Consensus guidelines on the testing and clinical management issues associated with HLA and non-HLA antibodies in transplantation. Transplantation. 2013;95:19-47. [PMID: 23238534 DOI: 10.1097/tp.0b013e31827a19cc]

Donor-specific antibodies in liver transplantation: challenges in diagnosis and determining clinical impact

PURPOSE OF REVIEW

Our understanding of the clinical impact of donor-specific antibodies in liver transplant recipients has evolved in recent years as outcomes for liver allografts have improved and advances in diagnostic testing have made recognition of antibody mediated rejection in transplant patients more sensitive.

RECENT FINDINGS

Two main types of donor-specific antibodies - preformed and de novo - have been reported in the literature to have a negative impact on graft survival, and researchers have been able to further identify subclasses of class II donor-specific antibodies as being the most clinically impactful. Furthermore, there is evidence that donor-specific antibody formation can augment cellular rejection in liver grafts and lead to worsened clinical outcomes. Recent data have shown a higher prevalence of donor-specific antibody formation than previously reported.

SUMMARY

This review explores the most recent literature regarding the clinical impact of both preformed and de-novo donor-specific antibodies and potential management guidelines for patients undergoing liver transplantation. The best practice guidelines for undergoing monitoring for donor-specific antibody formation and protocol biopsies in sensitized patients will depend on further multiinstitutional studies.

A Virtual Crossmatch-based Strategy Facilitates Sharing of Deceased Donor Kidneys for Highly Sensitized Recipients

BACKGROUND

It is estimated that 19.2% of kidneys exported for candidates with >98% calculated panel reactive antibodies are transplanted into unintended recipients, most commonly due to positive physical crossmatch (PXM). We describe the application of a virtual crossmatch (VXM) that has resulted in a very low rate of transplantation into unintended recipients.

METHODS

We performed a retrospective review of kidneys imported to our center to assess the reasons driving late reallocation based on the type of pretransplant crossmatch used for the intended recipient.

RESULTS

From December 2014 to October 2017, 254 kidneys were imported based on our assessment of a VXM. Of these, 215 (84.6%) were transplanted without a pretransplant PXM. The remaining 39 (15.4%) recipients required a PXM on admission using a new sample because they did not have an HLA antibody test within the preceding 3 months or because they had a recent blood transfusion. A total of 93% of the imported kidneys were transplanted into intended recipients. There were 18 late reallocations: 9 (3.5%) due to identification of a new recipient medical problem upon admission, 5 (2%) due to suboptimal organ quality on arrival, and only 4 (1.6%) due to a positive PXM or HLA antibody concern. A total of 42% of the recipients of imported kidneys had a 100% calculated panel reactive antibodies. There were no hyperacute rejections and very infrequent acute rejection in the first year suggesting no evidence for immunologic memory response.

CONCLUSIONS

Seamless sharing is within reach, even when kidneys are shipped long distances for highly sensitized recipients. Late reallocations can be almost entirely avoided with a strategy that relies heavily on VXM.

Discovery of non-HLA antibodies associated with cardiac allograft rejection and development and validation of a non-HLA antigen multiplex panel: From bench to bedside

We analyzed humoral immune responses to nonhuman leukocyte antigen (HLA) after cardiac transplantation to identify antibodies associated with allograft rejection. Protein microarray identified 366 non-HLA antibodies (>1.5 fold, P < .5) from a discovery cohort of HLA antibody-negative, endothelial cell crossmatch-positive sera obtained from 12 cardiac allograft recipients at the time of biopsy-proven rejection. From these, 19 plasma membrane proteins and 10 autoantigens identified from gene ontology analysis were combined with 48 proteins identified through literature search to generate a multiplex bead array. Longitudinal sera from a multicenter cohort of adult cardiac allograft recipients (samples: n = 477 no rejection; n = 69 rejection) identified 18 non-HLA antibodies associated with rejection (P < .1) including 4 newly identified non-HLA antigenic targets (DEXI, EMCN, LPHN1, and SSB). CART analysis showed 5/18 non-HLA antibodies distinguished rejection vs nonrejection. Antibodies to 4/18 non-HLA antigens synergize with HLA donor-specific antibodies and significantly increase the odds of rejection (P < .1). The non-HLA panel was validated using an independent adult cardiac transplant cohort (n = 21 no rejection; n = 42 rejection, >1R) with an area under the curve of 0.87 (P < .05) with 92.86% sensitivity and 66.67% specificity. We conclude that multiplex bead array assessment of non-HLA antibodies identifies cardiac transplant recipients at risk of rejection.

Clinical Outcomes of Lung Transplantation in the Presence of Donor-Specific Antibodies

Rationale: There is significant variation in approach to pre-lung transplant donor-specific antibodies (DSA), with some centers declining to cross any DSA. We implemented a protocol for transplantation for candidates with pretransplant DSA so long as a prospective complement-dependent cytotoxicity crossmatch was negative, regardless of number, specificity, class, or mean fluorescence intensity.Objectives: To compare post-transplant outcomes including overall survival, chronic lung allograft dysfunction-free survival, antibody-mediated rejection, and acute cellular rejection in lung transplant recipients where pretransplant DSA was and was not present.Methods: This was a single-center retrospective cohort study. For recipients with pretransplant DSA, if the prospective complement-dependent cytotoxicity crossmatch was negative, the donor offer was accepted and plasmapheresis was performed within 24 hours of transplantation and continued until retrospective crossmatch results returned. Immunosuppression and post-transplant management were not otherwise modified.Results: Of the 203 included recipients, 18 (8.9%) had pretransplant DSA. The median DSA mean fluorescence intensity was 4,000 (interquartile range, 2,975-5,625; total range, 2,100-17,000). The median number of DSA present per patient was one (interquartile range, 1-2). The presence of pretransplant DSA was not associated with increased mortality (hazard ratio, 1.2; 95% confidence interval [CI], 0.4-3.4) or decreased chronic lung allograft dysfunction-free survival (hazard ratio, 1.1; 95% CI, 0.6-2.1). Recipients with pretransplant DSA were more likely to require prolonged mechanical ventilation (adjusted odds ratio, 7.0; 95% CI, 2.3-21.6) and to have antibody-mediated rejection requiring treatment (adjusted odds ratio, 7.5; 95% CI, 1.0-55.8).Conclusions: A protocol of accepting donor offers for lung transplant candidates with preformed, complement-dependent cytotoxicity crossmatch-negative DSA is associated with increased need for prolonged mechanical ventilation and antibody-mediated rejection without affecting short-term overall or chronic lung allograft dysfunction-free survival.

