Primate lentiviral Nef proteins differentially upregulate cell surface TfR. (A) PBMC were infected with HIV-1 variants coexpressing 31 different primate lentiviral Nef proteins and GFP (compare Additional file 1: Table S1). TfR cell surface levels were measured by flow cytometry 48 hours post infection. Nef proteins were compared according to their phylogenetic relationship. Group 1 Nefs are derived from HIV-1 and its direct simian precursors whereas Group 2 Nefs represent all other lentiviruses including those which are non-pathogenic in their natural simian hosts. Each symbol represents the mean activity of a respective Nef protein in the function tested (please find the mean values ± SD in Additional file 1: Table S1). (B) THP-1 and monocyte derived macrophages (MDM) were infected with a subset of HIV-1 variants to analyse cell surface TfR modulation similar to the experiment described in (A). The functional activity of the Nef proteins in upregulation of TfR in PBMC was correlated to the results from the THP-1 and MDM infection experiments. (C) PBMC infected with the 31 HIV-1 variants described in (A) were also analysed for Nef-mediated downregulation of cell surface CD3, CD4, MHCI and CD28. The results were correlated to the functional activity of the respective Nef proteins in upregulation of TfR.