CD69+CD4+CD25− T Cells, a New Subset of Regulatory T Cells, Suppress T Cell Proliferation through Membrane-Bound TGF-β1

Abstract

The underlying mechanisms of tumor-induced immune suppression need to be fully understood. Regulatory T (Treg) cells have been shown to play an important role in tumor immune escape. Until now, many subsets of Treg cells have been described that can suppress T cell response via different mechanisms. CD69 is generally regarded as one of the activating markers; however, recent studies show that CD69 may exert regulatory function in the immune response. In this study, we have identified tumor-induced CD69+CD4+CD25− T cells as a new subset of CD4+ Treg cells. CD69+CD4+CD25− T cells increase dramatically along tumor progression, with up to 40% of CD4+ T cells in the advanced tumor-bearing mice. Distinct from the previously described CD4+ Treg cell subsets, CD69+CD4+CD25− T cells express high CD122, but they do not express Foxp3 and secrete IL-10, TGF-β1, IL-2, and IFN-γ. CD69+CD4+CD25− T cells are hyporesponsive and can suppress CD4+ T cell proliferation in a cell-cell contact manner. Interestingly, the fixed CD69+CD4+CD25− T cells still have suppressive activity, and neutralizing Abs against TGF-β1 can block their suppressive activity. We found that CD69+CD4+CD25− T cells express membrane-bound TGF-β1, which mediates suppression of T cell proliferation. Furthermore, engagement of CD69 maintains high expression of membrane-bound TGF-β1 on CD69+CD4+CD25− T cells via ERK activation. Our results demonstrate that CD69+CD4+CD25− T cells act as a new subset of regulatory CD4+ T cells, with distinct characteristics of negative expression of Foxp3, no secretion of IL-10, but high expression of CD122 and membrane-bound TGF-β1. Our data contribute to the better understanding of mechanisms for tumor immune escape.