Background:  The leukocyte surface marker CD38 docks to CD4 on the cell surface and can prevent HIV-1 infection in vitro through an N-terminal peptide (CD38_51-74). We here model the structural interactions of CD38 with CD4 and evaluate the specificity of CD38_51-74 as a potential antiretroviral agent.

Methods:  CD38 binding to CD4 was evaluated using biotinylated probes in acellular and flow-cytometric assays. CD4/CD38 interactions were evaluated by molecular dynamics. Peptide toxicity was evaluated in vitro by standard methods. HIV-1 glycoprotein mediated fusion was evaluated by syncytium assays. HIV-1 replication was assessed as p24 production in acutely infected MT-2 cells.

Results:  The HIV-1 inhibitory peptide CD38_51-74 bound recombinant human CD4 in acellular assays (p <0.05; t-test for regression) and selectively bound to CD4⁺ transfectants of the CD4^- murine cell line SR.D10 (p <0.01, Kolmogorov Smirnoff statistics). Accordingly, CD38_51-74 participates to the CD38/CD4 docking interface. The lowest-energy docking of CD38 to CD4 also involves an interaction of CD38_51-74 with the CD4 Phe43 residue fundamental for HIV-1 gp120/CD4 binding. In line with this model, CD38_51-74 dose-dependently antagonized gp120/CD4 binding and inhibited HIV-1 glycoprotein mediated fusion in the nanomolar range (p <0.05, t-test for regression). The EC₅₀ of CD38_51-74 on both fusion and acute infection assays was decreased to average values <100 nM when the peptide was subjected to lysine-PEGylation. Instead, CD38_51-74, and its PEGylated counterpart, at concentrations up to 1 mM did not inhibit viability of several cell types such as lymphoid cell lines, peripheral blood mononuclear cells (PBMC), HeLa cells, and human spermatozoa (p = 0.6 to 0.9), thus showing extremely high selectivity indexes (10³ to 10⁴). Despite CD38 is a marker of lymphocyte activation, the CD38_51-74 peptide neither induced lymphocyte activation, nor altered production of IL-2, IL-4, IL-5, IL-6, IL-10, or the chemokines MIP-1a and MIP-1β (p = 0.5 to 0.7).

Conclusions:  Human CD38 might represent a natural HIV-1 attachment inhibitor that might be exploited for the development of new and safe antiretrovirals or topical microbicides .