Donor-reactive HLA antibodies in renal allograft recipients: Considerations, complications, and conundrums

Impact of allele-specific anti-human leukocyte antigen class I antibodies on organ allocation

Technological advances in the field of histocompatibility have allowed us to define anti-human leukocyte antigen (HLA) antibody specificity at the allelic level. However, how allele-specific antibodies affect organ allocation is poorly studied. We examined allelic specificities of class I HLA antibodies in 6726 consecutive serum samples from 2953 transplant candidates and evaluated their impact on the corresponding crossmatch and organ allocation. Out of 17 class I HLA antigens represented by >1 allele in the LABScreen single antigen bead assay, 12 had potential allele-specific reactivity. Taking advantage of our unbiased cohort of deceased donor-candidate testing (123,135 complement-dependent cytotoxicity crossmatches between 2014 and 2017), we estimated that the presence of allele-specific antibody detected using a single antigen bead assay (median fluorescence intensity, >3000) against only the rare allele was a poor predictor of a positive complement-dependent cytotoxicity crossmatch, with a positive predictive value of 0% to 7%, compared with 52.5% in allele-concordant class I HLA antibodies against A or B locus antigens. Further, we confirmed allele-specific reactivity using flow crossmatch in 3 scenarios: A11:01/A11:02, A68:01/A68:02, and B44:02/B44:03. Our results suggest that allele-specific antibodies may unnecessarily exclude transplant candidates (up to 10%) from organ offers by overcalling unacceptable antigens; incorporation of selective reactivity pattern in allocation may promote precision matching and more equitable allocation.

A blueprint for electronic utilization of ambiguous molecular HLA typing data in organ allocation systems and virtual crossmatch

Virtual crossmatch (VXM) compares a transplant candidate's unacceptable antigens to the HLA typing of the donor before an organ offer is accepted and, in selected cases, supplant a prospective physical crossmatch. However, deceased donor typing can be ambiguous, leading to uncertainty in compatibility prediction. We have developed a prototype web application that utilizes ambiguous HLA molecular typing data to predict which unacceptable antigens are present in the donor HLA genotype as donor-specific antibodies (DSA). The application compares a candidate's listed unacceptable antigens to computed probabilities of all possible two-field donor HLA alleles and UNOS antigens. The VIrtual CrossmaTch for mOleculaR HLA typing (VICTOR) tool can be accessed at http://www.transplanttoolbox.org/victor. We reanalyzed historical VXM cases where a transplant center's manual interpretation of molecular typing results influenced offer evaluation. We found that interpretation of ambiguous donor molecular typing data using imputation could one day influence VXM decisions if the DSA predictions were rigorously validated. Standardized interpretation of molecular typing data, if applied to the match run, could also change which offers are made. HLA typing ambiguity has been an underappreciated source of immunological risk in organ transplantation. The VICTOR tool can serve as a testbed for development of allocation policies with the aim of decreasing offers refused due to HLA incompatibility.

Therapeutic Effect of Plasmapheresis and Intravenous Immunoglobulin in Recipients of Kidney Transplant With High Panel-reactive Antibody Levels: A Single-center Experience

OBJECTIVE

High panel-reactive antibody (PRA) levels limit patients' access to kidney transplantation from potential living donor candidates and decrease renal graft survival by causing acute antibody-mediated rejection (AAMR). In this article, we report our experiences about the efficiency of plasmapheresis (PP) and intravenous immunoglobulin (IVIG) in reduction of serum PRA levels in candidates for renal transplantation and in patients with AAMR.

METHODS

We examined retrospectively 47 patients with high PRA levels (18 for desensitization (DS) and 29 with AAMR) at Ankara University. The reduction in PRA class 1 and PRA class 2 levels before and after the PP, IVIG, and rituximab or eculizumab therapy were evaluated.

RESULTS

In the DS group, mean reduction in PRA class I ± SD was 28.0 ± 9.10 to 22.1 ± 8.14 (P <.05), and mean reduction in PRA class II ± SD was 40.3 ± 6.89 to 32.2 ± 5.68 (P < .05). In the AAMR group; mean reduction in PRA class I ± SD was 23.9 ± 9.56 to 17.8 ± 8.64 (P > .05), and mean reduction in PRA class II ± SD was 28.1 ± 8.37 to 26.7 ± 7.96 (P > .05). In total, mean reduction in PRA class I was 25.7 ± 6.66 to 19.7 ± 6.00 (P < .01). Mean reduction in PRA class II was 33.8 ± 5.93 to 29.2 ± 4.96 (P > .05). In the DS group, 3 (16.7%) patients were treated with rituximab. In the AAMR group, 9 (31.0%) patients were treated with rituximab, and 1 (5.5%) patient received eculizimab.In the DS group, the mean follow-up period in years ± SD was 5.06 ± 3.01 and no patient had graft loss. In the AAMR group, the mean follow-up period in years was 5.06 ± 2.74 and 6 (33.3%) patients had graft loss with acute rejection.