Human Leukocyte Antigen Examination Using Luminex®-Based Assays for Donor-Recipient Compatibility Assessment in Kidney Transplantation: Our Preliminary Experience

Background: Detection of anti-HLA antibodies (HLAabs) in organ transplantation recipients is vital to determine whether the recipient has specific HLAabs against the donor’s HLA molecules (donor-specific anti-HLA antibodies [DSA]). Methods: This preliminary study involved seven subjects: five without prior HLA sensitization and two with a history of possible HLA sensitization (kidney transplantation and pregnancy). Two of the subjects were siblings, two of the subjects were mother and son, and two of them were first cousins once removed. All the subjects underwent HLA typing, and four of the subjects underwent HLAabs assays using the sequence-specific oligonucleotide probes (SSOP) method. Results: There were 10/16 HLA matches between subjects who were siblings and between subjects who were mother and son, and 6/16 HLA matches were found between subjects who were first cousins once removed. Subjects with previous kidney transplantation developed 59 HLA-Abs toward class I and class II HLA (the mean fluorescence intensity (MFI) value range: 1,003.36 to 12,404.77). HLAabs were also found among subjects without prior HLA sensitization, albeit with relatively lower MFI values. Conclusions: HLA typing and HLAabs detection for donor-recipient compatibility assessment using the SSOP method offers more detailed, semi-quantitative results. Further research involving larger subjects in cohort settings will be useful for profiling MFI cut-off values for HLAabs.

Donor-specific antibody characteristics, including persistence and complement-binding capacity, increase risk for chronic lung allograft dysfunction

BACKGROUND

Chronic lung allograft dysfunction (CLAD) is the major complication limiting long-term survival in lung transplant recipients (LTRs), with those developing donor-specific anti-human leukocyte antigen (HLA) antibodies (DSAs) previously found to have increased risk for CLAD. However, as DSA responses vary in timing of development, specificity, breadth, persistence, and complement-binding capacity, we hypothesized that these characteristics would impact CLAD and survival outcomes.

METHODS

We retrospectively analyzed DSA characteristics and outcomes in a single-center cohort of 582 LTRs who had serum samples collected prospectively from 2010 to 2016. Luminex-based single antigen bead assays were performed to assess DSA.

RESULTS

DSAs were detected in 247 LTRs (42%), of which 124 (21.3%) were de novo DSAs and 53 (9.1%) were complement-binding (C1q+). CLAD developed in 208 LTRs (35.7%) during the follow-up period, with 67.8% determined as bronchiolitis obliterans syndrome phenotype and 32.2% as restrictive allograft syndrome phenotype. We found a shorter time to CLAD in LTRs with persistent DSAs (p = 0.04) and HLA-DQ-specific DSAs (p = 0.03). LTRs who developed C1q+ DSAs had significantly shorter time to CLAD (p < 0.001), with 100% of C1q+ DSAs being persistent and no differences between CLAD phenotypes. CLAD-free survival was significantly reduced in LTRs who developed C1q+ DSAs (p = 0.001), HLA-DQ-specific DSAs (p = 0.03), and multiple DSAs (p = 0.02).

CONCLUSIONS

Together, our findings demonstrate that DSA characteristics of persistence, HLA-DQ specificity, and C1q+ DSAs are associated with shorter time to CLAD. Additionally, C1q+, HLA-DQ-specific, and multiple DSAs are associated with decreased CLAD-free survival. These characteristics may improve DSA risk stratification for deleterious outcomes in LTRs.

Non-invasive cardiac allograft rejection surveillance: reliability and clinical value for prevention of heart failure

Allograft rejection-related acute and chronic heart failure (HF) is a major cause of death in heart transplant recipients. Given the deleterious impact of late recognized acute rejection (AR) or non-recognized asymptomatic antibody-mediated rejection on short- and long-term allograft function improvement of AR surveillance and optimization of action strategies for confirmed AR can prevent AR-related allograft failure and delay the development of cardiac allograft vasculopathy, which is the major cause for HF after the first posttransplant year. Routine non-invasive monitoring of cardiac function can improve both detection and functional severity grading of AR. It can also be helpful in guiding the anti-AR therapy and timing of routine surveillance endomyocardial biopsies (EMBs). The combined use of EMBs with non-invasive technologies and methods, which allow detection of subclinical alterations in myocardial function (e.g., tissue Doppler imaging and speckle-tracking echocardiography), reveal alloimmune activation (e.g., screening of complement-activating donor-specific antibodies and circulating donor-derived cell-free DNA) and help in predicting the imminent risk of immune-mediated injury (e.g., gene expression profiling, screening of non-HLA antibodies, and circulating donor-derived cell-free DNA), can ensure the best possible surveillance and management of AR. This article gives an overview of the current knowledge about the reliability and clinical value of non-invasive cardiac allograft AR surveillance. Particular attention is focused on the potential usefulness of non-invasive tools and techniques for detection and functional grading of early and late ARs in asymptomatic patients. Overall, the review aimed to provide a theoretical and practical basis for those engaged in this particularly demanding up-to-date topic.

Immunological follow-up of patients with renal transplants: A proposal for clinical practice in Colombia

Graft damage is a process that starts at the moment of transplantation, due to comorbidities of receptor, donor status, ischemia time, ischemia-reperfusion phenomenon, among others, those induce metabolic and immune factors that ultimately trigger clinical manifestations of graft dysfunction. However, the preclinical progression between the time of transplantation and the appearance of signs and symptoms of graft damage can take weeks to years. Therefore, the implementation of rational monitoring approaches during the post-transplantation period is critical and should include not only the clinical follow-up but also anticipate immunological graft damage. In the present essay, we propose an immunological monitoring algorithm for the post-renal transplantation period.

The Graz Liver Allocation Strategy-Impact of Extended Criteria Grafts on Outcome Considering Immunological Aspects

Background: Transplant centers are forced to use livers of extended criteria donors for transplantation due to a dramatic organ shortage. The outcome effect of extended donor criteria (EDCs) remains unclear. Thus, this study was designed to assess the impact of EDCs on outcome including immunological aspects after liver transplantation (LT).

Patients and Methods: Between November 2016 and March 2018, 49 patients (85.7% male) with a mean age of 57 ± 11 years underwent LT. The impact of EDCs on outcome after LT was assessed retrospectively using both MedOcs and ENIS (Eurotransplant Network Information System).

Results: About 80% of grafts derived from extended criteria donors. Alanine aminotransferase/aspartate aminotransferase (AST/ALT) levels elevated more than three times above normal values in organ donors was the only significant risk factor for primary dysfunction (PDF) and primary non-function (PNF)/Re-LT and early non-anastomotic biliary strictures (NAS). Balance of risk (BAR) score did not differ between EDC and non-EDC recipients. PDF (14.3% of all patients) and PNF (6.1% of all patients) occurred in 23.1% of EDC-graft recipients and in 10.0% of non-EDC-graft recipients (RR 2.31, p = 0.663). The 90-day mortality was 3.6%. There was no difference of early non-anastomotic biliary tract complications and biopsy proven rejections (BPR). There was no correlation of PDF/PNF with BPR and NAS, respectively; however, 66.7% of the patients with BPR also developed early NAS (p < 0.001).

Conclusion: With the Graz liver allocation strategy, excellent survival can be achieved selecting livers with no more than 2 not outcome-relevant EDCs for patients with MELD >20. Further, BPR is associated with biliary complications.

