Elucidating the basis for permissivity of the MT-4 T-cell line to replication of an HIV-1 mutant lacking the gp41 cytoplasmic tail

HIV-1 encodes an envelope glycoprotein (Env) that contains a long cytoplasmic tail (CT) harboring trafficking motifs implicated in Env incorporation into virus particles and viral transmission. In most physiologically relevant cell types, the gp41 CT is required for HIV-1 replication, but in the MT-4 T-cell line the gp41 CT is not required for a spreading infection. To help elucidate the role of the gp41 CT in HIV-1 transmission, in this study we investigated the viral and cellular factors that contribute to the permissivity of MT-4 to gp41 CT truncation. We found that the kinetics of HIV-1 production are faster in MT-4 than in the other T-cell lines tested, but MT-4 express equivalent amounts of HIV-1 proteins on a per-cell basis relative to cells not permissive to CT truncation. MT-4 express higher levels of plasma-membrane-associated Env than non-permissive cells and Env internalization from the plasma membrane is slower compared to another T-cell line, SupT1. Paradoxically, despite the high levels of Env on the surface of MT-4, two-fold less Env is incorporated into virus particles in MT-4 compared to SupT1. Cell-to-cell transmission between co-cultured 293T and MT-4 is higher than in co-cultures of 293T with most other T-cell lines tested, indicating that MT-4 are highly susceptible to this mode of infection. These data help to clarify the long-standing question of how MT-4 cells overcome the requirement for the HIV-1 gp41 CT and support a role for gp41 CT-dependent trafficking in Env incorporation and cell-to-cell transmission in physiologically relevant cell lines.Importance The HIV-1 Env cytoplasmic tail (CT) is required for efficient Env incorporation into nascent particles and viral transmission in primary CD4+ T cells. The MT-4 T-cell line has been reported to support multiple rounds of infection of HIV-1 encoding a gp41 CT truncation. Uncovering the underlying mechanism of MT-4 T-cell line permissivity to gp41 CT truncation would provide key insights into the role of the gp41 CT in HIV-1 transmission. This study reveals that multiple factors contribute to the unique ability of a gp41 CT truncation mutant to spread in cultures of MT-4 cells. The lack of a requirement for the gp41 CT in MT-4 is associated with the combined effects of rapid HIV-1 protein production, high levels of cell-surface Env expression, and increased susceptibility to cell-to-cell transmission compared to non-permissive cells.List of abbreviationsCTcytoplasmic tailVSvirological synapseC-Ccell-to-cellEnvenvelope glycoproteinSTRshort tandem repeatMFImedian fluorescence intensityPMplasma membraneRadio-IPradio-immunoprecipitationhPBMChuman peripheral blood mononuclear cellsRTreverse transcriptaseVSV-GVesicular stomatitis virus glycoproteinPBSphosphate buffered salineFBSfetal bovine serumWTwild typeHRPhorseradish peroxidaseERendoplasmic reticulumMAHIV-1 Matrix proteinEBVEpstein-Barr virusHTLVHuman T-cell leukemia virusALLAcute lymphocytic leukemiaATLAdult T-cell lymphomaATCCAmerican Type Culture CollectionNIH ARPNIH AIDS Reagent Programn.s.not statistically significantINHIV-1 Integrase proteinCAHIV-1 Capsid proteinRINRNA Integrity NumberView Full Text