ABO Genotyping finds more A2 to B kidney transplant opportunities than lectin-based subtyping.

ABO compatibility is important for kidney transplantation, with longer waitlist times for blood group B kidney transplant candidates. However, kidneys from non-A1 (eg, A2) subtype donors, which express less A antigen, can be safely transplanted into group B recipients. ABO subtyping is routinely performed using anti-A1 lectin, but DNA-based genotyping is also possible. Here, we compare lectin and genotyping testing. Lectin and genotype subtyping was performed on 554 group A deceased donor samples at 2 transplant laboratories. The findings were supported by 2 additional data sets of 210 group A living kidney donors and 124 samples with unclear lectin testing sent to a reference laboratory. In deceased donors, genotyping found 65% more A2 donors than lectin testing, most with weak lectin reactivity, a finding supported in living donors and samples sent for reference testing. DNA sequencing and flow cytometry showed that the discordances were because of several factors, including transfusion, small variability in A antigen levels, and rare ABO∗A2.06 and ABO∗A2.16 sequences. Although lectin testing is the current standard for transplantation subtyping, genotyping is accurate and could increase A2 kidney transplant opportunities for group B candidates, a difference that should reduce group B wait times and improve transplant equity.

Histocompatibility considerations for kidney paired donor exchange programs.

PURPOSE OF REVIEW: To highlight the role of histocompatibility testing in kidney paired donor (KPD) exchange programs as well as the new technological advances that may have an effect on KPD. RECENT FINDINGS: Technological advances in human leukocyte antigen (HLA) antibody identification using the Luminex single-antigen bead multiplexing platform have facilitated virtual cross-matching and the ability to accurately match donor/recipient pairs through KPD. A knowledge of the limitations of this assay is the key to proper interpretation of the data and maximization of this new technology. Novel assays such as C1q and Ig subclass identification may be useful in further determining which HLA antibodies are clinically relevant. SUMMARY: KPD is an established method for increasing access to transplantation for patients with incompatible live donors. Advances in histocompatibility testing have played a role in the success of KPD.

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.

Mapping molecular HLA typing data to UNOS antigen equivalents.

BACKGROUND: Histocompatibility labs must convert molecular HLA typing data to antigen equivalencies for entry into the United Network for Organ Sharing (UNOS) UNet system. While an Organ Procurement and Transplantation Network (OPTN) policy document provides general guidelines for conversion, the process is complex because no antigen mapping table is available. We present a UNOS antigen equivalency table for all IPD-IMGT/HLA alleles at the A, B, C, DRB1, DRB3/4/5, DQA1, and DQB1 loci. METHODS: An automated script was developed to generate a UNOS antigen equivalency table. Data sources used in the conversion algorithm included the World Marrow Donor Association (WMDA) antigen table, the HLA Dictionary, and UNOS-provided tables. To validate antigen mappings, we converted National Marrow Donor Program (NMDP) high resolution allele frequencies to antigen equivalents and compared with the UNOS Calculated Panel Reactive Antibodies (CPRA) reference panel. RESULTS: Normalized frequency similarity scores between independent NMDP and UNOS panels for 4 US population categories (Caucasian, Hispanic, African American and Asian/Pacific Islander) ranged from 0.85 to 0.97, indicating correct antigen mapping. An open source web application (ALLele to ANtigen ("ALLAN")) and web services were also developed to map unambiguous and ambiguous HLA typing data to UNOS antigen equivalents based on NMDP population-specific allele frequencies (http://www.transplanttoolbox.org). CONCLUSIONS: Computer-assisted interpretation of molecular HLA data may aid in reducing typing discrepancies in UNet. This work also sets a foundation for molecular typing data to be utilized directly in the UNet match run as well as the virtual crossmatch process at transplant centers.

Trends in HLA antibody screening and identification and their role in transplantation.

HLA testing has been a staple in transplantation since the recognition that antibodies, directed against lymphocytes, were associated with allograft failure. This seminal finding led to the discovery of the MHC and the appreciation of the importance of HLA testing in transplantation. Early approaches focused on the importance of HLA matching, and were an important aspect of deceased organ donor allocation. More recently, and as a direct result of improvements in immunosuppression, there has been a movement away from 'matching' as the driving force in organ allocation. By contrast, we are now challenged with selecting donor-recipient pairs based on acceptable mismatches. For patients devoid of HLA antibodies, this is not an issue. However, for patients with HLA alloantibodies, that is, the sensitized patient, we face significant challenges in assessing the repertoire of the HLA antibody reactivity they possess. Over the past several years, significant advances in HLA antibody detection have occurred. Solid-phase, multiplex testing platforms have replaced traditional cell-based assays, and have provided better sensitivity and specificity in antibody detection. As a direct result of improved antibody identification, many programs are moving into the realm of the 'virtual crossmatch'. The virtual crossmatch has proven to be successful in renal, cardiac and lung transplantation, and has resulted in a greater percentage of sensitized patients gaining access to transplantation. This review will be devoted to highlighting the latest developments in antibody assessments and discussing their utilization in transplant testing.

A virtual crossmatch protocol significantly increases access of highly sensitized patients to deceased donor kidney transplantation.

BACKGROUND: Patients with preexisting antihuman leukocyte antigen (HLA) antibodies (sensitized patients) are more likely to have a positive crossmatch with possible donors and have a lower likelihood of receiving a renal transplant with longer wait times. A virtual crossmatch protocol using solid-phase technology to determine the specificity of anti-HLA antibodies may improve the probability of identifying a crossmatch-negative compatible donor and increase access of sensitized patients to kidney transplantation. METHODS: A virtual crossmatch protocol was implemented on October 1, 2006 with solid-phase HLA antibody characterization for all sensitized patients on the waiting list. Transplant rates for the period from October 2006 to June 2008 were compared with Scientific Registry of Transplant Recipients (SRTR) data from 2006 to determine national transplant rates for sensitized patients. RESULTS: SRTR data for 2006 showed that nationally 590 of 10,659 transplants (5.5%) were in-patients with panel reactive antibody (PRA) more than or equal to 80%. During 2006 to 2008, after initiation of the virtual crossmatch protocol, we performed 122 deceased donor kidney transplants, of which 15 (12.3%) sensitized patients (PRA>or=80%) received transplants (P=0.004 compared with SRTR national data), with 9 (7.4%) patients having a PRA more than 90%. The virtual crossmatch protocol was predictive of a negative-final crossmatch and eliminated the use of preliminary cross-matching with attendant cost savings of more than $100,000. CONCLUSION: Initiation of a virtual crossmatch protocol using solid-phase histocompatibility techniques significantly increased access of sensitized patients to kidney transplantation and was more cost effective. Usage of a virtual crossmatch may facilitate greater sharing of kidneys to improve access to transplantation for sensitized recipients.