BSHI and BTS UK guideline on the detection of alloantibodies in solid organ (and islet) transplantation

Exploring the immunological relevance of pre-transplant donor-specific antibody in intestinal transplantation, with special consideration to the liver

Despite recent advances that have improved outcomes following intestinal transplantation (ITx), achieving long-term patient survival and rejection-free survival is still challenging. Understanding the relevance of pre-transplant human leukocyte antigen (HLA) donor-specific antibody (DSA) in ITx and the immunomodulatory potential of the liver within the allograft is crucial to providing an accurate assessment of pre-transplant immunological risk, which could influence and improve post-transplant outcomes further. This was the primary objective of this retrospective study of 95 adult ITx transplants which took place at Cambridge University Hospitals (United Kingdom) between 2007 and 2019. Two novel programs were developed and validated to identify DSA (tested by Luminex single antigen beads) in this dataset. Data analysis utilised Kaplan-Meier survival methods, and statistical analysis was performed using log-rank tests and adjusted Cox models. Fifty-four (57%) ITx cases contained a liver, and 36 (38%) were sensitised to HLA. Pre-transplant DSA > 500 mean fluorescent intensity appeared to negatively affect post-ITx patient survival and rejection outcomes. Additionally, liver-inclusive allografts seemed to show particular resistance to HLA class I DSA. Our data hints towards consistency with other ITx studies where deleterious effects of DSA have been demonstrated, and where liver inclusion is protective from HLA class I DSA. This is in line with current national guidelines for immunological risk. Our publicly available research programs could support future large or multicentre studies where statistically relevant data might be gained.

Predicting flow cytometry crossmatch results from single-antigen bead testing

The aim of this study was to devise an algorithm that would predict flow cytometry crossmatch (FCXM) results using single-antigen bead (SAB) mean fluorescent intensity (MFI) levels using samples received through the National External Quality Assurance Scheme (NEQAS) 2B external proficiency testing scheme between 2019 and 2023. A total of 159 serum samples were retrospectively screened using LABScreen Single Antigen Class I and II (SAB), and 40 peripheral blood samples were human leucocyte antigen (HLA) typed with LABType SSO. Donor-specific antibodies were identified for each cell-serum combination tested, and cumulative MFI values were calculated for each test before correlating the screening result with the consensus crossmatch results for this scheme. HLA Class I MFIs were combined to predict the T cell crossmatch. For the B cell crossmatch prediction, two options were considered: (i) HLA Class II MFI values alone and (ii) HLA Class I + Class II MFIs. Receiver operating characteristic analysis was carried out to identify the combined MFI threshold that predicted NEQAS consensus results with the greatest sensitivity and specificity. HLA Class I combined MFI >5000 predicted T cell crossmatch results with 96% sensitivity, 100% specificity, 100% positive predictive value (PPV) and 92% negative predictive value (NPV). For B cell results, HLA Class I + Class II combined MFIs >11,000 gave the best model, showing 97% sensitivity, 82% specificity, 96% PPV and 85% NPV. However, for samples with only HLA Class II sensitization, combined MFIs >13,000 improved the B cell crossmatch predictions: 92% sensitivity, 95% specificity, 96% PPV and 91% NPV. Using this model, combined MFI can be used to predict the immunological risk posed by donor-specific antibodies when it is not possible to carry out an FCXM.

Complement-dependent cytotoxicity crossmatch in solid organ transplantation: The gold standard or golden history?

Complement-dependent cytotoxicity crossmatch (CDC-XM) has been considered for many years the standard of practice for determining compatibility in solid organ transplantation (SOT). However, as this method is laborious, time intensive and lacks sensitivity and specificity, it has been replaced in many laboratories worldwide by flow cytometry crossmatch (FCXM) and/or virtual crossmatch (vXM). With this study we intend to show the relevance of performing CDC-XM in the era of virtual crossmatching. We retrospectively analyzed 1,007 consecutive T and B cell deceased donor (DD) CDC-XMs performed in parallel using non-treated and dithiothreitol (DTT) treated sera between May 2022 and January 2023 in waitlisted patients with no donor specific antibodies (DSA) against HLA-A, B and/or DR antigens. Thirty five of 1,007 (3.5%) T cell crossmatches and 132 of 1,007 (13.1%) B cell crossmatches were positive with non-treated sera. Correlation with the vXM demonstrated no DSA in any of the positive T cell crossmatches. DSA were also absent in 126/132 positive B cell crossmatches, indicating a high rate of false positive CDC-XM. Indeed, only 4/35 T cell and 13/132 B cell CDC-XM remained positive after treatment with DTT, confirming that false positive reactivity with non-treated sera is high. Class I HLA DSA against C locus antigens were present in 17/1,007 T cell crossmatches and none were detected by CDC-XM (sensitivity = 0%). Similarly, only 6/77 B cell crossmatches with DSA targeting HLA-C, DQ and/or DP antigens were CDC-XM positive (sensitivity = 7.8%). Furthermore, only 4/6 positive B cell CDC-XM were confirmed to have complement binding potential using the C1q assay, suggesting additional false positive reactivity in 2/6 of the positive CDC-XM. Our study demonstrates that CDC-XM exhibits poor sensitivity, high false positive reactivity (especially without DTT treatment) and does not meaningfully contribute to pre-transplant compatibility testing in the context of vXM based allocation. Furthermore, the use of CDC-XM can unnecessarily delay or even prevent safe and appropriate transplant allocation.