A Critical Role of MyD88 Signaling in Donor T Cells for Graft-Versus-Host Disease (GVHD) and Graft-Versus-Leukemia Effect (GVL) After Experimental Allogeneic Hematopoietic Stem Cell Transplantation (allo-HSCT)
Authors: Lim Ji-Young , Jeong Dae-Chul , Youn Hyewon , Choi Eun-Young , Min Chang-Ki 
Published: October 2019

Abstract
Abstract 1893
The therapeutic potential of allogeneic hematopoietic stem cell transplantation (allo-HSCT) relies on the graft-versus-leukemia effect (GVL) to eradicate residual tumor cells by immunologic mechanisms. However, graft-versus-host disease (GVHD) remains the major toxicity of allo-HSCT. Alloreactive donor T cells are important effector cells in the development of GVHD, and proinflammatory cytokines enhance the generation of donor antihost cytotoxic function. Myeloid differentiation factor (MyD88) is a cytoplasmic adaptor molecule essential for integrating and transducing the signals generated by the Toll-like receptor (TLR) family. TLR engagement on professional antigen-presenting cells induces their maturation, resulting in optimal T-cell activation. However, recent advances indicate that the adjuvant effects of certain TLR agonists may also be attributed to the activation of TLRs and MyD88 directly in T cells. Both CD4 and CD8 T cells express functional TLRs. It remains to be defined whether direct TLR signaling on donor T cells is critical for GVHD or GVL activity.
We used C57BL/6 (H-2b) → B6D2F1 (H-2b/d) experimental allo-HSCT model, which differs at major and minor histocompatibility loci, to address the role of donor T cell MyD88 signaling on GVHD and GVL. Lethally irradiated recipient mice were transplanted TCD-BM (5 × 106) together with either wild-type (WT) or MyD88 knock out (KO) mice spleen T cells (1 × 106) on day 0 and then host-type P815 mastocytoma or L1210 leukemia (H-2d) cells were injected either intravenously (3 × 103) or subcutaneously (1 × 106) on day 1 to generate a GVHD/GVL model. First of all, clinical GVHD scores were comparable between recipients of WT T cells and MyD88 KO T cells. At 70 days post-allo-HSCT, 50 % of allogeneic recipients of WT T cells died due to severe GVHD, but necropsy showed no evidence of tumor. In contrast, 83.5% of those of MyD88 KO T cells died with gross evidence of tumors (P<.05). Moreover, subcutaneous tumors in the allogeneic recipients receiving MyD88 KO T cells exhibited markedly increased growth in vivo compared to those receiving WT T cells (tumor volume on day 41, 15205.6 vs. 373.9 mm3, P<.01). GVHD mortality is critically dependent on donor CD4 T cells in this donor/recipient strain combination (B6 → B6D2F1) and CD8 T cells that mediate cytotoxicity are more potent effectors of GVL. The percentages of donor T cells to undergo proliferation or apoptosis in response to alloantigens in vivo between the two T cell types was examined; apoptosis of CD8 T cells in recipients of MyD88 KO T cells was significantly enhanced compared to those of WT T cell recipients (P<.01) whereas apoptosis of CD4 T cells was comparable between two groups. Resultingly, the percentages of CD8 T cells in recipients of MyD88 KO T cells were significantly lower (P<.01). We next examined the effects of MyD88 signaling in donor T cells on cytolytic activity to host antigens. Splenocytes harvested from WT mice showed stronger cytolytic activity against P815 targets compared to those from MyD88 KO mice (P<.01). After allogeneic mixed leukocyte reaction, responder T cells from MyD88 KO mice showed markedly reduced IFN-γ, MCP-1 and IL-17A production with a significant augmentation in IL-10 secretion. We further evaluated the effect of T-cell MyD88 deficiency on GVL mediated by the intensity of total body irradiation (TBI) conditioning (1300 vs. 900 cGy, Exp Hematol 2011; 39: 1018–29). Enhanced GVL in the allogeneic recipients receiving 1300 cGy TBI was not shown in the recipients of MyD88 KO T cells.
In summary, these results highlight a critical role for MyD88 signaling in T-cell activation and cytotoxicity, offering the opportunity for improving GVL activity by targeting TLR-MyD88 signaling within donor T cells. Furthermore, these data demonstrated that MyD88 deficiency in T cells can impair cytolytic function or subsequent GVL activity of CD8 T cells without significant change in the severity of CD4-dependent GVHD. This difference is attributed to the fact that MyD88 deficiency in T cells causes an enhanced apoptosis of donor CD8 T cells but not donor CD4 T cells in vivo after HSCT.

Disclosures:
No relevant conflicts of interest to declare.