The discovery that most CD4+ T cells killed during HIV infection die through a process known as pyroptosis may provide long-sought explanations for HIV-associated T-cell depletion and inflammation. See Article p.509
The first paper to describe AIDS reported that patients had very few CD4+ T cells in their blood1. Depletion of this crucial subset of immune cells is now known to be a key feature of the disease, but the mechanisms responsible for their loss have remained unclear. Particularly mysterious has been the observation that HIV-1 infection results not only in the death of activated, productively infected CD4+ T cells (those in which the virus successfully replicates) but also in 'bystander' CD4+ T cells that do not seem to be infected. On page 509 of this issue, Doitsh et al.2 show that most CD4+ T cells depleted during HIV-1 infection are abortively infected cells that die through pyroptosis — a cell-death mechanism that is distinct from apoptosis and necroptosis3.
HIV-1 replication in productively infected CD4+ T cells kills them quickly, within one to two days4,5. This direct killing is apparent during acute infection, when virus levels are high and massive depletion of CD4+ T cells occurs in the gastrointestinal tract6. However, in the absence of treatment, most of the CD4+ T-cell loss associated with the infection occurs during the prolonged asymptomatic phase between the acute stage and the development of AIDS. During this period, the number of activated, productively infected CD4+ T cells is low, suggesting that the infection may promote death of quiescent (non-activated) cells.
Levels of immune activation are high in untreated HIV-1 infection, perhaps reflecting the translocation of microbial products across a compromised gastrointestinal barrier7, and it is commonly assumed that this immune activation is responsible for CD4+ T-cell loss. Perhaps the best evidence for this comes from studies of simian immunodeficiency virus infections, in which there is high virus replication, but little immune activation or CD4+ T-cell depletion8. Nevertheless, the mechanistic link between immune activation and CD4+ T-cell depletion has remained unclear.
Doitsh and colleagues suggest that this link may lie in the manner of cell death. Using cultures of human cells isolated from the spleen or tonsils, they demonstrate that more than 95% of CD4+ T cells that die following HIV-1 infection are quiescent cells that undergo pyroptosis. Only a small proportion of the dying cells were activated, productively infected CD4+ T cells undergoing apoptosis (Fig. 1). Apoptosis depends on the activation of the cell-signalling molecule caspase-3, whereas pyroptosis is triggered by inflammasome-activated caspase-1. Inflammasomes are multiprotein cytoplasmic complexes that integrate pathogen-triggered signalling pathways and then recruit and activate inflammatory caspase molecules. Pyroptosis results in lysis of the cell and release of the cytoplasmic contents into the extracellular space, and is highly inflammatory.
Productive HIV-1 infection involves the virus binding to the T-cell surface and entering the cell. There, the viral RNA is reverse transcribed to DNA and integrated into the host-cell genome, resulting in replication of the virus. If this process is aborted before integration and viral replication occur, the infection is termed non-productive. Doitsh and colleagues previously demonstrated9 that there is selective depletion of CD4+ T cells in which incomplete viral DNA transcripts accumulate following abortive infection. The same research group also recently identified interferon-γ-inducible protein 16 (IFI16) as the host-cell DNA sensor that triggers this cell death10.
To verify that most CD4+ T-cell depletion occurring during HIV-1 infection is mediated by pyroptosis, the authors treated cells with inhibitors of caspase-3 or caspase-6 (important in apoptosis), or of receptor-interacting protein kinase enzymes (important in necroptosis), and found that these treatments did not prevent most of the CD4+ T-cell loss. Also consistent with pyroptosis and the associated release of intracellular contents into the extracellular milieu was the presence of the cytoplasmic enzyme lactate dehydrogenase in the cell-culture supernatants. In vivo evidence for pyroptosis came from the detection of caspase-1 in quiescent CD4+ T cells in the paracortical zone that surrounds the region of activated CD4+ T and B cells in HIV-1-infected lymph-node tissues. The authors did not detect caspase-1 in the zone of activated CD4+ T cells or in uninfected tissue.
Caspase-1 activation is known11,12 to induce secretion of the highly inflammatory cytokine proteins interleukin-1β (IL-1β) and IL-18, which contribute to inflammatory conditions such as atherosclerosis and metabolic syndromes11,12,13,14. Doitsh and colleagues show that IL-1β release also occurs after infection with HIV-1, and that this requires caspase-1 activation (Fig. 1). Finally, the authors show that pyroptosis induced by HIV-1 can be prevented with VX-765, a caspase-1 inhibitor that has previously been tested in people with chronic epilepsy and psoriasis, and found to be safe and well tolerated. VX-765 treatment inhibited caspase-1 activation, IL-1β secretion and CD4+ T-cell death in HIV-1-infected cell cultures.
These findings raise the possibility of reducing immune activation and inflammation in response to chronic viral infections through caspase-1 inhibition. The research also suggests two new approaches to improve HIV-1 therapy: the use of antiretroviral agents that act early in the viral life cycle to block abortive infection, and the use of agents that inhibit caspase-1. Combination therapy with multiple classes of antiretroviral drugs is the standard of care for patients infected with HIV-1, and this therapy effectively suppresses viral replication. Suppression of caspase-1 activation may not be necessary if combination therapy prevents abortive infection as well.
Although Doitsh et al. do not report IL-18 levels in their study, this cytokine is generally produced along with IL-1β after inflammasome activation, and elevated levels are associated with inflammatory conditions13,14,15. Serum IL-18 levels, which are known to be high in HIV-1 infection, are reduced by antiretroviral therapy14,15. Thus, it remains to be seen whether caspase-1 inhibitors will add to existing antiretroviral therapy for the treatment of HIV-1 infection. Either way, the implication of pyroptosis in CD4+ T-cell depletion is a new explanation for this 30-year-old mystery in HIV-1 pathogenesis.