Technical Aspects of Crossmatching in Transplantation

Development and performance of a next generation sequencing (NGS) assay for monitoring of dd-cfDNA post solid organ transplantation

The aim of this study was to evaluate the analytical performance of a novel NGS assay, intended for monitoring of donor-derived cell-free DNA (dd-cfDNA), and describe its validity in clinical plasma samples from kidney transplanted patients. Artificial and clinical samples with increasing amounts of patient DNA were evaluated using NGS analysis of indel markers. Monitoring of dd-cfDNA with the NGS assay presented herein demonstrated a sensitivity of ≥0.1% dd-cfDNA and excellent accuracy (R2 0.99) throughout an extensive range of dd-cfDNA (0.1-30%). The precision of the test was determined for two levels (0.1% (LoD) and 1%) of dd-cfDNA. The between run precision (CV%) for the respective level was 16% and 9% and the corresponding result for the within run precision was 19% and 7%. To evaluate performance of the assay in clinical samples, 507 retrospective monitoring samples from 21 patients transplanted either with kidneys from living or deceased donors were analyzed. Monitoring samples were sampled at multiple time points from 24 h up to 90 days post-transplantation. We show that in one patient, increase of dd-cfDNA preceded increase of creatinine caused by acute cellular rejection by several days. In conclusion, the NGS assay displayed a combination of high sensitivity with good accuracy and precision in both artificial and clinical dd-cfDNA samples.

Crossmatch assays in transplantation: Physical or virtual?: A review

The value of the crossmatch test in assessing pretransplant immunological risk is vital for clinical decisions, ranging from the indication of the transplant to the guidance of induction protocols and treatment with immunosuppressants. The crossmatch tests in transplantation can be physical or virtual, each with its advantages and limitations. Currently, the virtual crossmatch stands out for its sensitivity and specificity compared to the physical tests. Additionally, the virtual crossmatch can be performed in less time, allowing for a reduction in cold ischemia time. It shows a good correlation with the results of physical tests and does not negatively impact graft survival. Proper communication between clinicians and the transplant immunology laboratory will lead to a deeper understanding of each patient's immunological profile, better donor-recipient selection, and improved graft survival.

Utilizing proficiency testing survey data to create advanced educational content: the virtual crossmatch challenge model

Proficiency testing (PT) surveys include data from laboratories across the world and are ideal for creating advanced educational content, beyond just consensus grading. Educational challenges provide a unique opportunity to probe common laboratory practices and risk assessment, especially in cases where there is no "analyte" tested. Human leukocyte antigen (HLA) compatibility evaluation between donor and recipient pairs has been traditionally assessed using T-cell and B-cell physical crossmatches. However, advancements in our ability to identify and characterize HLA antibodies using solid phase assays, in combination with changing deceased donor allocation schemes and improved HLA typing, have shifted the paradigm from performing physical crossmatches to the use of the virtual crossmatch (VXM). VXM is a compatibility assessment relying on the interpretation of pre-transplant HLA laboratory data and as such, it is not an "analyte". However, VXM results are used in clinical decision-making. The VXM assessment depends on patient characteristics as well as laboratory and transplant center practices but must ensure safe transplantation outcomes while maintaining equity in access to transplantation. In this manuscript, we describe the American Society for Histocompatibility and Immunogenetics (ASHI) PT Educational VXM Challenge, as a model for creating educational content using PT survey data. We discuss the different components of the VXM Challenge and highlight major findings and learning points acquired from ASHI VXM Challenges performed between 2018-2022, such as the lack of correlation between the VXM and the physical crossmatch in the presence of low level donor-specific antibodies (DSA), or when the DSA were aimed against donor alleles that are not present on the antibody panel, and in the presence of an antibody to a shared eplet. Finally, we show that the VXM Educational Challenge serves as a valuable tool to highlight the strengths and pitfalls of the VXM assessment and reveals differences in testing and result interpretation among participating HLA laboratories.

Low Expression Loci and the Use of Pronase in Flow Cytometry Crossmatch

Pronase treatment of lymphocytes has been used to improve the specificity and sensitivity of flow cytometric crossmatch, especially B-cell flow cytometric crossmatch, due to the presence of Fc receptors on the cell surface. Some limitations have been reported in the literature: false negatives due to the reduction of major histocompatibility complex expression and false-positive T cells in HIV+ patients due to exposure to cryptic epitopes. This study aimed to evaluate the effect of pronase in our assays, using untreated and treated cells with 2.35 U/mL of pronase to improve flow cytometric crossmatch specificity and sensitivity. The study was carried out with donor-specific IgG antibodies (DSAs) to low expression loci (HLA-C, -DQ, or -DP) because, in our laboratory practice, patients with virtual crossmatch (LABScreen single antigen assays) to DSA against antigen HLA-A, B, and DR are excluded from cellular crossmatch. Our results showed that, for T-cell flow cytometry crossmatch (FCXM), a cutoff value of 1171 median fluorescence intensity (MFI), an area under the curve (AUC) of 0.926 (P < .0001), and 0.834 (P < .0001), a sensitivity of 100% and 85.7%, and a specificity of 77.5% and 74.4%, without and with pronase treatment, respectively. For B-cell FCXM without pronase treatment, the best cutoff was 2766 MFI, an AUC of 0.731 (P < .0001), a sensitivity of 69.6%, and a specificity of 66.7%, whereas for B cells treated with pronase, the cutoff value was 4496 MFI, an AUC of 0.852 (P < .0001), a sensitivity of 86.4%, and a specificity of 77.8%. Our analysis of 128 FCXM showed a better performance using the untreated lymphocytes for FCXM with the prerequisite of a higher cutoff value (≈5000 MFI) to reach a better sensitivity and specificity due to the loss of HLA expression.

