Principles of cancer immunotherapy

INTRODUCTION — The fields of immunology and oncology have been linked since the late 19^th century, when the surgeon William Coley reported that an injection of killed bacteria into sites of sarcoma could lead to tumor shrinkage [1]. Since that time, exponential advances in the understanding of the intersection between immune surveillance and tumor growth and development have led to broad therapeutic advances that are now being studied in all cancer types.

The basic immunology and the various approaches to immunotherapy for tumors are discussed here. The details of immunology's role in specific malignancies are discussed in the relevant tumor-oriented topics, and the toxicity of checkpoint inhibitor immunotherapy is discussed separately. (See "Toxicities associated with immune checkpoint inhibitors".)

TUMOR IMMUNOLOGY

Cell types involved in tumor recognition and rejection — An efficient and specific cytotoxic immune response against a tumor requires a complex, rapidly evolving interaction between various immune cell types in the adaptive and innate immune system.

●CD8+ lymphocytes and Th1/Th2 subclasses of CD4+ T lymphocytes, traditionally referred to as cytotoxic T cells and helper T cells. CD8+ and CD4+ lymphocytes initiate the distinction between self and non-self-antigens, through recognition at the "immune synapse." (See 'The "immune synapse"' below.)

●Natural killer (NK) cells do not require antigen presentation by the major histocompatibility complex (MHC) for cytotoxic activity. In fact, NK cells target cells with low MHC class 1 expression for destruction. Like T cells, NK cells express numerous inhibitory molecules as well, most notably various killer immunoglobulin-like receptor (KIR) subtypes [2].

●Additional cell types, such as FoxP3+ CD25+ CD4+ T regulatory (Treg) and myeloid derived suppressor cells (MDSCs) largely inhibit cytotoxic T lymphocyte activity [3,4]. Th17 cells, subsets of CD4+ T cells that secrete interleukin (IL)-17, are implicated in autoimmunity and cancer [5].

●Macrophages differentiate into at least two different phenotypes: M1 macrophages, which release interferon (IFN) gamma and are responsible for phagocytosis, and M2 macrophages, which release cytokines such as IL-4, IL-10, transforming growth factor beta (TGF-beta), and dampen inflammatory responses and foster tolerance [6].

The "immune synapse" — The most widely studied phenomenon in immunologic surveillance is the ability of T lymphocytes to distinguish self- versus non-self-antigens, which are presented by antigen-presenting cells (APCs) such as dendritic cells. Overall, the cytotoxic activity of a CD8+ T cell is regulated by the presence and spatial orientation of a set of stimulatory and inhibitory receptors whose expression is regulated by a myriad of cytokines. Together, this configuration is often referred to as the "immune synapse" (figure 1):

The T cell receptor (TCR) complex consists of three major components: the TCR itself, the CD4 or CD8 receptor, and the CD3 molecule:

●CD4 or CD8 receptor – The CD4 or CD8 receptor binds to the MHC. The CD4/CD8 protein in most T cells consists of a highly variable alpha subunit linked to a beta subunit (ab). These variable regions of the CD4/CD8 molecule resemble the variable fragment (Fab) of an antibody and are responsible for the specificity of a specific T cell for a particular antigen.

●CD3 molecule – The CD3 molecule encodes a nonvariable transmembrane protein complex with an intracellular tyrosine-based activation component that relays surface signals to intracellular downstream effectors [7].

The TCR binds specific short stretches of amino acids presented by MHC molecules [8]. MHC class 1 is expressed by all nucleated cells and is recognized by CD8+ T cells, while MHC class 2 molecules are constitutively expressed by APCs and are recognized by CD4+ T cells.

For efficient activation of a naïve CD8+ T cell, its TCR must bind to a peptide presented by the MHC in the presence of a second set of costimulatory signals (figure 1). This interaction leads to CD3 intracellular signaling that causes secretion of pro-inflammatory cytokines such as IL-12 and IFN gamma. In the absence of a costimulatory signal, a state of peripheral tolerance to the antigen ("anergy") develops (figure 2) [9].

The most important costimulatory signal in naïve T cells is CD28, which binds to B7-1 and B7-2 (CD80/86) on the APC (figure 1). This costimulatory process is tightly regulated by both "agonist" molecules (eg, GITR, OX40, ICOS) and inhibitory signals on both the APC and T cells, often collectively referred to as "immune checkpoint" molecules. Examples of co-inhibitory or "immune checkpoint" molecules include cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), programmed cell death 1 (PD-1), TIM3, and LAG3. Chronic recognition of an antigen (such as that present in a malignant clone or in a chronic viral infection) may lead to feedback inhibition of effector T cell function, resulting in a phenotype termed "exhaustion" [10].

Tumor evasion of immune surveillance — The prevailing theory of the immune system's influence on neoplastic progression is termed "cancer immunoediting," which proceeds in three phases (figure 3) [11]:

●The elimination phase consists of innate and adaptive immune responses to specific tumor-associated antigens and is characterized by T, B, and NK cell effector function, which is mediated by cytokines such as IFN alpha, IFN gamma, and IL-12 [12,13].

●The equilibrium phase is a balance between immune-mediated destruction by the adaptive immune system (eg, activated CD4+ and CD8+ T cells) and persistence of rare malignant clones.

●Immunologic escape describes the phase where malignant clones have acquired the ability to evade the adaptive immune system.

There are several posited mechanisms for escape from immune surveillance [14]. Established mechanisms include:

●Loss or alteration of specific antigens or antigenic machinery [15,16]. Tumors can lose major MHC class 1 expression or the intracellular machinery required to transport tumor antigens to the tumor surface for T cell recognition [17-20].

