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Background:  Defensins are cationic peptides employed by the innate immune system to protect against viral, fungal, and bacterial infection. a, β, and θ defensins all have anti-HIV-1 activity, but the mechanisms by which they inhibit HIV-1 replication have not been fully characterized.

Methods:  We measured loss of cell surface CXCR4 and CCR5 following incubation with HNP-1 and RTD-1 in a GHOST cell line uniformly expressing CCR5, CXCR4, and CD4. We assayed defensin effects on HIV-1 replication in GHOST and in PBMC cultures. HIV-1 infection was measured by the presence of the viral capsid protein p24 in tissue culture supernatant, by internal cell staining for p24 or by GFP expression. Defensins were incubated with cells and with virus at various times pre- and post-infection and at 4°C or 37°C. We used both R5 and X4 strains of HIV-1 for infections.

Results:  We observed down-modulation of CXCR4 by the human a-defensin, HNP-1, and the rhesus macaque θ-defensin, RTD-1. Defensin incubation at 37°C showed a dose-dependent loss of CXCR4, but not CCR5. We also found that RTD-1 inhibited both X4 and R5 HIV-1 infection in GHOST cells in a dose-dependent manner when added at the time of infection, but had no activity when added 4 hours later. In contrast, HNP-1 inhibited X4 and R5 HIV-1 replication in PBMC when added at the time of infection and as long as 16 hours later, but had no activity in GHOST cells. HBD-2 inhibited R5 and X4 HIV-1 replication in PBMC. RTD-1 inhibited anti-CXCR4 monoclonal antibody binding to CXCR4 indicating that RTD-1 may bind CXCR4. RTD-1 also directly inactivated X4 HIV-1 when RTD-1 was pre-incubated with HIV-1 prior to infection. HNP-1 directly inactivated both X4 and R5 HIV-1.

Conclusions:  Both HNP-1 and RTD-1 down-modulate cell surface CXCR4 expression, however, this requires higher defensin concentrations than are needed to inhibit HIV-1 infection. HNP-1 can inhibit viral replication when added both before and after infection of target cells, whereas, HBD-2 and RTD-1 inhibit infection when added to cells before or at the time of infection. HNP-1 and RTD-1 inactivate X4 HIV-1 directly when incubated with virus prior to infection; however, only HNP-1 effectively inactivates R5 HIV-1. Our results indicate that HNP-1 and RTD-1 inhibit HIV-1 infection by different mechanisms.

Keywords: Defensins; HIV-1; inhibition