Translation of therapeutic strategies to modulate B cell reponses from non-human primate models to human kidney transplantation

Using novel drugs targeting lymphocyte costimulation, cytokines, antibody, complement, and plasma cells, we have developed strategies in a non-human primate model to modulate the B cell response to incompatible kidney transplants. After more than two decades of research supported by mechanistic studies, this has resulted in clinically relevant approaches that are currently enrolling in clinical trials or preparing for such. In this manner, we aim to address the problems of HLA sensitization for very highly sensitized patients awaiting transplantation and the unmet need of effective treatment for antibody-mediated rejection.

Aspectos técnicos y clínicos de la prueba cruzada de histocompatibilidad en el trasplante de órganos sólidos

La presencia de anticuerpos dirigidos contra los antígenos leucocitarios humanos (Human Leukocyte Antigens, HLA) que se expresan en las células del donante, es uno de los factores de riesgo más importantes asociados con las complicaciones clínicas después del trasplante. La prueba cruzada es una de las pruebas de histocompatibilidad más eficaces para la detección de anticuerpos específicos contra el donante en los receptores de injertos. En los primeros métodos de la prueba cruzada, se utilizaba la citotoxicidad dependiente del complemento, que es útil para detectar dichos anticuerpos responsables del rechazo hiperagudo del injerto, pero carece de la sensibilidad adecuada. Por ello, se desarrollaron métodos de pruebas cruzadas más sensibles, entre ellas, la prueba cruzada por citometría de flujo que hoy se considera el método preferido.

En este artículo se revisa la evolución de la prueba cruzada y los factores más importantes que deben tenerse en cuenta al realizarla y al interpretar los resultados de esta prueba fundamental para la supervivencia a largo plazo del injerto.

Phasing out the pre-transplant cytotoxicity crossmatch: Are we missing something?

Introduction:

The anti-human globulin-enhanced complement-dependent cytotoxicity crossmatch (AHG-CDCXM) assay has been used to assess the presence of donor-specific antibodies (DSA) in recipient’s serum before kidney transplantation. The flow cytometric crossmatch (FCXM) assay was first introduced as an additional test. The aim of this study was to clinically validate the single use of the FCXM assay.

Methods:

This study compared the outcomes of a cohort of kidney transplant patients that underwent FCXM only (FCXM group) versus a cohort of kidney transplant patients that underwent AHG-CDCXM (control group).

Results:

Ninety-seven patients in the FCXM group and 98 controls were included. All crossmatches in the control group were negative. One patient in the FCXM group had a positive B cell crossmatch. One year after transplantation, there were no significant differences in patient survival (p = 0.591) and graft survival (p = 0.692) between the groups. Also, no significant difference was found in the incidence of Banff ≥ 1A acute cellular rejection episodes (p = 0.289). However, acute antibody-mediated rejections occurred in 3 controls (p = 0.028).

Conclusion:

The results showed that discontinuing the AHG-CDCXM assay does not modify the clinical outcomes in a 1-year follow-up.

Image cytometry as an alternative to flow cytometry for the transplant histocompatibility crossmatch assay

The lymphocyte crossmatch is currently the only cell-based compatibility assay performed by histocompatibility laboratories for transplant purposes. While in many transplant programs the complement-dependent cytotoxicity crossmatch (CDCXM) remains in use, when available, the flow cytometry crossmatch (FCXM) is the method of choice because of its superior sensitivity and specificity. Unfortunately, the maintenance and cost of a flow cytometer is a considerable limitation for small histocompatibility laboratories. Therefore, in this study, we evaluated the use of the Cellometer Vision CBA image cytometer (Nexcelom Bioscience LLC, Lawrence, Massachusetts) as an alternative instrument to perform the crossmatch assay. The 3-color FCXM protocol was modified into two separate 2-color panel image cytometry crossmatches (IXMs), one for T cells and one for B cells. After initial serum and cell incubation, a cocktail consisting of PE/Cy5-conjugated anti-human CD3 or CD19 and PE-conjugated anti-human IgG F(ab')₂ was added to the T cell and B cell panels, respectively. The final cell preparation was added to a separate counting chamber. Images were captured using the Cellometer Vision CBA, an image cytometer designed for cell counting, size analysis and fluorescence intensity measurement. Thirty-nine IXMs were performed and compared with the FCXM. We obtained a concordance sensitivity of 94.1% and 100% and specificity of 100% and 88.9% for T cells and B cells, respectively. The linearity of the system was verified using dilutions of a sample containing known donor-specific anti-HLA antibodies (DSA) against the target cells. This feasibility study demonstrates that the FCXM test could be easily adapted to the Cellometer Vision CBA image cytometer without compromising specificity and sensitivity. The low instrumentation cost, minimal maintenance, and simple operation allow for efficient implementation or transition from the FCXM to the IXM method.