●Tumors can promote an immune-tolerant microenvironment by manipulation of cytokines (increased secretion of IL-6, IL-10, and TGF-beta; consumption of IL-2) that encourage infiltration of Treg cells, myeloid derived suppressor cells (MDSCs), and other cell types that inhibit cytotoxic T cell function [19,21,22]. These cells can then actively suppress proliferation of CD4+ and CD8+ T lymphocytes that would otherwise recognize tumor antigens.

●Tumors can upregulate the expression of immune checkpoint molecules such as programmed cell death ligand 1 (PD-L1) that promote peripheral T cell exhaustion [23].

●Many oncogenic cell signaling pathways that were originally viewed as pure accelerators of cell division and growth are now understood to be mediators of immunologic escape. For example, constitutive KIT signaling in gastrointestinal stromal tumors leads to overexpression of indoleamine-2,3-dioxygenase (IDO), which enhances Treg infiltration that promotes tumor growth; this can be reversed in a CD8 T cell-dependent fashion with the KIT inhibitor, imatinib [24]. Melanomas with beta-catenin/Wnt signaling inhibit dendritic-cell mediated antigen presentation and exclude CD8+ T cell infiltration [25].

Understanding these mechanisms of immunologic escape can suggest mechanisms for immune-based therapies that may be broadly applicable across cancer types.

THERAPEUTIC APPROACHES — A number of therapeutic approaches are being studied to unleash the immune system and control malignancy. These approaches include cytokines, T cells (checkpoint inhibitors, agonism of costimulatory receptors), manipulation of T cells, oncolytic viruses, therapies directed at other cell types, and vaccines.

Cytokines — Initial approaches to immunotherapy harnessed the numerous downstream effects of cytokines and other substances that influence immune cell activity. Examples include:

●Interleukin (IL)-2 was initially discovered as T cell growth factor. IL-2 has pleiotropic effects on both cytotoxic T cell function as well as T regulatory (Treg) cell maintenance. The effects partially depend upon the dose and timing of IL-2 administration [26]. At higher doses, IL-2 promotes CD8+ effector T cell and natural killer (NK) cytolytic activity and promotes differentiation of CD4+ cells into T helper (Th)1 and Th2 subclasses [27]. At lower doses, IL-2 appears to preferentially expand Treg populations, probably due to the higher affinity of the trimeric IL-2 receptor (IL-2R, also known as CD25) on those cells, and inhibits the formation of Th17 cells implicated in autoimmunity [28-30].

Although IL-2 use has been largely supplanted by the use of checkpoint inhibitors, bolus, high-dose IL-2 achieved durable objective responses in a minority of patients with melanoma and renal cell carcinoma (RCC), serving as proof of principle that the immune system could eliminate cancer cells [31,32]. (See "Interleukin 2 and experimental immunotherapy approaches for advanced melanoma", section on 'Interleukin 2' and "Systemic therapy of advanced clear cell renal carcinoma", section on 'Interleukin 2 and other interleukins'.)

●Lenalidomide and pomalidomide are immunomodulatory agents that have prolonged survival in multiple myeloma. These agents mediate their antitumor effects largely via the cereblon-mediated destruction of Ikaros family proteins that inhibit IL-2 secretion [33-35].

●Interferon (IFN) alfa-2b promotes Th1-mediated effector cell responses such as IL-12 secretion via STAT-1 and STAT-2-mediated downstream signaling events [36,37]. IFN alfa has been used as adjuvant treatment of high-risk melanoma, although its long-term impact on overall survival is controversial. Subsequent data have established the role of immune checkpoint blockade as preferred adjuvant treatment [38,39]. (See "Adjuvant and neoadjuvant therapy for cutaneous melanoma".)

●Bacillus Calmette-Guerin (BCG), derived from attenuated mycobacterium bovis, induces a robust inflammatory response when injected in the bladder and is used for the treatment and secondary prevention of superficial bladder cancer [40]. (See "Treatment of primary non-muscle invasive urothelial bladder cancer".)

Checkpoint inhibitor immunotherapy

PD-1 and PD ligand 1/2 — Programmed cell death 1 (PD-1) is a transmembrane protein expressed on T cells, B cells, and NK cells (figure 4). It is an inhibitory molecule that binds to the programmed cell death ligand 1 (PD-L1; also known as B7-H1) and PD-L2 (B7-H2). PD-L1 is expressed on the surface of multiple tissue types, including many tumor cells, as well as hematopoietic cells; PD-L2 is more restricted to hematopoietic cells. The PD-1:PD-L1/2 interaction directly inhibits apoptosis of the tumor cell, promotes peripheral T effector cell exhaustion, and promotes conversion of T effector cells to Treg cells [41,42]. Additional cells such as NK cells, monocytes, and dendritic cells also express PD-1 and/or PD-L1.

In general, PD-1 and PD-L1/L2 are upregulated in the context of pro-effector cytokines such as IL-12 and IFN gamma, highlighting their role as a physiologic brake on unrestrained cytotoxic T effector function [43,44]. There are additional binding partners outside of the PD-1:PD-L1 axis; for example, PD-L1 has also been shown to inhibit CD80 [45], suggesting layers of interaction between cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), PD-1, and other pathways; these require further research to elucidate the context-dependent roles of each pathway in regulating T effector cell function.

Based upon prolonged overall survival in phase III trials and durable responses in phase I and II studies, antibodies inhibiting PD-1 (pembrolizumab, nivolumab, dostarlimab) and PD-L1 (atezolizumab, avelumab, durvalumab) have been approved for a number of clinical indications and are being evaluated in multiple other malignancies; these are discussed in specific disease-related topics.

