Evaluation and treatment of antibody-mediated lung transplant rejection

INTRODUCTION — Acute allograft rejection is a significant problem in lung transplantation; it is responsible for approximately 4 percent of deaths in the first 30 days following transplantation [1,2]. Despite advances in induction and maintenance immunosuppression regimens, more than a third of lung transplant recipients are treated for acute rejection in the first year after transplant [1,3]. The role of the humoral immune system in acute rejection of lung allografts is a source of ongoing investigation and debate [3-7].

The clinical manifestations, evaluation, treatment, and routine monitoring of antibody-mediated lung transplant rejection will be reviewed here. The immunobiology of transplantation, antibody-mediated rejection in other organ allografts, maintenance immunosuppression after lung transplantation, acute cellular lung transplant rejection, and chronic lung transplant rejection are discussed separately. (See "Transplantation immunobiology" and "Maintenance immunosuppression following lung transplantation" and "Evaluation and treatment of acute cellular lung transplant rejection" and "Chronic lung allograft dysfunction: Bronchiolitis obliterans syndrome" and "Kidney transplantation in adults: Prevention and treatment of antibody-mediated rejection".)

DEFINITIONS

Antibody-mediated rejection — Antibody-mediated rejection (AMR) of lung allografts is believed to be mediated by donor specific antibodies (DSA) against human leukocyte antigens (HLA) and other donor antigens. These antibodies may have been present in the recipient prior to transplant, although most appear to develop after transplantation.

Hyperacute rejection — Hyperacute rejection is a fulminant form of rejection occurring within minutes or hours of reperfusion of the allograft and is caused by preformed DSA [8]. Hyperacute rejection has become a rare form of rejection because screening for HLA antibodies has become more sensitive and more specific, thereby preventing HLA incompatible transplantation.

Acute antibody-mediated rejection — Acute AMR is thought to be a consequence of DSA that were present at a low titer prior to or developed after transplantation; DSA can cause clinical disease in the transplanted lung in the weeks to months following implantation. However, the development of DSA is not necessarily specific for AMR, as DSA are also associated with the development of acute cellular rejection and an increased risk of bronchiolitis obliterans [9].

Acute cellular rejection — Acute cellular rejection is the predominant type of acute lung transplant rejection and is mediated by T lymphocyte recognition of foreign major histocompatibility complexes (MHC), also known as HLA [3]. (See "Evaluation and treatment of acute cellular lung transplant rejection".)

EPIDEMIOLOGY — Hyperacute rejection has become rare in lung transplantation due to the use of virtual crossmatches prior to transplantation [10].

The exact frequency of acute antibody-mediated rejection (AMR) is more difficult to ascertain, as exact diagnostic criteria have not been agreed upon. In a single center series, acute antibody-mediated rejection was detected in 21 (4 percent) of 501 lung transplant recipients [11]. The median time of onset was 258 days (mean 364 +/- 402 days) after transplantation. Patients developing AMR tended to be slightly younger than those without; age 42 (+/- 16.2) years compared with 51.4 (+/- 14.1) years. Otherwise, recipients with or without AMR did not differ in their sex, pretransplant diagnosis, type of transplant (single, bilateral, or heart-lung), or cytomegalovirus seropositivity [11].

HYPERACUTE REJECTION — Hyperacute rejection is a fulminant form of rejection occurring within minutes to hours of reperfusion of the allograft and is caused by preformed donor-specific antibodies (DSA). The DSA have most commonly been directed at donor human leukocyte antigens (HLA) but may be directed at donor ABO blood group or endothelial antigens.

Pathology and pathogenesis — The pathologic findings described in hyperacute rejection include acute lung injury with neutrophilic infiltration and platelet and fibrin thrombi in alveolar septae with concomitant fibrinoid necrosis and hemorrhage (picture 1 and picture 2) [1,3,12-14]. The predilection of neutrophilic infiltration and fibrinoid necrosis of vessels in the alveolar septa illustrates that alveolar capillaries are the primary targets of antibody-mediated injury in hyperacute rejection. It is thought that antibody binding to its specific HLA on endothelial cells activates complement, which ultimately results in endothelial cell lysis. Endothelial cell damage results in exposure of the basement membrane and activation of the coagulation cascade leading to thrombosis and infarction. In addition, complement activation leads to the chemotaxis of neutrophils and macrophages, which propagate the acute lung injury.

Complement component deposition (C4d) in the capillary endothelium demonstrates direct immunopathological evidence for the action of antibodies. However, complement deposition is not necessarily specific for AMR. In some reports, deposition of C3d and C4d is present in lung biopsies from patients with infection and primary graft dysfunction without evidence of HLA antibodies, suggesting that this may be a nonspecific finding [15]. (See "Kidney transplantation in adults: Prevention and treatment of antibody-mediated rejection".)

Clinical manifestations — Hyperacute rejection is characterized by the rapid onset of profound hypoxemia, edema fluid emanating from the airways, and the appearance of diffuse opacities on radiographic imaging in the transplanted lung(s), all occurring in the first 24 hours following lung implantation [3].

