T Cell Toll-Like Receptors Play a Critical Role in Graft-Versus-Host Disease and Graft-Versus-Leukemia

Graft-versus-host disease (GVHD), the major complication of allogeneic hematopoietic stem cell transplantation, is mediated by donor T cells recognizing host alloantigens expressed on antigen-presenting cells (APCs). Total body irradiation (TBI) activates and damages host tissue, produces inflammatory cytokines and damage-associated molecular patterns, and allows translocation of microbial products into the systemic circulation. It has been assumed that these products bind to toll-like receptors (TLRs) on APCs and monocytes/macrophages, activate inflammatory pathways, and further amplify GVHD. On the other hand, TLRs are also expressed on T cells but their roles in the adoptive T cell response remain to be investigated in vivo. TLR signals are mediated by both MyD88-dependent and independent TRIF protein pathways. We evaluated the role of T cell TLR signaling in T cell alloreactivity in a mouse model of allogeneic bone marrow transplantation using mice with defective TLR signaling induced by deletions of the TLR adaptor proteins TRIF and MyD88 (TRIF/MyD88 DKO mice); in these mice, the TLR-dependent signaling pathway was eliminated. Lethally irradiated B6D2F1 mice were injected with 2 × 10⁶ T cells from wild-type (wt) or TRIF/MyD88 DKO mice together with 5 × 10⁶ T cell-depleted bone marrow (TCD-BM) from wt or DKO mice. Morbidity and mortality of GVHD was significantly less in recipients of TRIF/MyD88 DKO T cells than in recipients of wt T cells, irrespective of donor bone marrow type (Table). Reduction of GVHD was not seen in recipients of MyD88^−/− T cells or TRIF^−/− T cells, suggesting that inhibition of signaling through TRIF or MyD88 alone in donor cells is not sufficient to reduce GVHD. Histopathologic examination of the small intestine, liver, and skin showed significantly reduced GVHD pathology in recipients of DKO T cells (Table). Thus, effects of TLR signaling in donor cells on GVHD primarily reside in the T cell compartment of the donor graft, not in donor accessory cells. In recipients of TRIF/MyD88 DKO T cells, reduction of GVHD was associated with impairment of donor T cell expansion and production of TNF-□ and IFN-□. In marked contrast though, TRIF/MyD88 DKO T cells were activated equally to wt T cells in unirradiated B6D2F1 mice after adoptive transfer, thus activation of DKO T cells was impaired only in recipient mice heavily pretreated with TBI. These results suggest that T cell TLR signaling provides critical co-stimulation for T cells in an inflamed environment. T cell TLR signaling was also required for optimal graft-versus-leukemia activity. These results challenge the current paradigm that TLR functions are primarily attributable to their role in APCs and monocytes/macrophages in GVHD, and reveal a previously unrecognized critical role for T cell TLRs in inducing GVHD and GVL after hematopoietic stem cell transplantation.