CTLA-4 — Cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) was discovered in 1987 and implicated as a negative regulator of T cell activation in the mid-1990s [46-48]. CTLA-4 exerts its effect when it is present on the cell surface of CD4+ and CD8+ T lymphocytes, where it has higher affinity for the costimulatory receptors CD80 and CD86 (B7-1 and B7-2) on antigen-presenting cells (APCs) than the T cell costimulatory receptor CD28 (figure 5) [49]. The expression of CTLA-4 is upregulated by the degree of T cell receptor (TCR) activation and cytokines such as IL-12 and IFN gamma, forming a feedback inhibition loop on activated T effector cells. As a result, CTLA-4 can be broadly considered a physiologic "brake" on the CD4+ and CD8+ T cell activation that is triggered by APCs.

CTLA-4 was initially implicated in immune surveillance of cancer when inhibition of CTLA-4 in mouse models of sarcoma and colon adenocarcinoma led to tumor shrinkage [50]. The anti-CTLA-4 antibody ipilimumab was the first immune checkpoint inhibitor to be approved based upon its ability to prolong survival in patients with metastatic melanoma [51]. Ipilimumab has also been approved as adjuvant therapy for high-risk melanoma as an alternative to IFN.

LAG3 — Lymphocyte activation gene 3 (LAG3), which is expressed by B cells, some T cells, NK cells, and tumor-infiltrating lymphocytes (TILs), is used to regulate immune checkpoint pathways [52]. The LAG3 protein enhances Treg activity by binding major histocompatibility complex (MHC) class II and hampering T cell differentiation and proliferation [53]. LAG-3 blocking antibodies restore the effector function of exhausted T cells and increases their ability to attack tumor cells.

The combination of relatlimab, a LAG3-blocking antibody, and PD-1 blockade with nivolumab (nivolumab-relatlimab) has been evaluated in advanced and metastatic melanoma. (See "Systemic treatment of metastatic melanoma lacking a BRAF mutation", section on 'Nivolumab-relatlimab'.)

Other potential targets — Increased understanding of the underlying immunologic mechanisms is leading to the identification of several additional potential targets for checkpoint inhibition. Examples of these include the following, although all are in early stage clinical development:

●BTLA – B and T cell lymphocyte attenuator (BTLA) is a ligand of herpes virus entry mediator (HVEM) whose interaction leads to decreased production of cytokines and cell proliferation by CD4+ T cells [54,55]. It is expressed on B, T and NK cells as well as APCs. BTLA is induced during T cell activation, with persistent expression in TH1 but not TH2 cells [54]. Blockade of BTLA has been shown to enhance NY-ESO-1 specific CD8+ T cell function and add to anti-PD-1 efficacy [56].

●VISTA – V-domain Ig suppressor of T cell activation (VISTA), as connoted by its name, shares homology with PD-L1 and is a negative checkpoint ligand [57-59]. It is found in hematopoietic tissues and T cell-infiltrated structures, including tumors [57]. VISTA blockade has been shown to increase T cell infiltration and function in tumors, thereby reducing tumor growth [59].

●TIM-3 – T cell immunoglobulin and mucin domain 3 (TIM-3) is expressed by dendritic cells, monocytes, CD8 T cells, and T-helper-1 (Th1) cells [60,61]. When TIM-3 binds galectin-9, its ligand that is often found on tumors, this causes TH1 cell death; conversely, TIM-3 blockade causes TH1 cell hyperproliferation and cytokine release [62,63]. In combination with anti-CTLA4 or anti-PD-1, TIM-3 blockade led to tumor shrinkage in a mouse model [64].

●CD47 – The antigen CD47 may be expressed on tumor cells, protecting them from phagocytosis by macrophages, and is therefore a potential target for anticancer therapy. In a phase Ib study of 22 heavily pretreated patients with relapsed or refractory non-Hodgkin lymphoma (NHL; 15 with diffuse large B cell lymphoma and 7 with follicular lymphoma), Hu5F9-G4 (a humanized anti-CD47 monoclonal antibody) in combination with rituximab was associated with objective responses in half, including complete response in more than one-third [65]. Adverse events were generally mild. This study suggests that targeting CD47 may enhance the activity of tumor-directed antibodies in patients with NHL.

Agonism of costimulatory receptors — Multiple costimulatory receptors are involved in the immune response to tumors, and hence are potential targets for cancer immunotherapy. The following are either being studied in preclinical animal models or are in early phases of clinical development as noted:

●4-1BB (CD137) – 4-1BB is expressed on activated T cells, activated NK and NKT cells, as well as being expressed constitutively on some dendritic and Treg populations. When stimulated by 4-1BBL, its natural ligand, or agonist antibodies, this promotes activity of T cells, dendritic cells, monocytes, and neutrophils [66-68]. Preclinical data show tumor control with agonist anti-4-1BB antibodies alone [69,70] and in combination with other modalities [71-73].

A 4-1BB agonist antibody urelumab (BMS-663513) was originally developed as monotherapy, but development was suspended due to fatal liver toxicity when given at higher doses [74]. Urelumab is now being studied at lower doses both as monotherapy and in development with other agents. Another 4-1BB agonist antibody, PF-05082566, has also shown clinical activity in a phase I trial without toxicity at lower dose levels [75].

●OX40 (CD134) – OX40 is expressed on activated CD4+ T cells one to two days after activation, as well as on CD8+ T cells, dendritic cells, neutrophils, and Tregs [76,77]. OX40 is stimulated by the OX40 ligand (OX40L) found on APCs and activated T cells. Interaction with an agonist antibody can also reverse regulatory T cells' suppressive function [77-80]. In preclinical models, OX40 agonism has antitumor activity on its own [81] and in combination with various other chemo- and immunotherapies [82-85]. Studies of anti-OX40 and anti-OX40L antibodies are ongoing.

