Desensitization strategies enabling successful renal transplantation in highly sensitized patients

Abstract:  Currently, the number of highly sensitized patients awaiting a renal transplant is increasing on the waiting lists of different organ exchange organizations. Due to the presence of antibodies against a broad variety of human leukocyte antigen (HLA) specificities, highly sensitized patients have a markedly reduced chance of receiving a crossmatch-negative organ. It has long been recognized that hyperacute rejection is associated with the presence of donor-specific anti-HLA antibodies at the time of transplantation. Meanwhile treatment protocols have been developed to achieve successful transplantation across antibody barriers. Therefore, the presence of donor-specific anti-HLA antibodies and a positive serological crossmatch are no longer considered as an absolute contraindication to renal transplantation. Mainly, two desensitization protocols have been established in order to overcome a positive crossmatch or to enhance the chance of highly sensitized patients to receive a crossmatch-negative organ: high-dose intravenous immunoglobulin (IVIg) or low-dose IVIg in combination with plasmapheresis. Herein, we summarize the characteristics of these two treatment regimes along with other alternative approaches that are currently used for the management of kidney graft recipients with broad alloantibody reactivity against potential kidney donors.     

Successful renal transplantation despite low levels of donor-specific HLA class I antibody without IVIg or plasmapheresis

We prospectively transplanted 10 primary kidney recipients with deceased donor organs (nine kidney and one pancreas/kidney) when their flow cytometric T-cell IgG, HLA class I donor-specific crossmatch was positive but the AHG T-cell crossmatch was negative, with a median follow-up of 1.8 yr. No pre- or peri-operative IVIg or plasmapheresis was administered to any patient. All but one of the 11 organs transplanted into patients with a flow T(+)/AHG(-) crossmatch is currently functioning despite the continued presence of circulating low levels of HLA class I antibody. Flow HLA class I antigen-coated beads showed the presence of at least one donor-specific HLA class I antibody at transplantation in each of the 10 cases. No rejections were observed in seven of the 10 cases (70%). Six rejection episodes, four cellular and two humoral, occurred in three patients. Each rejection was successfully treated. The only graft loss occurred in a kidney recipient on day 667 secondary to ischemia to the kidney because of cardiac surgery. Thus, short-term (one to two years) graft survival in primary transplants was not influenced by low levels of donor-specific HLA class I antibody present at transplantation and no prophylactic treatment such as IVIg, plasmapheresis, anti-CD20 or splenectomy was needed peri-operatively.     

Desensitization in kidney transplantation: review and future perspectives

More than half of the kidney transplant candidates awaiting transplantation are sensitized to human leukocyte antigens (HLAs). Desensitization to HLAs involves treatment with immunomodulating therapies designed to reduce levels of anti‐HLA antibodies in order to make kidney transplantation possible. Over the last two decades, desensitization therapies have been limited to plasmapheresis (PP), immunoadsorption (IA), intravenous immunoglobulins (IVIg), and rituximab. Review of reported experiences with desensitization in kidney transplant candidates revealed that PP or IA alone is inadequate to achieve durable reductions in HLA antibodies. Increasing evidence has accumulated indicating that high‐dose IVIg has limited ability to reduce HLA antibodies, but a few centers have reported success with high‐dose IVIg+rituximab in non‐randomized trials. Overall experience in multiple centers, however, has shown high antibody‐mediated rejection (AMR) rates, particularly in patients with the highest degrees of HLA sensitization. Low‐dose IVIg combined with alternate day PP in living donor transplant candidates has been shown to provide enhanced survival over dialysis. However, low‐dose IVIg/PP regimens also continue to be associated with unacceptable AMR rates. Recent experiences with plasma cell‐targeted therapies based on the proteasome inhibitor bortezomib are relatively small but may represent an important alternative to non‐deletional strategies with IVIg.     

A predictive test for the efficacy of intravenous immunoglobulin in the inhibition of alloantibodies

Preformed recipient HLA-specific antibodies can cause hyperacute rejection of a transplanted kidney if they are directed against mismatched donor HLA antigens. To avoid hyperacute rejection it is essential that recipient antibodies be identified during patient workup for transplantation and HLA antigens to which a patient is sensitized then be avoided when selecting a kidney donor for them. Absence of donor-specific reactivity is then confirmed by a pretransplantation crossmatch test.     

