CD4 and HIV infectious disease: To measure the infection rate of thymocytes with R5- and X4-tropic HIV strains, and considering the known downregulation of CD4 upon HIV infection (Kwon et al., 2020), the percentages of p24+ cells within FoxP3+ CD3highCD8- and FoxP3-CD3highCD8- thymocytes were assessed for co-expression of FoxP3 and p24 (Figure 2A) at day 2 post-infection.