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Background:  Defensins are small, cysteine-rich endogenous peptides that contribute to host defense against several microorganisms and enhance certain adaptive immune responses. The θ-defensins are tetracyclic peptides with 18 residues with 3 disulfide bonds. Both naturally occurring and synthetic forms have been described. A synthetic form derived from the human pseudo-gene sequence (Retrocyclin-1) acts as a lectin, binding N- and O-linked carbohydrates. Retrocyclin-1 (RC100) and some analogues (RC100b, RC112) have been shown to protect cells from HIV-1 infection most likely due to binding to glycosylated residues on gp120 and CD4. In addition, the naturally occurring rhesus macaque θ-defensins, RTD-1 and RTD-2, have been shown to inhibit HIV-1 isolates as well. The objective of this study was to analyze the activity of diverse θ-defensins against several primary HIV-2 and SIV isolates.

Methods:  Binding studies of RC100 and RC100b (retrocyclin-2) to SIVgp130 was done by surface plasmon resonance. Selected HIV-2 and SIV isolates were tested using the synthetic peptides, RC100, RC100b, RC112, RTD-1, and RTD-2, in an inhibition assay using a reporter cell line, JC53-BL. IC₅₀ values were calculated and statistical comparison by paired t-test was done.

Results:  Binding studies using SIVgp130-generated affinity constants—Kds (nM)—of 151 and 117 for RC100 and RC100b, respectively. These values were higher than the previously published binding constants for HIV-1gp120, which were 35.4 and 9.41 for the same peptides. These results indicate the θ-defensins studied form less stable complexes with SIV envelop. It was previously shown the synthetic defensins (RC100, RC100b, RC112, RTD-1, and RTD-2) were highly effective at inhibiting HIV-1 entry. However, no consistent blocking activity was observed with most HIV-2 and SIV isolates, even at peptide concentrations higher than those previously reported to be effective against HIV-1 isolates.

Conclusions:  Results from these inhibition assays support previous findings that suggest differences between HIV-1 and HIV-2 entry. The reported decrease in binding and inhibition may be due to the different glycosylation patterns observed between HIV-1 and HIV-2/SIV isolates. Furthermore, these results suggest θ-defensins are not likely to be good therapeutics for HIV-2 infections.

Keywords: Theta-defensins; Inhibitors; Viral Entry