Analysis of T and B Cell Epitopes to Predict the Risk of de novo Donor-Specific Antibody (DSA) Production After Kidney Transplantation: A Two-Center Retrospective Cohort Study

Risk prediction of de novo donor specific antibody (DSA) would be very important for long term graft outcome after organ transplantation. The purpose of this study was to elucidate the association of eplet mismatches and predicted indirectly recognizable HLA epitopes (PIRCHE) scores with de novo DSA production. Our retrospective cohort study enrolled 691 living donor kidney transplantations. HLA-A, B, DRB and DQB eplet mismatches and PIRCHE scores (4 digit of HLA-A, B, DR, and DQ) were determined by HLA matchmaker (ver 2.1) and PIRCHE-II Matching Service, respectively. Weak correlation between eplet mismatches and PIRCHE scores was identified, although both measurements were associated with classical HLA mismatches. Class II (DRB+DQB) eplet mismatches were significantly correlated with the incidence of de novo class II (DR/DQ) DSA production [8/235 (3.4%) in eplet mismatch ≤ 13 vs. 92/456 (20.2%) in eplet mismatch ≥ 14, p < 0.001]. PIRCHE scores were also significantly correlated with de novo class II DSA production [26/318 (8.2%) in PIRCHE ≤ 175 vs. 74/373 (19.8%) in PIRCHE ≥ 176, p < 0.001]. Patients with low levels of both class II eplet mismatches and PIRCHE scores developed de novo class II DSA only in 4/179 (2.2%). Analysis of T cell and B cell epitopes can provide a beneficial information on the design of individualized immunosuppression regimens for prevention of de novo DSA production after kidney transplantation.

Alloimmunity in pancreas transplantation

PURPOSE OF REVIEW

Despite significant improvement in pancreas allograft survival, rejection continues to be a major clinical problem. This review will focus on emerging literature related to the impact of pretransplant and de-novo DSA (dnDSA) in pancreas transplant recipients, and the diagnosis and treatment of T-cell-medicated rejection (TCMR) and antibody-mediated rejection (ABMR) in this complex group of patients.

RECENT FINDINGS

Recent data suggest that pretransplant DSA and the emergence of dnDSA in pancreas transplant recipients are both associated with increased risk of ABMR. The pancreas allograft biopsy is essential for the specific diagnosis of TCMR and/or ABMR, distinguish rejection from other causes of graft dysfunction, and to guide-targeted therapy. This distinction is important especially in the setting of solitary pancreas transplants but also in simultaneous pancreas-kidney transplants where solid evidence has now emerged demonstrating discordant biopsy findings. Treatment of rejection in a functioning pancreas can prolong allograft survival.

SUMMARY

The accurate and timely diagnosis of active alloimmune destruction in pancreas transplant recipients is paramount to preserving graft function in the long term. This review will discuss new, rapidly evolving information that is valuable for the physician caring for these patients to achieve optimal immunological outcomes.

Pre-existing Alloreactive T and B Cells and Their Possible Relevance for Pre-transplant Risk Estimation in Kidney Transplant Recipients

In allogeneic transplantation, genetic disparities between patient and donor may lead to cellular and humoral immune responses mediated by both naïve and memory alloreactive cells of the adaptive immune system. This review will focus on alloreactive T and B cells with emphasis on the memory compartment, their role in relation to kidney rejection, and in vitro assays to detect these alloreactive cells. Finally, the potential additional value of utilizing donor-specific memory T and B cell assays supplementary to current routine pre-transplant risk assessment of kidney transplant recipients will be discussed.

Impact of ABO-Incompatible Living Donor Kidney Transplantation on Patient Survival

RATIONALE & OBJECTIVE

Compared with recipients of blood group ABO-compatible (ABOc) living donor kidney transplants (LDKTs), recipients of ABO-incompatible (ABOi) LDKTs have higher risk for graft loss, particularly in the first few weeks after transplantation. However, the decision to proceed with ABOi LDKT should be based on a comparison of the alternative: waiting for future ABOc LDKTs (eg, through kidney paired exchange) or for a deceased donor kidney transplant (DDKT). We sought to evaluate the patient survival difference between ABOi LDKTs and waiting for an ABOc LDKT or an ABOc DDKT.

STUDY DESIGN

Retrospective cohort study of adults in the Scientific Registry of Transplant Recipients.

SETTING & PARTICIPANTS

808 ABOi LDKT recipients and 2,423 matched controls from among 245,158 adult first-time kidney-only waitlist registrants who did not receive an ABOi LDKT and who remained on the waitlist or received either an ABOc LDKT or an ABOc DDKT, 2002 to 2017.

EXPOSURE

Receipt of ABOi LDKT.

OUTCOME

Death.

ANALYTICAL APPROACH

We compared mortality among ABOi LDKT recipients versus a weighted matched comparison population using Cox proportional hazards regression and Cox models that accommodated for changing hazard ratios over time.

RESULTS

Compared with matched controls, ABOi LDKT was associated with greater mortality risk in the first 30 days posttransplantation (cumulative survival of 99.0% vs 99.6%) but lower mortality risk beyond 180 days posttransplantation. Patients who received an ABOi LDKT had higher cumulative survival at 5 and 10 years (90.0% and 75.4%, respectively) than similar patients who remained on the waitlist or received an ABOc LDKT or ABOc DDKT (81.9% and 68.4%, respectively).

LIMITATIONS

No measurement of ABO antibody titers in recipients; eligibility of participants for kidney paired donation is unknown.

CONCLUSIONS

Transplant candidates who receive an ABOi LDKT and survive more than 180 days posttransplantation experience a long-term survival benefit compared to remaining on the waitlist to potentially receive an ABOc kidney transplant.

Anti-Hla donor-specific antibody monitoring in pancreas transplantation: Role of protocol biopsies

In kidney transplantation, de novo donor-specific antibodies (DSA) correlate with poor graft survival, and Consensus Guidelines recommend a protocol biopsy. In pancreas transplantation, DSA are also associated with poor graft outcomes; however, there are no recommendations on protocol biopsies. We started an antibody screening protocol on pancreas transplant patients at 0, 3, 6, 12 months, and yearly. Patients with DSA or high MFI non-DSA were considered for protocol biopsies of both organs. Results: 143 pancreas recipients were screened. 84 patients had negative antibodies throughout the study, 11 patients were found to have antibodies at graft dysfunction, and 48 patients had positive antibodies at screening without acute organ dysfunction (study group). Among the 30 non-DSA patients, 9 had protocol simultaneous pancreas and kidney biopsies performed with negative results in all of them. In contrast, among the 18 DSA patients, 15 had these biopsies performed, and 47% presented with subclinical rejection of the kidney, the pancreas, or both. In addition, some of the DSA patients without a protocol biopsy presented with rejection during the first 15 months of follow-up. Conclusion: We conclude that protocol biopsies of both grafts may play a role in the follow-up of pancreas transplant patients with de novo DSA appearance.