Successful Renal Transplantation across Simultaneous ABO Incompatible and Positive Crossmatch Barriers

ABO incompatibility and human leukocyte antigen (HLA) sensitization remain the two largest barriers to optimal utilization of kidneys from live donors. Here we describe the first successful transplantation of patients who were both ABO incompatible and crossmatch positive with their only available donor. A preconditioning regimen of plasmapheresis (PP) and low-dose CMV hyperimmune globulin (CMVIg) was delivered every other day until donor-specific antibody (DSA) titers were reduced to a safe level and isoagglutinin titers were < or =16. Each patient received quadruple sequential immunosuppression, splenectomy and three protocol post-transplant PP/CMVIg treatments. There was no hyperacute rejection. Two of the three patients had a persistent positive cytotoxic crossmatch on the day of transplant and eliminated their DSA subsequently. Antibody-mediated rejection (AMR) in one patient was reversed by reinitiating PP/CMVIg and anti-CD20. The patients are more than 9 months post-transplant with excellent graft function. Preconditioning with PP/CMVIg results in a durable suppression of DSA and permits accommodation of the allograft to a discordant blood type. The ability to cross these two barriers simultaneously is clinically important as sensitized patients have often exhausted their blood type compatible living donors during previous transplants.     

Persistence of low levels of alloantibody after desensitization in crossmatch-positive living-donor kidney transplantation

BACKGROUND:: Desensitization protocols have been developed to allow successful kidney transplantation in sensitized recipients. However, a detailed analysis of the impact of these protocols on alloantibody has not been performed. METHODS:: We studied 12 living-donor kidney-transplant recipients with positive antihuman globulin-enhanced complement dependent cytotoxicity (AHG-CDC) crossmatches against their donors. Using a variety of crossmatch techniques and single-antigen flowbeads (SAFBs), we characterized the specificity and amount of alloantibody at baseline before desensitization, after desensitization (using plasmapheresis followed by 100 mg/kg intravenous immunoglobulin, and anti-CD20 antibody), and 4 months after transplantation (after splenectomy and on maintenance immunosuppression). RESULTS:: All 12 patients with a positive baseline AHG-CDC crossmatch were AHG-CDC crossmatch negative at the time of transplant (after desensitization). However, despite desensitization, the majority of patients had low-level donor-specific alloantibodies demonstrable on the day of transplantation by both flow crossmatch (FXM 8/12) and SAFBs (10/11). Four months after transplantation, no patient had a positive AHG-CDC crossmatch, but again the majority had persistent low levels of donor-specific alloantibodies by FXM (6/12) and SAFBs (9/11). No patient experienced hyperacute rejection, and the persistence of low levels of donor-specific alloantibodies did not correlate with the development of humoral rejection in the early posttransplant period. CONCLUSIONS:: Despite desensitization, a majority of positive crossmatch transplant recipients demonstrate low levels of donor-specific alloantibodies both on the day of transplant and 4 months after transplantation. The impact of these antibodies appears to be minimal early after transplant, but their long-term significance bears further study.     

Liver transplantation and the lymphocyte crossmatch

Hyperacute rejection related to donor-specific antibodies rarely occurs in liver transplantation, probably because of the role of Kupffer cells, sinusoidal cells, and the dual blood supply. However, the long-term outcome of liver grafts, transplanted in the context of a positive lymphocytotoxic crossmatch, is approximately 20% lower than that of grafts with a negative crossmatch. Without recipient selection based on the crossmatch, which is currently the generally accepted policy, 11% to 24% of the liver transplants will be performed with a positive crossmatch. Using the level of panel reactive antibodies, (eg > 30%) as an indicator for sensitization, a lymphocytotoxic crossmatch can be performed before transplantation on a limited number of patients, to avoid a positive crossmatch. Sometimes the medical urgency does not allow the patient to wait for a negative crossmatch. Therefore, plasmapheresis should be performed before transplantation to diminish the antibody level and the posttransplant use of prostaglandin-E and high-dose steroids should be considered to avoid the short-term immunologic complications. These measures require more study before they can become standard practices. Possibly, Mycophenolate Mofetil might be effective in preventing chronic rejection. The introduction of the flowcytometry crossmatch procedure will not only provide a more sensitive, but also a faster procedure (~4 hours), allowing a wider use of the pretransplant crossmatch test in liver transplantation. However, the increased sensitivity and speed are balanced by the much lower specificity of the positive flowcytometric crossmatch test. The most appropriate way to use flow techniques for lymphocyte crossmatching has not yet been clearly determined because of the specificity problem. In the setting of other solid organ transplantation, the presence of preformed donor-specific antibodies is linked with occurrence of hyperacute or accelerated rejection, and is likely to result in graft failure in a short period of time. Therefore, a positive crossmatch directed against donor HLA antigens is a concern in organ transplantation, especially in kidney and heart transplantation. A positive IgG crossmatch test precludes transplantation in most cases. In liver transplantation, the picture is not so clear, but probably represents a relative contraindication to transplantation.     

