First, it has been suggested that cell-to-cell transfer of HIV-1 from infected T cells to astrocytes may overcome entry barriers to cell-free infection, allowing astrocyte infection (Do et al., 2014, Luo and He, 2015, Nath et al., 1995), potentially via a CD4-independent but CXCR4-dependent mechanism (Li et al., 2015). Here, CD4 is linked to infection.