●GITR (CD357) – Glucocorticoid-induced tumor necrosis factor (TNF)-like receptor (GITR) expression increases on CD4+ and CD8+ T cells one to three days after stimulation and is then sustained. T cell proliferation and effector function, as well as prevention of activation-induced cell death (AICD), results from GITR stimulation [86,87]. GITR may also play a role in leukocyte adhesion and transmigration [88]. Furthermore, GITR can reverse suppression by Tregs [89,90], as well as induce memory, contributing to protection from tumor rechallenge in mouse models [91]. Administration of the murine anti-GITR antibody DTA-1 impairs Treg function and improves antitumor activity of cytotoxic T cells [92,93].

●ICOS – Inducible T cell co-stimulator (ICOS) is expressed on activated T cells and has multiple functions, including a role in isotype switching, germinal center formation, and effector and regulatory CD4+ T cell responses [94]. ICOS ligand engagement of ICOS in combination with CTLA-4 blockade enhanced efficacy of the latter [95].

●CD40 – CD40 is a costimulatory molecule present on APC and necessary for dendritic cell activation that binds its ligand, CD40L, expressed on T helper cells [96]. Two agents, dacetuzumab and lucatumumab, have been studied in hematologic malignancies.

●CD28 – CD28 is constitutively expressed on T cells and is a costimulatory receptor for CD80 (B7.1) and CD86 (B7.2). In a phase I trial in which CD28 was targeted by a monoclonal antibody, TGN1412, the six treated patients all suffered organ damage and critical illness secondary to cytokine release [97]. CD28 is used as a stimulator of T cells ex vivo, but targeted antibodies are no longer under development due to toxicity.

Combination immune checkpoint blockade strategies — Checkpoint inhibitor immunotherapy agents have regulatory approval for multiple clinical indications. Concurrent PD-1 and CTLA-4 blockade (eg, nivolumab plus ipilimumab) is furthest along in standard clinical use. As examples:

●Metastatic melanoma – In patients with metastatic melanoma, the combination of nivolumab plus ipilimumab (figure 6) improved overall survival compared with ipilimumab monotherapy in metastatic melanoma. Further details are discussed separately. (See "Systemic treatment of metastatic melanoma lacking a BRAF mutation", section on 'Nivolumab plus ipilimumab (preferred)'.)

●Renal cell carcinoma – Similarly, in patients with treatment-naïve RCC, the combination of nivolumab plus ipilimumab improved overall survival compared with sunitinib. Further details of this study are discussed separately. (See "Systemic therapy of advanced clear cell renal carcinoma", section on 'Nivolumab plus ipilimumab'.)

●Non-small cell lung cancer – In patients with advanced non-small cell lung cancer (NSCLC), nivolumab plus ipilimumab also improved overall survival compared with chemotherapy. Further details are discussed separately. (See "Initial management of advanced non-small cell lung cancer lacking a driver mutation", section on 'Nivolumab plus ipilimumab, with or without chemotherapy'.)

●Other malignancies – Nivolumab plus ipilimumab also has clinical efficacy in hepatocellular carcinoma and microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR) colorectal cancer. (See "Systemic treatment for advanced hepatocellular carcinoma", section on 'Nivolumab plus ipilimumab' and "Tissue-agnostic cancer therapy: DNA mismatch repair deficiency, tumor mutational burden, and response to immune checkpoint blockade in solid tumors", section on 'Nivolumab with or without ipilimumab'.)

Based on these data, the combination of nivolumab plus ipilimumab has been approved by the US Food and Drug Administration (FDA) for melanoma, RCC, and NSCLC, among other malignancies. Of note, the rate of grade ≥3 adverse events was higher with combined nivolumab plus ipilimumab versus nivolumab or ipilimumab monotherapy. (See "Toxicities associated with immune checkpoint inhibitors".)

Clinical trials combining multiple other immunotherapy targets are being investigated in multiple cancers.

Manipulating T cells — Adoptive T cell transfer broadly refers to the practice of manipulating patient-specific T cells ex vivo to make them more reactive to specific antigens.

CAR-T cells — Chimeric antigen receptor (CAR)-T cells are genetically modified autologous immunotherapy. The patient's own (autologous) T cells are collected from blood and modified ex vivo to express a tumor antigen-specific CAR (specifically, the antigen-binding domain from a B cell receptor that is fused to the intracellular domain of a CD3 TCR [CD3-zeta]). Following ex vivo expansion, the CAR-T cells are then reinfused back into the patient. As a result of this therapy, recognition of a specific tumor/cell surface antigen activates T cell response independently of MHC recognition. Various modifications can enhance CAR-T cell effector function (eg, co-expression of intracellular costimulatory domains such as CD28 or 4-1BB [CD137] or pro-effector cytokines such as IL-12) [98,99].

CAR-T cell therapy is used to treat certain hematologic malignancies including diffuse large B cell lymphoma (DLBCL), acute lymphoblastic leukemia (ALL), and multiple myeloma. Examples that are used in clinical practice include CAR-T cells directed against CD19 (axicabtagene ciloleucel, brexucabtagene autoleucel, lisocabtagene maraleucel, loncastuximab tesirine, tisagenlecleucel) and BCMA (ciltacabtagene autoleucel, idelcabtagene vicleucel). (See "Diffuse large B cell lymphoma (DLBCL): Suspected first relapse or refractory disease in patients who are medically fit" and "Treatment of relapsed or refractory acute lymphoblastic leukemia in adults" and "Multiple myeloma: Treatment of third or later relapse".)

