Bone marrow-derived monocyte-to-fibroblast transition is a key step in renal fibrosis pathogenesis, which is regulated by the inflammatory microenvironment. However, the mechanism by which the inflammatory microenvironment regulates this transition is not fully understood. In this study, we examined how the CD8(+) T cell/IFN-gamma microenvironment regulates the monocyte-to-fibroblast transition in renal fibrosis. Genetic ablation of CD8 promoted a monocyte-to-fibroblast transition and increased renal interstitial fibrosis, whereas reconstitution of CD8 knockout (KO) mice with CD8(+) T cells decreased fibrosis. However, depletion of CD4(+) T cells in CD8 KO mice also reduced fibrosis. To elucidate the role of CD4(+) T cells in mediating CD8-regulated monocyte-to-fibroblast transition, CD4(+) T cells were isolated from obstructed kidneys of CD8 KO or wild-type mice. CD4(+) T cells isolated from CD8 KO obstructed kidney expressed more IL-4 and GATA3 and less IFN-g and T-bet and showed increased monocyte-to-fibroblast transition in vitro compared with those isolated from wild-type obstructed kidney. To examine the role of IFN-gexpressing CD8(+) T cells, we reconstituted CD8 KO mice with CD8(+) T cells isolated from IFN-g KO mice. The IFN-g KO CD8(+) cells had no effect on IL-4, GATA3, IFN-g, and T-bet mRNA expression in obstructed kidneys or renal fibrosis. Taken together, our findings identify the axis of CD8(+) T cells and IFN-gamma-CD4(+) T cells as an important microenvironment for the monocyte-to-fibroblast transition, which negatively regulates renal fibrosis.