Progressive CD4+ T-cell depletion is the hallmark of AIDS, yet most dying CD4+ T cells are not productively infected, so the death mechanisms responsible remain incompletely accounted for. A 2024 Cell study showed that the CARD8 inflammasome is activated by the protease packaged in incoming HIV-1 virions immediately after viral entry, driving rapid pyroptosis of resting CD4+ T cells; genetic CARD8 ablation in humanized mice delayed but only partially prevented CD4+ T-cell loss. The open problem left by that study and its preprint is to determine, in physiologically relevant in vivo settings, (i) the quantitative division of responsibility between CARD8-dependent pyroptosis and other cell-death mechanisms or additional inflammasome sensors — including the proposed but unvalidated IFI16- and NLRP3-dependent pyroptosis — for CD4+ T-cell loss; and (ii) whether CARD8-driven pyroptosis contributes to the persistent inflammation and immune activation of chronic, ART-suppressed HIV infection and to progressive immune deficiency. As of mid-2026, later work has deepened the in vitro mechanism (cell-to-cell transmission, macrophage IL-1-beta release, viral evasion during assembly) while both in vivo attribution questions remain open.

Background

Infection with HIV-1, the virus causing AIDS, is defined immunologically by the progressive loss of CD4+ T cells (the helper T cells that coordinate adaptive immunity), yet only a small fraction of the cells that die harbor productive viral replication — the majority are 'bystander' cells that die without making new virus. A major proposed death route is pyroptosis: a highly inflammatory, lytic form of cell death triggered when intracellular multiprotein complexes called inflammasomes activate the protease caspase-1, which cleaves gasdermin D (GSDMD) to form membrane pores and matures the pro-inflammatory cytokines IL-1-beta and IL-18. Doitsh and colleagues (Nature, 2014) showed that lymphoid-tissue CD4+ T cells undergoing abortive (non-productive) HIV infection die by caspase-1-dependent pyroptosis, and Monroe and colleagues (Science, 2014) implicated the DNA-sensor protein IFI16 in this death; however, Munoz-Arias and colleagues (Cell Host & Microbe, 2015) found that blood-derived CD4+ T cells naturally resist pyroptosis during abortive infection, and Zhang and colleagues (J Clin Invest, 2021) reported the NLRP3 inflammasome (a sensor activated by diverse cellular stress signals) as a driver of CD4+ T-cell loss in chronically infected patients. Whether IFI16 and NLRP3 can actually drive pyroptosis of human T cells in vivo, and how much CD4+ T-cell loss each pathway accounts for, has remained unresolved; the source study notes that these two inflammasomes are reported to be functional only in certain subsets of CD4+ T cells. CARD8 (caspase recruitment domain-containing protein 8) is an inflammasome-forming sensor kept inactive in cells by the protease DPP9; work from the Shan laboratory established that HIV-1 protease, packaged in incoming virions, directly cleaves and activates CARD8 immediately after viral entry, causing rapid pyroptosis of resting (quiescent) CD4+ T cells without productive infection, while T-cell activation abolishes CARD8 function. In the 2024 Cell study (Wang et al.), humanized mice — immunodeficient mice reconstituted with a human immune system — whose human cells lacked CARD8 showed delayed CD4+ T-cell depletion despite high viremia, but the protection was incomplete; the same study found loss-of-function mutations in CARD8 in 'natural hosts' of SIV (the primate lentivirus counterpart of HIV, e.g. sooty mangabeys and African green monkeys) that typically tolerate high viremia without CD4+ T-cell loss, and in vivo SIV studies of rhesus macaques had independently shown that the early CD4+ T-cell loss in that pathogenic model occurs predominantly by pyroptosis (Barouch et al., Cell, 2016). The study's discussion and limitations, echoed by the authors' 2024 review, explicitly flag what remains undetermined: the residual CD4+ T-cell loss seen when CARD8 is absent might reflect incomplete genome editing, limitations of humanized-mouse models, or the possible involvement of other cell-death mechanisms, including the activation of additional inflammasome sensors; the contribution of this mechanism of CD4+ T-cell depletion to progressive immune deficiency, and its relationship with previously described, non-mutually exclusive mechanisms of disease progression, remains to be determined; and the role of CARD8 in the chronic inflammation that persists in people on antiretroviral therapy (ART, the standard suppressive HIV treatment) is unknown — notably, CARD8-mediated pyroptosis of T cells releases little IL-1-beta, and a 2025 eLife study proposed instead that macrophage-derived IL-1-beta after cell-to-cell HIV transmission could be the inflammatory route. These questions concern the in vivo behavior of the CARD8-dependent pathway in physiologically relevant systems, and a quantitative, mechanistic in vivo attribution of the competing death pathways in CD4+ T-cell loss is not yet available.

Problem Statement

Determine the in vivo contribution of CARD8 inflammasome activation to CD4+ T-cell depletion and disease progression in pathogenic HIV/SIV infection, as one mechanistic accounting with two linked components: (1) the quantitative division of responsibility for CD4+ T-cell loss in vivo between CARD8-dependent pyroptosis and other cell-death mechanisms or additional inflammasome sensors — including whether IFI16- and NLRP3-dependent pyroptosis genuinely operates in human T cells in vivo — explaining the incomplete protection seen when CARD8 is ablated, and distinguishing genuine additional death pathways from the source-acknowledged confounders of incomplete genome editing and the limitations of humanized-mouse models; and (2) whether CARD8-driven pyroptosis (directly, or indirectly via macrophage IL-1-beta release) contributes to the persistent inflammation and immune activation of chronic infection, including under suppressive antiretroviral therapy, and to progressive immune deficiency. No in vivo measurement of this division is yet available, so the division of responsibility for in vivo CD4+ T-cell loss between CARD8-dependent pyroptosis and the residual — apportioned among the additional sensors and other non-inflammasome death mechanisms where the cited evidence permits — is bounded by, and not resolved beyond, the precision the cited in vivo evidence permits.

