows the loss of potassium increases the mean cell Hb S concentration and Hb S polymerization. (See "Pathophysiology of sickle cell disease", section on 'Effects on the RBC'.)

Parasite growth is normal in RBCs that have a low potassium concentration induced by incubation with ouabain, which inhibits the Na-K-ATPase pump [175,176]. This observation suggests that water loss with promotion of polymerization of Hb S, not a low potassium content, is of primary importance for impairing parasite growth.

•RBC microRNAs – A subset of erythrocyte miRNAs translocates into the parasite and reduces the growth of the parasites; this includes miRNAs miR-451 and let-7i, which are enhanced in RBCs containing Hb AS. In experimental systems, transfection of miR-451 and let-7i inhibits parasite growth; these miRNAs form chimeric transcripts with the 5' end of parasite mRNAs and inhibit translation of targeted parasite mRNAs [154].

•Increased phagocytosis – There is conflicting experimental evidence for enhanced phagocytosis or retention of ring-stage-infected Hb AS RBCs compared with ring-stage-infected Hb AA RBCs.

-In some experimental models, malaria-infected RBCs containing Hb AS are also susceptible to enhanced phagocytosis of ring forms of the parasite [157].

Membrane-bound hemichromes, autologous immunoglobulin G (IgG) and complement C3c fragments, aggregated band 3, and phagocytosis by human monocytes were greater in rings developing in Hb AS RBCs than in controls, and phagocytosis of ring-parasitized Hb AS RBCs was predominantly complement-mediated. Similar findings were reported for RBCs in beta thalassemia trait and Hb H disease, but not for alpha thalassemia trait erythrocytes [30]. (See 'Thalassemia' below.)

-However, other experimental models examining retention of malaria-infected RBCs by microspheres in vitro or human spleens ex vivo have not shown that ring-stage parasites in Hb AS red cells are retained more than ring-stage parasites in Hb AA red cells [157].

•Altered actin polymerization – There is substantial evidence for altered actin polymerization in malaria-infected Hb AS cells.

Hbs S and C affect the trafficking system that directs parasite-encoded proteins to the surface of infected erythrocytes. Hemoglobin oxidation may inhibit actin polymerization and disrupt the actin cytoskeleton and Maurer's clefts (a parasite-derived organelle) [158]. Maurer's clefts moved significantly faster in infected RBCs containing the Hb variants S and C than in infected wild-type erythrocytes due to an impaired actin network in infected erythrocytes [159]. (See 'Hemoglobin C' below.)

Reduced actin polymerization resulted in reduced protein trafficking from the Maurer's clefts, which appears to be due to redox imbalance [160]. Similar reductions in actin polymerization can be shown when Hb AA erythrocytes are exposed to a transient moderate oxidant stress before parasite invasion, suggesting that irreversibly oxidized products, such as ferryl hemoglobin, hemichrome radicals, or ferryl heme interfere with actin reorganization and cytoadhesion [160]. (See 'G6PD deficiency' below.)

•Reduced cytoadherence – It is possible that reduced adhesion of mature infected RBCs contributes protection in Hb AS RBCs.

Infected RBCs from sickle trait individuals demonstrate reduced in vitro adhesion to microvascular endothelial cells and blood monocytes relative to adhesion of infected Hb AA RBCs. Hb AS RBCs also show reduced expression on their surface of the variant antigen Pf-EMP-1 (P. falciparum erythrocyte membrane protein-1), the parasite's major cytoadherence ligand and virulence factor, and reduced adhesion to CD36 and endothelial protein C receptor (EPCR) [161,177]. Sickle RBCs display altered endothelial cell adhesion, and decreased adhesion resulted in less endothelial cell activation [178].

The relative importance of these phenomena in patients with malaria is unclear.

•Immune mechanisms – A series of intriguing observations suggests that the protection of people with sickle cell trait may be mediated, at least in part, by enhancing the acquired immune response to malaria [163,179,180]. Such an explicit immune-mediated mechanism(s) would explain observations that the effect of Hb AS on parasite density was strongly age-dependent, only beginning after two years [163].

