CCR5-tropic (R5) immunodeficiency virus type 1 (HIV-1) strains are highly transmissible during the early stage of infection in humans, whereas CXCR4-tropic (X4) strains are less transmissible. This study aimed to explore the basis for early phase R5 and X4 HIV-1 infection invivo by using humanized mice dually challenged with R5 HIV-1NLAD8-D harboring DsRed and X4 HIV-1NL-E harboring EGFP. Whereas R5 HIV-1 replicated well, X4 HIV-1 caused only transient viremia with variable kinetics; however, this was distinct from the low level but persistent viremia observed in mice challenged with X4 HIV-1 alone. Flow cytometric analysis of HIV-1-infected cells revealed that X4 HIV-1 infection of CCR5+CD4+ T cells was significantly suppressed in the presence of R5 HIV-1. X4 HIV-1 was more cytopathic than R5 HIV-1; however, this was not the cause of restricted X4 HIV-1 infection because there were no significant differences in the mortality rates of CCR5+ and CCR5- cells within the X4 HIV-1-infected cell populations. Taken together, these results suggest that restricted infection of CCR5+CD4+ T cells by X4 HIV-1 (occurring via a still-to-be-identified mechanism) might contribute to the preferential transmission of R5 HIV-1 during the early phase of infection.
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