Predictive value of mixed antigen screen beads in pre-transplant assessment of HLA immunization in solid organ transplant recipients

Pre-transplant serum screening of anti-HLA antibodies is recommended for solid organ transplantations. Many laboratories use the less expensive bead-based screening assay as the main technique and, if positive, turn to single-antigen beads (SAB). We studied the correlations between these two immunoassays. We re-analyzed the raw data of the two assays in 3030 first organ transplant recipients, explored with the two tests. We performed a ROC curve analysis of the screening ratio to predict a positive SAB assay. The AUC were 0.72 and 0.64 for class I and class II. The optimal thresholds of screening ratios were 3.28 (class I) and 2.11 (class II). Whatever the class, the negative predictive value was low, around 40%, with 36% of discordant sera, as defined by negative screening and positive SAB. Testing class I discordant sera on acid-treated SAB showed that 54% of antibodies reacted against denatured HLA molecules. However, these screening-negative sera may contain donor-specific antibodies in 13.9% and 28.7% of cases for class I and class II, respectively, involved in antibody-mediated rejection with the same frequency as non-discordant sera. Given the low predictive value of screening, both assays should be performed at least once on the same serum before transplantation.

What have we learned about how to prevent and treat antibody-mediated rejection in kidney transplantation?

Antibody-mediated rejection (ABMR) in kidney transplantation is a major cause of late graft loss, and despite all efforts to date the "standard of care" remains plasmapheresis, IVIg, and steroids, which itself is based on low quality evidence. This review focuses on the risk factors leading to memory and de novo donor-specific antibody (DSA)-associated ABMR, the optimal prevention strategies for ABMR, and advances in  adjunctive and emerging therapies for ABMR. Because new agents require regulatory approval via a Phase 3 randomized control trial (RCT), an overview of progress in innovative trial design for ABMR is provided. Finally, based on the insights gained in the biology of ABMR, current knowledge gaps are identified for future research that could significantly affect our understanding of how to optimally treat ABMR.

Human leukocyte antigen eplet mismatches and long-term clinical outcomes in pediatric renal transplantation: A pragmatic, registry-based study

BACKGROUND

HLA epitope-based matching offers the potential to improve immunological risk prediction and management in children receiving renal allografts; however, studies demonstrating the association between systems for defining epitope mismatches and clinical end-points are lacking in this population.

METHODS

We conducted a pragmatic, retrospective, registry-based study of pediatric recipients of primary renal allografts in Victoria, Australia between 1990 and 2014 to determine the association between HLA EpMM and clinical outcomes including graft failure, re-transplantation and dnDSA formation.

RESULTS

A total of 196 patients were included in the analysis with a median age of 11 years. Median follow-up period was 15 years during which time 108 (55%) primary grafts failed and 72 patients were re-transplanted. HLA class I but not class II EpMM was a significant predictor of graft failure on univariate analysis but not in adjusted models. EpMM was associated with reduced likelihood of re-transplantation in univariate but not adjusted analysis. Within the limitations of the study, class-specific EpMM was a strong predictor of dnDSA formation. Associations were stronger when considering only the subset of antibody-verified EpMM.

CONCLUSION

Associations between HLA EpMM and clinical outcomes in pediatric renal allograft recipients seen on univariate analysis were attenuated following adjustment for confounders. These findings are inconclusive but suggest that HLA EpMM may provide one tool for assessing long-term risk in this population while highlighting the need for further clinical studies.

The association between human leukocyte antigen eplet mismatches, de novo donor-specific antibodies, and the risk of acute rejection in pediatric kidney transplant recipients

BACKGROUND

The longitudinal relationship between HLA class I and II eplet mismatches, de novo donor-specific antibodies (dnDSA) development, and acute rejection after transplantation in childhood is unknown.

METHODS

Eplet mismatches at HLA class I and II loci were calculated retrospectively for each donor/recipient pair transplanted between 2005 and 2015 at a single Australian center. Logistic regression analyses were conducted to determine the association between the number of eplet mismatches, dnDSA, and acute rejection.

RESULTS

The cohort comprised 59 children (aged 0-18 years) who received their first kidney allograft and were followed for median (interquartile range) 4.5 (± 2.6) years. Overall, 32% (19/59) developed dnDSA (class I 3% (2/59), class II 14% (8/59), 15% class I and II (9/59)), and 24% (14/59) developed biopsy-proven acute rejection. Every unit increase in class I and II eplet mismatches corresponded to an increase in risk of class I (odds ratio (OR) 1.22, 95% CI 1.07-1.39, p < 0.01) and class II (OR 1.06, 95% CI 1.01-1.11, p = 0.02) dnDSA development. Compared with recipients without dnDSA, class I and II dnDSA were associated with direction of effect towards increased risk of acute cellular rejection (class I: OR 5.87, 95% CI 0.99-34.94, p = 0.05; class II: OR 12.00, 95% CI 1.25-115.36, p = 0.03) and acute antibody-mediated rejection (class I: OR 25.67, 95% CI 3.54-186.10, p < 0.01; class II: OR 9.71, 95% CI 1.64-57.72, p = 0.01).

CONCLUSIONS

Increasing numbers of HLA class I or II eplet mismatches were associated with the development of dnDSA. Children who developed dnDSA were also more likely to develop acute rejection compared with children without dnDSA.

Relevance of donor-specific antibody monitoring after kidney transplantation: Findings from the Collaborative Transplant Study and the Heidelberg Transplant Center

Monitoring of donor-specific HLA antibodies (DSA) has become part of the clinical routine in kidney transplantation. This paper gives a brief overview on data from the Collaborative Transplant Study (CTS) and the Heidelberg Transplant Center on the clinical relevance of post-transplant DSA monitoring in patients undergoing renal transplantation. The obtained findings underline the importance of DSA monitoring in the post-operative course in immunologically high-risk patients and patients with deterioration of graft function. Especially in patients with a pre-activated immune system, a gap in the immunosuppressive therapy appear to lead to persistence, reappearance or de novo occurrence of strong, complement-activating DSA, resulting in severe antibody-mediated rejection (AMR) and, without timely intervention, in AMR-related graft loss.

Comparison Between Phenotypic Bead Assay and Single Antigen Bead Assay for Determining Specificity of HLA Antibodies in Kidney Transplant Waiting List

OBJECTIVES

The determination of unacceptable antigens in patients on kidney transplant waiting list is a critical laboratory investigation in sensitized patients. The Luminex single antigen bead (SAB) assay has high sensitivity and accuracy. However, several countries have not yet implemented SAB testing for waitlisted patients because of limited financial resources. In Thailand, specificities of HLA antibodies are identified by using a phenotypic bead assay. The aim of this study was to evaluate the performance of the phenotypic bead assay for determining HLA antibody specificities when compared with the SAB assay.

METHODS

A total of 254 sera from patients awaiting kidney transplantation were included. Of 254 sera, 206 and 171 were positive for HLA class I and II antibodies, respectively. Antibody specificities of sera that were tested with both phenotypic and SAB assay were analyzed. The performances of the phenotypic bead assay were compared with those of the SAB assay as the gold standard by using estimation of pooling sensitivity, specificity, and accuracy.

