PD-L1 and PD-L2 Protect The Heart In a T-Cell Receptor Transgenic Model Of Graft-Versus Host Disease

Graft-versus-host disease (GVHD) is a complication of allogeneic stem cell transplantation (alloSCT). In murine models of alloSCT, naive T cells (T_N) cause GVHD while effector memory T cells (T_EM) do not. To determine why T_EM fail to cause GVHD, we generated a novel T-cell receptor transgenic GVHD model. In this model CD4+ TS1 T cells, which recognize an epitope of influenza hemagglutinin (HA), are transferred, along with syngeneic bone marrow, into irradiated transgenic recipients that express HA in all tissues (HA104 Tg mice). We found that TS1 T_N induced early and prolonged weight loss and caused GVHD-like pathology in the skin, liver and colon. In contrast, TS1 T_EM induced mild, transient weight loss and minimal pathology, demonstrating that T_EM have repertoire-independent characteristics that limit their ability to induce GVHD. Post transplant analysis revealed that TS1 T_EM progeny, relative to TS1 T_N progeny, produced less IFN-γ, proliferated and accumulated less in the colon, and expressed higher levels of the inhibitory molecule PD-1.

To investigate whether PD-1 was responsible for limiting pathogenesis by T_EM, we used hosts and donor bone marrow lacking both PD-L1 and PD-L2. The absence of PD-L1/2 did not enable TS1 T_EM to cause early weight loss. However, between 35 and 60 days post transplant, TS1 T_EM recipients lacking PD-L1/2 rapidly began losing weight and approximately 50% died. Weight loss in T_EM recipients was dependent upon lack of PD-L1/2 expression on both donor bone marrow and host cells, including radioresistant stromal cells, suggesting a possible role for PD-L1/2 expressed in tissues. Indeed, global absence of PD-L1 alone, which (in contrast to PD-L2) is expressed on parenchymal tissues, also resulted in late weight loss in recipients given T_EM.

To determine the reason for late weight loss, we surveyed tissue histopathology. Surprisingly, in the absence of PD-L1/2, T_EM recipients did not develop exacerbated colon pathology but instead developed mononuclear infiltrates and mycocyte necrosis in the heart, accompanied by heart block and decreased cardiac output. Interestingly, heart disease was also seen in PD-L1/2 deficient T_N recipients that survived to later time points, indicating that the protective role of PD-L1/2 applied more generally to GVHD induced by CD4 T cells.

Strikingly, the extensive infiltrates in affected hearts were mostly comprised of non-TS1 T cells, including both CD4 and CD8 cells. These cells are likely host-derived, as severe cardiac infiltrates were seen when Rag-deficient donor BM was used to reconstitute host hematopoiesis. We therefore hypothesize that in GVHD PD-L1/2 normally prevent “allogeneic” T cell mediated damage but also protect from subsequent syngeneic T cell-mediated pathogenesis that could contribute to prolonged disease. This effect is tissue specific and could in part be due to parenchymal expression of PD-L1 in certain organs. It is possible that such mechanisms could explain more chronic phases of GVHD, which differs from acute GVHD. Ongoing depletion experiments will determine the relative contributions of donor TS1 T cells, donor bone marrow derived T cells and host T cells.