Overcoming immunologic incompatibility: transplanting the difficult to transplant patient

Immunologic incompatibilities between donor and recipient have limited the access to renal transplantation for many patients. Previously the presence of donor-specific alloantibodies directed against donor major histocompatibility complex (MHC) antigens or natural antibodies directed against donor ABO blood group antigens was considered an absolute contraindication to renal transplantation. However, with the current understanding of humoral immune responses, superior immunosuppressive agents, and improved diagnosis and treatment of antibody-mediated rejection, renal transplantation can be safely performed with outstanding results despite the presence of donor-specific antibody. In this review we discuss the biology of antibody-mediated rejection and sensitization. We discuss the diagnostic tests necessary to characterize the type, affinity, and avidity of the donor-directed antibodies. Current methods for performing renal transplants across ABO and human leukocyte antigen (HLA)-sensitized barriers are covered, including the potential morbidities. The rest of the review focuses on advances in managing these antibodies to increase the likelihood of receiving a deceased donor kidney or allow transplantation from a living donor against whom one has a prohibitive antibody.     

Detection of alloantibodies against non-HLA antigens in kidney transplantation by flow cytometry

The presence of alloantibodies against human leucocyte antigen (HLA)-I and HLA-II antigens has been associated with hyperacute and accelerated graft rejections. However, occasionally these rejections occur in patients without donor-specific anti-HLA antibodies, suggesting the presence of other antigenic complexes that are shared by the graft and other cell populations. Usually, these antibodies are not routinely studied and their role in graft rejection is poorly understood. For this reason, we tested, by flow cytometry, the presence of panel-reactive alloantibodies (PRA) using different cell populations in 30 pre-transplant sera of kidney graft recipients. The patients studied had or had not hyperacute and accelerated rejection episodes (HARE) and did not have alloantibodies against HLA of their specific donors. We found that IgG and IgM alloantibodies directed against HLA-I antigens, different to the HLA-I antigens of the specific donors, as well as IgG against endothelium/monocyte antigens, IgM against platelets, and IgM against T cells are significantly associated with HARE, independently of the percentage of PRA. Our findings suggest that the detection of antibodies by flow cytometry against non-major histocompatibilty complex antigens may be useful as a pre-transplant crossmatch in living related donor kidney transplants to diminish the incidence of HARE.     

Activated HLA class I-reactive cytotoxic T lymphocytes associated with a positive historical crossmatch predict early graft failure

BACKGROUND: Preexisting alloantibodies against the mismatched HLA class I antigens of the donor, when present in current sera, are believed to be detrimental for kidney graft survival. The relevance of a positive crossmatch with historical sera only is still a matter of debate. Previous studies showed a correlation between the presence of alloantibodies and the presence of alloactivated cytotoxic T lymphocytes (CTLs). We wondered whether the persistence of activated CTLs might explain the poor results in a proportion of patients with a historical positive crossmatch. METHODS: We tested 10 sensitized patients to determine whether activated CTLs persist when the antibodies disappear. Limiting dilution assays were performed in the presence and absence of cyclosporine (CsA) to distinguish between activated (primed) (CsA resistant) and naive (CsA sensitive) CTLs. To test the clinical relevance of the persisting CTLs, eight sensitized patients, who underwent a kidney transplantation across a positive historical crossmatch, were retrospectively tested for the presence or absence of activated donor-specific CTLs at the day of transplantation. RESULTS: In the first group, four patients had CsA-sensitive CTLs, three patients had CsA-resistant CTLs, and three other patients had CsA-sensitive CTLs for a particular HLA antigen and CsA-resistant CTLs for another HLA antigen. In the transplant group, four patients with CsA-sensitive CTLs at the day of transplantation were found to have a good graft function. In the other four patients, the presence of CsA-resistant donor-specific CTLs was associated with rejection and early graft loss. CONCLUSION: The present study suggests that determining the activation state of CTLs specific for the HLA mismatch against which antibodies were present in historical sera, may be relevant to transplant outcome in patients who undergo transplantation across a positive historical crossmatch.     