Evaluation — For lung recipients presenting with declining oxygenation and diffuse radiographic opacities in the allograft within the first 24 hours after transplantation, the evaluation includes assessment for AMR, fluid overload, aspiration, primary graft dysfunction (PGD), and vascular anastomotic complications. (See "Noninfectious complications following lung transplantation", section on 'Vascular anastomotic complications' and "Primary lung graft dysfunction", section on 'Diagnostic evaluation'.)

Laboratory — Laboratory testing in suspected hyperacute rejection is focused on immunologic tests for HLA antibodies but should also include testing for other processes in the differential diagnosis (eg, complete blood count and differential, blood cultures, brain natriuretic peptide [BNP], troponin).

Current and historical HLA antibody screening tests and the specificity of any pre-formed HLA antibodies should be reviewed. In addition, the results of the direct crossmatch performed at the time of transplantation are reviewed to determine whether the recipient is sensitized to the specific donor antigens and ultimately whether the transplant is HLA-compatible or not. In general, donor and recipient HLA-compatibility is determined by the results of both recipient HLA antibody screening and the direct donor-recipient crossmatch.

●Virtual crossmatch prior to transplantation – Potential lung transplant recipients are screened for pre-existing HLA antibodies to identify HLA antigens that will need to be avoided in any potential donors for that recipient [1,10]. The older complement-dependent cytotoxicity (CDC) assay has largely been replaced by highly sensitive, solid-phase assays that test the potential recipient's serum against beads coated with single or multiple purified HLA antigens. Using this approach, the presence and specificity of any HLA antibodies in the potential recipient's serum can be determined. The mean fluorescence intensity (MFI) for any HLA antibody that is detected provides a measure of the antibody's avidity to its respective HLA molecule. The results of the HLA antibody assessment are entered into the United Network for Organ Sharing (UNOS) database at the time of listing for transplantation. (See "Human leukocyte antigens (HLA): A roadmap".)

Any HLA antibodies above an arbitrary MFI threshold, which varies among centers, are used in a "virtual crossmatch" with the HLA typing of any potential donors to avoid HLA incompatibility and hyperacute rejection. The "virtual crossmatch" is the primary reason that hyperacute rejection has become rare.

Despite a negative "virtual crossmatch," occasional patients have DSA that are below the level of detection of the assay or against antigens not contained in the assay (eg, non-HLA antigens), but still place them at risk for hyperacute rejection [1,8]. (See "Kidney transplantation in adults: Clinical features and diagnosis of acute kidney allograft rejection", section on 'Active antibody-mediated rejection'.)

●Direct crossmatch at the time of transplantation – At most centers, a direct crossmatch is performed at the time of transplantation using fresh recipient serum and donor lymphocytes from peripheral blood or lymph node-derived cells. Again, a variety of methods may be used, including CDC and flow cytometry. In a flow cytometry crossmatch, recipient serum is incubated with donor lymphocytes (T and B cells). After incubation, antibodies bound to the donor cells are tagged with immunofluorescent anti-immunoglobulin-G antibodies, and flow cytometry is used to assess the number of tagged cells [16,17]. The flow cytometry method has largely replaced the CDC method, due to greater sensitivity.

Imaging — Virtually all patients presenting with acute deterioration in gas exchange following lung transplantation undergo conventional chest radiography. In patients with hyperacute lung transplant rejection, the chest radiograph typically shows diffuse reticular and ground glass opacities in the transplanted lung(s) [18].

A bedside nuclear perfusion scan can be a helpful imaging study to evaluate for vascular anastomotic complications, although the findings are sometimes non-specific. Computed tomography (CT) angiography is a more sensitive and specific imaging study to detect vascular anastomotic complications, but it requires transporting a critically ill patient to the radiology department and administration of intravenous contrast, which may increase the risk of kidney injury.

Transesophageal echocardiography can assess flow through the pulmonary veins into the left atrium and can be safely done at the bedside.

Bronchoscopy with bronchoalveolar lavage — Patients with declining oxygenation and diffuse pulmonary opacities developing within the first 24 hours after lung transplantation typically undergo flexible bronchoscopy with bronchoalveolar lavage to examine the bronchial anastomosis, assess for alveolar hemorrhage, and obtain samples for microbiologic stains and culture. Transbronchial lung biopsies can add important diagnostic information, but the risk of the procedure is sometimes prohibitive. The techniques of bronchoscopy and bronchoalveolar lavage are discussed separately. (See "Flexible bronchoscopy in adults: Overview" and "Basic principles and technique of bronchoalveolar lavage".)

Diagnosis and differential diagnosis — The diagnosis of hyperacute rejection is suspected when the recipient develops allograft dysfunction and diffuse radiographic opacities within the first 24 hours after transplantation. Confirmation of the diagnosis depends upon the presence of the following two immunologic findings [3,5]:

●A positive direct cross-match between the recipient's serum and donor cells indicating immunologic incompatibility.

