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Background:  It has long been believed that CCR5 signaling triggered by R5 virions had no effect on the HIV-1 replicative cycle. Yet, the studies on which this dogma was based had been performed on transformed cell lines or on activated peripheral blood mononuclear cells (PBMC). We have recently shown by using signaling-defective CCR5 mutants and by blocking Gai protein activation either with pertussis toxin or with specific siRNA, that in the course of R5 HIV-1 infection, Gai1 signaling boosted virus replication in nonactivated PBMC. In the present study we looked for activation pathways, downstream of Gai, that could be involved in the facilitation of HIV-1 replication.

Methods:  The phosphorylation of the p42/p44 MAPK ERK1/2 secondary to the exposition of resting human PBMC to the R5 HIV-1 strain Ada-M was analyzed by Western blot in the presence or absence of 1 ng/mL of pertussis toxin. The effect of U0126 (10mM) on Ada-M infection of nonactivated PBMC was followed by measuring gag p24 concentration in the culture supernatant.

Results:  We observed the phosphorylation of ERK1/2, but not of the MAPK JUNK and p38, consecutively to the exposition of PBMC to the R5 HIV-1 strain. This phosphorylation was inhibted by treating the cells with pertussis toxin. Moreover, incubation of PBMC in the presence of the ERK1/2 inhibitor U0126 inhibited HIV-1 infection.

Conclusions:  Our data show that ERK1/2 are activated via CCR5 and the Gai pathway during R5 HIV-1 infection and that this activation increases the efficiency of the virus replication. Unveiling the activation pathways contributing to the optimal lentivirus replication in primary cells sheds a new light on HIV-1 pathogenesis and treatment.

Keywords: cell activation; coreceptor; virus life cycle