Noncytotoxic Suppression of Human Immunodeficiency Virus Type 1 Transcription by Exosomes Secreted from CD8+ T Cells

CD8(+) T cells display a noncytotoxic activity that suppresses transcription of human immunodeficiency virus type 1 (HIV-1) in an antigen-independent and major histocompatibility complex-unrestricted manner. To date, the precise cellular and molecular factors mediating this CD8(+) T-cell effector function remain unsolved. Despite evidence indicating the dependence of the activity on cell-cell contact, the possibility of a membrane-mediated activity that represses transcription from the viral promoter remains unexplored. We therefore investigated whether this inhibition of HIV-1 transcription might be elicited by a membrane-bound determinant. Using a CD8(+) T-cell line displaying potent noncytotoxic HIV-1 suppression activity, we have identified a membrane-localized HIV-1-suppressing activity that is concomitantly secreted as 30-to 100-nm endosomederived tetraspanin-rich vesicles known as exosomes. Purified exosomes from CD8(+) T-cell culture supernatant noncytotoxically suppressed CCR5-tropic (R5) and CXCR4-tropic (X4) replication of HIV-1 in vitro through a protein moiety. Similar antiviral activity was also found in exosomes isolated from two HIV-1-infected subjects. The antiviral exosomes specifically inhibited HIV-1 transcription in both acute and chronic models of infection. Our results, for the first time, indicate the existence of an antiviral membrane-bound factor consistent with the hallmarks defining noncytotoxic CD8(+) T-cell suppression of HIV-1.