CONCLUSIONS

PP and IVIG treatment provide significant reduction in PRA levels and can be used in DS protocols.

Virtual crossmatch in kidney transplantation

BACKGROUND

The Luminex Single-Antigen Beads (LSA) assay allows an accurate detection and characterization of preexisting donor-specific antibodies (DSA) in kidney transplant candidates. But the ability of LSA to detect quite low levels of antibodies makes it hard to correctly predict crossmatch results in donor selection. In this study we retrospectively analyzed the accuracy of our virtual crossmatch (v-XM) protocol, which was used for selection of potential kidney transplant recipients, in predicting the results of actual crossmatch (a-XM) in cadaver-donor renal transplantation. We also investigated correlation between negative a-XM results and strength/specificity of preformed DSA.

METHODS

The correlation between negative v-XMs and a-XMs performed in 2007-2012 at the Regional Transplant Center of the Lazio Region, Italy, was analyzed. In carrying out v-XM, the donor HLA molecules against which patients showed LSA-detected DSA with normalized mean fluorescence intensity (MFI)≥5,000 were considered to be "unacceptable DSA," and LSA-DSA showing MFI<5,000 were defined as "acceptable DSA." All cadaver donors had been typed for HLA-A, -B, -DR, and -DQB molecules by sequence-specific primer methods. On the basis of a negative v-XM, we performed 507 a-XMs between serum samples from 256 renal transplant candidates and T/B lymphocytes from 302 cadaver donors with the use of both complement-dependent cytotoxicity (CDC) and flow cytometry (FC) methods.

RESULTS

The v-XM negative results showed good correlation with both CDC and FC a-XMs (97% and 90%, respectively). The sensitivity of v-XM was 100%; this high value was related to the lack of false-negative DSA results. The limited specificity with both techniques (CDC-XM, 74%; FC-XM, 79%) was due to the presence of "acceptable" and/or anti-DQA/DPB DSA in some patient sera used to perform the a-XMs. During the study period, 171 (67%) of the 256 sensitized patients received a kidney transplant: 30% of these had "acceptable DSA" and/or anti-DQA/DPB DSA. No antibody-mediated rejection due to preformed HLA-DSA was observed.

CONCLUSIONS

Our v-XM protocol showed high sensitivity in predicting donor-recipient immunologic compatibility. The results of this study also demonstrated the importance of evaluating DSA strength for implementing v-XM results in the selection of kidney transplant recipients. Moreover, the finding of anti-DQA/DPB DSA, especially in serum samples that gave positive results with the use of both CDC and FC a-XMs, highlights the importance of defining all of the donor HLA molecules to perform an accurate v-XM.

Donor-Specific Anti-HLA Antibodies in Huntington's Disease Recipients of Human Fetal Striatal Grafts

Fetal grafting in a human diseased brain was thought to be less immunogenic than other solid organ transplants, hence the minor impact on the efficacy of the transplant. How much prophylactic immune protection is required for neural allotransplantation is also debated. High-sensitive anti-HLA antibody screening in this field has never been reported. Sixteen patients with Huntington's disease underwent human fetal striatal transplantation in the frame of an open-label observational trial, which is being carried out at Florence University. All patients had both brain hemispheres grafted in two separate robotic-stereotactic procedures. The trial started in February 2006 with the first graft to the first patient (R1). R16 was given his second graft on March 2011. All patients received triple immunosuppressive treatment. Pre- and posttransplant sera were analyzed for the presence of anti-HLA antibodies using the multiplexed microsphere-based suspension array Luminex xMAP technology. Median follow-up was 38.5 months (range 13-85). Six patients developed anti-HLA antibodies, which turned out to be donor specific. Alloimmunization occurred in a time window of 0–49 months after the first neurosurgical procedure. The immunogenic determinants were non-self-epitopes from mismatched HLA antigens. These determinants were both public epitopes shared by two or more HLA molecules and private epitopes unique to individual HLA molecules. One patient had non-donor-specific anti-HLA antibodies in her pretransplant serum sample, possibly due to previous sensitization events. Although the clinical significance of donor-specific antibodies is far from being established, particularly in the setting of neuronal transplantation, these findings underline the need of careful pre- and posttransplant immunogenetic evaluation of patients with intracerebral grafts.

