Macropinosomes are a site of HIV-1 entry into primary CD4 ⁺ T cells

HIV-1 has been observed to enter target cells at both the plasma membrane and endosomes. However, which pathways mediate its entry into primary CD4 ⁺ T cells, the major targets of this virus, remains unclear. Here, we show that HIV-1 can enter primary CD4 ⁺ T cells through macropinocytosis, a form of endocytosis. We found that HIV-1 can enter primary CD4 ⁺ T cells at both the plasma membrane and internal compartments, while entry into common T cell lines occurred primarily at the plasma membrane. Inhibition of macropinocytosis suppressed HIV-1 internalization into and subsequent fusion with primary CD4 ⁺ T cells regardless of the viral coreceptor usage. Microscopic analysis of viral contents exposed to the cytosol confirmed that HIV-1 fusion occurs at the macropinosomal membrane. Finally, the inhibition of macropinocytosis blocked HIV-1 infection of primary CD4 ⁺ T cells. Altogether, this study identifies macropinocytosis as one pathway for HIV-1 entry into primary CD4 ⁺ T cells.