Preformed cytotoxic antibodies in potential allograft recipients: recent data

The results of HLA-incompatible kidney transplantation according to pre-transplant crossmatch tests: Donor-specific antibody as a prominent predictor of acute rejection

BACKGROUND

Crossmatching (XM) between organ donors and recipients is correlated with clinical outcomes. This study evaluates the results of HLA-incompatible kidney transplant (HLA-i KT) according to pre-transplant XM modalities.

METHODS

This study included 731 consecutive patients. HLA-i KT was defined as a transplant under conditions of complement-dependent cytotoxicity (CDC) XM positivity, flow-cytometric XM (FCXM) positivity, and/or maximal donor-specific antibody (DSA) mean fluorescence intensity (MFI) ≥5000.

RESULTS

The incidence of antibody-mediated rejection (AMR) within 1 year after transplant was significantly higher in the HLA-i group than in the HLA compatible (HLA-c) group (15 vs 9 patients, 14.2% vs 1.4%; P < 0.01). Multivariate analysis indicated that a DSA MFI ≥5000 (odds ratio [OR] = 2.63; 95% confidence interval [CI], 1.00-6.98; P = 0.05) was significantly associated with acute rejection (AR), whereas CDC (OR = 2.09; 95% CI, 0.55-7.99; P = 0.28) and FCXM positivity (OR = 2.07; 95% CI, 0.73-5.87; P = 0.17) were not. Similarly, DSA MFI ≥ 5000 (OR = 4.14; P = 0.02) was the only significant factor affecting the risk of AMR.

CONCLUSIONS

Of the various XM tests, DSA MFI ≥5000 was the most prominent predictor of AR in patients undergoing HLA-i KT.

HLA Class Ia and Ib Polyreactive Anti-HLA-E IgG2a Monoclonal Antibodies (TFL-006 and TFL-007) Suppress Anti-HLA IgG Production by CD19+ B Cells and Proliferation of CD4+ T Cells While Upregulating Tregs

The anti-HLA-E IgG2a mAbs, TFL-006 and TFL-007, reacted with all HLA-I antigens, similar to the therapeutic preparations of IVIg. Indeed, IVIg lost its HLA reactivity, when its HLA-E reactivity was adsorbed out. US-FDA approved IVIg to reduce antibodies in autoimmune diseases. But the mechanism underlying IVIg-mediated antibody reduction could not be ascertained due to the presence of other polyclonal antibodies. In spite of it, the cost prohibitive high or low IVIg is administered to patients waiting for donor organ and for allograft recipients for lowering antiallograft antibodies. A mAb that could mimic IVIg in lowering Abs, with defined mechanism of action, would be highly beneficial for patients. Demonstrably, the anti-HLA-E mAbs mimicked several functions of IVIg relevant to suppressing the antiallograft Abs. The mAbs suppressed activated T cells and anti-HLA antibody production by activated B cells, which were dose-wise superior to IVIg. The anti-HLA-E mAb expanded CD4+, CD25+, and Foxp3+ Tregs, which are known to suppress T and B cells involved in antibody production. These defined functions of the anti-HLA-E IgG2a mAbs at a level superior to IVIg encourage developing their humanized version to lower antibodies in allograft recipients, to promote graft survival, and to control autoimmune diseases.

Methodological aspects of anti-human leukocyte antigen antibody analysis in solid organ transplantation

Donor human leukocyte antigen (HLA)-specific antibodies (DSA) play an important role in solid organ transplantation. Preexisting IgG isotype DSA are considered a risk factor for antibody mediated rejection, graft failure or graft loss. The post-transplant development of DSA depends on multiple factors including immunogenicity of mismatched antigens, HLA class II typing of the recipient, cytokine gene polymorphisms, and cellular immunoregulatory mechanisms. De novo developed antibodies require special attention because not all DSA have equal clinical significance. Therefore, it is important for transplant clinicians and transplant immunologists to accurately characterize DSA. In this review, the contemporary immunological techniques for detection and characterization of anti-HLA antibodies and their pitfalls are described.