Two studies have found significantly raised IgG levels in young Hb AS children [181,182]. Children with the Hb AS phenotype had higher levels of anti-sporozoite antibodies and agglutinating antibodies to variant surface antigens [144]. However, the antibody response against a microarray of P. falciparum proteins showed no significant difference in antibody responses in children with Hb AS and Hb AA controls [183]. It remains possible that Hb AS and Hb AC may modulate other protective immune responses.

Significantly increased lymphoproliferative responses to malaria antigens have also been seen [184].

The mechanism of an enhanced immunologic response has not been established but may be due to enhanced removal of ring-stage parasites and/or reduced adhesion of late-stage or mature infected RBCs [185-187].

•Mouse models – Mice are susceptible to several species of Plasmodia, and a transgenic mouse model recapitulating sickle cell disease (SCD) has allowed new approaches to dissecting the protective mechanisms of malaria [31,188-190]. In studies of sickle transgenic mice, P. chabaudi adami, P. berghei, and the P. yoelii strains were used in nonlethal and lethal infections.

When these mice were infected with two species of rodent malaria, they showed a diminished and delayed increase in parasitemia compared with controls [31]. This benefit in P. chabaudi adami infection was completely prevented by splenectomy, suggesting that enhanced removal of malaria-infected Hb AS cells by the spleen is required to mediate protection from malaria.

Sickle cell disease — People with sickle cell anemia (Hb SS), a form of sickle cell disease (SCD), remain susceptible to malaria and may die from volume depletion, acidosis, or cytokine release. (See "Diagnosis of sickle cell disorders", section on 'Terminology'.)

It is widely believed that malaria is a major risk factor for death among children with SCD, although there is no clear evidence that children with SCD are at greater risk from malaria than children without SCD [191,192]. Children with Hb SS are no more likely to have uncomplicated malaria than controls. Nevertheless, mortality was considerably higher among children with Hb SS than controls with Hb AA or Hb AS children hospitalized with malaria [193]. Children with Hb SS are at higher risk of readmission to the hospital after severe malaria than children with Hb AA [194].

Similar results were reported from Tanzania, where parasitemia during the hospitalization of children with SCD was associated with both severe anemia and death [195]. Intermittent preventive treatment with sulfadoxine-pyrimethamine in patients with Hb SS may significantly reduce patients' symptoms and anemia but not vaso-occlusive pain episodes or hospitalization [196,197].

Hemoglobin C — Hb C is a beta globin (HBB) variant that substitutes lysine for glutamic acid at amino acid 7 (figure 2). (See "Hemoglobin variants including Hb C, Hb D, and Hb E", section on 'Hb C'.)

●Epidemiologic data – There is good epidemiological evidence for a protective effect of the heterozygous (Hb AC) state, especially in regard to cerebral malaria [198,199]:

•Hb C is seen at maximum frequency in northern Burkina Faso; the prevalence decreases concentrically in surrounding areas. A 2001 case-control study involving 4348 Mossi individuals from Burkina Faso suggested a 29 percent reduction in risk of clinical malaria in individuals who were heterozygous for Hb C (Hb AC) and a 93 percent reduction in individuals who were homozygous (Hb CC) [200].

•A case-control study from Ghana reported that children with Hb AC and Hb CC genotypes were protected from severe malaria [201].

•The data for protection against mild malaria for Hb AC heterozygotes are less clear. Two prospective cohort studies from Mali have shown inconsistent results.

-A study from Bamako in Mali showed that mild malaria incidence was slightly higher in Hb AC children than in Hb AA children (adjusted IRR 1.15, 95% CI 1.01-1.32) [202].

-A study in the town of Bandiagara in a younger-aged cohort showed fewer episodes of clinical malaria in children with Hb AC compared with Hb AA controls [199]. Hb AC was associated with higher newborn birthweight among following pregnancy-associated malaria, conferring a survival advantage for newborns from Hb AC mothers [203].

●Mechanistic data – In vitro studies in oxygenated Hb CC cells have shown the following results [169]:

•No abnormality in parasite invasion.

