Cutting edge: CD4-independent development of functional FoxP3+ regulatory T cells.

The CD4 coreceptor is mandatory for the differentiation and function of conventional MHC class II-restricted T cells, but little is known about its contribution in regulatory T cells (Tregs). We thus investigated the Treg compartment in mice lacking CD4. CD3+CD8-FoxP3+ cells were readily detected in the periphery of CD4(-/-) mice, where their percentages were even increased as compared with wild-type animals. These cells had a classical CD25+CD152+GITR+ Treg phenotype, were enriched in memory-type Tregs, and displayed a diversified TCR repertoire. Functionally, CD4(-/-) Tregs were equally as suppressive as CD4(+/+) Tregs in vitro as well as in vivo. Hence, the CD4 coreceptor is dispensable for the generation and function of FoxP3+ Tregs. Furthermore, CD3+CD8-FoxP3+ Tregs were also found to develop in the absence of both CD4 and MHC-II molecules, demonstrating that the generation of Tregs can occur independently of MHC-II recognition.