SAMHD1 and HIV-1 infection: To investigate the mechanism by which S1P signaling inhibition reduces HIV-1 infection of primary CD4 T cells, we examined expression and surface density of CD4, CXCR4, and CCR5, as well as expression of the innate HIV-1 restriction factor SAMHD1, within uninfected CD4 T cells treated from days 10 to 13 with DMS, SKI II, ABC294640, or FTY720 (14).