Background:  HIV-1 binding with CXCR4 is associated with functional responses (including apoptosis induction) in different cellular target cells, but the role of this co-receptor is poorly understood in monocytes/macrophages (M/M), that are a crucial reservoir of HIV-1. M/M similarly express both HIV co-receptors, CXCR4 and CCR5, yet productive infection occurs only with CCR5-using (R5) viruses.

Methods:  M/M were obtained from PBMCs of healthy seronegative donors and infected in vitro with several CXCR4-using (X4) or R5 HIV-1 strains. HIV-1 p24 viral Ag production was monitored at different time-points. DNA fragmentation was detected by propidium iodide incorporation measured by flow cytometric analysis. Gene activation was detected through microarray, and MAPK activation by western blotting assays.

Results:  Stable and massive viral production was obtained in R5 HIV-infected M/M, while abortive infection occurred in all M/M cultures infected with X4 strains. Apoptosis in M/M cultures was induced by X4 strains in a time dependent fashion, starting from day 4 (25,3%) with a peak of 50% (p <0.001) at day 10 after infection (only 50 to 60% of M/M become infected by HIV-1 in vitro). Apoptosis in uninfected and R5 infected M/M was 4.2% and 4.1% at day 10 after infection, respectively. All X4, but not R5 strains induced p38 MAPK and Erk2 MAPK activation at 10 min. and 30 min. after infection in M/M. Apoptosis and MAPK activation were counteracted by the specific CXCR4 inhibitor AMD3100. Microarray analysis showed that HIV-1 X4 strains activate in M/M several pro-apoptotic genes and modulate cancer-related genes in a time dependent manner, starting at 24 hours after infection. On the contrary, R5 viruses modulate expression of genes not correlated with apoptotic pathways.

Conclusions:  Our results shed light on the biological mechanism leading to M/M survival during HIV-1 infection. The abortive infection of M/M by X4 strains, and the consequent depletion of this virus from persistent reservoirs, may explain the greater pathogenetic role of R5-viruses in all (but terminal) phases of the disease.

Keywords: macrophage; co-receptor; apoptosis