•Normal growth during the first growth cycle but a markedly reduced number of ring forms following the schizont stage, with degenerated schizonts observed on day 4. This defect was not affected by deoxygenation or an increase in the extracellular potassium concentration, excluding a potassium leak-dependent mechanism.

•Marked resistance to osmotic lysis compared with parasitized normal cells. Thus, these cells cannot burst and release merozoites in a normal fashion.

•Hb C modulates the cell surface properties of P. falciparum-infected erythrocytes by reducing expression of the variant antigen (Pf-EMP-1) [204]. As a result, infected RBCs with Hb AC or Hb CC show reduced adhesion to endothelial cells expressing CD36 and intercellular adhesion molecule-1 (ICAM-1), reduced rosetting with uninfected RBCs.

•In malaria-infected RBCs, the presence of Hb C may inhibit actin polymerization and the production of Maurer's clefts, both of which are necessary for directing parasite-encoded proteins to the surface of infected RBCs [158].

Further studies have extended these findings and shown that the kinetics of protein trafficking from the parasite to different host cell compartments is delayed, slower, and less effective in parasitized Hb AS and Hb AC erythrocytes [205].

These data strongly suggest that sequestration of Hb C-containing erythrocytes would be diminished compared with Hb AA-infected RBCs and thus represent a potent mechanism of protection.

Hemoglobin SC disease — It is likely that individuals with Hb SC disease have significant resistance to malaria.

In culture, oxygenated Hb SC cells are indistinguishable from Hb AA cells as a host for Plasmodia. However, the parasite rapidly dies when Hb SC cells are partially deoxygenated [206]. Why this occurs is unclear.

Possible mechanisms whereby patients with Hb SC disease might be protected against malaria include [207]:

●Lowering of the intraerythrocyte pH by parasite metabolism, inducing more deoxy Hb S polymerization in a red cell with high mean corpuscular hemoglobin concentration (MCHC) due to the presence of Hb C.

●Increased crystallization of Hb C induced by the presence of Hb S, creating an inadequate substrate for the parasite's proteases.

Thalassemia — Both alpha thalassemia and, to a lesser degree, beta thalassemia, are protective against malarial infection, although fewer studies support the selective advantage of thalassemia compared with some other RBC polymorphisms.

●Epidemiologic evidence – The initial evidence supporting malaria protection by alpha and beta thalassemia was derived from population genetic studies. Overall, gene frequencies >0.10 are the norm in populations living in tropical climates, whereas frequencies are somewhat lower in the subtropics and rare in the temperate zones.

•Alpha thalassemia epidemiology

-While both sickle cell trait and alpha thalassemia offer protection against malaria, this protective effect is lost when alpha thalassemia and the sickle cell variant are coinherited. This phenomenon could explain the failure of alpha thalassemia to reach fixation in any population in sub-Saharan Africa [150]; it could also explain why the sickle cell variant is uncommon in the Mediterranean, where alpha thalassemia is common [208].

-Isolated examples of extreme frequencies have been described in particular groups, particularly in certain tribes in India and Nepal [209-212]. In the Tharu people of Nepal, the alpha thalassemia gene frequency reaches 0.78 [213].

-Dramatic data from communities throughout Melanesia showed a gradient in the alpha thalassemia haplotype frequency both from north to south and with increasing altitude, each of which are paralleled by a gradient in the historical incidence of malaria [214,215].

-A case control study from Africa found lower rates of malaria in individuals who were heterozygous for alpha thalassemia but not in those who were homozygous [216].

-There is epidemiologic evidence for protection from malaria by alpha thalassemia trait. In a study from the South Pacific and two studies from Africa, the relative risk of severe malaria was significantly reduced in carriers of alpha thalassemia trait [150,216,217]. In Papua New Guinea, severe malaria was less common in individuals homozygous for an alpha thalassemia variant compared with controls without alpha thalassemia (OR 0.40, 95% CI 0.22-0.74) and in individuals heterozygous for an alpha thalassemia variant (OR 0.66, 95% CI 0.37-1.20) [217]. The risk of hospital admission with infections other than malaria was reduced in children who were homozygous for alpha thalassemia variants (OR 0.36, 95% CI 0.22-0.60) and children who were heterozygous (OR 0.63, 95% CI 0.38-1.07) [217].

