Cancer cells accelerate exhaustion of persistently activated mouse CD4⁺ T cells

Most exhaustion studies have focused on CD8⁺ T cells. Here, we demonstrated reciprocal growth inhibition of CD4⁺ T cells and colorectal cancer cells, which induced the expression of PD-1, PD-L1, and PD-L2 in CD4⁺ T cells. The accelerated exhaustion of CD4⁺ T cells was evidenced by the reduced secretion of several cytokines, including IL-2, IFN-γ, or TNFα, and elevated secretion of CXCL family chemokines. Progressive expression of PD-L1, CTLA4, and IDO1 exhaustion markers occurred concomitantly with tumor growth in vivo in a mouse model. The pattern of CD4⁺ T cell exhaustion was analogous to that observed in CD8⁺ T cells, although with altered dynamics. The PD-L1-high phenotype can be induced by co-culture with tumor cells and is mediated by secreted factors in addition to cell contact. Our findings revealed that IFN-γ receptor knockout T cells exhibited PD-L1 protein expression when cultured with tumor cells, suggesting that PD-L1 expression is not fully dependent on IFN-γ. The TIL population undergoing exhaustion due to persistent antigen stimulation in the presence of cancer cells gradually acquires an immunosuppressive phenotype. The accumulation of inhibitory signals exerted by both cancer cells and T cells, which had converted to a suppressive phenotype, accelerated T cell exhaustion.