Clinical impact of H-Y alloimmunity

H-Y antigens are a group of minor histocompatibility antigens encoded on the Y-chromosome with homologous H-X antigens on the X-chromosome. The disparate regions of the H-Y antigens are highly immunogenic and play an important role in understanding human alloimmunity. In this review, we investigate the history of H-Y antigen discovery along with their critical contributions in transplantation and pregnancy. In hematopoietic cell transplantation, male recipients with female donors who become seropositive for B-cell responses as H-Y antibodies following transplantation have increased rates of chronic graft-versus-host disease and decreased rates of relapse. Conversely, female patients who receive male kidney allografts are more likely than other gender combinations to develop H-Y antibodies and reject their allografts. Finally, in the setting of pregnancy, mothers who initially gave birth to boys are more likely to have subsequent pregnancy complications, including miscarriages, in association with H-Y antibody development. H-Y antigens continue to serve as a model for alloimmunity in new clinical scenarios. Our development of more sensitive antibody detection and next-generation DNA sequencing promises to further advance our understanding and better predict the clinical consequences of alloimmunity.

Preformed and de novo donor specific antibodies in visceral transplantation: long-term outcome with special reference to the liver

Despite improvement in early outcome, rejection particularly chronic allograft enteropathy continues to be a major barrier to long-term visceral engraftment. The potential role of donor specific antibodies (DSA) was examined in 194 primary adult recipients. All underwent complement-dependent lymphocytotoxic crossmatch (CDC-XM) with pre- and posttransplant solid phase HLA-DSA assay in 156 (80%). Grafts were ABO-identical with random HLA-match. Liver was included in 71 (37%) allografts. Immunosuppression was tacrolimus-based with antilymphocyte recipient pretreatment in 150 (77%). CDC-XM was positive in 55 (28%). HLA-DSA was detectable before transplant in 49 (31%) recipients with 19 continuing to have circulating antibodies. Another 19 (18%) developed de novo DSA. Ninety percent of patients with preformed DSA harbored HLA Class-I whereas 74% of recipients with de novo antibodies had Class-II. Gender, age, ABO blood-type, cold ischemia, splenectomy and allograft type were significant DSA predictors. Preformed DSA significantly (p < 0.05) increased risk of acute rejection. Persistent and de novo HLA-DSA significantly (p < 0.001) increased risk of chronic rejection and associated graft loss. Inclusion of the liver was a significant predictor of better outcome (p = 0.004, HR = 0.347) with significant clearance of preformed antibodies (p = 0.04, OR = 56) and lower induction of de novo DSA (p = 0.07, OR = 24). Innovative multifaceted anti-DSA strategies are required to further improve long-term survival particularly of liver-free allografts.

Antibodies, isotypes and complement in allograft rejection

PURPOSE OF REVIEW

Classical complement activation is a key step in the process of antibody-mediated rejection. Emphasizing novel diagnostic strategies, this study will discuss recent studies highlighting the particular relevance of alloantibodies with complement-fixing ability.

RECENT FINDINGS

Reinforcing the pivotal role of complement, numerous studies have shown tight associations of capillary C4d deposition, a 'footprint' of alloantibody-triggered complement activation, with the occurrence of allograft injury. Distribution patterns of immunoglobulin isotypes or subclasses, which strongly differ in their ability to activate complement, may not adequately reflect the actual pathogenetic relevance of detected allosensitization. This fact may be explained by the finding that other variables, such as antibody-binding density or a synergism of antibodies against different epitopes of the same antigen, may contribute to complement activation. An attractive approach to distinguish between complement-fixing and presumably less harmful noncomplement-fixing alloreactivities could be the detection of C4d deposition in vitro. Applying such techniques, recent studies have shown that human leukocyte antigen reactivity with C4d-fixing ability, in contrast to noncomplement-fixing sensitization, may strongly predict antibody-mediated rejection and inferior graft survival.

SUMMARY

Considering the pivotal role of complement, technologies that uncover the complement-fixing ability of alloantibodies may be of particular interest for the selective detection of deleterious sensitization.

The clinical significance of human leukocyte antigen antibody development in kidney transplantation

BACKGROUND

This retrospective study uses the LAT-M (One Lambda Inc., Calif) screen assay to reexamine the impacts (a), of pretransplant human leukocyte antigen (HLA) antibody on long-term graft survival; (b) posttransplant HLA antibody on long-term graft survival and (c) immunosuppressive regimen on posttransplant HLA antibody development.

PATIENTS AND METHODS

Pretransplant sera from 222 renal transplant recipients and posttransplant sera from 216 renal transplant recipients were studied for the impact of HLA antibody on long-term graft survival.