●The presence of pretransplant DSA in the recipient's serum. The DSA may target non-HLA antigens.

The differential diagnosis of hyperacute AMR includes primary graft dysfunction, pulmonary edema from volume overload or myocardial dysfunction, bacterial pneumonia (figure 1), aspiration, and technical problems, such as vascular anastomotic complications. These possibilities are evaluated by assessing volume status with central venous or pulmonary artery catheterization, assessing cardiac function with an echocardiogram, obtaining computed tomography with pulmonary angiography (CTPA), and obtaining cultures of peripheral blood, and bronchoalveolar lavage, as well as reviewing donor culture results. (See "Noninfectious complications following lung transplantation" and "Primary lung graft dysfunction", section on 'Diagnostic evaluation'.)

A general approach to the evaluation of postoperative hypoxemia is provided separately. (See "Overview of the management of postoperative pulmonary complications".)

Treatment — As hyperacute rejection has been an uncommon complication after lung transplantation, data to support a specific treatment regimen are lacking. Nonetheless, for patients with suspected hyperacute rejection, the overall goals of treatment are to deplete circulating antibodies and antibody-producing cells and to block antibody binding to the endothelium. Due to the severity of respiratory impairment, several immunosuppressive therapies (eg, therapeutic plasma exchange [TPE], intravenous immune globulin [IVIG], rituximab, bortezomib) may be initiated simultaneously. While hyperacute rejection can be fatal [8,19], successful salvage of the allograft has been reported.

Therapeutic plasma exchange (TPE; plasmapheresis) rapidly removes anti-donor antibodies from the peripheral blood to prevent further allograft damage [14,20]. A case report described a patient with hyperacute rejection of a single lung transplant who had a positive retrospective crossmatch with DSA to HLA A2 [14]. The patient was treated with TPE, antithymocyte globulin, and cyclophosphamide. Marked clinical improvement was noted by the fifth posttransplant day, at which time a repeat crossmatch showed substantially less anti-donor reactivity. Implementation of TPE is described separately. (See "Therapeutic apheresis (plasma exchange or cytapheresis): Indications and technology".)

IVIG causes B cell apoptosis, blocks binding of donor-reactive antibodies, and may inhibit complement activation. IVIG has been used widely in desensitization protocols for renal transplant recipients. A reasonable dose of IVIG is 0.5 to 2 g/kg, using ideal body weight for patients with obesity. Premedication (eg, acetaminophen 650 mg orally or rectally, diphenhydramine 50 mg orally or intravenously) is needed 30 to 60 minutes prior to each dose. Administration of IVIG is described separately. (See "Overview of intravenous immune globulin (IVIG) therapy".)

Rituximab, a B lymphocyte depleting monoclonal antibody, is often included in the regimen at 375 mg/m², given intravenously as a one-time dose. As a practical matter, TPE removes drugs such as IVIG and rituximab, so administration of these agents should occur after TPE sessions. (See "Therapeutic apheresis (plasma exchange or cytapheresis): Complications", section on 'Removal of/reduction in medications'.)

Bortezomib is a proteasome inhibitor that has pro-apoptotic effects on plasma cells, thus decreasing antibody production. Case reports have described using bortezomib at a dose of 1.3 mg/m², intravenously or subcutaneously [21,22]. Bortezomib is not cleared by TPE. Carfilzomib, a second-generation proteasome inhibitor that binds irreversibly to the proteasome, is an alternative to bortezomib, although head-to-head studies comparing the two agents' efficacy and safety have not been done.

Eculizumab is an anticomplement C5 antibody that was successfully used (along with other therapies) in at least one patient. In a case report, eculizumab (1200 mg on day 1, and 600 mg on days 6, 8, and 9) was administered to a patient with a pretransplant PRA of 99 percent who developed progressive respiratory failure due to hyperacute rejection after bilateral lung transplantation despite treatment with IVIG, plasma exchange, rituximab, and bortezomib [22]. While it is difficult to know the relative contribution of the various therapies to the patient’s improvement, the time course of improvement paralleled addition of eculizumab.

Patients are already receiving high-dose systemic glucocorticoids, as part of the usual induction of immunosuppression following lung transplantation. However, glucocorticoids are not felt to be particularly beneficial in treating hyperacute AMR.

Empiric systemic antibiotic therapy is typically administered pending culture results (figure 1). (See "Bacterial infections following lung transplantation", section on 'Pneumonia'.)

ACUTE ANTIBODY-MEDIATED REJECTION — Historically, antibody-mediated rejection (AMR) after lung transplantation has been a difficult diagnosis to establish [6,18,23,24], and until recently, there was no widely accepted definition. In 2016, the International Society for Heart and Lung Transplantation (ISHLT) developed a consensus definition for AMR based on findings in renal and cardiac AMR and case series in lung transplantation [25].

