Modulation of receptor expressed by human PBMC by SIVsmm nef alleles. (A) PBMCs (panels A-D) were transduced with HIV-1 NL4-3 constructs coexpressing the indicated nef alleles and GFP and assayed for surface expression of CD4, CD3, CD28 and MHC-I. The ranges of eGFP expression used to calculate receptor modulation are indicated. (B) Quantitative assessment of SIVsmm Nef-mediated down-modulation of the indicated cellular receptors. The animal specifications and SIVsmm lineages are indicated and the activities obtained using the SIVmac Nef are shown for comparison. Values give averages ± SD derived from three independent experiments. The asterisks denote a statistical significance (P < 0.05) between SIVmac239 and SIVsmm Nef activity. All SIV nef alleles were significantly (P < 0.001) more active than that of HIV-1 in modulating CD3, whereas the HIV-1 nef was more active (P < 0.05) than all SIVsmm nefs in down-regulation of MHC-I.