RESULTS

Among the patients who did not display pretransplant HLA antibodies, 85% enjoyed 5-year and 59% 10-year graft survival, whereas the patients who tested positive were 83% and 83% (P = .5596). Among the patients who did not show posttransplant HLA antibodies, 99% enjoyed 5-, 91% 10-, and 65% 15-year graft survival, whereas for the 44 patients who tested positive they were 59%, 44%, and 30%, respectively (P < .0001). Patients prescribed cyclosporine + myfortic (odds ratio 0.17, P = .05) or FK + Cellcept (odds ratio 0.36, P = .04) showed the lowest posttransplant HLA antibody development.

CONCLUSION

Both regimens improve graft survival.

Clinical relevance of preformed C4d-fixing and non-C4d-fixing HLA single antigen reactivity in renal allograft recipients

Donor-specific alloantibodies (DSA), especially those fixing complement, may pose a particular immunologic risk to transplant recipients. To assess the clinical impact of C4d- or non-C4d-fixing (IgG) HLA sensitization, pretransplant sera obtained from 338 kidney allograft recipients prescreened by FlowPRA were retrospectively evaluated by Luminex single antigen (SA) testing using a novel fluorescent-labeled anti-C4d reagent for detection of antibody-triggered C4d deposition in addition to IgG binding. Recipients with [IgG]DSA (n = 39) showed a substantially higher rate of C4d positive rejection (33%) than 16 patients with [IgG] non-DSA (0%) or 283 antibody-negative patients (4%, multivariate analysis excluding retransplantation because of high co-linearity: P < 0.0001), and adversely affected 5-year death-censored graft survival (74% vs. 81% and 90%, respectively, multivariate model: P < 0.05). [C4d] DSA (n = 21) and [C4d] non-DSA (n = 25) increased rates of C4d positive rejections to a similar extent (24% and 28% vs. 4% in recipients without C4d-fixing reactivity; multivariate analysis: P <or= 0.002) with a trend towards adverse 5-year graft survival (76% and 76% vs. 90%; P <or= 0.2). In conclusion, Luminex-based characterization of HLA sensitization may be a useful strategy for risk stratification. Possibly as a result of intensified immunosuppression in presensitized recipients, identification of C4d-fixing DSA was not associated with a further increase of rejection and graft loss rates.

Detection of immunoglobulin G human leukocyte antigen-specific alloantibodies in renal transplant patients using single-antigen-beads is compromised by the presence of immunoglobulin M human leukocyte antigen-specific alloantibodies

BACKGROUND

Luminex-based single-antigen human leukocyte antigen (HLA) antibody detection beads (SAB) are a major advance for the characterization of HLA-specific antibodies but their clinical utility is limited unless the analysis is performed and interpreted optimally. Here, we identify problems encountered in routine monitoring of antibody levels that may give rise to misleading results, and describe how these can be overcome to provide more meaningful clinical information.

METHODS AND RESULTS

HLA class I specific antibody-binding levels were determined using SAB in the sera of 42 highly sensitized patients awaiting renal transplantation. Normalization of the results against the HLA class I specific monoclonal antibody W6/32 overcame the problems caused by variation in antigen density on SAB and also suggested the presence of alloantibodies directed against multiple HLA class I epitopes of a given specificity. Routine analysis using undiluted sera gave an incomplete assessment of antibody levels. On serum dilution, three patterns of antibody binding became apparent: most sera showed a sequential reduction in immunoglobulin G (IgG) binding levels but several sera displayed antibody binding which remained unchanged (suggesting antigen saturation) or increased IgG binding on serum dilution (suggesting inhibition of IgG binding using neat serum). The presence of immunoglobulin M (IgM) HLA-specific antibodies in sera correlated with inhibition of IgG antibody binding for the corresponding specificity and treatment of sera with dithiothreitol to eliminate IgM HLA-specific blocking antibodies restored maximum IgG antibody binding levels.

CONCLUSION

When using SAB to monitor HLA-specific antibody binding levels, sera should be pretreated with dithiothreitol to eliminate blocking IgM HLA-specific antibodies that may mask clinically relevant allosensitization.

Tailoring antibody testing and how to use it in the calculated panel reactive antibody era: the Northwestern University experience

BACKGROUND

Patients with human leukocyte antigen antibodies constitute a significantly disadvantaged population among those awaiting renal transplantation. We speculated that more understanding of the patients' antibody makeup would allow a more "immunologic" evaluation of crossmatch data, facilitate the use of virtual crossmatch (XM), and lead to more transplantability of these patients.