-Ghanaian children with alpha thalassemia were protected from severe malaria; heterozygous alpha thalassemia showed protection from severe malaria (OR 0.74, 95% CI 0.56-0.98) [216]. In Kenya, alpha thalassemia also conferred protection from severe malaria (adjusted OR 0.73, 95% CI 0.57-0.94 for heterozygous alpha thalassemia and OR 0.57, 95% CI 0.40-0.81 for homozygous alpha thalassemia) [218].

-A 2006 study from Kenya showed that severe malarial anemia was reduced in children heterozygous or homozygous for alpha thalassemia compared with Hb AA controls (IRR 0.33, 95% CI 0.15-0.73) [219]; these findings were confirmed in a larger study [27].

•Beta thalassemia epidemiology

-In a population survey in northern Liberia, there was an increasing frequency of beta thalassemia trait with increasing age, suggesting that carriers had increased survival [142]. Using a parasite density of ≥1 x 10⁹/L to indicate a potentially lethal infection, the relative risk was 0.45 in children between the ages of one and four with beta thalassemia trait when compared with those without thalassemia.

-In a series of classic studies conducted in the 1960s, a gradient (also called a cline) in the population frequencies of beta thalassemia in Sardinia correlated with altitude [220]. Although malaria was no longer endemic in Sardinia, historically, the incidence of malaria was known to have correlated closely with altitude. The favored hypothesis was that the cline represented selection for the thalassemic beta globin gene under pressure from malaria. The gene frequency for beta thalassemia followed a similar correlation with altitude in Papua New Guinea [221].

●Mechanisms of protection – Enhanced phagocytosis of ring-parasitized RBCs may represent the common mechanism for malaria protection in nonimmune individuals with beta globin variants, while individuals with alpha thalassemia trait are likely protected by a different mechanism. Increased Hb F may also be protective in individuals with thalassemia, which is associated with a delayed postnatal switch from Hb F to Hb A.

•Oxidant injury – Parasitized RBCs are under substantial oxidant challenge [222]. It has been proposed that oxidant injury is a prominent factor in parasitized alpha and beta thalassemic RBCs and contributes to the protection against severe malarial infection, an effect that may be enhanced by the release of superoxide anions by effector T cells upon binding to the infected RBCs [223,224].

In addition, the RBC membrane in thalassemic cells undergoes oxidative damage initiated by the binding of excess globin chains, mostly in the form of hemichromes. The combination of intrinsic thalassemic injury and the effect of malarial infection produces an RBC with reduced viability. These changes permit the preferential destruction and removal of thalassemic parasitized RBCs. (See "Pathophysiology of thalassemia".)

•Reduced intracellular growth

-It is likely that thalassemic RBCs have reduced intraerythrocytic multiplication of P. falciparum, potentially delaying the development of life-threatening parasite densities [225].

-Parasite growth impairment is less prominent in cells from individuals who are heterozygous for beta thalassemia than individuals who are heterozygous for alpha thalassemia or who have other thalassemia syndromes (Hb H/Constant Spring or beta thalassemia/Hb E) [226,227]. The mechanisms involved in diminished parasite growth in beta thalassemic cells are not well understood. Oxidant injury may promote the preferential removal of parasitized thalassemic red cells, and persistence of Hb F may contribute in some patients [228,229]. (See 'Fetal hemoglobin' below.)

-Beta thalassemic mice have lower and delayed peak parasitemia after infection with P. chabaudi adami, which has no preference for immature red cells, but not with P. berghei, which preferentially invades reticulocytes [230].

-In addition to reduced parasite growth, beta thalassemic RBCs infected with P. falciparum show marked reductions in cytoadherence and rosette formation [93]. Infected thalassemic RBCs have decreased expression of surface targeted parasite antigens that mediate these properties.

-Malaria-infected RBCs from people with beta thalassemia trait are susceptible to enhanced phagocytosis of ring forms of the parasite. Membrane-bound hemichromes, autologous IgG and complement C3c fragments, aggregated band 3, and phagocytosis by human monocytes were increased in ring forms of the parasite developing in beta thalassemia trait erythrocytes compared with control infected erythrocytes [30].