Critical role of sensitized serum in rejection of allogeneic bone marrow cells

Objective: Humoral immunity has been clearly implicated in solid organ transplantation, but little is known about the relationship between humoral immunity and hematopoietic stem cell transplantation. This study was designed to investigate that relationship.

Materials and Methods: Sensitized serum was obtained from a sensitized murine model established by allogeneic splenocyte transfusion. Sensitized serum was incubated with allogeneic bone marrow cells (BMCs) in vitro and the cytotoxicity was evaluated by the complement-dependent cytotoxicity method. Mice were transplanted with allogeneic BMCs incubated with sensitized serum after lethal irradiation. The engraftment was assayed by hematopoietic recovery and chimera analysis. Moreover, mice received passive transfer of sensitized serum 1 day prior to transplantation. Mortality was scored daily after bone marrow transplantation.

Results: The in vitro experiments showed that sensitized serum was capable of impairing allogeneic BMCs through the complement-dependent cytotoxicity pathway. The animal studies showed that BMCs incubated with sensitized serum failed to rescue mice from lethal irradiation. The engraftment assay showed that the allogeneic BMCs incubated with sensitized serum were rejected with time in the recipients. Furthermore, the mice died of marrow graft rejection by transfer of sensitized serum prior to transplantation.

Conclusion: Taken together, our results indicated that sensitized serum played a critical role in graft rejection during hematopoietic stem cell transplantation.

Different impact of pretransplant anti-HLA antibodies detected by Luminex in highly sensitized renal transplanted patients

It is well know that anti-HLA antibodies are an important obstacle in kidney transplantation. Our aim was to study the clinical impact of pretransplant donor specific anti-HLA antibodies (HLA-DSA), in highly sensitized (HS) patients. We analyzed retrospectively the day-of-transplant sera by Luminex Single Antigen Assay (LSA) in HS patients, and the results were correlated with episodes of humoral and cellular rejection as well as with graft and patient survival. All HS subjects received the same induction therapy and rejection episodes were biopsy proven. Thirteen patients (56.5%) preformed HLA-DSA, and we observed higher incidence of acute rejection in aforementioned patients than in the pre-transplant negatives DSA recipients (77% versus 30%, P = 0.03). The one-year graft survival was significantly reduced in positive pre-transplant HLA-DSA patients (60% versus 100%, P = 0.01 Breslow). The positive predicted value of HLA-DSA in relation to rejection reached 100% if patients lost their previous graft in the first year after transplant. Among anti-HLA antibodies present in patients before transplant, HLA-DSA were significantly associated with high risk of acute humoral and cellular rejection and reduced graft survival in posttransplant outcome. The negative impact of these antibodies was even higher when patients suffered an early loss of the previous transplant.

Occurrence of Fatal and Nonfatal Adverse Outcomes after Heart Transplantation in Patients with Pretransplant Noncytotoxic HLA Antibodies

HLA antibodies (HLA ab) in transplant candidates have been associated with poor outcome. However, clinical relevance of noncytotoxic antibodies after heart transplant (HT) is controversial. By using a Luminex-based HLA screening, we retested pretransplant sera from HT recipients testing negative for cytotoxic HLA ab and for prospective crossmatch. Out of the 173 consecutive patients assayed (52 ± 13y; 16% females; 47% ischemic etiology), 32 (18%) showed pretransplant HLA ab, and 12 (7%) tested positive against both class I and class II HLA. Recipients with any HLA ab had poorer survival than those without (65 ± 9 versus 82 ± 3%; P = 0.02), accounting for a doubled independent mortality risk (P = 0.04). In addition, HLA-ab detection was associated with increased prevalence of early graft failure (35 versus 15%; P = 0.05) and late cellular rejection (29 versus 11%; P = 0.03). Of the subgroup of 37 patients suspected for antibody mediated rejection (AMR), the 9 with pretransplant HLA ab were more likely to display pathological AMR grade 2 (P = 0.04). By an inexpensive, luminex-based, HLA-screening assay, we were able to detect non-cytotoxic HLA ab predicting fatal and nonfatal adverse outcomes after heart transplant. Allocation strategies and desensitization protocols need to be developed and prospectively tested in these patients.