METHODS

We retrospectively compared the transplantability and transplant outcome of two consecutive patient populations transplanted in our center. Group I (n=374) was evaluated using solid-phase base testing for determination of percentage panel reactive antibody ("PRA screen") with limited antibody identification testing. Group II (n=333) was tested in a more comprehensive manner with major emphasis on antibody identification, antibody strength assignment, and the use of pronase for crossmatch.

RESULTS

Given this approach, 49% (166/333) of the transplanted patients in group II were sensitized compared with 40% (150/374) of the recipients in group I; P=0.012. Transplant outcome at 1-year posttransplant was similar in both groups.

CONCLUSIONS

We conclude that comprehensive evaluation of human leukocyte antigen sensitization and application of immunologic in analyzing compatibility between donor and recipient can increase the transplantability of sensitized patients while maintaining similar outcome. Our approach is in line with United Network for Organ Sharing new guidelines for calculated panel reactive antibody and virtual XM analysis. We hope this report will prompt additional transplant programs to consider how they will use the new United Network for Organ Sharing algorithms.

Differential effects of donor-specific alloantibody

Alloantigen exposure typically provokes an adaptive immune response that can foster rejection of transplanted organs, and these responses present the most formidable biological barrier to kidney transplantation. Although most cellular alloimmune responses can be therapeutically controlled with T-cell-specific immunosuppressants, humoral alloimmune responses remain relatively untamed. Importantly, humoral immunity, typically manifesting as allospecific antibody production, is increasingly recognized for its variable appearance after kidney transplantation. Indeed, the appearance of alloantibody can herald the onset of rapid and destructive antibody-mediated rejection or have no demonstrable acute effects. The factors determining the end result of alloantibody formation remain poorly understood. This review will discuss the breadth of alloantibody responses seen in clinical kidney transplantation and provide an overview of potential factors explaining the phenotypic variability associated with humoral alloimmunity. We propose several avenues ripe for future investigation including the influence of innate immune components and the potential influence of heterologous immune responses in determining the ultimate clinical import of an alloantibody response.

Relevance of MICA and other non-HLA antibodies in clinical transplantation

The clinical importance of HLA-specific antibodies for organ allograft outcome is well established. In the past few years, there has been an increasing interest in non-HLA antigens as targets of injury in organ transplant recipients. This increased interest has been spurred by the fact that HLA-identical kidney transplants also undergo immunological rejections. Polymorphisms within non-HLA genes associated with evoking an immune response to alloantigens are currently being studied for their association with transplant outcome. Non-HLA antigens, such as the polymorphic MHC class I-related chain A (MICA), expressed on endothelial cells have been implicated in the pathogenesis of hyperacute, acute and chronic organ allograft rejections. Use of endothelial cells as targets may clarify the specificities of other clinically relevant non-HLA antibodies in graft rejections. This review summarizes past and current knowledge of the clinical importance and specificities of non-HLA antibodies, and mechanisms by which these antibodies may contribute to graft destruction in clinical transplantation. The aims of current research into the role of non-HLA antigens and their genetics in predicting outcome are to develop an improved insight into the basic science of transplantation and to develop a risk or prognostic index for use in the clinical setting. Non-HLA antibody responses are receiving increasing interest in acute and chronic rejection and specificity, affinity, and pathogenicity need to be investigated to estimate their contribution. Undoubtedly, this will continue to be an area of interest in terms of fully understanding the role of non-HLA antigens as targets of immune-mediated injury and the potential for clinical intervention.

Vision improvement in retinal degeneration patients by implantation of retina together with retinal pigment epithelium

PURPOSE

To demonstrate efficacy and safety of the implantation of neural retinal progenitor cell layers (sheets) with its retinal pigment epithelium (RPE) in retinitis pigmentosa (RP) and dry age-related macular degeneration (AMD) patients with 20/200 or worse vision in the surgery eye.

DESIGN

Interventional nonrandomized clinical trial.

METHODS

Ten patients (six RP, four AMD) received retinal implants in one eye and were followed in a phase II trial conducted in a clinical practice setting. Early Treatment Diabetic Retinopathy Study (EDTRS) was the primary outcome measure. All implant recipients and nine of 10 tissue donors were deoxyribonucleic acids typed.

