*Purpose: T cell reconstitution after lymphoablation in allograft recipients may lead to acute rejection and poor graft outcome. Using a mouse model of heart transplantation and murine anti-thymocyte globulin (mATG) we showed that while T cell recovery depends on B cells, B cell MHC expression is dispensable. The goal of this study was to investigate the role of B cell derived IL-1β and IL-6 in T cell homeostatic reconstitution after mATG lymphoablation.

*Methods: B6 (H-2^b) recipients of BALB/c (H-2^d) heart allografts were treated with mATG (d. 0, 4). The NanoString gene expression analysis showed that mATG upregulated IL-1β and IL-6 expression in recipient B cells. To validate NanoString findings, IL-1β and IL-6 expression was measured by intracellular flow cytometry. To test the role of IL-6 and IL-1β during T cell recovery, we treated recipients with mATG plus neutralizing anti-IL-1β mAb or anti-IL-6 mAb or used IL-1R1, caspase-1, MyD88 or IL-6 KO recipients. To test the requirement for B cell derived IL-1β or IL-6, caspase-1 KO or IL-6 KO B cells were adoptively transferred into B cell deficient μMT heart allograft recipients treated with mATG. The possibility of IL-1β or IL-6 direct effect on T lymphocytes was assessed by adoptive co-transfer of WT and either IL-1R or IL-6R KO T cells into heart allograft recipients and comparing their reconstitution after mATG treatment.

*Results: Following heart allograft placement and mATG treatment, B cells are the main source of IL-1β and IL-6, and these cytokines mutually regulate each other’s production. Either treatment with anti-IL-1β mAb or the use of IL-1R1 KO, caspase-1 KO and MyD88 KO recipients diminished both CD4⁺ and CD8⁺ T cell recovery along with improved heart allograft survival and decreased donor-reactive IFN-γ producing cells after mATG treatment. Similarly, in vivo IL-6 neutralization with mAb or using IL-6 KO recipients delayed CD4⁺ and CD8⁺ T cell recovery, markedly prolong heart allograft survival and decreased donor specific T cell responses. Adoptive B cell transfers into μMT mice showed that B cell derived IL-1β and IL-6 supported CD8⁺ T cells homeostatic proliferation. The absence of either IL-1R or IL-6R on CD8⁺ T cells markedly impaired their homeostatic recovery following mATG depletion.

*Conclusions: These findings reveal the essential role of B cell-derived IL-1β and IL-6 during homeostatic T cell expansion in a clinically relevant model of lymphoablation.

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