•Alpha thalassemia-specific mechanisms

-While there is evidence for reduced parasite growth in individuals who are heterozygous for alpha thalassemia or who have other thalassemia syndromes, some epidemiological and clinical studies suggest that alpha thalassemia may confer protection by additional mechanisms. These studies do not necessarily preclude similar mechanisms in the beta thalassemia syndrome, but equivalent studies have not been performed.

-The protection against severe malaria associated with alpha thalassemia may be mediated in part by increased susceptibility to infection with the nonlethal P. vivax, particularly in young children, thereby inducing limited cross-species protection against subsequent severe P. falciparum infection [227,231,232]. This possibility was illustrated in a study of the epidemiology of malaria in Vanuatu in the northeast Pacific [232]. The prevalence of uncomplicated malaria (P. falciparum and P. vivax) and of splenomegaly (commonly associated with malaria infection) was increased among children <5 years old with alpha thalassemia; this was not seen in older children.

-Alpha thalassemia is associated with reduced expression of red cell complement receptor 1 (CR1), responsible for red cell rosetting of infected and noninfected red cells. Low CR1 expression on reticulocytes reduces the ability of P. vivax to invade [106]. (See 'Knops blood group (complement receptor 1)' above.)

-Infected RBCs from people with alpha thalassemia appear to have reduced expression of the variant antigen Pf-EMP-1 and subsequently reduced adherence of infected RBCs to microvascular endothelial cells (MVECs) and monocytes in vitro [233]. Such a mechanism may reduce sequestration and the progression to severe disease.

-Some of the protective effect of homozygous alpha thalassemia on severe anemia may be explained by the increased erythrocyte count and microcytosis, allowing a greater reduction in erythrocyte count than children of normal genotype during malaria infection before the hemoglobin level falls to cause severe anemia (eg, total hemoglobin concentration <5 g/dL) [234,235]. This is likely to apply to beta thalassemia as well, although similar studies have not been performed with beta thalassemic RBCs.

●Interaction with haptoglobin genotypes – It is unknown if variation in genotype for HP, which encodes haptoglobin, alters the risk for severe malaria in individuals with thalassemia, with studies producing conflicting results.

One study uncovered a significant epistatic interaction between alpha thalassemia and HP genotype that may explain why studies that do not examine both genotypes could give conflicting results. Whereas Hp2-1, inherited in combination with al^pha⁺ thalassemia, offered significant (37 percent) protection against severe malaria (37 percent), such protection was considerably reduced (13 percent protection) when inherited in combination with a normal alpha globin genotype [236]. Moreover, in those with alpha thalassemia, protection was lost altogether when inherited in combination with Hp2-2. The data require confirmation in other groups and/or further mechanistic studies before firm conclusions can be drawn.

Hemoglobin E — Hb E is a beta globin (HBB) variant that reduces beta globin expression (a beta thalassemic phenotype). It is due to a point mutation that substitutes lysine for glutamic acid at amino acid 26 of the beta globin chain (Glu26Lys). (See "Hemoglobin variants including Hb C, Hb D, and Hb E", section on 'Hb E'.)

●Epidemiologic evidence

•There is no direct evidence for the protective effect of Hb E in malaria, but early epidemiological studies suggested a connection [237]. The pattern of linkage disequilibrium surrounding the beta globin locus in Thailand suggests a single origin of Hb E arising between 1200 to 4400 years ago and a rapid increase in allelic frequency [3].

•Hb E is the most frequent hemoglobin structural variant in Southeast Asia [237]. The highest frequency of this gene is observed in the "Hb E triangle" where the frontiers of Cambodia, Laos, and Thailand converge.

•In the central region of Indochina, genetic changes in RBC proteins including Hb E, thalassemia, and glucose-6-phosphate dehydrogenase (G6PD) are so common that only approximately 15 percent of the population has RBCs that lack a malaria-protective variant [238]. (See 'G6PD deficiency' below.)

•In one study, individuals with Hb AE and EE had significantly higher levels of antimalarial antibodies than individuals with Hb AA from the same geographic areas [239].