RESULTS

Seven patients (three RP, four AMD) showed improved EDTRS visual acuity (VA) scores. Three of these patients (one RP, two AMD) showed improvement in both eyes to the same extent. Vision in one RP patient remained the same, while vision in two RP patients decreased. One RP patient has maintained an improvement in vision from 20/800 to 20/200 ETDRS for more than five years; at the six-year examination, it was still maintained at 20/320 while the nonsurgery eye had deteriorated to hand motion vision. This patient also showed a 22.72% increase in light sensitivity at five years compared to microperimetry results at two years; the other patients showed no improved sensitivity. Although no match was found between donors and recipients, no rejection of the implanted tissue was observed clinically.

CONCLUSIONS

Seven (70%) of 10 patients showed improved VA. This outcome provides clinical evidence of the safety and beneficial effect of retinal implants and corroborates results in animal models of retinal degeneration.

Four stages and lack of stable accommodation in chronic alloantibody-mediated renal allograft rejection in Cynomolgus monkeys

The etiology of immunologically mediated chronic renal allograft failure is unclear. One cause is thought to be alloantibodies. Previously in Cynomolgus monkeys, we observed a relationship among donor-specific alloantibodies (DSA), C4d staining, allograft glomerulopathy, allograft arteriopathy and progressive renal failure. To define the natural history of chronic antibody-mediated rejection and its effect on renal allograft survival, we now extend this report to include 417 specimens from 143 Cynomolgus monkeys with renal allografts. A subset of animals with long-term renal allografts made DSA (48%), were C4d positive (29%), developed transplant glomerulopathy (TG) (22%) and chronic allograft arteriopathy (CAA) (19%). These four features were highly correlated and associated with statistically significant shortened allograft survival. Acute cellular rejection, either Banff type 1 or 2, did not correlate with alloantibodies, C4d deposition or TG. However, endarteritis (Banff type 2) correlated with later CAA. Sequential analysis identified four progressive stages of chronic antibody-mediated rejection: (1) DSA, (2) deposition of C4d, (3) TG and (4) rising creatinine/renal failure. These new findings provide strong evidence that chronic antibody-mediated rejection develops without enduring stable accommodation, progresses through four defined clinical pathological stages and shortens renal allograft survival.

HLA class II-like antiidiotypic antibodies from highly sensitized patients inhibit T-cell alloresponses

The purpose of this study is to identify factors in the sera of highly sensitized (HS) patients (pts) that inhibit T-cell alloresponses. An in vitro assay was used to measure HLA class I and class II-like antiidiotypic antibodies (anti-ids). The stimulation index (SI) was used to measure PBL and T-cell responses to alloantigens. All HS sera (32 pts) and the IgG fraction inhibited PBL and CD4(+) T-cell responses to alloantigens. The SI with HS IgG was 7.9 +/- 1.7 as compared to 31.5 +/- 5.9 with normal IgG (p = 0.0003). In a subset of pts who were transiently sensitized, the SI was 6.6 +/- 1.0 with a high panel reactive antibody (PRA), but when their PRA was zero, the SI was 17.8 +/- 1.3 (p = 0.0000001). Anti-ids were found in 100% of 17 pts with a high PRA. The T-cell inhibitory factors reduced CD4(+) T-cell responses of HS pts to alloantigens in the presence of autologous anti-ids, were MHC restricted and were inactivated by in vitro generated antibodies to HLA class II-like anti-ids. The HLA class II-like anti-id IgG molecules bind to the TCR of CD4(+) T cells and may impair their ability to help in the downregulating antibody response to anti-ids.

Renal transplantation in patients with pre-transplant donor-specific antibodies and negative flow cytometry crossmatches

The clinical significance of pre-transplant donor-specific antibodies (DSA), despite negative cytotoxicity and flow cytometry crossmatches (FCXMs), is unknown. We performed a retrospective cohort study of 60 living donor renal transplant recipients, all with pre-transplant cytotoxicity and T-cell and B-cell FCXMs that were negative. Twenty recipients had pre-transplant DSA detected by enzyme-linked immunosorbent assays (ELISA) and/or microbead methods. Forty contemporaneous DSA-negative controls were selected. In the DSA-positive group, after a median follow-up of 8.2 months (25-75% range, 5.4-22.8 months), patient survival was 100% and allograft survival was 95.0%. Acute humoral rejection (AHR) developed in four patients (20.0%). Three of the AHR episodes occurred within the first month post-transplant. Median serum creatinine at last follow-up was 1.3 mg/dL (25-75% range, 1.0-1.6 mg/dL), versus 1.1 mg/dL (25-75% range, 0.9-1.4 mg/dL) in the DSA-negative controls (p = 0.29). Only one of the 40 controls developed AHR (2.5%). Pre-transplant DSA was associated with a significantly increased incidence of AHR (p = 0.02 by log-rank test). In conclusion, despite negative pre-transplant cytotoxicity and FCXMs, renal transplant recipients with pre-transplant DSA detected by solid-phase methods may have an increased incidence of AHR and require close monitoring post-transplant.