Pathology and pathogenesis — Specific morphologic features of acute AMR have been difficult to identify, although capillaritis is sometimes seen [11,26-28]. The pathology in many reported cases has been nonspecific, demonstrating features of acute lung injury including intra-alveolar fibrin deposition and diffuse alveolar damage or acute and organizing pneumonitis [11,27,29]. The nonspecific features contrast with acute cellular rejection where the diagnosis rests firmly on the pathology. (See "Evaluation and treatment of acute cellular lung transplant rejection", section on 'Pathology' and "Chronic lung allograft dysfunction: Bronchiolitis obliterans syndrome", section on 'Histopathology'.)

While capillaritis is sometimes seen as a feature of acute AMR, it is not a specific finding. In a study of capillaritis in lung allograft biopsies, none of the seven patients with pure capillaritis had donor specific antibodies (DSA) [9]. This raises the possibilities that non-HLA anti-donor antibodies can cause capillaritis or that capillaritis is not specific for AMR. Confounding matters further is the unknown inter-reader reliability of capillaritis in lung allograft biopsies. In addition, a non-specific pathology of acute lung injury or diffuse alveolar damage is often seen [11,29]. Ultimately, the histopathologic findings in acute AMR are usually non-specific and the diagnosis requires a multidisciplinary evaluation including testing for DSA.

The mechanism for cell injury (capillaritis) in AMR is thought to be antibody recognition via DSA, followed by complement-mediated cytotoxicity, which is supported by the finding of C4d deposition as a prominent feature in antibody-mediated rejection in kidney allografts. However, the relationship between DSA and acute AMR is complex, as DSA can be seen in patients with acute cellular rejection and chronic lung transplant rejection with bronchiolitis obliterans, as well as acute AMR [30-33]. In a study of lung biopsies obtained for allograft dysfunction, the histopathologic findings of 23 patients who had concurrent DSA were compared with 26 patients who did not have DSA [9]. The predominant finding among those with DSA was high-grade acute cellular rejection; 17 of 23 patients had acute cellular rejection (2 with grade A2, 14 with grade A3, and 1 with grade A4) [9]. The remaining five patients with DSA had acute and organizing lung injury without capillaritis. Of note, capillaritis was noted only in a minority of cases and was associated with other features of cellular rejection.

Deposition of immunoglobulins and complement products (particularly C4d) in the alveolar septum has been reported in patients with HLA antibodies and may be a manifestation of AMR [27], although this is controversial and lung allograft injury may occur in the absence of complement activation [3,4,27,34,35].

●In a case series of 22 lung transplant recipients who developed capillaritis, complement deposition was associated with septal capillary necrosis and deposition of complement components and immunoglobulin [24]. Serum panel reactive antibodies (anti-HLA antibodies) were absent, possibly due to the relatively insensitive assay used at the time, but anti-endothelial antibodies were noted.

●C4d deposition did not correlate well with serum DSA in the 23 lung transplant recipients who underwent lung biopsy, as described above, although it was noted more frequently among patients with DSA than without [9].

C4d deposition is neither sensitive nor specific for acute AMR. Among 17 patients who had acute cellular rejection and DSA, 13 had C4d deposition, but this was focal and inconspicuous [9]. Furthermore, C4d deposition was noted in 6 of the 26 patients who had cellular rejection but no DSA, and the pattern of deposition was similar to those with DSA. In addition, C4d deposition has been an inconsistent finding in some case series of AMR after lung transplantation. In one series of 10 patients with AMR, two of five had C4d deposition in the capillary endothelium [29]. In a separate series of nine patients with AMR, only two of eight had C4d deposition [36]. (See "Kidney transplantation in adults: Prevention and treatment of antibody-mediated rejection".)

In a study that compared 28 patients who had acute AMR with C4d deposition and 45 patients who did not have C4d deposition, similar histologic features were noted, but the C4d-positive group was more likely to have capillaritis [37]. Where testing was available, all patients who had C4d-positive AMR had complement-fixing DSA, whereas 6 of 18 patients with C4d-negative AMR did not have complement-fixing DSA. While the sample size is small, this suggests that acute AMR can occur via complement-independent pathways. Additionally, the similarity of clinical outcomes between those with C4d-positive AMR and with C4d-negative AMR suggests that C4d-deposition may not be a necessary criterion for the diagnosis of AMR [37].

Clinical manifestations — The clinical manifestations of acute AMR are nonspecific and are characterized by the acute onset of respiratory symptoms in the weeks or months following transplantation that are typically severe enough to require hospitalization [11,18]. In a case series, the time from transplant to diagnosis of acute AMR ranged from one week to over one year; the majority were diagnosed between 1 and 12 months following transplant [18]. However, some patients present years after transplantation [11,29]. Presenting symptoms and signs included dyspnea (100 percent), cough, fever, hemoptysis (25 percent), hypoxemia, and respiratory failure (18 percent). In a separate case series, 14 of 21 required mechanical ventilation [11].