Side effects are substantial in certain patients and include signs of the cytokine release syndrome (CRS) such as fever, hypotension, altered mental status, and seizures, with some patients requiring intensive care. (See "Cytokine release syndrome (CRS)".)

Ex vivo expansion of tumor-infiltrating lymphocytes — Tumor-infiltrating lymphocytes (TILs) represent an immune cell population that recognizes tumor antigen but may have developed an exhausted phenotype due to the tumor microenvironment.

Ex vivo expansion of TILs utilizes freshly resected tumor tissue to extract TILs and co-culture with IL-2 to stimulate in vitro TIL expansion. Prior to reinfusion of expanded TILs, the patient receives nonmyeloablative chemotherapy regimens such as cyclophosphamide or total body irradiation, which functions to deplete inhibitory Treg cells and other lymphocytes in the patient to improve the rate of in vivo expansion of the stimulated TILs [100]. The in-vitro-stimulated TILs, largely comprised of CD8+ and to a lesser extent CD4+ T lymphocytes, are then reintroduced into patients at high doses, together with HD IL-2, where they can recognize specific tumor antigens in a microenvironment that is now less prone to induce tolerance [101].

In a series of highly selected patients with advanced melanoma prior to the era of checkpoint inhibition, 56 percent of those who received the T cell infusion had an objective response [102]. The major limitations of this approach are that it requires fresh tissue, the expansion process that takes several weeks, and not all TILs are guaranteed to expand in vitro. (See "Interleukin 2 and experimental immunotherapy approaches for advanced melanoma".)

Adoptive cell therapy with the TIL product lifileucel (LN-144) has demonstrated clinical efficacy in patients with melanoma refractory to prior PD-1 inhibitor therapy and, in some cases, BRAF plus MEK inhibitor therapy. (See "Systemic treatment of metastatic melanoma with BRAF and other molecular alterations", section on 'Adoptive cell therapy'.)

Lifileucel is also being investigated in clinical trials for several other diseases, including cervical carcinoma, and in combination with PD-1 blockade.

CD3-directed therapies

Bispecific T cell engagers — Conceptually, bispecific T cell engager antibodies function as linkers between T cells and specific target antigens in an MHC-subtype independent manner. They consist of a protein fragment containing two separate single-chain variable regions. One end recognizes CD3, which is expressed on all T cells, and one end recognizes the target antigen. Bispecific antibodies thus aim to induce cytotoxic T cell-mediated tumor eradication.

Because bispecific antibodies are not MHC-specific, they can be administered to all patients regardless of human leukocyte antigen (HLA) type and do not require patient-specific processing. One consequence of this more broadly applicable approach is its relative lack of specificity in T cell recruitment when compared with the more labor-intensive method of adoptive T cell transfer. Because many different T cell subtypes express CD3, bispecific antibodies recruit polyclonal cytotoxic T cells, Th1 and Th2 CD4+ cells, and Tregs.

Examples of bispecific antibodies used in clinical practice include:

●Blinatumomab – Blinatumomab has specificity for CD19 antigen found on many B cell malignancies and the Fc region of the CD3 receptor found on T lymphocytes. Blinatumomab is used to treat relapsed or refractory Philadelphia-chromosome negative and Philadelphia-chromosome positive B-ALL. (See "Treatment of relapsed or refractory acute lymphoblastic leukemia in adults", section on 'Blinatumomab'.)

●Teclistamab – Teclistamab has specificity for B cell maturation antigen (BCMA) and the CD3 receptor on T-cells. This BiTE is used to treat refractory multiple myeloma. (See "Multiple myeloma: Treatment of third or later relapse", section on 'Bispecific antibodies'.)

Immune-mobilizing monoclonal TCRs against cancer — Immune-mobilizing monoclonal TCRs against cancer (ImmTACs) are similar to bispecific antibodies in the sense that they aim to link T cells and specific target antigens. Unlike bispecific antibodies, however, they utilize an HLA-A specific engineered MHC class 1 molecule on one end and a single-chain variable region on the other end. This can theoretically target both intracellular and extracellular expressed proteins like monoclonal TCRs. (See 'Monoclonal TCRs' below.)

One clinical example of an ImmTAC is tebentafusp, an HLA-A 02:01 restricted agent targeting the melanocytic antigen gp100 with a CD3 variable chain fragment. Tebentafusp is used to treat metastatic uveal melanomas. (See "Metastatic uveal melanoma", section on 'Tebentafusp'.)

Clinical trials are ongoing investigating the use of ImmTACs for additional targets across various cancer types.

Monoclonal TCRs — Another approach to increasing effector T cell function against a particular antigen is engineering a soluble TCR (CD8) to recognize a particular antigen target and fusing this to the variable fragment that recognizes an effector target, such as CD3 [103]. The ability to engineer a TCR rather than an antibody fragment can lead to higher affinity for a given peptide chain and allow for targeting of intracellular peptide fragments [104]. This approach must be engineered using a specific MHC class 1 molecule, and complications have occurred through TCR cross-recognition of other antigens [105]. MHC A*02-restricted TCRs are furthest along in clinical development because these are the most common alleles (50 percent) in people of Western European descent. See the figure for a simplified schematic representation of the key differences in the above four T cell-directed therapies (figure 7).

