Differential HIV-1 endocytosis and susceptibility to virus infection in human macrophages correlate with cell activation status.

HIV-1 is an enveloped virus that enters target cells by fusion either directly at the plasma membrane or at the endosomal membrane. The latter mechanism follows a rapid engulfment of HIV-1 after its receptor engagement at the cell surface, and its scale depends on cellular endocytosis/degradation rates and virus fusion kinetics. HIV-1 has recently been shown to exploit a novel Pak1-dependent macropinocytosis mechanism as a way to productively infect macrophages. However, macrophages are highly heterogeneous cells that can adapt functionally to their changing environment, and their endosomal/lysosomal pathway is highly regulated upon cell activation. These changes might impact the ability of HIV-1 to exploit endocytosis as a way to productively infect macrophages. In this study, we compared HIV-1 endocytosis/degradation rates in nonactivated, M1-activated, and M2a-activated monocyte-derived macrophages (MDMs). We found that the rate of HIV-1 endocytosis was increased in M1-activated but decreased in M2a-activated MDMs. However, both M1 and M2a activations of MDMs led specifically to a greater clathrin-mediated endocytosis of HIV-1, which was independent of CD4 and CCR5 binding. Furthermore, clathrin-mediated endocytosis is unlikely to result in productive HIV-1 infection, given that it leads to increased viral degradation. Therefore, we suggest that viral fusion following endocytosis is restricted in activated macrophages.