It is possible that AMR may be clinically occult as most cases of acute cellular rejection are clinically silent. However, there have been no reported cases of circulating DSA and a characteristic pathology of AMR in the absence of clinical signs or symptoms. So, it is unclear if clinically silent AMR exists and what its clinicopathological features would be.

Development of DSA may be a risk factor for acute AMR. In a series of 441 lung transplant recipients, 139 (32 percent) developed detectable antibodies to HLA; of these, 54 (39 percent) developed antibodies specific to donor HLA, which was associated with poorer survival [38].

Evaluation — When lung transplant recipients develop respiratory symptoms weeks to months after transplantation, the evaluation requires a combination of tests to determine the cause of allograft dysfunction, such as infection, airway anastomotic complications, acute cellular rejection, bronchiolitis obliterans, cardiac dysfunction, and AMR. This evaluation typically includes laboratory assessment for DSA, spirometry, imaging, and flexible bronchoscopy.

Laboratory — At our center, we test for donor specific HLA antibodies (DSA) using a single antigen assay (eg, LABScreen Single Antigen assay) when a lung transplant recipient presents with worsening respiratory symptoms or a decline in spirometry [11,29,39]. DSA has been the key diagnostic component of AMR, but it is possible that non-DSA HLA antibodies and non-HLA antibodies may cause AMR [11,24,29,40]. Indeed, some recipients develop HLA antibodies that are not donor specific, and in one study, 11 of 22 patients with non-DSA HLA antibodies developed AMR compared to 0 of 22 patients without HLA antibodies [29].

Complement activation is considered the central event in the pathogenesis of AMR, although not all antibodies fix complement. The C1q assay identifies HLA antibodies that bind the first component of complement and can be used to predict AMR and late graft failure [36,41]. In a majority of cases, DSA bind and activate complement; however in a minority of patients, DSA do not bind complement and there is no C4d-deposition, suggesting that AMR may occur via complement-independent pathways [11,37]. (See 'Laboratory' above and 'Pathology and pathogenesis' above.)

Recent advances indicate that donor-derived cell-free DNA (cfDNA) detected in peripheral blood is a highly sensitive marker of allograft injury, and the amount of circulating cfDNA correlates well with the severity of allograft injury and the likelihood of AMR [42,43]. In addition, elevated levels of cfDNA are detectable months before the diagnosis of AMR making this an appealing screening assay for AMR [42,44]. However, elevated levels of cfDNA are nonspecific and may be seen in the setting of acute cellular rejection and infection [44,45]. Threshold levels may be improved by adjustment for differences in transplanted lung mass (eg, single versus bilateral lung transplants). One study that compared cfDNA levels in single lung recipients with bilateral recipients identified a threshold for detecting acute rejection of 0.54 percent in single lung recipients and 1.1 percent in bilateral recipients [46].

Additional tests typically include a complete blood count with differential looking for evidence of infection, blood and sputum cultures, cytomegalovirus (CMV) blood polymerase chain reaction (PCR), brain natriuretic peptide (BNP), and possibly troponin. Blood levels of the medications in the patient's immunosuppressive regimen (eg, calcineurin inhibitors) are obtained, as appropriate. Data are limited regarding the utility of the galactomannan antigenemia assay, the beta-D-glucan assay, or PCR for the diagnosis of invasive aspergillosis in lung transplant recipients.

Pulmonary function tests — In addition to routine screening spirometry, diagnostic spirometry is typically performed in lung transplant recipients to assess new onset respiratory symptoms. Based on limited data, patients with acute AMR often have a reduced forced vital capacity (FVC) and/or forced expiratory volume in one second (FEV₁), although many are too ill to perform spirometry [11]. In a case series of patients with a syndrome suggestive of AMR and biopsy evidence of pulmonary capillaritis, a greater than 20 percent decrease from the baseline FEV₁ was noted in 62 percent [18]. The reduction in FEV₁ is a nonspecific finding that is also seen in acute and chronic cellular rejection and airway complications, such as anastomotic stenosis and airway granulation tissue.

Imaging — Virtually all lung transplant recipients presenting with new onset respiratory symptoms or deterioration in gas exchange undergo conventional chest radiography. Diffuse pulmonary opacities are typical findings of AMR on chest radiography [11], but less severe cases may only have subtle findings. High resolution computed tomography (HRCT) is usually performed in addition to assess for complications such as recurrent primary disease, airway anastomotic problems, infection, and development of malignancy, as radiographic evidence of rejection is nonspecific.

Isolated reports of HRCT findings in acute AMR describe diffuse septal thickening with areas of ground-glass opacification within the apices and bases [27].

Flexible bronchoscopy — For patients presenting with unexplained respiratory symptoms in the weeks to months following lung transplantation, most centers perform flexible bronchoscopy with bronchoalveolar lavage (BAL) and transbronchial biopsy in order to establish a specific diagnosis, unless the patient is too unstable. At the time of bronchoscopy, the airway is surveyed, with particular attention to the bronchial anastomotic site. Bronchoalveolar lavage is performed in an area of radiographic abnormality to obtain samples for microbiologic stains, immunofluorescence, polymerase chain reaction, and culture to assess for bacterial, fungal, or viral pneumonia (table 1)[47].

