CTLA4Ig Differentially Suppresses De Novo and Recall Alloantibody Responses in a Mouse Model of HLA.A2 Allosensitization.

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Background CTLA4Ig, a potent inhibitor of T cell activation, has been approved by the FDA for antirejection in kidney transplantation. However, the efficacy and mechanism(s) by which CTLA4Ig modulates T-dependent alloantibody responses are poorly understood. Methods We examined the effects of CTLA4Ig on de novo alloantibody responses in C57BL/6 mice sensitized with HLA.A2+ skin allografts (SG). Mice received 5 IP injections of abatacept (500μg/dose) while control mice received sterile water for injection (SWFI, 100μl/dose). For the study of recall antibody responses, mice receiving primary SG were re-immunized with a second HLA.A2+ SG at Day 90 Ptx. Mice were then divided into 4 treatment groups: (1) CTLA4Ig combined with antiCD20 mAb; (2) antiCD20 mAb alone; (3) CTLA4Ig alone, and (4) control (SWFI). Blood samples, collected at designated times, were examined for donor-specifi c antibody (DSA) levels using an antibody binding assay. T, B and plasma cells were analyzed in flow cytometry. Results In de novo response groups all the control mice developed peak levels of HLA.A2-specific IgM (35 ±7 MFI) at day 14 PTx while the CTLA4Ig treated mice had low DSA IgM (12.8±0.5 MFI), indicating suppression of primary IgM response (p=0.007). Control mice developed DSA IgG antibodies at day 14 (582.09±144.26 MFI) and reached peak levels at day 21 (714.9±217.6 MFI). In contrast, mice in the treatment group had low levels of DSA IgG at day 14 (11.77±0.64 MFI, p=2.6E-10 vs. control), day 21 (26.15±16.39 MFI, p=9.2E-9) and day 28 (18.2±9.7 MFI, p=2.44E-8), indicating a potent suppression of de novo IgG alloantibodies. The recall antibody suppression study showed a moderate attenuation of DSA IgG in the CTLA4Ig group (212.7±167.9 MFI vs. control: 453.5±191.5 MFI, p=0.039) and the CTLA4Ig/antiCD20 group (127.5±61 MFI vs. antiCD20: 397±238 MFI, p=0.018). Conclusion CTLA4Ig is a potent inhibitor of de novo alloantibody responses and a moderate attenuator of recall alloantibody responses. The differential effects of CTLA4Ig in de novo versus recall responses indicate that CTLA4Ig is most effective in primary T-dependent B-cell responses. Memory T-cells are activated through CD28 independent mechanisms and thus less responsive to CTLA4Ig. Alternatively, CTLA4Ig may mitigate recall B cell responses by unknown mechanism(s) independent of CD28 co-stimulation.