CARD8 inflammasome activation upon HIV-1 infection

Abstract

Inflammasomes are host cytosolic innate immune complexes that assemble upon detection of diverse pathogen-associated cues and play a crucial role in host defense but can also contribute to inflammatory pathogenesis. In prior work, the inflammasome-forming sensor CARD8 was reported to recognize the enzymatic activity of the protease of human immunodeficiency virus type 1 (HIV-1). I demonstrated that human CARD8 has a unique motif among hominoids and Old World monkeys that renders it susceptible to cleavage by HIV-1 protease (HIV-1PR). Furthermore, the protease from the precursor to HIV-1, SIVcpz, can cleave human CARD8, but not chimpanzee CARD8. This indicates that the precursor viruses to HIV-1 were poised to cleave human CARD8 prior to cross-species transmission into human. In addition, I show that CARD8 sensing can happen during acute HIV-1 infection, using multiple modes of infection in cancer and primary cell lines, in a manner dependent on HIV-1PR cleavage of the human-specific motif in CARD8, resulting in a lytic form of cell death called pyroptosis and the release of proinflammatory cytokines. Genetic knockout of the inflammasome adaptor protein ASC suggests that cell death associated with HIV-dependent inflammasome activation is primarily CARD8-dependent whereas cytokine release may be amplified through secondary modulation by the NLRP3 inflammasome. Additionally, I identified mutant HIV-1 proteases from a panel of protease inhibitor resistant HIV-1 strains that differentially cleave and activate CARD8 compared to wildtype HIV-1. HIV-1 arose from multiple cross-species transmissions from simian immunodeficiency virus (SIV) infecting other primates. While some non-human primates infected with SIVs exhibit inflammatory pathologies, humans present with the most severe inflammatory pathogenesis, progressing to acquired immunodeficiency syndrome (AIDS) without anti-retroviral therapy. The findings outlined in this thesis suggest a model whereby human-specific activation of the CARD8 inflammasome may contribute to chronic immune activation and may partially explain the heightened pathogenesis of HIV-1 in humans relative to SIV in other non-human primate reservoirs.