Oncolytic viruses — Oncolytic viruses mediate antitumor effects in several ways. Viruses can be engineered to efficiently infect cancer cells preferentially over normal cells, to promote presentation of tumor-associated antigens, to activate "danger signals" that promote a less immune-tolerant tumor microenvironment, and to serve as transduction vehicles for expression of immune modulatory cytokines [106]. (See "Overview of gene therapy, gene editing, and gene silencing", section on 'Cancer therapy'.)

Talimogene laherparepvec (T-VEC) utilizes an attenuated herpes simplex virus 1 virus to overexpress granulocyte macrophage colony-stimulating factor (GM-CSF), which promotes dendritic cell mediated antigen presentation. T-VEC improved durable response rates compared with intratumoral GM-CSF alone in patients with unresectable, injectable advanced melanoma. This agent is approved by the US FDA for unresectable or advanced melanoma with an injectable skin or lymph node metastasis with limited visceral disease. Further data on the clinical efficacy of T-VEC in this patient population are discussed separately. (See "Cutaneous melanoma: Management of local recurrence", section on 'Talimogene laherparepvec'.)

The combination of oncolytic viruses such as T-VEC with immune checkpoint inhibitors is discussed separately. (See "Systemic treatment of metastatic melanoma lacking a BRAF mutation", section on 'Investigational agents'.)

Numerous other virus backbones are under clinical or preclinical investigation, including adenovirus [107-110], reovirus [111], Newcastle disease virus [112], and others.

Therapies directed at other cell types in tumor microenvironment — In tumor immunology, cell types other than tumor-specific and circulating T cells contribute to an effective versus a suppressed immune response, and thus represent additional targets for immunotherapy beyond T cells.

Natural killer cells — The biology of natural killer (NK) cells is complex [113]. NK cell infiltrates in solid tumors, and metastases have been associated with an improved prognosis [114-116], although the use of CD56 or CD57 markers also expressed on T cell subsets (eg, CD8+ T cells) and some tumors may somewhat confound these findings. The killer immunoglobulin-like receptor (KIR) genes are expressed by NK cells and bind to HLA molecules on normal host cells. Cancer cells, which often lose HLA expression, are recognized by NK cells as missing their HLA molecules and are consequently destroyed by the NK cells [113].

Anti-KIR antibodies have shown preclinical efficacy in lymphoma [117], multiple myeloma [118], and acute myelogenous leukemia (AML) [119]. In clinical trials, a phase I study of IPH2101 was safe in patients with AML in first remission [120]. There were no objective responses as a single agent in patients with relapsed/refractory multiple myeloma [121], but results were more promising when administered in conjunction with lenalidomide in this population [122]. Lirilumab (BMS986015) is the other anti-KIR antibody undergoing early stage trials alone and in combination in hematologic malignancies and solid tumors.

Macrophages — The presence of intratumoral macrophages can portend a poor prognosis. Although an oversimplification, the general categorization of macrophages into classically activated phenotype (M1) and alternatively activated (M2) suggests that in the context of malignancy, M2 macrophages play a pro-tumoral role due to their involvement in immunosuppression, angiogenesis, and tumor cell activation [123].

Intratumoral macrophages are largely recruited by C-C chemokine ligand 2 (CCL2) or colony-stimulating factor 1 (CSF-1), and pre-clinical and clinical data have focused on targeting the CSF-1/CSF-1 receptor axis. The antitumor impact of CSF-1 receptor (CSF-1R) inhibition in pre-clinical models varies [124], but there are promising data in combination with other modalities such as chemotherapy [125,126], radiation therapy [127], angiogenic inhibitors [128], adoptive cell transfer [129], as well as when used in conjunction with CTLA-4 and PD-1 blockade in the challenging setting of pancreatic ductal adenocarcinoma [130].

Examples of CSF-1R inhibitors with clinical activity include emactuzumab (RG7155) and pexidartinib (PLX3397) [131-133]. Pexidartinib has regulatory approval for the treatment of unresectable tenosynovial giant cell tumor, a locally aggressive neoplasm that overexpresses CSF-1. (See "Treatment for tenosynovial giant cell tumor and other benign neoplasms affecting soft tissue and bone", section on 'Pexidartinib (CSF1R inhibitor)'.)

IDO — Indoleamine 2,3-dioxigenase 1 (IDO1) catalyzes the rate-limiting step in the conversion from the essential amino acid L-tryptophan (Trp) into L-kynurenine (Kyn). IDO1 expression by tumors can promote evasion of immune surveillance by suppressing T cell function [134] and impairing immune surveillance [135].

Although potentially promising activity was seen in early phase trials [136], a randomized phase 3 study of pembrolizumab with or without epacadostat did not show benefit compared with the combination [137]. This negative result has resulted in diminished interest in clinical development of IDO inhibitors.

Vaccines — There is a long history of attempting to harness the adaptive immune recognition of a cancer-related antigen to effect antitumor responses. Vaccine methods range widely, and a full review is outside the scope of this article.

A simplistic way to view vaccine development method is that varying types of antigens, administration schedules, and accompanying immune adjuvants can influence an adaptive immune response. Antigen choices range from simple peptides, which are easy to administer but affect a narrow antigen spectrum and are often restricted by specific HLA class 1 molecule expression that allows efficient antigen presentation, to whole cell preparations that offer a broader range of antigens but are more costly and time-consuming to prepare [138].

The only approved vaccine-based therapy for advanced cancer is sipuleucel-T, which is an autologous dendritic-cell preparation engineered to target prostatic acid phosphatase (PAP) that demonstrated an overall survival benefit in men with castrate-resistant prostate adenocarcinoma [139]. Patient blood undergoes leukapheresis and is exposed ex vivo to PAP fused to GM-CSF. Theoretically, GM-CSF fosters maturation of dendritic cells and other APCs to present PAP to the patient's T cells, which then recognize the PAP. However, the degree of PAP-specific T cell proliferation at week 6 did not correlate with survival in the study, suggesting that additional immunologic mechanisms may explain this survival benefit. (See "Immunotherapy for castration-resistant prostate cancer", section on 'Therapeutic vaccination with Sipuleucel-T'.)