RESULTS

The sensitivity, specificity, and accuracy of the phenotypic bead assay for determining HLA class I antibodies was 53.9%, 93.0%, and 78.1%, respectively. The sensitivity, specificity, and accuracy of the phenotypic bead assay for determining HLA class II antibodies were 57.3%, 94.9%, and 81.4% respectively.

CONCLUSION

In waitlisted kidney transplant patients, the phenotypic bead assay had high specificity and moderate accuracy when compared with the SAB assay. However, the low sensitivity of the test suggests that the use of the phenotypic assay for determining HLA specificities should be applied with caution in sensitized patients.

HLA-specific memory B-cell detection in kidney transplantation: Insights and future challenges

Humoral alloimmunity mediated by anti‐human leucocyte antigen (HLA) antibodies is a major challenge in kidney transplantation and impairs the longevity of the transplanted organ. The immunological risk of an individual patient is currently mainly assessed by detection of HLA antibodies in the serum, which are produced by long‐lived bone marrow‐residing plasma cells. However, humoral alloimmunity is complex, and alloreactive memory B cells constitute an additional factor in the interplay of immune cells. These recirculating “silent” cells are responsible for the immunological recall response by differentiating into antibody‐producing cells upon antigen re‐encounter. Historically, due to the lack of appropriate and routinely applicable assays to determine the presence and HLA specificity of alloreactive memory B cells, their contribution to the humoral alloimmune response has clinically often been suspected but could not be determined. In this review, we give an overview of recent advances in techniques to detect alloreactive memory B cells and discuss their strengths and limitations. Furthermore, we summarize experiences with these techniques in alloimmunized individuals and transplant recipients, thereby emphasizing unmet needs to be addressed in future studies.

Development and outcomes of de novo donor-specific antibodies in low, moderate, and high immunological risk kidney transplant recipients

De novo donor-specific antibodies (dnDSA) play an important role in antibody-mediated rejection (ABMR) and graft failure, yet their development in kidney transplant recipients (KTx) of higher immunological risk has not been characterized. We prospectively determined the incidence of dnDSA at 3 and 12 months posttransplant and assessed their associations with outcomes in recipients stratified by low, moderate, and high immunological risk. Adult KTx were screened for DSA pretransplant, months 3 and 12 posttransplant, and when clinically indicated. Outcomes included incidence of dnDSA, death-censored graft survival (DCGS), and ABMR. Of 371 recipients, 154 (42%) were transplanted across a pretransplant DSA that became undetectable by 12 months posttransplant in 78% of cases. dnDSA were detected in 16% (95% confidence interval [CI]: 12-20%) by 3 months and 23% (95% CI: 18-29%) by 12 months posttransplant. Incidence at 12 months was higher in the moderate (30%) and high-risk groups (29%) compared to the low-risk group (16%). dnDSA were associated with an increased risk of ABMR (hazard ratio [HR] 2.2; 95% CI: 1.1-4.4; P = .04) but were not an independent risk factor for DCGS. In conclusion, dnDSA were more frequent in transplant recipients of higher immune risk and associated with an increased risk of ABMR.

Immune Algorithm Optimization for Organ Transplantation in Poland

The lack of a uniform method for determining unacceptable HLA mismatches (UAMs) for organ transplantation worldwide has resulted in many different algorithms for donor-recipient matching. Here we present our proposal for changes to the current algorithm for immune evaluation of potential kidney recipients in Poland based on the experience of various transplantation centers. The most important finding of this article is an algorithm that stratifies the pretransplant immunologic risk based on strict laboratory criteria, enabling harmonization between transplant centers in Poland. This is because of a step-by-step algorithm for alloantibody assessment using solid-phase assays (SPA) and clearly defined technical issues, as well as cutoffs for reporting UAMs. Our novel approach focuses on a laboratory testing extension in the scope of HLA typing; detection and characterization of alloantibodies before transplantation; desensitization; and post-transplant monitoring. The proposed changes will allow for the assessment of clinically relevant anti-HLA antibodies with complement binding properties; the determination of UAMs in the potential donor; the calculation of virtual panel reactive antibodies (vPRA); the calculation of the recipient's immunologic rejection risk stratification; the assessment of the donor-recipient virtual cross-match (vXM); and the determination of the final recipient's selection for the biological cross-match testing. Collectively, the optimized algorithm permit for UAM verification is based on laboratory proofed data and will firmly improve organ allocation and transplant outcomes in Poland. We hope that this novel approach also improves the individual patient's risk stratification and future personalized treatment.

Luminex screening first vs. direct single antigen bead assays: Different strategies for HLA antibody monitoring after kidney transplantation

MAIN PROBLEM

Luminex panel and single antigen beads (SAB) are used for screening and DSA specificity determination respectively. The cost of SAB may limit its general use, so some labs perform SAB tests only after positive screening.

METHODS

We compared both strategies: 1) SAB only if positive screening with kits from manufacturer A, and 2) direct SAB from manufacturer B, and correlate their sensitivity with histological findings.

RESULTS

We selected 118 kidney transplant recipients with a normal biopsy (n = 19), histological antibody-mediated damage (ABMR, n = 52) or interstitial fibrosis/tubular atrophy (IFTA, n = 47) following Banff 2015 and 2017 classification. Direct SAB detected DSA in 13 patients missed by screening. Strategy 1 detected DSA in 0% normal, 61.5% ABMR and 8.5% IFTA patients; percentages with strategy 2 were 5.2%, 78.8% and 14.8% (p=0.004). Strategy 2 identified DSA allowing full ABMR diagnosis in 17% cases missed by strategy 1. Thereafter, direct SAB from manufacturer A confirmed DSA in 46% DSA-positive cases with strategy 2 (55.5% ABMR cases).

CONCLUSIONS

Luminex screening failed to identify clinically relevant HLA antibodies, hampering DSA detection in patients with possible ABMR. Direct SAB testing should be the chosen strategy for post-transplantation monitoring, albeit direct SAB from the two existing manufacturers may diverge in as much as 50% of cases.

Epitope matching in kidney transplantation: recent advances and current limitations

PURPOSE OF REVIEW

Evolution of human leukocyte antigen (HLA) molecular typing techniques has progressively enabled more accurate determination of the three-dimensional building blocks that form the antibody accessibility and binding sites of each HLA allele. These immunogenic HLA regions known as epitopes are composed of polymorphic sequences of amino acid residues termed eplets. This review provides a critical appraisal of the current understanding of epitope compatibility in kidney transplantation.