Precision diagnostics in transplantation: from bench to bedside

The Canadian and American Societies of Transplantation held a symposium on February 22, 2012 in Quebec City focused on discovery, validation and translation of new diagnostic tools into clinical transplantation. The symposium focused on antibody testing, transplantation pathology, molecular diagnostics and laboratory support for the incompatible patient. There is an unmet need for more precise diagnostic approaches in transplantation. Significant potential for increasing the diagnostic precision in transplantation was recognized through the integration of conventional histopathology, molecular technologies and sensitive antibody testing into one enhanced diagnostic system.

Transnational validation of the Australian algorithm for virtual crossmatch allocation in kidney paired donation

An independent pool of 16 incompatible live donor-recipient pairs registered at the Vienna transplant unit was applied to test whether virtual crossmatch allocation used in the Australian kidney paired donation (KPD) program reliably predicts negative crossmatches. High resolution HLA data were entered into the computer-matching algorithm and allocation was performed excluding any DSA>2000MFI. CDC and flow crossmatch data of recipients against any of the donors were available for 112 crossmatch combinations. The computer program identified 19 possible pairings in 2-way or 3-way chains in multiple combinations. The top ranked combination included one 3-way and two 2-way ABO-compatible chains. Where crossmatches were available all recipients were CDC crossmatch negative with the computer-matched donor. Excluding allocation of KPD donors in the presence of DSA>2000MFI had a negative predictive of 99.9% for CDC and 96.4% for flow crossmatch. In the 12 pairings with ⩾1 DSA against crossmatched donors there was a negative CDC and flow crossmatch. These results show excellent correlation between matching using virtual crossmatch and actual crossmatch results. Using the 2000MFI cut-off the number of potentially unacceptable CDC and flow crossmatch positive pairings identified by virtual crossmatching is low, but some potential crossmatch negative pairings are missed.

Effect of implementing anti-HLA antibody detection by Luminex in the kidney transplant program in Chile

The development of new highly sensitive, specific technologies to detect HLA antibodies has allowed a better definition of the profile of non-permitted antigens for patients awaiting kidney transplantation. The use of calculated or virtual panel reactive antibodies (CPRA or vPRA) seeks to improve the prediction of positive crossmatches (XM), but increases the proportion of sensitized patients on the waiting list. In 2008-2009, we implemented detection of antibodies using Luminex technology and applied vPRA since 2009. The objective of this study was to evaluate the impact of these innovations in defecting patient sensitization on kidney transplant waiting lists for deceased donors and among transplanted patients. We analyzed the waiting list for 2007 through 2009 and the first semester of 2010, including the patients transplanted in these periods and the XM with deceased donors. We observed an increase in the mean peak PRA of transplanted patients from 7.2% in 2007 to 17.1% in 2010 (P = .001), and in the proportion of patients transplanted with a peak PRA > 50% from 2.8% in 2007 to 15.7% in 2010 (P = .0001), with no increase in the proportion of this population on the waiting lists. There was a concurrent decrease in positive XM among patients with a peak PRA > 50%. The use of vPRA and Luminex permitted a greater number of transplants of patients with peak PRA > 50% and was a good predictor of a positive XM.

Clinical relevance of HLA donor-specific antibodies detected by single antigen assay in kidney transplantation

BACKGROUND

Clinical relevance of donor-specific antibodies (DSAs) detected by a single antigen Luminex virtual crossmatch in pre-transplant serum samples from patients with a negative cytotoxicity-dependent complement crossmatch is controversial. The aim of this study was to analyse the influence of a pre-transplant positive virtual crossmatch in the outcome of kidney transplantation.

METHODS

A total of 892 patients who received a graft from deceased donors after a negative cytotoxicity crossmatch were included. Presence of anti-human leucocyte antigen (HLA) antibodies was investigated using a Luminex screening assay and anti-HLA specificities were assigned performing a Luminex single antigen assay.

RESULTS

Graft survival was significantly worse among patients with anti-HLA DSA compared to both patients with anti-HLA with no DSA (P = 0.001) and patients without HLA antibodies (P < 0.001) using a log-rank test. No graft survival differences between anti-HLA with no DSA and no HLA antibodies patient groups were observed (P = 0.595). Influence of both anti-Class I and anti-Class II DSA was detected (P < 0.0001 in both cases). When the fluorescence values were stratified in four groups, no significant differences in graft survival were observed among the groups with fluorescence levels >1500 (global P > 0.05).