●Mechanistic evidence

•Initial in vitro studies of the relationship between P. falciparum and Hb E-containing cells demonstrated a moderate decrease in growth of P. falciparum in homozygous Hb E RBCs but not Hb E trait RBCs [240]. A subsequent report showed diminished growth of parasites in both Hb E trait and Hb E disease cells that was most pronounced as the concentration of Hb E increased; this effect was maximal at 20 percent oxygen and decreased but present at 5 percent oxygen [239].

•Additional studies suggest that RBCs from individuals with Hb E trait are relatively resistant to invasion by P. falciparum, which may reduce parasite burden during infection [241]. In addition, Hb E is somewhat unstable and is capable of generating free radicals and inducing oxidative damage to the RBC membrane [242]. The oxidant activity of Hb E may explain the greater rate of monocyte phagocytosis of parasitized RBCs from individuals homozygous for Hb E and heterozygous (Hb E trait) relative to individuals with Hb AA [28].

Fetal hemoglobin — Fetal hemoglobin (Hb F; a tetramer of alpha globin and gamma globin [alpha2gamma2]) is the main hemoglobin in fetal RBCs. At birth, it accounts for approximately 60 percent of total hemoglobin; by six months of age, it accounts for <1 percent of total hemoglobin. (See "Fetal hemoglobin (Hb F) in health and disease", section on 'Hemoglobin switching and downregulation of Hb F expression' and "Fetal hemoglobin (Hb F) in health and disease", section on 'Hereditary persistence of fetal hemoglobin (HPFH)'.)

●Epidemiologic evidence – There is a correlation between reduced risk of malaria and increased Hb F levels in early infancy, but there is not strong epidemiologic evidence that Hb F is protective.

Resistance to malaria during the first six months of life can be explained by two mechanisms: increased malaria invasion of Hb F-containing cells, resulting in fewer parasites available for invasion of the Hb A-containing cells; and slowed growth within the Hb F cells [229]. Protection during the first six months of life is critical, since humoral and cellular defenses are not yet effective, although transferred maternal antibodies may confer relative protection from malaria in infancy.

●Mechanistic evidence

•Some studies have found that growth of P. falciparum is diminished in red cells containing fetal hemoglobin (Hb F), even though parasite invasion may be increased in umbilical cord red cells, which have a high concentration of Hb F [228,229].

Reduced parasite growth can be demonstrated in cord blood cells containing a majority of Hb F, F cells from infants (which contain about 20 percent Hb F), and RBCs from adults who are homozygous for hereditary persistence of fetal hemoglobin (HPFH), which contain nearly 100 percent Hb F [229]. However, other groups have not found such reduced parasite growth in cells with more Hb F [243].

•Initial studies suggest that the decrease in parasite growth associated with the presence of Hb F may be mediated by an increase in intraerythrocytic oxidative stress [244]. This mechanism was studied directly in transgenic mice with 40 to 60 percent Hb F [245].

•In mice infected with P. yoelii 17XL, which produces a syndrome resembling cerebral malaria caused by P. falciparum in humans, light microscopy revealed that intraerythrocytic parasites developed more slowly in Hb F-containing erythrocytes [246]. Hb F was digested only one-half as well as Hb A by malarial hemoglobinases (recombinant plasmepsin II), which could explain the slowing of parasite growth.

RBC ENZYME DEFICIENCIES

G6PD deficiency — Glucose-6-phosphate dehydrogenase (G6PD) is an enzyme that protects cells against oxidant damage. G6PD deficiency is the most common inherited RBC enzyme abnormality, affecting 400 million people worldwide. G6PD deficiency is an X-linked disorder, with males more likely to be affected than females. (See "Genetics and pathophysiology of glucose-6-phosphate dehydrogenase (G6PD) deficiency" and "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency".)

●Epidemiologic evidence – The global pattern of G6PD deficiency parallels areas where malaria was once endemic, including sub-Saharan Africa, South America, South Asia, India, and the Mediterranean region (figure 3). (See "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency", section on 'Epidemiology'.)