RECENT FINDINGS

There is a tendency to suggest that epitope matching is likely to be superior to broad antigen HLA matching such that the allocation of donor kidneys to patients with a more favorable epitope compatibility profile may lead to better allograft outcomes. A growing body of work has highlighted the association between a greater number of eplet mismatches and adverse allograft outcomes, and approaches using eplet matching have been successfully implemented in organ allocation programs. However, our understanding of epitope compatibility remains in its infancy, requiring further and more in-depth evaluation. Critically, it remains unclear how best to translate findings derived at the population level to the care of individual patients. Questions that need to be answered include a lack of consensus in the definition and interpretation of epitope compatibility, are class I and II compatibility of similar clinical importance, how best to define predetermined mismatch thresholds for utilization in organ allocation, and whether other properties such as differences in electrostatic potential between donor and recipient HLA alleles are also important in determining immunological compatibility.

SUMMARY

Epitope matching likely represents a valid progression in understanding donor-recipient HLA compatibility. However, more clinical data and a better understanding about differences in methods to determine epitope compatibility are required before the approach can be widely applied in clinical practice.

Inactivation of IgM Antibodies as a Crucial Element of Diagnostics in Sensitized Patients Awaiting Kidney Transplant

BACKGROUND

The positive result of complement-dependent cytotoxicity crossmatch (CDC XM) may not constitute a contraindication to renal transplantation, unless it is mediated by IgM antibodies, particularly of the autologous type. The current work presents the evaluation of the frequency of reactive IgM antibodies in sensitized kidney patients in Poland and their influence on panel-reactive antibodies (PRAs) readout and allocation status.

PATIENTS AND METHODS

Results of PRA CDC assay with and without dithiothreitol were elaborated in 53 prospective recipients with historic PRA of ≥50%. Delta PRA (dPRA) was calculated. Retrospective analysis of the results in the context of age, sex, transplant number, and cause of end-stage renal failure was performed.

RESULTS

Reactive IgM antibodies were detected in 81% of patients. Panel reactivity completely disappeared in 4% and in 51% of recipients PRA decreased by 2 to 77 percentage points. In 14 patients, PRA increased, and in 10, its level did not change. The allocation was altered in 36% of recipients. Priority status was lost in 8 and gained in 3 cases. Additional points for high sensitization were obtained in 1 and lost in 7 patients. dPRA was significantly greater in patients awaiting the first transplant compared with the second (P = .007) and third (P = .002). Higher dPRA was also symptomatic for autoimmune patients (P = .025).

CONCLUSIONS

Reactive IgM antibodies affected PRA level and allocation in a considerable group of sensitized patients in Poland. Discrimination of IgM from IgG antibodies should be introduced to the recipient qualification algorithm.

The clinical impact of donor-specific antibodies on antibody-mediated rejection and long-term prognosis after heart transplantation

PURPOSE OF REVIEW

Outcomes after cardiac transplantation have improved over past decades, but long-term graft survival remains limited in part because of uncertainty regarding clinical implications of donor-specific antibodies (DSAs). The purpose of this review is to consolidate recent advances in knowledge on the topic of DSA and their potential to impact long-term prognosis after heart transplantation.

RECENT FINDINGS

The presence of persistent DSA increases the risk of poor outcome after heart transplantation, including development of antibody-mediated rejection (AMR), graft failure, cardiac allograft vasculopathy, and mortality. Importantly, different DSA vary in clinical significance. DSA capable of activating the complement cascade portend a higher risk of developing AMR. human leukocyte antigen class I and class II antigens are expressed differently within the heart, and so, clinical manifestations of class I and class II DSA vary accordingly. Further, compared with class I, class II DSA carry an increased risk of graft loss and mortality. When comparing preexisting DSA with formation of de-novo DSA, de-novo DSA are associated with worse outcome.

SUMMARY

DSAs are generally associated worse long-term prognosis after heart transplantation but vary in their clinical significance. Recognition of specific risk profiles is essential for guiding posttransplant antibody management.

The meaning of donor-specific antibodies after heart transplant

PURPOSE OF REVIEW

Antibody-mediated rejection (AMR) is a major contributor of impaired long-term survival after heart transplantation (HTx). The presence of circulating donor-specific antihuman leukocyte antigen (HLA) antibodies (DSAs) is considered as a mandatory criterion for AMR after HTx. DSA are known prognostic biomarkers of outcome, for example, recipients with de-novo DSA have a three-fold increased risk of mortality.

RECENT FINDINGS

Although the awareness of the impact and prognosis of DSA on the survival has been increased in the HTx community over the last decade, the management of DSA pre and posttransplant varies among centers and is mainly based on the experience of transplant physicians. Thus, firm consensus strategies for each HTx recipient should be established by a center advisory board of experts in the field of HLA genetics, transplantation immunology, and HTx to evaluate the immunological risk preoperatively and also continuously during the posttransplant course. Consequently, the recent advances of invasive and noninvasive diagnostic tools should be applied, according to the risk, laboratory findings, and clinical events of the recipient. Such individual strategy will result in tailored therapeutic options.

SUMMARY

Novel standards for the management of DSA in HTx recipients on the basis of an interdisciplinary approach of experts will improve diagnostics for personalized medicine.

Transplanting the Elderly: Mandatory Age- and Minimal Histocompatibility Matching

Worldwide over 40% of patients receiving renal replacement therapy (RRT) are aged 65 years or older, a number that is still increasing. Renal transplantation is the preferred RRT, providing substantial survival benefit over those remaining on dialysis, including the elderly. Only 3% of patients aged 65 years or older accepted on the waiting list actually received a kidney transplant offer within the Eurotransplant allocation region. To increase the chance for elderly to receive a timely kidney transplant, the Eurotransplant Senior Program was introduced. The ESP supports local allocation of older kidneys to older donors in order to decrease cold ischemia time, while disregarding former exchange principles based on matching for HLA antigens. As a consequence, more elderly received a kidney transplant and a relative higher incidence of acute rejection resulted in additional courses of high steroids and/or depleting antibody therapy. Since death with a functioning graft due to infections is the dominant reason of graft loss in elderly, more intense clinical immunosuppression to prevent or treat acute rejection is not a very attractive option. Therefore in elderly kidney transplant candidates, we advocate reintroduction of minimal histocompatibility criteria (i.e., HLA-DR matching) followed by age-matching with mandatory local/regional allocation to also facilitate short cold ischemia.

Serum dilutions as a predictive biomarker for peri-operative desensitization: An exploratory approach to transplanting sensitized heart candidates

Antibody-mediated rejection (AMR) of cardiac allografts mediated by anti-HLA Donor Specific Antibodies (DSA) is one of the major barriers to successful transplantation for the treatment of end-stage heart failure. Therapeutic plasma exchange (TPE) is a first-line treatment for pre-transplant desensitization. However, indications for treatment regimens and treatment end-points have not been well established. In this study, we investigated how sera dilutions could guide TPE regimens for effective peri-operative desensitization and early AMR treatment. Our data show that 1:16 dilutions of EDTA-treated sera and 1.5 volume TPE reduce anti-HLA class I and class II antibody levels in the same manner and, therefore, allows to predict which antibodies would respond to peri-operative TPE. We successfully applied this approach to transplanting three highly sensitized cardiac recipients (CPRA 85-93%) with peri-operative desensitization based on a virtual crossmatch performed on 1:16 diluted serum. Furthermore, we have used sera dilutions to guide DSA treatment post-transplant. Although these findings have to be confirmed in a larger prospective study, our data suggest that serum dilutions can serve as a predictive biomarker to guide peri-operative desensitization and post-transplant immunologic management.