CONCLUSIONS

The presence of preformed HLA DSA in transplanted patients with a negative cytotoxicity crossmatch is associated with a lower allograft survival. The detection of anti-HLA with no DSA has no influence in the graft outcome. Finally, there were no demonstrable effects of mean fluorescence intensity (MFI) values >1500 on graft survival.

Antibody-mediated rejection--an ounce of prevention is worth a pound of cure

The presence of preformed, donor-specific alloantibodies inpatients undergoing renal transplantation is associated with a high risk of hyperacute and acute antibody-mediated rejection (ABMR), and often limits potential recipients' access to organs from living and deceased donors. Over the last decade, understanding of ABMR has improved markedly and given rise to numerous, diverse strategies for the transplantation of allosensitized recipients. Antibody desensitization programs have been developed to allow renal transplant recipients with a willing but antibody-incompatible living donor to undergo successful transplantation, whereas kidney paired exchange schemes circumvent the antibody incompatibility altogether by finding suitable pairs to donors and recipients. Recognizing the complexity of ABMR and the recent developments that have occurred in this important clinical research field, the Roche Organ Transplantation Research Foundation (ROTRF) organized a symposium during the XXIII Congress of The Transplantation Society in Vancouver, Canada, to discuss current understanding in ABMR and ways to prevent it. This Meeting Report summarizes the presentations of the symposium, which addressed key areas that included the interactions between alloantibodies and the complement system in mediating graft injury, technological advancements for assessing antibody-mediated immune responses to HLA antigens, and the potential benefits and challenges of desensitization and kidney paired donation schemes.

The evolution and clinical impact of human leukocyte antigen technology

PURPOSE OF REVIEW

As human leukocyte (HLA) antigen and antibody testing has evolved, typing, screening, and crossmatching assays have become exquisitely specific and sensitive. How best to use this information is a focal point of the transplant community and of this review.

RECENT FINDINGS

Driven by advances in technology, two major changes in the transplant community recently occurred. Firstly, the United Network for Organ Sharing mandated the use of cPRA to replace the more subjective panel reactive antibody value. Secondly, the virtual crossmatch (vXM) became a reality. The vXM categorizes sensitized patients as compatible or incompatible with their donors by predicting the likelihood of a positive crossmatch. Although the use of cPRA and vXM represents major progress in organ allocation algorithms, there are compelling issues that must be addressed. These include defining appropriate threshold values for antibody assignment and recognizing the relevance of antibodies to HLA-Cw*, HLA-DQ*, and HLA-DP*.

SUMMARY

Undoubtedly, molecular HLA-typing and solid-phase antibody-detection assays have revolutionized clinical histocompatibility testing. These new testing methodologies have had a major impact in both allocation and outcome of transplanted kidneys. However, even these new methodologies are not flawless and pose new challenges to the transplant community.

Clinical cytometry and progress in HLA antibody detection

For most solid organ and selected stem cell transplants, antibodies against mismatched HLA antigens can lead to early and late graft failure. In recognition of the clinical significance of these antibodies, HLA antibody identification is one of the most critical functions of histocompatibility laboratories. Early methods employed cumbersome and insensitive complement-dependent cytotoxicity assays with a visual read-out. A little over 20 years ago flow cytometry entered the realm of antibody detection with the introduction of the flow cytometric crossmatch. Cytometry's increased sensitivity and objectivity quickly earned it popularity as a preferred crossmatch method especially for sensitized recipients. Although a sensitive method, the flow crossmatch was criticized as being "too sensitive" as false positive reactions were a know drawback. In part, the shortcomings of the flow crossmatch were due to the lack of corresponding sensitive and specific HLA antibody screening assays. However, in the mid 1990s, solid phase assays, capable of utilizing standard flow cytometers, were developed. These assays used microparticles coated with purified HLA molecules. Hence, the era of solid-phase, microparticle technology for HLA antibody detection was born permitting the sensitive and specific detection of HLA antibody. It was now possible to provide better correlation between HLA antibody detection and the flow cytometric crossmatch. This flow-based technology was soon followed by adaptation to the Luminex platform permitting a mutltiplexed approach for the identification and characterization of HLA antibodies. It is hoped that these technologies will ultimately lead to the identification of parameters that best correlate with and/or predict transplant outcomes.