Assignment of C1q-binding HLA antibodies as unacceptable HLA antigens avoids positive CDC-crossmatches prior to transplantation of deceased donor organs

Soon, a virtual crossmatch shall replace the complement-dependent cytotoxicity (CDC) allocation crossmatch in the Eurotransplant region. To prevent positive CDC-crossmatches in the recipient centre, careful definition of unacceptable antigens is necessary. For highly sensitized patients, this is difficult by CDC alone. Assignment of all antibodies detected by sensitive assays, however, could prevent organ allocation. To assess the usefulness of the Luminex C1q-assay to prevent positive CDC-crossmatches, all CDC-crossmatches performed prior to deceased kidney transplantation in a 16-month-period were reviewed. Sera causing positive crossmatches were investigated by the C1q-assay. 31 out of 1432 crossmatches (2.2%) were positive. Sera involved in 26 positive crossmatches were available. C1q-binding donor-specific antibodies were detected in 19 sera (73.1%). The other sera were from recipients without any HLA antibodies detectable by CDC or common solid phase assays. Three patients had known Non-HLA antibodies causing positive CDC-results. Four crossmatches were only weak positive. Therefore, avoidance of donors with HLA antigens against whom C1q-binding antibodies were detected would have prevented all positive crossmatches due to HLA antibodies. Provided that all HLA specificities against which antibodies are detected by the Luminex C1q-assay are considered as unacceptable antigens, CDC-crossmatches prior to transplantation might safely be omitted in many patients. They should be maintained in highly immunized patients, however, for whom assignment of all C1q-positive antibodies as unacceptable antigens could lead to a significant delay or even prevention of transplantation.     

Immunological status in three patients, thirty years after living related renal transplantation: antibody production in long-term survivors

There have been few studies of the immune status of long-term follow-up patients after living-donor kidney transplantation. We investigated the immune status from the immunologic and pathologic standpoint of three long surviving recipients who had received renal grafts more than 30 years earlier. Anti-HLA antibodies that had not been present before transplantation were detected in one recipient with three HLA mismatches. One recipient with identical HLA showed positive for the crossmatch test, but not for the panel reactive antibody test (PRA), thus showing that this patient had HLA antibodies against HLA minor histocompatibility antigens, etc. Only one patient with established microchimerism was stable without any antibody production. Pathologically, chronic allograft nephropathy with C4d staining suggestive of antibody-mediated rejection was observed in both patients with HLA antibodies. Physicians should clinically manage patients by always bearing in mind the presence of anti-donor antibodies during long-term regular follow-up of transplanted kidneys.     

The Utility of Splenectomy as Rescue Treatment for Severe Acute Antibody Mediated Rejection

Antibody-mediated rejection (AMR) after desensitization for a positive crossmatch (+XM) live donor renal transplant can be severe and result in sudden onset oliguria and loss of the allograft. Attempts to rescue these kidneys using plasmapheresis (PP) and IVIg may be ineffective due to the magnitude of antibody burden that must be controlled to prevent renal thrombosis or cortical necrosis. We review our experience using splenectomy combined with PP/IVIg as rescue therapy for patients experiencing an acute deterioration in renal function and a rise in donor-specific antibody within the first posttransplant week after desensitization for a +XM. Five patients underwent immediate splenectomy followed by PP/IVIg and had return of allograft function within 48 h of the procedure. Emergent splenectomy followed by PP/IVIg may be an effective treatment for reversing severe AMR.     

The association of antivascular endothelial cell antibody with hyperacute rejection: a case report

Early graft loss almost always occurs when recipients of a renal allograft develop antibody directed against antigens specific for donor vascular endothelial cells (VECs) and peripheral blood monocytes. In studies involving recipients of human leukocyte antigen identical, living-related grafts exhibiting preformed antibody to the VEC antigens of their donors, the median onset of rejection was 3 days after transplantation. Although preformed antibody to VEC antigens has been related in numerous articles to early graft loss, there has never been a published report of anti-VEC antibody leading to hyperacute rejection. We report a patient who hyperacutely rejected a renal allograft after undergoing a donor-specific transfusion protocol with her mother in which the kidney was removed in less than 24 hours. Nine months later the patient had a retransplantation with an allograft from a cadaveric donor. The cadaveric graft was again hyperacutely rejected, and this kidney was removed immediately. Anti-VEC/monocyte antibody directed against both donors was detected in the patient's pretransplant sera. With the exception of a positive B-lymphocyte crossmatch with her mother, all the standard crossmatches were negative.     

