Background: Anti-thymocyte globulin (ATG) is the preferred induction for kidney transplant recipients at high risk of antibody-mediated rejection (AMR). However, evolution of B cell phenotype has never been compared in patients treated with ATG or with anti-IL2 receptor. Methods: We prospectively monitored peripheral B cell phenotype during the first year post-transplantation in 29 kidney transplant recipients treated with rabbit ATG (ATG group, n=14) or basiliximab (IL2R group, n=15). Maintenance immunosuppression consisted of steroids, tacrolimus, and mycophenolate mofetil. Phenotyping of lymphocyte subsets was performed on freshly drawn blood using the Bm classification (IgD/CD38 double staining performed on CD19+ cells), at day-0, month-3 and month-12 post-transplantation. At month-12, we tested by ELISA the serum levels of three cytokines potentially involved in B cell homeostasis, BAFF, IL7 and IL21.

Results: As expected, at month-3 and -12, patients in the ATG group had significantly reduced counts of CD3+ and CD19+ cells. In both groups, Bm2 proportions transiently decreased at month-3 and returned to baseline at month-12, whereas memory B cells remained unchanged. Patients in the IL2R group displayed a persistent reduction of transitional cells at month-3 and month-12. In the rATG group, transitional cells decreased at month-3 but increased from month-3 to month-12 and were significantly higher at both times compared with the IL2R group (month-3: 1.5 ± 2.0 versus 0.3 ± 0.4%, p=0.012; month-12: 3.9 ± 5.5 versus 0.4 ± 1.2, p=0.022). Within the whole group (n=29), we observed an inverse correlation between CD4+ cells counts and percentage of transitional cells at month-12 (r=-0.387, p=0.038). BAFF serum levels did not correlate with B cell counts, but with transitional cells percentage (r=0.67, p<0.0001), and with CD4+ cells count (r=-0.43, p=0.02). We did not observe such correlation for IL21 or IL7 serum levels.

Conclusion. B-cell repopulation after ATG induction is characterized by an increase in transitional B cells and BAFF serum levels. This situation represents a theoretical risk factor for selection of alloreactive B cell clones and has to be further explored.

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