Antibody-mediated lymphocyte depletion is used as induction therapy in sensitized transplant recipients. Following lymphoablation, peripheral lymphopenia triggers homeostatic T cell proliferation (HP) and enhanced thymopoiesis. However, the relative contribution of these pathways to T cell recovery and allograft rejection is unknown. Using a mouse model of heterotopic heart transplantation and a murine analog of Thymoglobulin (mATG), we have reported that the recovery of CD8T cells following mATG depletion is associated with allograft rejection and requires the depletion-resistant memory CD4 T cells and CD40 expressing B cells. The goal of the current study was to investigate the role of thymus during T cell reconstitution in mATG treated recipients. Limiting CD4 T cell help by CD4 T cell depletion or CD154 blockade significantly reduced the numbers of single positive CD8 thymocytes generated in mATG treated mice suggesting a potential role for residual memory CD4 T cells in thymopoiesis. To definitively test whether thymus is required for T cell reconstitution, C57Bl/6 thymectomized and euthymic mice were transplanted with BALB/c heart allografts and treated with mATG (25 mg/kg i.p.) on days 0 and 4 posttransplant. The recovery of CD4 and CD8 T cells in thymectomized recipients was markedly impaired compared to euthymic mice (10%, 13% vs. 1.7%, 1.9% CD4 and 3%, 7% vs 0.06%, 2% CD8 on average on days 12, 30 posttransplant). The transplantation of thymi from B6 WT or RAG-/- mice under the kidney capsule of the thymectomized mice at the time of heart transplantation restored rapid T cell reconstitution following mATG depletion. Furthermore, adoptive transfer of CD4 T cells (10×10⁶ i.v. on d. -1 posttransplant) or treatment with agonistic anti-CD40 antibody (0.1mg i.v. on d 0,2) restored T cell recovery in thymectomized mice suggesting that the generation of sufficient numbers of helper CD4 T cells in the thymus is a necessary step preceding CD8 T cell reconstitution. This step can be bypassed by providing strong signal to CD40 expressing B cells. These results demonstrate that depletion-resistant memory CD4 T cells and thymus are both required for effective CD8 T cell recovery. We are currently testing the possibility that the residual memory CD4 T cells traffic into the thymus and facilitate thymopoiesis through CD40-CD40L signaling. Our findings may facilitate the development of future therapies specifically targeting the recovery of naïve vs memory T cells following lymphoablative therapies in transplant recipients.

CITATION INFORMATION: Ayasoufi K, Fan R, Valujskikh A. Thymus Is Required for T Cells Reconstitution Following Antibody-Mediated Lymphoablation. Am J Transplant. 2016;16 (suppl 3).

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