Single-peptide vaccines continue to be tested extensively, especially in "immunogenic" cancers such as melanoma. They have largely shown disappointing efficacy in preventing recurrence or prolonging survival [138]. As an example, in a randomized phase III trial of 185 patients, those who received the combination of IL-2 plus the HLA-A*0201- MHC-specific vaccine against the surface glycoprotein gp100 had a higher response than those who received IL-2 alone (22 versus 10 percent), and there was a nonsignificant trend toward improved survival (17 versus 11 months) [140]. However, in the randomized clinical trial that demonstrated a survival benefit for ipilimumab with or without this gp100 vaccine, the vaccine did not improve survival over ipilimumab alone [141].

Given the increasing understanding of the importance of immune recognition of multiple patient-specific, tumor-specific antigens, efforts to develop therapeutic vaccines against cancer are beginning to explore the use of individualized pooled antigens. This suggests that patient-specific vaccination approaches may be feasible, particularly in immunogenic tumors such as melanoma, non-small cell lung cancer, mismatch-repair deficient colorectal carcinoma, and bladder carcinoma [142,143].

IMMUNOTHERAPY RESPONSE CRITERIA

Patterns of response — Evaluation of the effectiveness of immune checkpoint inhibitors and other forms of immunotherapy requires an understanding of the potentially different patterns of response seen with these classes of agents [144]. The patterns of response to treatment with these immunotherapy agents can differ from those with molecularly targeted agents or cytotoxic chemotherapy in several important respects [145]:

●Some patients may experience pseudoprogression, which is a transient worsening of disease, manifested either by progression of known lesions or the appearance of new lesions, before the disease stabilizes or tumor regresses (image 1). Therefore, some caution should be taken in abandoning therapy early. However, these delayed responses are generally not observed in patients with symptomatic deterioration [146], so continuing therapy beyond progression is not recommended in these patients.

●Hyperprogressive disease is a rare event where patients experience an unexpected, accelerated pace of disease following initiation of checkpoint inhibitor immunotherapy [147]. It results in dramatic progression of tumor growth that is measurable on imaging and associated with worsening disease-related symptoms and biomarkers. The mechanism of hyperprogressive disease is unclear, and it is not known whether hyperprogression reflects unexpected tumor biology or is specific to the immune checkpoint inhibitor therapy administered. Patients who present with hyperprogressive disease should be treated similarly to those with true progressive disease and switched to alternative therapy.

●Responses can take longer to become apparent compared with cytotoxic therapy.

●Some patients who do not meet criteria for objective response can have prolonged periods of stable disease that are clinically significant.

Response criteria — Immune-related response criteria (irRC) have been proposed to properly recognize the nontraditional patterns of response occasionally seen with checkpoint inhibitors and some other immunotherapies [145].

●Immune-related complete response – Complete resolution of all measurable and nonmeasurable lesions, with no new lesions. A complete response must be confirmed by a second, consecutive assessment at least four weeks later.

●Immune-related partial response – A decrease in the total tumor burden of 50 percent or more compared with baseline, which must be confirmed by a second, consecutive assessment at least four weeks later. This category allows for the inclusion of progression of some lesions or the appearance of new lesions as long as the total tumor burden meets the response criterion.

●Immune-related stable disease – Not meeting the criteria for either a partial or complete response or for progressive disease.

●Immune-related progressive disease – An increase in tumor burden of 25 percent or more relative to the minimum recorded tumor burden. This must be confirmed by a second, consecutive assessment no fewer than four weeks after the initial documentation of an increase in tumor.

Use of these immune-related response criteria is important because the application of traditional Response Evaluation Criteria In Solid Tumors (RECIST) criteria in patients treated with checkpoint inhibitors may lead to premature discontinuation of treatment in a patient who will eventually respond to treatment or have prolonged disease.

Consensus-based criteria for response to immunotherapy (iRECIST) have been developed for use in trials testing immunotherapy (table 1) [148]. They may also be applicable to patients receiving immunotherapy in a non-trial setting.

These criteria are based upon RECIST 1.1, with some modifications, and are prefixed with an "i" (immune). These iRECIST criteria build upon the previously described response criteria (irRC), which were also based upon RECIST and World Health Organization (WHO) guidelines for response assessment in patients treated with chemotherapy [145]. The major modifications are:

●The definitions of measurable and nonmeasurable disease; numbers and sites of target disease are the same as for RECIST 1.1.

●The biggest difference from RECIST 1.1 (and similar to the irRC) is that the development of new lesions during therapy is classified as immune unconfirmed progressive disease (iUPD); immune confirmed progressive disease (iCPD) is only assigned if at the next assessment, additional new lesions appear or there is an increase in the size of the new lesions (≥5 mm for the sum of the new lesion targets or any increase in a new lesion nontarget); the appearance of new lesions when none have previously been recorded can also confirm iCPD.

●The response assignment categories are immune complete response (iCR), immune partial response (iPR), iUPD, iCPD, and immune stable disease (iSD).

●Time point responses are defined according to whether or not there was a prior iUPD in any category (table 2).

Immune-modified Response Evaluation Criteria In Solid Tumors (imRECIST) have also been developed to utilize a unidimensional measurement system based upon the RECIST 1.1 system (table 3) [149]. The imRECIST may offer advantages compared with RECIST by recognizing the potential benefits from treatment in patients who have a transient progression after initiation of immunotherapy.

