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Background:  It has been known that LPS inhibits HIV infection in human macrophage, probably through Toll-like receptor 4 (TLR4). So far, 10 TLR have been identified that recognize structurally conserved bacterial and viral components by pathogen-associated molecular patterns. We explored the hypothesis that activation of TLR3 with dsRNA would have the same anti-HIV effects on primary human macrophages.

Methods:  Primary human macrophages pretreated by poly I:C or LPS were infected by HIV-1 Ba-L or pseudotyped HIV-1_vsv and HIV infection were determined by ELISA of p24 antigen or luciferase activity respectively. Post-integration inhibition of TLR ligands was also examined when treatment of poly I:C and LPS began after the completion of HIV-1_vsv integration in  macrophages through 72 hours post infection culture. To study the mechanism of HIV inhibition, expression of HIV-1 co-receptor CCR5 on activated macrophage surface was analyzed by flow cytometry, mRNA of reporter gene and IFNa, b were analyzed by real-time PCR, blockage of IFN cytokines were performed using neutralization antibodies, and activation of STAT1 was detected by Western blotting.

Results:  Activation of both TLR3 and TLR4 on primary human macrophages significantly inhibited the infection of replication-competent HIV-1 Ba-L and replication-incompetent HIV-1_vsv. A decrease of CCR5 on the surface of macrophages in response to TLR ligands contributes to HIV-1 suppression. Type I IFN mRNA level and activation of STAT1 increased. During the post-integration phase, poly I:C and LPS inhibited HIV-1 expression by inhibiting transcription. Type I IFN did not play a significant role in this inhibition as there was no effect on HIV-1 suppression in the presence of IFN neutralized antibodies.

Conclusions:  Our data indicate that activation of TLR3 in primary human macrophages results in anti-HIV-1 activity similar to activation of TLR4. Down-regulation of HIV-1 co-receptor CCR5, induction of type I IFN, and direct transcriptional inhibition contribute to TLR ligands-mediated HIV-1 inhibition. This study provides evidence that innate immunity induced by TLR3 ligand can inhibit HIV infection in primary human macrophages in vitro.

Keywords: Toll like receptor; HIV; innate immunity