Virtual Crossmatching in Kidney Transplantation: The Wait Is Over

BACKGROUND

Flow cytometric crossmatching is currently the method of choice for most transplantation programs before kidney transplantation. In July of 2017, our program implemented the virtual crossmatch, without a prospective physical crossmatch, for the majority of patients in the setting of a new kidney allocation system implemented by the United Network for Organ Sharing.

STUDY DESIGN

A retrospective review was conducted to determine whether virtual crossmatching could reduce cold ischemia time (CIT). Secondary outcomes included the incidence of delayed graft function and 1-year patient and allograft failure.

RESULTS

A total of 825 patients received a kidney transplant between December 1, 2014 and July 1, 2018; 505 were in the pre-implementation group and 227 were in the post-implementation group. The CIT decreased between the pre-implementation era to post implementation era from 16.67 ± 8.7 hours to 14.5 ± 8.2 hours (p = 0.002). On univariate analysis, delayed graft function (DGF) rates were similar between the 2 eras (19% vs 17%; p = 0.415), despite having more donations after cardiac death and higher Kidney Donor Profile Index donors in the post-implementation era. There was no difference in biopsy-proven acute rejection (n = 28 [5.6%] vs n = 8 [3.5%]; p = 0.226), 1-year graft loss (4% vs 3%; p = 0.304), or patient death (2% vs 1%; p = 0.567) rate between groups. On multivariable modeling for mean CIT and incidence of DGF, patients receiving transplants in the post-implementation era had an adjusted reduction in CIT of an estimated 2.35 hours (95% CI, 1.15 to 3.55; p < 0.001). Patients in the post-implementation era also had 26% lower odds of DGF developing (odds ratio 0.74; 95% CI, 0.48 to 1.14; p = 0.170), after adjusting for covariates.

CONCLUSIONS

Kidney transplantation can be safely performed with virtual crossmatching, without a prospective physical crossmatch with improved CIT and potentially reduced DGF rate without increased risk of rejection.

Donor-specific antibodies detected by single antigen beads alone can help risk stratify patients undergoing retransplantation across a repeat HLA mismatch

Whether reexposure to mismatched HLA antigens (RMM) in the setting of a negative crossmatch is associated with increased immunological risk remains an area of uncertainty. This is due to evidence derived predominantly from registry data, which lacks comprehensive information on alloantibody and rejection. In this study, we analyze the impact of low-level preformed donor-specific antibodies (DSA) against an RMM on transplant outcomes. From 1988 consecutive renal transplant recipients, we analyzed 179 patients undergoing retransplantation, of whom 55 had a RMM. All patients were crossmatch negative and preformed DSA were detected by single antigen beads alone. Multivariate analysis revealed that patients with preformed DSA against an RMM were independently at risk of antibody-mediated rejection (HR 8.70 [3.42-22.10], P < .0001) and death-censored allograft loss (HR 3.08 [1.17-8.14], P = .023). In addition, prior transplant nephrectomy (HR 2.04 [1.00-4.17], P = .0495) was also associated with allograft failure, whereas receiving a retransplant that was matched at HLA class II was associated with a favorable outcome (HR 0.37 [0.14-0.99], P = .047). In the absence of preformed DSA, an RMM was not associated with de novo DSA development, rejection, or allograft loss. In conclusion, an RMM portends increased immunological risk only in the presence of a preformed DSA in patients undergoing retransplantation.

The effect of serum pretreatment regimens for the detection of HLA class I antibodies in platelet-refractory patients

BACKGROUND

Single antigen bead (SAB) assays are used to identify human leukocyte antigen (HLA) antibodies in patients with platelet refractoriness due to HLA Class I alloimmunization. Some laboratories use serum pretreatment regimens to eliminate interference from immunoglobulin M antibodies and complement. These modifications may contribute to interlaboratory variability, which is a recognized problem with the SAB assay.

STUDY DESIGN AND METHODS

Five patients' sera were overnight shipped to 12 laboratories in the United States and internationally. Recipients used their lab's SAB procedure to identify HLA Class I antibodies. The resultant mean fluorescence intensity (MFI) data were compared by instrumentation, bead lot, and pretreatment regimens. Laboratory-specific cutoffs for positive antibodies were applied to the results.

RESULTS

Interlaboratory variability for MFI values appears to be associated with different pretreatment regimens. The coefficient of variation (CV) of MFI from samples pretreated with ethylenediaminetetraacetic acid, dithiothreitol, or heat inactivation (EDHI) were similar, ranging from 14% to 56% (mean, 22%). For samples with no pretreatment, the CVs were significantly higher than EDHI-treated samples, ranging from 25% to 74% (mean, 39%; 95% confidence interval, 12.10-21.90; p < 0.0001). An intralaboratory comparison of pretreatment regimens confirmed these findings. Some positive antibody specificities present in EDHI-treated samples were negative in corresponding samples with no pretreatment when laboratory-specific cutoffs for positive antibodies were applied.

CONCLUSION

Our results show that greater interlaboratory precision can be achieved when samples are pretreated with EDHI as opposed to no pretreatment, likely because these pretreatments eliminate interference from inhibitors. Inhibitors may mask antibodies, leading to missed (or uncalled) specificities when no pretreatment is used.

Biomarkers of rejection in kidney transplantation

PURPOSE OF REVIEW

To provide an update of the literature on the use of new biomarkers of rejection in kidney transplant recipients.

RECENT FINDINGS

The kidney allograft biopsy is currently considered the gold standard for the diagnosis of rejection. However, the kidney biopsy is invasive and could be indeterminate. A significant progress has been made in discovery of new biomarkers of rejection, and some of them have been introduced recently for potential use in clinical practice including measurement of serum donor-derived cell free DNA, allo-specific CD154 + T-cytotoxic memory cells, and gene-expression 'signatures'. The literature supports that these biomarkers provide fair and reliable diagnostic accuracy and may be helpful in clinical decision-making when the kidney biopsy is contraindicated or is inconclusive.

SUMMARY

The new biomarkers provide a promising approach to detect acute rejections in a noninvasive way.

Antibodies against ARHGDIB are associated with long-term kidney graft loss

The clinical significance of non-HLA antibodies on renal allograft survival is a matter of debate, due to differences in reported results and lack of large-scale studies incorporating analysis of multiple non-HLA antibodies simultaneously. We developed a multiplex non-HLA antibody assay against 14 proteins highly expressed in the kidney. In this study, the presence of pretransplant non-HLA antibodies was correlated to renal allograft survival in a nationwide cohort of 4770 recipients transplanted between 1995 and 2006. Autoantibodies against Rho GDP-dissociation inhibitor 2 (ARHGDIB) were significantly associated with graft loss in recipients transplanted with a deceased-donor kidney (N = 3276) but not in recipients of a living-donor kidney (N = 1496). At 10 years after deceased-donor transplantation, recipients with anti-ARHGDIB antibodies (94/3276 = 2.9%) had a 13% lower death-censored covariate-adjusted graft survival compared to the anti-ARHGDIB-negative (3182/3276 = 97.1%) population (hazard ratio 1.82; 95% confidence interval, 1.32-2.53; P = .0003). These antibodies occur independently from donor-specific anti-HLA antibodies (DSA) or other non-HLA antibodies investigated. No significant relations with graft loss were found for the other 13 non-HLA antibodies. We suggest that pretransplant risk assessment can be improved by measuring anti-ARHGDIB antibodies in all patients awaiting deceased-donor transplantation.

