*Purpose: Lymphoablation with anti-thymocyte globulin (ATG) is a common therapy in solid organ transplantation. However homeostatic proliferation of pre-existing memory T cells undermines the graft-prolonging benefits of lymphoablation. We have reported that B cells activated by depletion-resistant CD4 T cells produce proinflammatory cytokines, and are essential mediators of CD8 T cell homeostatic reconstitution. Our current goal is to identify B cell subsets mediating T cell reconstitution.

*Methods: C57BL/6 (B6) recipients were transplanted with BALB/c heart allografts and treated with mATG (1mg i.p. on d.0 and d.4). The numbers and location of B220⁺CD21/35⁺IgM^int follicular (FO) and B220⁺CD21/35^hiIgM^hi marginal zone (MZ) B cells in the spleen and their ability to produce IL-6 and TNFα were evaluated by flow cytometry and immunohistochemistry. To test whether MZ B cells are required for optimal T cell reconstitution, recipients were treated with anti-integrin mAb (aVLA4 and aLFA1α, 100ug i.p. each on d.-4 and d.-2) to displace B cells from the marginal zone. We also used CD19^wt/cre Notch2^fl/fl B6 mice with impaired MZ B cell development as recipients (ΔMZB mice).

*Results: On d. 5 posttransplant, we found a prominent decrease in the B220+ cells within the marginal zone in allograft recipients treated with mATG. The numbers of spleen MZ B cells were decreased by 36% vs 54% in FO B cell subset. These changes in MZ B cells were not observed in untreated allograft recipients, isograft recipients, or mATG treated non-transplanted mice. Spleen marginal zone was repopulated by B cells by d. 12 posttransplant, and the proportion of cytokine-producing MZ B cells was higher than that in FO B cells (5% IL-6+ and 18% TNFα+ MZ B cells vs. 3% and 6% FO B cells). Anti-integrin mAb treatment displaced B cells from marginal zone prior to transplantation. The heart allograft recipients treated with anti-integrin mAb had impaired recovery of CD4 and CD8 T cells in the peripheral blood and spleen. In parallel experiments, ΔMZB heart allograft recipients treated with mATG had significantly diminished CD8 T cell recovery in the spleen compared to CD19^wt/cre littermates.

*Conclusions: Our data suggest that MZ and FO B cells are differentially affected by mATG lymphoablation in heart allograft recipients and that MZ B cells act as sensors of allograft-induced inflammation to facilitate homeostatic T cell reconstitution.

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