Applying MyD88 signaling in CD8+ T cells to enhance anti-tumor immunity

Abstract

T cell immunotherapy is a promising strategy for the treatment of patients with advanced cancers. Despite promising results in patients with hematological malignancies, response rates to solid tumors remain low. One of the biggest challenges is effectively activating and sustaining anti-tumor T cell responses in the context of an immunosuppressive tumor environment. Several approaches are currently being explored to improve the efficacy of adoptive T cell transfer, including the activation of co-stimulatory signaling pathways in T cells. Toll-like receptor (TLR) engagement on T cells is a potent co-stimulatory signal that increases anti-tumor activity by enhancing T cell proliferation, effector function, and T cell survival. We developed and tested two genetic engineering strategies to exploit the co-stimulatory effects of TLR signaling in CD8+ T cells. First, CD8+ T cells were modified to express and secrete the TLR5 ligand (TLR5L), flagellin, as a means to deliver this immune adjuvant to the tumor for enhanced anti-tumor activity. TLR5L-secreting T cells exhibited improved proliferation, cytokine secretion, and anti-tumor activity in both xenogeneic and syngeneic models of melanoma. The anti-tumor activity of TLR5L-secreting T cells was associated with decreased numbers of phenotypically exhausted T cells and fewer myeloid-derived suppressor cells. Second, we designed and characterized a synthetic fusion protein composed of the T cell co-receptor CD8α and the TLR adaptor protein MyD88, termed CD8α:MyD88. The expression of CD8α:MyD88 on T cells increased T cell responses to low concentrations of tumor antigens as well as augmented the expression of effector molecules and co-stimulatory proteins. These effects were antigen-dependent and accompanied by elevated levels of TLR signaling-related proteins. The enhanced anti-tumor activity of CD8α:MyD88-expressing T cells in tumor-bearing mice was associated with a unique tumor cytokine/chemokine signature, improved T cell infiltration, elevated levels of antigen presentation, and fewer macrophages with an immunosuppressive phenotype. The use of T cells as vehicle to deliver TLR5L to the tumor site and the co-stimulation of T cells through a synthetic CD8α:MyD88 receptor represent two novel and versatile approaches for modulating the tumor microenvironment to enhance anti-tumor immunity.