University of Florida Health Shands Hospital, Gainesville
Body: Flow cytometry crossmatching is a crucial technique for assessing transplant compatibility. However, unexpected positive results can complicate transplantation assessments, prompting investigation into underlying factors. With the increasing prevalence of antibody therapeutic agents, the identification and mitigation of interference in crossmatches have become more pressing concerns for transplant centers. We present a case study of a patient with an unexpected positive T cell flow crossmatch.
The patient is a 30yo male with renal failure. He had a red cell transfusion 4 months before and no prior transplants. Pre-transplant solid phase testing was negative for anti-HLA antibodies; last tested two months before the kidney offer. Virtual crossmatch was negative, and he was transplanted without prospective physical crossmatch as per our protocol. Post-transplant flow crossmatch was T cell positive (ratio 49.8; cut-off 2.0) and B cell negative (ratio 2.0; cut-off 6.3). Solid phase testing (FlowPRA) on the current pre-transplant serum was performed, and was negative for anti-HLA antibodies. Subsequent investigation revealed the patient had been treated with ibalizumab. Ibalizumab is a humanized IgG4 monoclonal antibody used in the treatment of HIV. The drug binds to the CD4 receptor on human T cells blocking HIV entrance.
To further investigate, we conducted additional flow crossmatches using the patient's serum and two non-donor test cells. As expected, these were T cell positive when gated on CD3. But, we also included CD8 for gating along with the usual CD3 and CD20. We theorized that ibalizumab interference would be limited to CD4 T cells, and gating on CD8+ T cells alone might yield useful results. As hoped, the CD3+/CD8+ T cells were crossmatch negative while the CD3+/CD8neg T cells (presumably CD4+) were strongly positive. This suggests that valid T cell flow crossmatching might still be possible in the presence of ibalizumab using this alternate gating strategy.
Conclusion: Monoclonal antibody treatments leading to false positive crossmatch results may deny patients transplant opportunities. While this particular patient was unsensitized, these treatments can be particularly challenging for those with anti-HLA antibodies who may heavily depend on timely, accurate crossmatch testing. Crossmatch interference due to ibalizumab may be avoidable by utilizing CD8 T cell gating.