Pre- and posttransplantation allosensitization in heart allograft recipients: major impact of de novo alloantibody production on allograft survival

The involvement of humoral response in allograft rejection has been suggested by both immunologic and histochemistry studies. In the present study, we explored the role of alloantibodies in a large cohort of heart allograft recipients followed for 15 years. Sequential samples of sera were obtained from 950 recipients of heart allografts before and after transplantation at the time when protocol endomyocardial biopsies were performed. The presence of anti-human leukocyte antigen (HLA) antibodies was investigated using complement mediated cytotoxicity and solid phase assay (SPA). Our data reveal an inverse correlation between the development of alloantibodies after transplantation and heart allograft survival. The 15-year graft survival was highest in patients who never developed alloantibodies (70%) or who displayed them only before transplantation (71%); graft survival in recipients who showed antibodies both before and after transplantation (56%), or only after transplantation (47%), was lower. The deleterious effect of antibodies on graft survival started 8 years after transplantation, suggesting that the production of de novo antibodies may have been triggered by some later event. We found that patients with de novo antibodies appearing more than 1 year after transplantation had the poorest survival. Furthermore, the development of de novo antibodies was preceded in 76% of these patients by cellular rejection grade 3 or higher, according to the International Society for Heart Transplantation (ISHT) grading criteria. Development of antibody-mediated rejection (AMR) had a significant negative impact on graft survival (16% in AMR(+) vs 63% in AMR(-) patients, p = 0.0008). Of the 23 patients with AMR, 21 displayed cytotoxic donor-specific antibodies (DSA) at the time of diagnosis, and in 18 of these cases SPA showed that they were directed against the donors' HLA. The data demonstrate that the detection of alloantibodies permits a better definition of AMR in heart allograft recipients. Identification of patients at risk for developing AMR is of great importance for early treatment of rejection episodes.

Clinical relevance of circulating donor-specific HLA antibodies

PURPOSE OF REVIEW

For a long period of time complement-dependent cytotoxicity was the standard assay to demonstrate clinically relevant HLA antibodies in the serum of patients before transplantation. The introduction of more sensitive solid-phase assays to detect HLA antibodies had led to a lot of discussion on the clinical relevance of the presence of these antibodies before transplantation and the appearance of these antibodies after transplantation. This review discusses the current controversies.

RECENT FINDINGS

The old dogma that the presence of donor-specific HLA antibodies before transplantation is a contra-indication for transplantation does not exist anymore. The correlation between the presence of HLA antibodies and clinical outcome is less strict although the presence of circulating donor-specific antibodies, both before and after transplantation, should be considered a risk factor. It is clear, however, that the opinion on the clinical relevance of donor-specific HLA antibodies is very different amongst the transplant centers.

SUMMARY

The clinical relevance of the presence of donor-specific HLA antibodies before transplantation and the appearance of these antibodies after transplantation is a controversial issue. The lack of consensus between different centers is partly due to the heterogeneity of the HLA antibodies involved. Standardization is essential and future studies must focus on the further characterization of the antibody titers, the immunoglobulin (sub)classes of the antibodies and the epitopes recognized. It remains to be established in which cases the HLA antibodies are the direct cause of or just associated with (chronic) graft failure.

Subtypes of immunoglobulin (Ig)-G antibodies against donor class II HLA and cross-match results in three kidney transplant candidates

Preexisting donor-specific antibodies (DSA) play a critical role in the success of solid-organ transplantation. Cross-match (CM) between donor lymphocytes and recipient serum is a pivotal methodology for detecting these antibodies. Luminex platform based solid-phase methodology for anti-human leukocyte antigen (HLA) antibody analysis has revolutionized the approach to antibody detection and HLA specificity identification. In this study, we have reported three cases of successful living donor kidney transplantations performed against strongly positive B lymphocyte flow cytometry (FC) CM owing to highly reactive DSA directed to HLA class II. IgG solid-phase subtype analysis showed that more than 50% of these antibodies were represented by non-complement binding IgG2/IgG4 subtypes. These findings account for antibody mediated rejection (AMR) free long-term post-transplant course in these patients despite, the high level of DSA. Thus, we conclude that routine application of single HLA-coated beads (SAB) IgG subtype assay may provide new insights regarding transplantation or desensitization of patients presenting with negative B-cell complement dependent cytotoxic (CDC) and positive FC CM.