Antibody-mediated rejection of a unilateral donor lung in bilateral living-donor lobar lung transplantation: report of a case

We report a case of antibody-mediated rejection (AMR) of a unilateral donor lung in the presence of newly formed donor-specific antibodies, 10 months after living-donor lobar lung transplantation (LDLLT). Of note is that the AMR occurred in the unilateral lung. Furthermore, the lung graft was from her husband and HLA analysis on the recipient's daughter revealed the same donor-specific HLA antigens, which strongly suggested pre-sensitization before lung transplantation. Fortunately, we could perform direct crossmatch even 1 year after lung transplantation because of the living donors.     

Hyperacute rejection in ABO-incompatible kidney transplantation: Significance of isoagglutinin subclass

IntroductionABO-incompatible transplantation has expanded the limited donor pool for kidney transplantation. Despite the successful desensitization protocols and immunosuppression, undesirable cases of hyperacute rejection occurs.ObjectiveFlow cytometry was used to measure isoagglutinin titer and its IgG subclasses in assessment of the cause of hyperacute rejection in ABO-incompatible kidney transplantation.Materials and methodsThe recipient was admitted for kidney transplantation due to end-stage renal disease. Pre-transplantation work-up for ABO-incompatible kidney transplantation included blood group typing, HLA DNA typing and HLA antibody analyses. HLA crossmatch analysis was conducted using donor lymphocytes and anti-HLA antibody assay using Luminex panel reactive antibody test (One Lambda, Inc., Canoga Park, CA). Desensitization protocol was composed of therapeutic plasma exchange sessions and rituximab.ResultsDespite negative HLA crossmatch results, a case of hyperacute rejection occurred after living donor kidney transplantation. Rejection resulted in immediate removal of graft, and the patient later received a second kidney transplantation. Retrospective evaluation of isoagglutinin titer and its subclasses using flow cytometry identified the cause of rejection to increased IgG1 subclass. Desensitization protocol for ABO-incompatible kidney transplantation now implements further caution for blood group O recipients.DiscussionHyperacute rejection resulting from increased IgG1 isoagglutinin subclass has not been previously confirmed using flow cytometry. Unfortunate outcome of this rejection case provides insight to how we should approach and ensure successful ABO-incompatible kidney transplantation.     

Donor Kidney Exchanges

Kidney transplantation from live donors achieves an excellent outcome regardless of human leukocyte antigen (HLA) mismatch. This development has expanded the opportunity of kidney transplantation from unrelated live donors. Nevertheless, the hazard of hyperacute rejection has usually precluded the transplantation of a kidney from a live donor to a potential recipient who is incompatible by ABO blood type or HLA antibody crossmatch reactivity. Region 1 of the United Network for Organ Sharing (UNOS) has devised an alternative system of kidney transplantation that would enable either a simultaneous exchange between live donors (a paired exchange), or a live donor/deceased donor exchange to incompatible recipients who are waiting on the list (a live donor/list exchange). This Regional system of exchange has derived the benefit of live donation, avoided the risk of ABO or crossmatch incompatibility, and yielded an additional donor source for patients awaiting a deceased donor kidney. Despite the initial disadvantage to the list of patients awaiting an O blood type kidney, as every paired exchange transplant removes a patient from the waiting list, it also avoids the incompatible recipient from eventually having to go on the list. Thus, this approach also increases access to deceased donor kidneys for the remaining candidates on the list.     

