CXCR4 and HIV infectious disease: Upon thymocyte infection with the R5- and X4-tropic HIV strains, a very low infection rate was observed in CD3highCD8- thymocytes co-expressing FoxP3 and p24 (Figures 2A,B), suggesting that despite expressing high levels of the HIV co-receptors CXCR4 and CCR5, FoxP3+ thymocytes are less susceptible to in vitro HIV infection.