HLA Alloimmunization Following Ventricular Assist Device Support Across the Age Spectrum

BACKGROUND

Ventricular assist device (VAD) therapy has become an important tool for end-stage heart failure. VAD therapy has increased survival but is associated with complications including the development of human leukocyte antigen (HLA) antibodies. We sought to determine the incidence of HLA antibody development post-VAD insertion, across the age spectrum, in patients receiving leukocyte-reduced blood products, with standardized HLA antibody detection methods and to investigate factors associated with antibody development.

METHODS

This was a retrospective analysis of all patients who underwent durable VAD placement between 2005 and 2014. Inclusion criteria included availability of pre- and post-VAD HLA antibody results. Associations between HLA antibody development in the first-year postimplant and patient factors were explored.

RESULTS

Thirty-nine adult and 25 pediatric patients made up the study cohort. Following implant, 31% and 8% of patients developed new class I and class II antibodies. The proportion of newly sensitized patients was similar in adult and pediatric patients. The class I HLA panel reactive antibody only significantly increased in adults. Pre-VAD sensitization, age, sex (pediatrics), and transfusion were not associated with the development of HLA antibodies.

CONCLUSIONS

In a cohort of VAD patients receiving leukocyte-reduced blood products and standardized HLA antibody testing, roughly one-third developed new class I antibodies in the first-year postimplant. Adults showed significantly increased class I panel reactive antibody following VAD support. No patient-related factors were associated with HLA antibody development. Larger prospective studies are required to validate these findings and determine the clinical impact of these antibodies following VAD insertion.

Xenotransplantation: Progress Along Paths Uncertain from Models to Application

For more than a century, transplantation of tissues and organs from animals into man, xenotransplantation, has been viewed as a potential way to treat disease. Ironically, interest in xenotransplantation was fueled especially by successful application of allotransplantation, that is, transplantation of human tissue and organs, as a treatment for a variety of diseases, especially organ failure because scarcity of human tissues limited allotransplantation to a fraction of those who could benefit. In principle, use of animals such as pigs as a source of transplants would allow transplantation to exert a vastly greater impact than allotransplantation on medicine and public health. However, biological barriers to xenotransplantation, including immunity of the recipient, incompatibility of biological systems, and transmission of novel infectious agents, are believed to exceed the barriers to allotransplantation and presently to hinder clinical applications. One way potentially to address the barriers to xenotransplantation is by genetic engineering animal sources. The last 2 decades have brought progressive advances in approaches that can be applied to genetic modification of large animals. Application of these approaches to genetic engineering of pigs has contributed to dramatic improvement in the outcome of experimental xenografts in nonhuman primates and have encouraged the development of a new type of xenograft, a reverse xenograft, in which human stem cells are introduced into pigs under conditions that support differentiation and expansion into functional tissues and potentially organs. These advances make it appropriate to consider the potential limitation of genetic engineering and of current models for advancing the clinical applications of xenotransplantation and reverse xenotransplantation.

The five dimensions of B cell tolerance

B cell tolerance has been generally understood to be an acquired property of the immune system that governs antibody specificity in ways that avoid auto-toxicity. As useful as this understanding has proved, it fails to fully explain the existence of auto-reactive specificities in healthy individuals and contribution these may have to health. Mechanisms underlying B cell tolerance are considered to select a clonal repertoire that generates a collection of antibodies that do not bind self, ie tolerance operates more or less in three dimensions that largely spare autologous cells and antigens. Yet, most B lymphocytes in humans and probably in other vertebrates are auto-reactive and absence of these auto-reactive B cells is associated with disease. We suggest that auto-reactivity can be embodied by extending the concept of tolerance by two further dimensions, one of time and circumstance and one that allows healthy cells to actively resist injury. In this novel concept, macromolecular recognition by the B cell receptor leading to deletion, anergy, receptor editing or B cell activation is extended by taking account of the time of development of normal immune responses (4th dimension) and the accommodation (or tolerance) of normal cells to bound antibody, activation of complement, and interaction with inflammatory cells (fifth dimension). We discuss how these dimensions contribute to understanding B cell biology in health or disease.

The role of the immune system in lung transplantation: towards improved long-term results

Over the past 35 years, lung transplantation has evolved from an experimental treatment to the treatment of choice for patients with end-stage lung disease. Beyond the immediate period after lung transplantation, rejection and infection are the leading causes of death. The risk of rejection after lung transplantation is generally higher than after other solid organ transplants, and this necessitates more intensive immunosuppression. However, this more intensive treatment does not reduce the risk of rejection sufficiently, and rejection is one of the most common complications after transplantation. There are multiple forms of rejection including acute cellular rejection, antibody-mediated rejection, and chronic lung allograft dysfunction. These have posed a vexing problem for clinicians, patients, and the field of lung transplantation. Confounding matters is the inherent effect of more intensive immunosuppression on the risk of infections. Indeed, infections pose a direct problem resulting in morbidity and mortality and increase the risk of chronic lung allograft dysfunction in the ensuing weeks and months. There are complex interactions between microbes and the immune response that are the subject of ongoing studies. This review focuses on the role of the immune system in lung transplantation and highlights different forms of rejection and the impact of infections on outcomes.

Donor-specific antibodies in a pediatric kidney transplant population-Prevalence and association with antiproliferative drug dosing

Prevalence and implications of anti-HLA class I and class II antibodies are beginning to be better characterized in pediatric kidney transplant recipients. dnDSA formation is predictive of AMR and downstream diminished graft function and survival. However, risk factors for the development of dnDSA are not well defined in this patient population. After introducing DSA surveillance into our pediatric kidney transplant program, we are reporting the prevalence of class I and class II DSA in 67 otherwise stable recipients. Secondary end-points included risk factors for DSA development and assessment of graft function. Significantly, lower median daily MMF doses were observed in patients with DSAs compared to patients without DSAs (371 vs 617 mg/m² /d, respectively; P = 0.035). Class II DSA formation was more common, with a prevalence of 17.9%, as compared to 10.4% for class I DSA. Estimated glomerular filtration rate was also decreased in patients with positive DSA vs those with negative titers (71, SD 25 vs 78, SD 29 mL/min/1.73 m² , respectively; P = 0.034). We conclude that reduced-dose MMF is associated with dnDSA and DSA is associated with diminished graft function in stable pediatric kidney transplant recipients.