Transbronchial biopsies (typically 10) are obtained from a lower lobe or an area of radiographic disease. The optimal number of transbronchial biopsies is not known, but most centers try to achieve five specimens adequate for histopathologic interpretation. Immunohistochemistry for C4d deposition is performed using an immunofluorescence (IF) or immunoperoxidase (IP) assay [25]. The biopsies should be interpreted by a pathologist familiar with the International Society for Heart Lung Transplantation (ISHLT) guidelines (picture 3) [26]. (See "Evaluation and treatment of acute cellular lung transplant rejection", section on 'Flexible bronchoscopy'.)

The techniques of BAL and transbronchial biopsy are discussed separately [47]. (See "Basic principles and technique of bronchoalveolar lavage" and "Role of bronchoalveolar lavage in diagnosis of interstitial lung disease" and "Flexible bronchoscopy in adults: Overview" and "Flexible bronchoscopy in adults: Associated diagnostic and therapeutic procedures", section on 'Transbronchial biopsy'.)

Diagnosis and differential diagnosis — According to the ISHLT, the diagnosis of definite AMR consists of the following criteria [25]:

●Acute allograft dysfunction

●Circulating DSA in the recipient's serum

●Histologic evidence of acute lung injury (picture 3)

●Sub-endothelial C4d deposition in alveolar capillaries (picture 4)

●Exclusion of other potential causes of allograft dysfunction

While C4d deposition is an inconsistent finding in pulmonary AMR [29,40,48] after the immediate posttransplant period, the simultaneous findings of DSA in the peripheral blood and pathologic findings of capillaritis with positive immunohistochemistry staining for complement components are considered diagnostic of definite AMR if other potential causes of allograft dysfunction are excluded (picture 4) [1,3,25]. The samples for pathologic analysis are typically obtained via transbronchial biopsy [18]. Cases fulfilling all criteria for definite AMR are uncommon. (See 'Laboratory' above and 'Flexible bronchoscopy' above.)

Cases of acute allograft dysfunction with circulating DSA, histologic evidence of acute lung injury without C4d deposition, and exclusion of other causes of allograft dysfunction would be considered probable AMR [25]. According to the ISHLT definition, a “confident” diagnosis of AMR can be made in the absence of C4d deposition if all other criteria are met [25].

Occasionally, when transbronchial biopsies are nondiagnostic and an alternate diagnosis has not been identified, video-assisted lung biopsy is performed to obtain surgical lung biopsy specimens [18]. (See 'Laboratory' above and 'Pathology and pathogenesis' above.)

The differential diagnosis of AMR includes acute and chronic cellular rejection, viral infection, airway complications (eg, bronchial stenosis at the anastomotic site, airway granulation tissue), myocardial dysfunction, pulmonary thromboemboli, and recurrent primary disease (figure 1). These processes are discussed separately. (See "Noninfectious complications following lung transplantation" and "Airway complications after lung transplantation" and "Evaluation and treatment of acute cellular lung transplant rejection".)

Treatment — The optimal treatment for acute AMR after lung transplantation is not known due to an absence of clinical trial data, and no medications are approved for treatment of lung allograft AMR. However, combination antibody-directed therapy is virtually always initiated due to the severity of respiratory insufficiency and associated poor prognosis.

Approach — Acute AMR is generally refractory to conventional immunosuppression including high-dose glucocorticoids, but sometimes it responds favorably to a combination of agents directed at antibody reduction, such as intravenous immune globulin (IVIG), therapeutic plasma exchange (plasmapheresis), rituximab, bortezomib, carfilzomib, and eculizumab [5,21,27,39,49,50]. Medication choices are partially derived from experience in the treatment of antibody-mediated kidney allograft rejection and clinical experience. We typically individualize treatment based upon the severity of illness, clinical course, and response to therapy [11]. However, rejection is refractory in many cases, resulting in graft failure and death. (See "Kidney transplantation in adults: Prevention and treatment of antibody-mediated rejection", section on 'Treatment of active antibody-mediated rejection'.)

In a single-center series of 21 patients with acute AMR, 20 received IVIG, 18 received rituximab, 8 received plasmapheresis, 3 bortezomib, and 1 each antithymocyte globulin or eculizumab; 15 survived to discharge [11]. In a series of nine lung transplant recipients with acute AMR, combination therapy with high-dose intravenous glucocorticoids, intravenous immune globulin, plasmapheresis, and rituximab was associated with a decrease in the DSA by 17 percent, while clinical outcomes were variable [40].

Empiric antibiotic therapy is often initiated pending results of microbial studies and histopathology. (See "Bacterial infections following lung transplantation", section on 'Pneumonia'.)

