We have previously characterized a model for immune activation of HIV-1 in which viral proteins as well as infectious virus are induced in HIV-1 transgenic (Tg) mice by infection with intracellular protozoan and bacterial pathogens commonly encountered by AIDS patients. Viral expression in this system appears to be restricted in situ and in vitro to antigen presenting cells (APC) with T cells failing to respond even after polyclonal stimulation. As reported last year, we have found that toll like receptors (TLR) play a major role in the triggering of transgenic HIV responses by co-infecting pathogens such as mycobacteria. During the present report period, we examined a strategy for preventing microbial induced immune activation of HIV involving TLR remodelling. This process, which was initially termed """"""""endotoxin tolerance"""""""", refers to the induction of a hyporesponsive state in macrophages following primary exposure to microbial ligands such as lipopolysaccharide (LPS), bacterial lipoprotein, lipoteichoic acid, and oligo CpG?DNA, now known to signal through TLRs. The resulting tolerant state is associated with a marked decrease in proinflammatory cytokine production following secondary challenge with these ligands.Because of the growing evidence that microbial ligand-TLR interactions can play a role in driving HIV-1 expression, it was possible that reprogramming, mediated by TLR, could be used as a means of reducing HIV viral loads in coinfected individuals. Unexpectedly, Tg splenocytes and macrophages, rendered tolerant in vitro to TLR2, TLR4, and TLR9 ligands as assessed by proinflammatory cytokine secretion and nuclear factor-B activation, showed enhanced rather than decreased HIV-1 p24 production. A similar enhancement was observed in splenocytes tolerized and then challenged with heterologous TLR ligands. Moreover, TLR2- and TLR4-ligand homotolerized mice demonstrated significantly increased plasma p24 production in vivo despite lower levels of TNF-alpha.Together, these results demonstrate that HIV-1 expression is enhanced in TLR-reprogrammed host cells, arguing against the use of TLR-reprogramming as a strategy for preventing microbial induced viral immune activation. It is possible that the observed enhancement rather than suppression of HIV gene expression may reflect a mechanism used by the virus to escape the effects of microbial-induced tolerance during natural infection in vivo.