●Mechanistic evidence – Enhanced phagocytosis of ring or the early intraerythrocytic form of the parasite may be an explanation for protection in G6PD deficiency [156].

Pyruvate kinase deficiency — Pyruvate kinase (PK) catalyzes the final step in the glycolytic pathway, involving transfer of a phosphate group from phosphoenolpyruvate to adenosine diphosphate. PK deficiency causes hemolytic anemia. (See "Pyruvate kinase deficiency".)

PK deficiency is protective against malaria in mouse models, but protection has not been demonstrated in human populations. PK-deficient mice are protected from malaria [247]. Furthermore, P. falciparum parasites showed reduced growth in RBCs from individuals with PK deficiency relative to control RBCs [248].

Some genetic variants that cause PK deficiency have reached polymorphic frequencies in malaria-endemic areas, although these variants have not been demonstrated to confer resistance to severe malaria [249].

SUMMARY

●Mechanistic insights – Malarial invasion of the red blood cell (RBC) is a complex multistep process. Evolutionary pressure that began thousands of years ago has selected for gene variants that protect against malaria by blocking one or more of these steps; the geographic distribution of these gene variants parallels the regions where Plasmodium falciparum or P. vivax is or was endemic. (See 'Evolutionary selection pressure from malaria' above.)

•Merozoite entry into the RBC via surface proteins that act as malaria ligands

•Intracellular parasite growth

•Lysis of the RBC as the parasite matures

•Immune response affecting sequestration and clearance of infected RBCs

●Parasite invasion – Host variants of RBC surface proteins can reduce parasite invasion. Many of these surface proteins are blood group antigens. (See 'RBC surface proteins' above and "Red blood cell antigens and antibodies".)

•Duffy-null RBCs are protected against P. vivax. (See 'Duffy blood group system' above.)

•Variation in the ABO, MNS, Gerbich, Knops, and Ok blood group systems may confer protection by reducing invasion and/or parasite growth. (See 'ABO blood group system' above and 'MNS blood group system (glycophorins A and B)' above and 'Gerbich blood group (glycophorin C)' above and 'Knops blood group (complement receptor 1)' above and 'Ok blood group antigen (basigin)' above.)

•Variants affecting RBC membrane channels ATP2B4, ferroportin, and Piezo1 may be protective. (See 'Membrane channels' above.)

•Variants affecting cytoskeletal proteins that may be protective include Southeast Asian ovalocytosis (SAO), hereditary elliptocytosis, and hereditary spherocytosis. (See 'RBC cytoskeletal changes' above and "Hereditary elliptocytosis and related disorders" and "Hereditary spherocytosis".)

●Parasite growth – Variant hemoglobins (especially the sickle cell variant and Hb C (figure 2)), thalassemias, and increased fetal hemoglobin (Hb F) have evolved in malaria-endemic areas. (See 'Hemoglobinopathies' above.)

Hemoglobinopathies are protective in the heterozygous state, and heterozygosity is clinically well tolerated, but homozygosity or compound heterozygosity can cause serious disease. (See "Sickle cell trait" and "Hemoglobin variants including Hb C, Hb D, and Hb E" and "Diagnosis of thalassemia (adults and children)".)

Glucose-6-phosphate dehydrogenase (G6PD) protects RBCs against oxidant injury. G6PD deficiency is seen throughout malaria endemic regions of the world (figure 3). The mechanism by which G6PD deficiency provides protection against malaria appears to be an increase in oxidant stress in infected RBCs with enhanced phagocytosis of ring or early intraerythrocytic forms of the parasite. (See 'G6PD deficiency' above and "Genetics and pathophysiology of glucose-6-phosphate dehydrogenase (G6PD) deficiency" and "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency".)

Pyruvate kinase (PK) deficiency is protective in experimental murine malaria, but protection has not been demonstrated in human populations. (See "Pyruvate kinase deficiency", section on 'Possible protection from malaria'.)

ACKNOWLEDGMENT — UpToDate gratefully acknowledges Stanley L Schrier, MD (deceased), who contributed as Section Editor on earlier versions of this topic and was a founding Editor-in-Chief for UpToDate in Hematology.