In addition to the therapy directed at AMR, the maintenance immunosuppression regimen is usually intensified [18]. The specific agents used for intensification depend on the underlying maintenance immunosuppression regimen. (See "Maintenance immunosuppression following lung transplantation", section on 'Monitoring and adjusting maintenance therapy'.)

Intravenous immune globulin — Intravenous immune globulin (IVIG) has been used in combination with rituximab in lung transplant recipients who developed donor specific HLA antibodies, but did not have clinical AMR, and also to treat acute AMR [39]. As noted above, 15 of 20 patients with acute AMR who were treated with IVIG survived to discharge [11].

A reasonable dose of IVIG for acute AMR is 0.5 to 2 g/kg, using ideal body weight for patients with obesity, given intravenously. Doses >1 g/kg are generally divided over two days. Premedication is needed 30 to 60 minutes prior to each dose and generally includes acetaminophen 650 mg orally or rectally, diphenhydramine 50 mg orally or intravenously, and methylprednisolone 50 mg intravenously (if the patient is not already on pulse glucocorticoids). Depending on the response to therapy, the dose may be repeated. It is important to note that the optimal dose and the number of doses of IVIG are unknown, and various doses have been used with mixed results. (See "Overview of intravenous immune globulin (IVIG) therapy" and "Intravenous immune globulin: Adverse effects".)

Therapeutic plasma exchange — Despite the absence of clinical trials, therapeutic plasma exchange (plasmapheresis) is considered an effective therapy in acute AMR complicating lung transplantation, based on a response in 12 of 18 (67 percent) of the patients with acute AMR who were refractory to glucocorticoids [1,3,18]. However, it is usually reserved for more severe cases as it is invasive and labor intensive. In addition, therapeutic plasma exchange removes some important components of AMR treatment, including IVIG and rituximab, making appropriate dosing and treatment schedules critical. (See "Therapeutic apheresis (plasma exchange or cytapheresis): Indications and technology" and "Therapeutic apheresis (plasma exchange or cytapheresis): Complications", section on 'Removal of/reduction in medications'.)

The usual regimen for therapeutic plasma exchange in lung transplant acute AMR is to replace 1 to 1.5 plasma volumes with an equal part of 5 percent human serum albumin and fresh frozen plasma on five consecutive days or every other day for three sessions [18].

Rituximab — Rituximab, an anti-CD20 monoclonal antibody, depletes B lymphocytes that express CD20 on their surface, thus interfering with antibody production. Rituximab has been used as an adjunctive agent in kidney transplant recipients with antibody mediated rejection. (See "Kidney transplantation in adults: Prevention and treatment of antibody-mediated rejection", section on 'Approach to initial therapy'.)

The evidence in favor of rituximab in acute lung transplant AMR includes the series of 21 patients with acute AMR, described above [11]; of the 18 who received rituximab, 12 survived to discharge. In addition, indirect evidence suggesting that rituximab would be effective in acute AMR comes from a series of 44 lung transplant recipients with posttransplant development of DSA (but without a clinical syndrome of acute AMR) [39]. Treatment with rituximab and IVIG led to clearing of antibodies in 27 (64 percent). Clearance of DSA was associated with better long-term survival. While pre-emptive treatment of patients with new onset DSA without clinical acute AMR requires further study, this observation lends credence to the use of rituximab as part of antibody-directed therapy.

A typical dose of rituximab for treating acute AMR is 375 mg/m² given intravenously as a one-time dose after completion of IVIG infusion [11]. Rituximab requires premedication, as described for IVIG above. If DSA have not cleared after two weeks, the dose of rituximab may be repeated.

Bortezomib — Bortezomib is a proteasome inhibitor that has pro-apoptotic effects on plasma cells, thus decreasing antibody production. In a few but not all cases of acute AMR, bortezomib has been associated with a favorable outcome [11,21,49]. A standard dose has not been established, but the dose used in case reports is 1.3 mg/m², given intravenously or subcutaneously [21,51].

Carfilzomib — Carfilzomib is a second-generation proteasome inhibitor that binds irreversibly to the proteolytic core leading to cell cycle arrest and apoptosis. In an observational study, 14 patients with AMR were treated with the combination of carfilzomib (20 mg/m² on days 1, 2, 8, 9, 15, and 16), plasma exchange, and IVIG [50]. A favorable response was defined as loss of C1q binding by the DSA, and 10 of the 14 patients were deemed carfilzomib responders. Responders were less likely to develop bronchiolitis obliterans syndrome (BOS) after the episode of AMR, but had no survival advantage over non-responders. Further study is needed to determine the efficacy and safety of carfilzomib for lung allograft AMR.

Systemic glucocorticoids — Unlike acute cellular rejection, which responds to systemic glucocorticoids, the response rate of acute AMR to glucocorticoids is limited. Among 40 cases of pulmonary capillaritis, only 43 percent improved with administration of intravenous glucocorticoids [18]. A number of patients who were refractory to glucocorticoid therapy responded to other, antibody-directed therapies. Nonetheless, high-dose glucocorticoids are often given in combination with antibody-directed therapy (eg, IVIG, rituximab, bortezomib, therapeutic plasma exchange).

