Regulatory T cells characterized by low Id3 expression are highly suppressive and accumulate during chronic infection.

Foxp3(+) regulatory T (Treg) cells are broadly divided into naive-like and activated Treg cells, however recent studies suggest further Treg cell heterogeneity. Treg cells contribute to impaired T cell responses in chronic infections, but the role of specific Treg cell subpopulations in viral infections is not well defined. Here, we report that activated Treg cells are separated into two transcriptionally distinct subpopulations characterized by low or high expression of the transcriptional regulator Id3. Id3(lo) Treg cells are a highly suppressive Treg cell subpopulation, expressing elevated levels of immunomodulatory molecules and are capable of broadly targeting T cell responses. Viral infection and interleukin-2 promote the differentiation of Id3(hi) into Id3(lo) Treg cells and during chronic infection Id3(lo) Treg cells are the predominant Treg cell population. Thus, our report provides a framework, in which different activated Treg cell subpopulations specifically affect immune responses, possibly contributing to T cell dysfunction in chronic infections.