CD4 and HIV-1 infection: Furthermore, HIV-1-infected THP1-CD4-sh-β-TrCP cells did not downregulate surface BST2 (Fig. S4B, compare dot plots j and k), which is usually reduced from the cell surface through a Vpu/β-TrCP-related mechanism following HIV-1 infection (Fig. S4B, compare dot plots d and e) (17, 18).