Acute antibody-mediated rejection following heart transplantation

Acute antibody-mediated rejection (AMR) in heart transplantation is often associated with hemodynamic compromise, and is associated with increased mortality and development of accelerated transplant coronary artery disease (TCAD). The diagnosis of AMR has historically been controversial and outcomes with aggressive immunosuppressive therapy including plasmapheresis and cyclophosphamide are poor. Advances in diagnostic techniques like the demonstration of immunopathologic evidence for antibody-mediated rejection by deposition of the complement split product C4d in tissue and detection of anti-HLA antibodies by flow cytometry will assist in further characterizing AMR. Immunosuppression targeting B-lymphocytes and use of m-TOR inhibitors to alter the predilection to develop TCAD and improve survival in AMR remains to be proven.

HLA-specific B cells: II. Application to transplantation

BACKGROUND

Differences in the antibody response to allogeneic transplantation exist between groups defined by race or gender. These differences may reflect differences in immune competency and/or exposure to alloantigens. We have investigated the frequencies and phenotypes of HLA-specific B cells to address those possibilities.

METHODS

HLA-specific B cells were identified by staining with HLA tetramers (tet) as described previously and the distribution of CD27 and CD38 among those cells were measured in groups defined by various parameters. Possible correlation between frequencies of HLA-specific B cells and production of HLA-specific antibody after transplantation was also investigated.

RESULTS

We found no correlation between the frequencies of CD27+tet+ (33%-44% vs. 34%-36%) or CD38+tet+ (57%-65% vs. 59%-66%) B cells and a previous mismatch for the HLA antigen of the tetramer. However, there was an increase in CD38+tet+ B cells among patients making antibody to the tetramer antigen (67%-72% vs. 53%-56%). Blacks had lower frequencies of CD27+ B cells than did whites (11.8% vs. 28.9%, P=0.003), but had greater increases of these cells among tet+ cells than did whites. There was a higher frequency of tet+ B cells among patients who developed "new" antibody to the HLA antigen (3.9%-8.6%) of the tetramer after transplantation than among those who did not (1.1%-3.7%).

CONCLUSIONS

The phenotype of HLA-specific B cells reflects current or historic sensitization to HLA and may reflect inherent differences between groups defined by race and/or gender. The frequencies of HLA-specific B cells may predict patients at risk for production of donor-specific antibody after transplantation.

HLA-specific B cells: I. A method for their detection, quantification, and isolation using HLA tetramers

BACKGROUND

The development of highly sensitive and specific assays for detecting and characterizing HLA-specific antibodies has contributed to an appreciation of the extensive involvement of those antibodies in graft injury and dysfunction. However, understanding the regulatory processes of the humoral response to transplantation and the mechanisms underlying therapeutic agents and protocols for preventing and treating sensitization requires a way to study HLA-specific B cells.

METHODS

Lymphocyte preparations enriched for B cells were stained with one or more of three different HLA tetramers. Tetramer-positive (tet+) B cells were enumerated and evaluated for an association of their frequencies with known sensitization. In some cases, tet+ B cells were isolated and placed in culture with supplements known to activate B cells in a nonspecific fashion.

RESULTS

For all tetramers used, the frequencies of tet+ B cells were significantly higher (4.1%-5.5%) among sensitized patients than among nonsensitized patients (1.6%-3.2%, P<0.001). Binding of the tetramers occurred by the surface immunoglobulin antigen receptor with little or no binding to antibody captured in the Fc receptor. Cultured tet+ B cells produced antibodies specific for epitopes of the tetramer antigen. There appeared to be a certain amount of crossreactivity in the binding of tetramers. The frequency of CD27+ cells among tet+ B cells was higher, on average (34.4%-38.8%) than among all B cells (26.2%) whereas the frequencies of CD38 were comparable in the two groups.

