Molecular, metabolic and functional CD4 T cell paralysis impedes tumor control.

CD4 T cells are important effectors of anti-tumor immunity, yet the regulation of CD4 tumor-specific T (T ) cells during cancer development is still unclear. We demonstrate that CD4 T cells are initially primed in the tumor draining lymph node and begin to divide following tumor initiation. Distinct from CD8 T cells and previously defined exhaustion programs, CD4 T cell proliferation is rapidly frozen in place and differentiation stunted by a functional interplay of T regulatory cells and both intrinsic and extrinsic CTLA4 signaling. Together these mechanisms paralyze CD4 T cell differentiation, redirecting metabolic and cytokine production circuits, and reducing CD4 T cell accumulation in the tumor. Paralysis is actively maintained throughout cancer progression and CD4 T cells rapidly resume proliferation and functional differentiation when both suppressive reactions are alleviated. Strikingly, Treg depletion alone reciprocally induced CD4 T cells to themselves become tumor-specific Tregs, whereas CTLA4 blockade alone failed to promote T helper differentiation. Overcoming their paralysis established long-term tumor control, demonstrating a novel immune evasion mechanism that specifically cripples CD4 T cells to favor tumor progression.