A human-specific motif facilitates CARD8 inflammasome activation after HIV-1 infection

From the Emerman and Mitchell labs, Cancer Basic Biology Programs, Cancer Consortium and UW Microbiology.

Over the past century, several pathogenic viruses have entered the human population, including human immunodeficiency virus type 1 (HIV-1). HIV-1 became a pandemic following the successful transmission of simian immunodeficiency viruses (SIVs) that naturally infect chimpanzees (SIVcpz). In the same way, SIVcpz itself is a result of a cross-species transmission of a recombinant SIV that infected the red-capped mangabeys (SIVrcm) and mustached monkeys (SIVmus). However, despite infections with these related lentiviruses, non-human primates generally suffer much less pathogenesis from these viral infections than do human infected with HIV-1. The present study is part of a larger question to ask why HIV-1 is so pathogenic in humans. 

During viral infection, our immune system alerts our body of an invading threat. Recently, it was reported that the caspase recruitment domain family member 8 (CARD8), an inflammasome sensor, plays a role in detecting HIV-1 infection. CARD8 is a cytosolic protein that forms part of the immune system and is activated during viral infection. CARD8 activation leads to cell death and promotes inflammatory responses to prevent pathogenesis. Jessie Kulsuptrakul, a graduate student mentored by Drs. Michael Emerman (Fred Hutch) and Patrick Mitchell (UW Microbiology), sought to identify if the cleavage of CARD8 by lentiviral protease, leading to activation of an inflammatory response and cell death, is unique to humans. The results of her study were recently posted on BioRxiv.  

The CARD8 protein consists of two domains: FIIND (N terminus) and CARD (C terminus), the FIIND domain is cleaved during HIV infection, allowing the free CARD domain to trigger cell death and promote inflammatory responses. The HIV-1 protease directly cleaves between amino acids at positions 59-60. In humans, phenylalanine is found at both positions and this cleavage site is unique to human CARD8.  While the phenylalanine at position 59 is conserved across hominids, gibbons, and Old-World Monkeys, the amino acid at position 60 is not. To understand the impact of this species-specific variation at position 60, the authors constructed two versions of the human CARD8 protein in which the human-specific mutation was replaced with the amino acid found in other primates (S60 or L60 instead of F60). Indeed, they found that substitution of F60 prevents HIV from cleaving CARD8. Moreover, they showed that the protease from the simian immunodeficiency virus (SIV)-precursor of HIV-1-, SIVcpz from chimpanzees, was able to cleave human CARD8, but not chimpanzee CARD8. They went on to knock-out CARD8 in a lymphocyte cell line and show that activation of the inflammasome after HIV-1 infection depends on having the human-specific CARD8 gene. “Humans are unique in that their CARD8 is able to sense the protease encoded by these viruses” Jessie explains. Importantly, “This suggests the viral precursor to HIV-1, SIVcpz in chimpanzees, was already poised to activate the inflammasome response (CARD8) before the virus was transmitted to humans”.  These findings “may explain, at least in part, why SIV-infected primates exhibit reduced or no overt pathogenesis compared to HIV-1 infection in humans”.

Human CARD8 senses HIV infection promoting cell death and inflammation. HIV or SIVcpz cannot activate chimpanzee CARD8.
Human CARD8 senses HIV infection promoting cell death and inflammation. HIV or SIVcpz cannot activate chimpanzee CARD8. Image provided by Jessie Kulsuptrakul.

Taken together, their findings support that “human CARD8 can sense HIV protease in a site-specific manner during viral infection,” Jessie explains. However, “whether or not CARD8 sensing of HIV occurs in more physiologically relevant cell types remains an open question.” Future work aims “to expand our research into primary cells to elucidate whether CARD8 sensing of HIV protease occurs in targets of HIV like CD4+ T cells and macrophages and investigate how activation of the CARD8-dependent inflammasome affects HIV replication in a population of infected cells”.

The spotlighted research was funded by the National Institutes of Health, Mallinckrodt Foundation and University of Washington Cellular and Molecular Biology Training Grant.

Fred Hutch/UW/ Cancer Consortium member Michael Emerman contributed to this research.

J Kulsuptrakul, EA Turcotte, M Emerman and PS. Mitchell. 2022. A human-specific motif facilitates CARD8 inflammasome activation after HIV-1 infection. bioRxiv. Online ahead of print.