CONCLUSIONS

Staining with HLA tetramers provides a means for identifying, quantifying, and isolating HLA-specific B cells.

Determinants of the complement-fixing ability of recipient presensitization against HLA antigens

BACKGROUND

The presence of preformed alloantibodies with the ability to activate complement may pose a particular risk for kidney allograft rejection. The aim of this study was to evaluate variables that determine the complement-fixing capability of human leukocyte antigen (HLA) sensitization.

METHODS

Sixty-five sensitized patients with > or =10% pretransplant panel-reactive antibody (PRA) levels uncovered by immunoglobulin G [IgG]FlowPRA HLA class I and/or class II screening were included. Applying modified FlowPRA screening, sera were evaluated for patterns of alloreactive IgG subclasses and IgM, and, in parallel, for their complement-activating ability assessed by flow cytometric detection of human complement split product deposition ([C4d]FlowPRA).

RESULTS

Approximately two-thirds (68%) of tested sera were found to contain complement-fixing alloreactivity (> or =10%[C4d]FlowPRA). IgG1 type panel reactivity was predominant (detectable HLA class I and II reactivity in 93% and 91% of IgG-positive sera), followed by IgG3 (49%/44%), IgG2 (44%/27%), and IgG4 (19%/11%). Applying partial correlation we found an independent correlation of both %[IgG1]FlowPRA and %[IgG3]FlowPRA with %[C4d]FlowPRA reactivities (P< or =0.01). In addition, for IgG1 a contribution of the amount of bound alloantibody to complement-fixation was observed. Complement-fixation was also favored by the simultaneous presence of alloreactive IgG1, IgG3, and IgM. Previous grafting, but not pregnancy and transfusion, was independently associated with complement-fixing sensitization (P<0.05), presumably due to increased IgG1 type reactivity.

CONCLUSIONS

Anti-HLA antibody-triggered complement activation is dependent on both the pattern of Ig reactivities and the amount of bound antibody. Previous transplantation represents a major risk factor for the development of complement-fixing sensitization.

Prolonged cardiac allograft survival in presensitized rats after a high activity Yunnan-cobra venom factor therapy

Complement-dependent antibody-mediated acute humoral rejection is the major obstacle of clinical transplantation across ABO incompatibility and human leukocyte antigen presensitization. We previously demonstrated that Yunnan-cobra venom factor (Y-CVF) could almost completely abrogate complement activity and successfully prevent hyperacute rejection in some xenotransplant models without any obvious toxicity. In this study we investigated whether depletion of complement by Y-CVF prevented acute humoral allograft rejection in presensitized rats thereby prolonging graft survival.

METHODS

Presensitization was achieved in Lewis rats by sequential grafting of three full-thickness skin pieces from Brown Norway rats. Serum cytotoxic alloantibody titers were determined by a modified in vitro complement-dependent microcytotoxicity assay. After presensitization, each Lewis rat received a heterotopic Brown Norway cardiac allograft. Fifteen recipients were divided into two groups: (1) no treatment control (n = 7); (2) Y-CVF therapy group (86 u/kg, IV, day -1) (n = 8). After cessation of the heart beat, allograft rejection was confirmed by pathologic as well as IgG and C3 immunohistochemical examinations.

RESULTS

The mean graft survival time was significantly prolonged to 99.50 +/- 38.72 hours among rats that received Y-CVF vs 12.71 +/- 13.94 hours in nontreated controls (P < .001). Upon pathological and immunohistochemical examination, acute humoral rejection was mainly exhibited in the control group, whereas acute cellular rejection was mainly displayed in the Y-CVF therapy group.

CONCLUSIONS

Our study demonstrated that complement depletion by Y-CVF significantly inhibited acute humoral allograft rejection in presensitized rats. As a therapeutic immunointervention tool for complement, Y-CVF has shown potential efficacy across ABO incompatible and positive cross-match barriers.

Histocompatibility Testing for Highly Sensitized Transplant Candidates

HLA antibodies can develop for many different reasons and are often present in the serum of patients awaiting renal transplantation. These antibodies can limit the patient's ability to find a compatible donor and are a risk factor for poor graft outcome. Detection of HLA antibodies is important for identifying compatible donors, assessing the pretransplantation risk for rejection, and diagnosing humoral rejection posttransplantation. The results of antibody screens are also useful when interpreting crossmatch results. Assays used for detecting panel-reactive and donor-specific HLA antibodies, including cytotoxicity, flow cytometry, and solid phase assays, are described in this review.

