Highly Efficient HIV-1 Entry Mediated by Nonspecific Virion-Cell Interactions Quantified by Real-Time Single Particle Imaging

Abstract

The apparent low infectivity of HIV-1 is purportedly explained by virion-cell interactions that take place prior to viral entry in which the virus must first diffuse to the cell surface and bind to specialized receptors; these interactions are speculated, from in vitro studies, to be the rate-limiting step involved in the infection process yet the underlying mechanisms are not completely understood. Furthermore, receptor binding, in addition to nonspecific interactions limiting infectivity, may dictate the subsequent steps, i.e. entry pathways, of infection. Using single-virus tracking methods with spatial and temporal resolutions of ∼20 nm and 40 ms, respectively, we have quantified the dynamics of mCherry labeled HIV-1 virions with varied envelope (Env) glycoprotein incorporation interacting with TZM-bl cells, a HeLa-derived cell line expressing CD4 and co-receptors at high surface densities. The number of viral-cell touching events and corresponding contact lifetimes of most trajectories suggest few and transient interactions with the cell surface before permanent dissociation. The fraction of virions that become immobilized is heavily influenced by nonspecific interactions in both the presence and, to a lesser extent, absence of DEAE-dextran, contributing to receptor-independent endocytic entry as revealed by time-lapse and confocal imaging studies. Measured internalization efficiencies as high as 80% appear to be independent of Env content despite proportionally higher infectivities; thus, nonspecific binding and subsequent endocytosis may actually lower the apparent HIV-1 infectivity. Moreover, we directly observed a prevalence of these internalized virions to undergo recycling and/or transcytosis implicating this pathway as being involved, albeit indirectly, with infection. (Supported by NIH 1DP2OD008693).