Rab11-FIP1C is dispensable for HIV-1 replication in primary CD4+ T cells but its role is cell-type-dependent in immortalized human T-cell lines

The HIV-1 envelope glycoprotein (Env) contains a long cytoplasmic tail harboring highly conserved motifs that direct Env trafficking and incorporation into virions and promote efficient virus spread. The cellular trafficking factor Rab11a family interacting protein 1C (FIP1C) has been implicated in the directed trafficking of Env to sites of viral assembly. In this study, we confirm that siRNA-mediated depletion of FIP1C in HeLa cells modestly reduces Env incorporation into virions. To determine whether FIP1C is required for Env incorporation and HIV-1 replication in physiologically relevant cells, CRISPR/Cas9 technology was used to knock out the expression of this protein in several human T-cell lines - Jurkat E6.1, SupT1 and H9 - and in primary human CD4+ T cells. FIP1C knock-out caused modest reductions in Env incorporation in SupT1 cells but did not inhibit virus replication in SupT1 or Jurkat E6.1 T-cell lines. In H9 cells, FIP1C knock-out caused a cell-density-dependent defect in virus replication. In primary CD4+ T cells, FIP1C knock-out had no effect on HIV-1 replication. Furthermore, HTLV-I transformed cell lines that are permissive for HIV-1 replication do not express FIP1C. Mutation of an aromatic motif in the Env cytoplasmic tail - Y795W - implicated in FIP1C-mediated Env incorporation impaired virus replication independently of FIP1C expression in SupT1, Jurkat E6.1, H9, and primary T cells. Together, these results indicate that while FIP1C may contribute to HIV-1 Env incorporation in some contexts, additional and potentially redundant host factors are likely required for Env incorporation and virus dissemination in T cells. ImportanceThe incorporation of the HIV-1 envelope (Env) glycoproteins, gp120 and gp41, into virus particles is critical for virus infectivity. Gp41 contains a long cytoplasmic tail that has been proposed to interact with host cell factors, including the trafficking factor Rab11a family interacting protein 1C (FIP1C). To investigate the role of FIP1C in relevant cell types - human T-cell lines and primary CD4+ T cells - we used CRISPR/Cas9 to knock out FIP1C expression and examined the effect on HIV-1 Env incorporation and virus replication. We observed that in two of the T-cell lines examined (Jurkat E6.1 and SupT1) and in primary CD4+ T cells, FIP1C knockout did not disrupt HIV-1 replication, whereas FIP1C knockout reduced Env expression and delayed replication in H9 cells. The results indicate that while FIP1C may contribute to Env incorporation in some cell lines, it is not an essential factor for efficient HIV-1 replication in primary CD4+ T cells.