Prediction of crossmatch outcome of highly sensitized patients by single and/or multiple antigen bead luminex assay

BACKGROUND

We used a solid-phase assay to identify human leukocyte antigen (HLA) Class I and II specificities in highly reactive sera, and applied this information to predict crossmatch outcome with greater than 90% accuracy.

METHODS

Sera from 20 highly sensitized end-stage renal disease patients reactive to 70-100% of HLA Class I and II antigen panel were analyzed by single and/or multiple antigen flow microparticle bead assay using Luminex platform (Luminex assay). These sera contained antibodies directed against high frequency public HLA class I and/or II epitopes accounting for 70-100% of serum's total reactivity. More than 2,000 complement dependent cytotoxicity (CDC) and 200 flow crossmatches (FLXM) were performed using sera from these patients and deceased donor T and B lymphocytes.

RESULTS

Luminex serum analysis was able to correctly predict outcomes of 95% of T and B cell FLXM. In contrast, predictive values for the CDC T and B cell crossmatches by Luminex serum analysis were only 77% and 75%, respectively. The use of serum analysis in donor selection would have reduced the total number of required FLXM by more than 50%. The frequency of negative T cell FLXM was 56% when donors were selected randomly; however, if serum antibody analysis had been used to preselect the donors by excluding donors that were likely to be positive, the frequency of corresponding negative crossmatches would have increased up to 93%.

CONCLUSION

This approach to donor selection may allow wider geographic sharing of cadaver donor organs without actually performing the crossmatch.

Pivotal role of complement-fixing HLA alloantibodies in presensitized kidney allograft recipients

Recipient presensitization represents a major risk factor for kidney allograft loss. Complement fixation may be a critical attribute of deleterious alloantibodies. We investigated clinical impact of complement-fixing HLA presensitization employing [C4d]FlowPRA, a novel assay permitting selective detection of HLA panel reactive antibody (PRA)-triggered C4 complement split product deposition. A cohort of 338 kidney transplants was evaluated for presensitization applying [C4d]FlowPRA together with [IgG]FlowPRA and complement-dependent cytotoxicity (CDC)-PRA. Analysis of HLA class I alloreactivities revealed a high incidence of C4d-positive graft dysfunction in [IgG]FlowPRA(+)/[C4d]FlowPRA(+) and [IgG]FlowPRA(+)/[C4d]FlowPRA(-) recipients (23% and 22% vs. 3% in [IgG]FlowPRA(-) patients). Only patients with complement-fixing HLA class I immunization had inferior graft survival [75% (3 years) vs. 91% and 89%, respectively (p=0.036)]. Despite frequent finding of capillary C4d deposition (28%), complement-fixing HLA class II immunization was not associated with inferior survival rates. This may have been due to reduction of clinical effects by intense immunosuppression in presensitized patients. Evaluating CDC, 29% of CDC-PRA(+)/[C4d]FlowPRA(+) recipients had C4d-positive graft dysfunction. For these patients 3-year graft survival was worst, followed by CDC-PRA(+)/[C4d]FlowPRA(-) and CDC-PRA(-) patients (76% vs. 81% vs. 90%, p=0.014). Results highlight a strong impact of complement-fixing HLA presensitization. Discerning complement-activating abilities of HLA alloantibodies, [C4d]FlowPRA may help identify recipients at particularly high risk for graft rejection and loss.

Diagnostic value of C4d in renal biopsies

PURPOSE OF REVIEW

Capillary C4d is now an established marker of antibody-mediated rejection in graft biopsies. The technique is widely used to further define the clinical relevance of humoral alloreactivity in various patient subgroups. These include highly sensitized patients, recipients with late graft failure and also some with 'stable' graft function.

RECENT FINDINGS

The C4d technique compares favourably with other techniques that are explored, for example detection of C3d. Capillary C4d can be associated with any graft pathology, including transplant glomerulopathy. C4d is related to circulating alloantibodies but not autoantibodies, and is probably not derived from local sources. Presensitization and de-novo sensitization are important scenarios of humoral alloreactivity that require refined analysis and treatment.

SUMMARY

Detection of C4d in graft biopsies has emerged as an important tool that could substantiate the clinical significance of antibody-mediated rejections. The comprehensive analysis of humoral alloreactivity in the posttransplantation period is still ongoing and will hopefully result in improved patient care and better long-term graft survival.