CD4 and cancer: found a group of fibroblasts specifically expressing tryptophan 2,3-dioxygenase (TDO2) in the distant part of cancer nests, while CD4 + T cells and CD8 + T cells were enriched around these fibroblasts, demonstrating that TDO2 has a chemotactic capacity for T cells, inducing the transformation of CD4 + T cells into Treg cells, leading to functional impairment of CD8 + T cells.