In clinical practice, patients receiving any immune-based therapy and whose tumors show initial growth should be assessed carefully for signs and symptoms of clinical benefit or progression; the majority of patients will have true progressive disease [150]. In the absence of symptomatic progression, however, a short-term repeat scan is reasonable prior to considering immune-based therapy a failure.

PREDICTORS OF RESPONSE TO IMMUNE-BASED THERAPY — As immune checkpoint blockade and other immune-based therapy approaches lead to broad treatment advances among patients with advanced cancer, an important consideration is how to best select patients whose tumors will respond to these therapies.

Programmed cell death ligand 1 — Programmed cell death ligand 1 (PD-L1) is a biomarker that has been extensively studied in clinical trials evaluating immune checkpoint inhibitors. There is no absolute value for PD-L1 that can be assigned to an individual patient. The thresholds that separate "positive" and "negative" PD-L1 expression can vary greatly depending on the antibody used, the tissues being examined for PD-L1 expression, and the tumor type. Furthermore, PD-L1 is often heterogeneously expressed; it is also a dynamic marker that can change in relation to local cytokines and other factors. Despite these major limitations, the majority of clinical trials have shown an association between PD-L1 "positive" tumors and clinical response to PD-1 pathway blockade [151-154]. (See 'PD-1 and PD ligand 1/2' above.)

Diagnostic tests — Diagnostic tests for PD-L1 include:

●PD-L1 immunohistochemistry assays – PD-L1 expression can be detected using immunohistochemistry (IHC) assays for various antibody clones to the PD-L1 protein. PD-L1 IHC 22C3 and PD-L1 IHC 28-8 are approved by the US Food and Drug Administration (FDA) as companion diagnostic for programmed cell death 1 (PD-1)/PD-L1 targeted immunotherapies [155]. Other commercially available PD-L1 IHC assays include SP142 and SP263 [156].

●Combined positive score – Combined positive score (CPS) is defined as the total number of tumor cells and inflammatory cells (ie, lymphocytes and macrophages) that are positive for PD-L1 on IHC divided by the total number of viable tumor cells, then multiplied by 100 [157].

●Tumor proportion score – Tumor proportion score (TPS) is defined as the number of total tumor cells that are positive for PD-L1 on IHC divided by the total number of viable tumor cells, then multiplied by 100 [157].

Use of PD-L1 biomarker by tumor type — Programmed cell death ligand 1 (PD-L1) is used as a predictive biomarker to determine the need for immunotherapy in various malignancies. However, each tumor type can vary for the specific type of assay, cell expression quantification, and threshold for use, which are discussed separately.

●Non-small cell lung cancer (see "Initial management of advanced non-small cell lung cancer lacking a driver mutation")

●Breast cancer (see "ER/PR negative, HER2-negative (triple-negative) breast cancer")

●Head and neck cancer (see "Treatment of metastatic and recurrent head and neck cancer")

●Gastric cancer (see "Second and later-line systemic therapy for advanced unresectable and metastatic esophageal and gastric cancer")

●Esophageal cancer (see "Initial systemic therapy for locally advanced unresectable and metastatic esophageal and gastric cancer")

●Cervical cancer (see "Management of recurrent or metastatic cervical cancer")

●Bladder cancer (see "Treatment of metastatic urothelial carcinoma of the bladder and urinary tract")

●Melanoma and renal cell carcinoma – For melanoma and renal cell carcinoma (RCC), the absolute value of PD-L1 expression used to select patients for treatment with immune checkpoint inhibitors is not established, as significant treatment responses to immune checkpoint inhibitors are also seen in these tumors that are negative for PD-L1 expression. (See "Systemic treatment of metastatic melanoma with BRAF and other molecular alterations", section on 'Tumor mutational burden' and "Systemic therapy of advanced clear cell renal carcinoma".)

Tumor mutational burden — Tumor mutational burden (TMB) is another predictive biomarker. Tumors with high levels of TMB (eg, sun-exposed cutaneous melanoma, NSCLC, bladder cancer, and microsatellite-unstable colorectal carcinomas) can benefit from immunotherapy such as pembrolizumab, among other agents. This approach is discussed in more detail separately. (See "Tissue-agnostic cancer therapy: DNA mismatch repair deficiency, tumor mutational burden, and response to immune checkpoint blockade in solid tumors".)

Additional gene-expression-based signatures for immune response are also under active investigation [158].

SUMMARY

●Tumor immunology – The pace of discovery in the fields of immunology and cancer biology is rapidly accelerating. Continued progress in the treatment of malignancy is likely as our understanding evolves regarding the role of the immune system in tumor initiation, progression, and metastasis. (See 'Tumor immunology' above.)

●Therapeutic approaches – Checkpoint inhibitor immunotherapy has become a primary treatment modality for patients with a broad diversity of cancers, and prolongs survival in some malignancies. Clinical trials exploring a wide variety of immunotherapy combinations in other malignancies are in progress. (See 'Therapeutic approaches' above.)

●PD-L1 biomarkers – Programmed cell death ligand 1 (PD-L1) is used as a predictive biomarker to determine the need for immunotherapy in various malignancies. Diagnostic tests include PD-L1 immunohistochemistry (IHC) assays, which are generally reported as a combined positive score (CPS) or tumor proportion score (TPS).

ACKNOWLEDGMENTS — The UpToDate editorial staff acknowledges Michael Postow, MD; Alexandra Snyder Charen, MD; Jedd Wolchok, MD, PhD; and Matthew Hellmann, MD, who contributed to earlier versions of this topic review.