Background: The aim of this study is to determine the incidence of early acute rejection and graft outcomes in living donor renal transplants with an isolated positive B-Cell Flow-Cytometry Crossmatch (B-FCXM).

Methods: We performed a retrospective analysis of graft outcomes in 152 adults who received living donor renal transplants between January 2019 and March 2021 at Mount Sinai Hospital in New York CIty. Pre-transplant histocompatibility was assessed with flow cytometric crossmatch (FCXM) and Luminex single-antigen beads. Positive B- FCXM is was defined as median channel shift of ≥ 40. A mean fluorescence intensity (MFI) ≥ 1400 defined positive donor-specific HLA Class I and/or II antibody (DSA). Most patients received induction immunosuppression consisting of rabbit anti-thymocyte globulin, steroids and intravenous immunoglobulin. Estimated glomerular filtration rate (eGFR) and biopsy-proven rejection at six months’ post-transplant were compared between patients with and without a positive B-FCXM, and positive and negative DSA.

Results: Of 152 patients, 89 (59%) were males and 63 (41%) females; the mean age was 44 years (SD ± 13.8). 45 donor B-FCXMs (30%) were positive and 107 (70%) were negative. 42 (28%) positive donor B-FCXM had no DSA and 3 (2%) had DSA. 91 (85%) negative donor B-FCXMs did not have DSA, and 16 (15%) had DSA. Thirty-six (80%) out of 45 patients with positive donor B-FCXM, including the 3 patients with DSA, received intravenous-immunoglobulin (IVIG) as part of induction immunosuppression. None of the patients with both positive donor B-FCXM and DSA had rejection. eGFR and rejection rate at 6 months were similar between patients with positive and negative donor B-FCXM [median 58.9 (IQR= 50.2-70.8) vs 57.6 (48.8-76.6), p=0.88; 6.8% vs 5.6%, p=0.7, respectively]. Comparison by combination of donor B-FCXM and DSA status (B-FCXM- DSA-, B-FCXM- DSA+, B-FCXM+ DSA-, B-FCXM+ DSA+) did not show statistically significant differences in eGFR or rejection rate at 6 months [median 57.4 (IQR= 48.5-74.3) vs 59.3 (52.6-83.3) vs 58.2 (50.1-70.8) vs 76.6 (58.3-102.5) p=0.4; 3.3% vs 18.7% vs 7.3% vs 0% p=0.09, respectively].

Conclusion: In our cohort, there was no statistical difference in early acute rejection and graft function at 6 months between living donor transplant recipients with positive and negative donor B-FCXM even after accounting for DSA status. Peri-transplant treatment with IVIG for a positive donor B-FCXM might be a helpful adjunct in this setting.