References

  1. Qiankun Wang, Kolin M. Clark, Ritudhwaj Tiwari, Nagarajan Raju, Gregory K. Tharp, Jeffrey Rogers, R. Alan Harris, Muthuswamy Raveendran, Steven E. Bosinger, Tricia H. Burdo, Guido Silvestri, Liang Shan. 'The CARD8 inflammasome dictates HIV/SIV pathogenesis and disease progression.' Cell 187, 1223-1237 (2024). DOI: 10.1016/j.cell.2024.01.048. https://doi.org/10.1016/j.cell.2024.01.048
  2. Qiankun Wang, Liang Shan. 'The CARD8 inflammasome drives CD4+ T-cell depletion in HIV-1 infection.' bioRxiv preprint (2023). DOI: 10.1101/2023.03.18.533283. https://doi.org/10.1101/2023.03.18.533283
  3. Qiankun Wang, Liang Shan. 'Role of the CARD8 inflammasome in HIV pathogenesis.' Cell Insight 3, 100193 (2024). DOI: 10.1016/j.cellin.2024.100193. https://doi.org/10.1016/j.cellin.2024.100193
  4. Jessie Kulsuptrakul, Michael Emerman, Patrick S. Mitchell. 'CARD8 inflammasome activation during HIV-1 cell-to-cell transmission.' eLife (2025). DOI: 10.7554/eLife.102676. https://doi.org/10.7554/eLife.102676
  5. Ivy K. Hughes, James B. Hood, Andrés A. Quiñones-Molina, Hisashi Akiyama, Suryaram Gummuluru. 'Evasion of CARD8 activation during HIV-1 assembly.' Science Advances (2025). DOI: 10.1126/sciadv.adz1069. https://doi.org/10.1126/sciadv.adz1069
  6. Gilad Doitsh, Nicole L. K. Galloway, Xin Geng, Zhiyuan Yang, Kathryn M. Monroe, Orlando Zepeda, Peter W. Hunt, Hiroyu Hatano, Stefanie Sowinski, Isa Muñoz-Arias, Warner C. Greene. 'Cell death by pyroptosis drives CD4 T-cell depletion in HIV-1 infection.' Nature 505, 509-514 (2014). DOI: 10.1038/nature12940. https://doi.org/10.1038/nature12940
  7. Kathryn M. Monroe, Zhiyuan Yang, Jeffrey R. Johnson, Xin Geng, Gilad Doitsh, Nevan J. Krogan, Warner C. Greene. 'IFI16 DNA sensor is required for death of lymphoid CD4 T cells abortively infected with HIV.' Science 343, 428-432 (2014). DOI: 10.1126/science.1243640. https://doi.org/10.1126/science.1243640
  8. Chao Zhang, Jin-Wen Song, Hui-Huang Huang, Xing Fan, Lei Huang, Jian-Ning Deng, Bo Tu, Kun Wang, Jing Li, Ming-Ju Zhou, CuiXian Yang, Qi-Wen Zhao, Tao Yang, Li-Feng Wang, Ji-Yuan Zhang, Ruo-Nan Xu, Yan-Mei Jiao, Ming Shi, Feng Shao, Rafick-Pierre Sékaly, Fu-Sheng Wang. 'NLRP3 inflammasome induces CD4+ T cell loss in chronically HIV-1-infected patients.' Journal of Clinical Investigation 131, e138861 (2021). DOI: 10.1172/JCI138861. https://doi.org/10.1172/JCI138861
  9. Isa Muñoz-Arias, Gilad Doitsh, Zhiyuan Yang, Stefanie Sowinski, Debbie Ruelas, Warner C. Greene. 'Blood-derived CD4 T cells naturally resist pyroptosis during abortive HIV-1 infection.' Cell Host & Microbe 18, 463-470 (2015). DOI: 10.1016/j.chom.2015.09.010. https://doi.org/10.1016/j.chom.2015.09.010
  10. Dan H. Barouch, Khader Ghneim, William J. Bosche, Yuan Li, Brian Berkemeier, Michael Hull, Sanghamitra Bhattacharyya, Mark Cameron, Jinyan Liu, Kaitlin Smith, Erica Borducchi, Crystal Cabral, Lauren Peter, Amanda Brinkman, Mayuri Shetty, Hualin Li, Courtney Gittens, Chantelle Baker, Wendeline Wagner, Mark G. Lewis, Arnaud Colantonio, Hyung-Joo Kang, Wenjun Li, Jeffrey D. Lifson, Michael Piatak, Rafick-Pierre Sekaly. 'Rapid inflammasome activation following mucosal SIV infection of rhesus monkeys.' Cell 165, 656-667 (2016). DOI: 10.1016/j.cell.2016.03.021. https://doi.org/10.1016/j.cell.2016.03.021
  11. Alexandra Flemming. 'CARD8 kills CD4+ T cells in response to HIV entry.' Nature Reviews Immunology 24, 232 (2024). DOI: 10.1038/s41577-024-01021-9. https://doi.org/10.1038/s41577-024-01021-9