Prognosis — Acute AMR can be a fulminant and sometimes fatal form of lung transplant rejection, and survivors have an increased risk of developing chronic lung allograft dysfunction (CLAD). In a series of 21 lung transplant recipients with AMR, the diagnosis was made at a median of 258 days following lung transplantation [11]. Six patients (29 percent) died during the index hospitalization, while 13 of 14 recipients who survived to discharge developed CLAD in the ensuing months without regard to whether they cleared DSA. The median survival after the diagnosis was 593 days. Of the 14 patients who required mechanical ventilation, eight (57 percent) were able to discontinue ventilator support and be discharged home.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Lung transplantation".)

SUMMARY AND RECOMMENDATIONS

●Antibody-mediated rejection (AMR) of lung transplant allografts is mediated by antibodies directed against donor human leukocyte antigens (HLA) and other donor epitopes. The more common type of acute rejection in lung transplantation, acute cellular rejection, is mediated by T lymphocyte recognition of foreign HLA epitopes. (See 'Definitions' above.)

●Hyperacute rejection is a recognized syndrome of AMR that occurs in the first minutes to hours after reperfusion of the lung allograft and is caused by pre-existing donor specific antibodies (DSA). Hyperacute rejection is rare, as "virtual crossmatch" of previously identified HLA antibodies with potential donor HLA epitopes prior to transplantation avoids HLA incompatibility. (See "Evaluation and treatment of acute cellular lung transplant rejection", section on 'Definitions' and 'Laboratory' above.)

●Hyperacute rejection is associated with profound hypoxemia, diffuse noncardiogenic pulmonary edema, and alveolar hemorrhage. The histopathologic findings attributed to hyperacute AMR include small vessel neutrophilic vasculitis or "capillaritis," intra-alveolar hemorrhage, and diffuse alveolar damage. (See 'Pathology and pathogenesis' above and 'Clinical manifestations' above.)

●The diagnosis of hyperacute rejection is based upon the timing of acute allograft dysfunction in a recipient with pre-transplant DSA (recognizing HLA or non-HLA epitopes) and a positive direct cross-match between the recipient's serum and donor cells indicating immunologic incompatibility. Other processes in the differential diagnosis, such as primary graft dysfunction, pulmonary edema from volume overload or myocardial dysfunction, bacterial pneumonia, aspiration, pulmonary thromboembolism, and anastomotic problems must be excluded. (See 'Evaluation' above and 'Diagnosis and differential diagnosis' above.)

●The optimal treatment for hyperacute rejection is not known. Due to the severity of respiratory compromise associated with hyperacute AMR, treatment directed at antibody depletion is typically initiated. Most regimens include intravenous immune globulin (IVIG), rituximab, and therapeutic plasma exchange (plasmapheresis). Additional agents (eg, bortezomib, carfilzomib, eculizumab) may be added if the patient fails to improve. Empiric antibiotics are often administered pending culture results. (See 'Diagnosis and differential diagnosis' above.)

●A small number of lung transplant recipients develop acute AMR in the weeks to months following transplantation. Acute AMR is associated with nonspecific symptoms, such as dyspnea, cough, fever, and hemoptysis, that can progress rapidly to respiratory failure. (See 'Clinical manifestations' above.)

●The evaluation of potential acute AMR requires a combination of tests to look for evidence of AMR and to exclude alternate diagnoses, such as infection, airway anastomotic problems, acute cellular rejection, bronchiolitis obliterans, cardiac dysfunction, and fluid overload. Testing usually includes laboratory studies for DSA, appropriate blood and sputum tests for infection, assessment of fluid status and cardiac function, chest imaging to clarify location and extent of abnormalities, spirometry, and flexible bronchoscopy with bronchoalveolar lavage and transbronchial biopsy. (See 'Evaluation' above and 'Diagnosis and differential diagnosis' above.)

●Criteria for the diagnosis of acute AMR after lung transplantation have been established by the International Society for Heart Lung Transplantation (ISHLT): presence of DSA, acute lung injury pattern on histopathology (picture 3), sub-endothelial C4d deposition in alveolar capillaries (picture 4), and overt allograft dysfunction, with the exclusion of other potential causes of allograft dysfunction. (See 'Diagnosis and differential diagnosis' above.)

●The optimal treatment for acute AMR complicating lung transplantation has not been determined. However, combination therapy to reduce antibody production is virtually always initiated due to the severity of respiratory insufficiency and associated poor prognosis. We typically initiate therapy with a combination of intravenous immune globulin (IVIG) and rituximab. In cases of severe allograft dysfunction or incomplete response to IVIG and rituximab, therapeutic plasma exchange and/or bortezomib may be added. (See 'Treatment' above.)