Living donor kidney transplantation in crossmatch-positive patients enabled by peritransplant immunoadsorption and anti-CD20 therapy

Living donor kidney transplantation in crossmatch-positive patients is a challenge that requires specific measures. Ten patients with positive crossmatch results (n = 9) or negative crossmatch results but strong donor-specific antibodies (DSA; n = 1) were desensitized using immunoadsorption (IA) and anti-CD20 antibody induction. IA was continued after transplantation and accompanied by HLA antibody monitoring and protocol biopsies. After a median of 10 IA treatments, all patients were desensitized successfully and transplanted. Median levels of mean fluorescence intensity (MFI) of Luminex-DSA before desensitization were 6203 and decreased after desensitization and immediately before transplantation to 891. Patients received a median of seven post-transplant IA treatments. At last visit, after a median follow-up of 19 months, 9 of 10 patients had a functioning allograft and a median Luminex-DSA of 149 MFI; serum creatinine was 1.6 mg/dl, and protein to creatinine ratio 0.1. Reversible acute antibody-mediated rejection was diagnosed in three patients. One allograft was lost after the second post-transplant year in a patient with catastrophic antiphospholipid syndrome. We describe a treatment algorithm for desensitization of living donor kidney transplant recipients that allows the rapid elimination of DSA with a low rate of side effects and results in good graft outcome.     

Formation of donor-specific human leukocyte antigen antibodies after kidney transplantation: correlation with acute rejection and tapering of immunosuppression

BACKGROUND: Before kidney transplantation, a serological crossmatch is routinely performed between donor and recipient to prevent hyperacute rejection by donor-specific anti-human leukocyte antigen (HLA) antibodies. After transplantation, the presence of these antibodies is not routinely monitored. We wanted to know whether donor-specific anti-HLA antibodies are detectable during acute rejection (AR), before or after reduction of immunosuppression in kidney transplant recipients who were converted from cyclosporine A (CsA) to the less nephrotoxic azathioprine (AZA) or mycophenolate mofetil (MMF) at 1 year after transplantation. METHODS: Plasma samples were collected before transplantation, at several time points after transplantation, and during AR. Antibodies were measured in 29 patients: 5 patients with AR during the first year after transplantation (before conversion), 14 patients with AR after conversion or dose-reduction of AZA or MMF, and a control group of 10 patients without AR during a follow-up of 2 years (1 year before and 1 year after conversion of immunosuppression). Antibodies were measured by complement-dependent cytotoxicity assay, enzyme-linked immunosorbent assay (ELISA), and flow-cytometry in a crossmatch with donor spleen cells. RESULTS: Donor-specific antibodies were not detectable after transplantation in the control group without AR, nor in patients with AR shortly after transplantation during CsA therapy. After conversion from CsA to AZA or MMF, antibodies appeared only in one patient after graft failure followed by transplantectomy and in patients during AR on AZA but not on MMF therapy. CONCLUSION: In this patient group, we could not detect donor-specific antibodies during CsA treatment, not even at the time of AR using three different techniques. Donor-specific antibodies were primarily present during AR in patients converted from CsA to AZA and were not found in the sera from patients converted to MMF.     

Sharing kidneys across donor-service area boundaries with sensitized candidates can be influenced by HLA C

Bryan CF, Luger AM, Smith JL, Warady BA, Wakefield M, Schadde E, Murillo D, Nelson PW. Sharing kidneys across donor-service area boundaries with sensitized candidates can be influenced by HLA C.
Clin Transplant 2010: 24: 56–61. © 2009 John Wiley & Sons A/S.
Abstract:  The United Network for Organ Sharing (UNOS) implemented the virtual crossmatch system in UNet as a way to improve the likelihood of a negative crossmatch when kidneys are shared with HLA-sensitized candidates across donor service area (DSA) boundaries. The role of HLA C in that process is not universally appreciated. We recently experienced an unexpected positive flow T and B cell crossmatch for an imported, HLA zero-mismatched kidney because of donor-specific HLA C antibodies and transplanted it into the backup candidate. HLA C locus antigens were not typed by the OPO's laboratory that sent the kidney so the UNet virtual crossmatch could not "strike" our candidate from the UNOS match run. HLA C locus typing data of donors for kidneys our DSA imported from other DSAs revealed that C typing was not performed in 23% (14/60) and was discrepant with our molecular type for 10% (6/60) and was concordant in 67% (40/60) of cases. The rate of positive donor-specific crossmatches was higher (83%) for HLA C discrepantly typed donors than for concordantly typed donors (44%). Sensitization for HLA C (42%) is less frequent than for A (80%) or B (83%) locus antigens but the immunogenicity of C locus antigens in patients who make C locus antibodies is equivalent in black and white patients. Finally, the transplant rate of imported kidneys into class I-sensitized candidates was 24%, and C locus-sensitized candidates comprised 55% of those transplanted.