Blood-Derived CD4 T Cells Naturally Resist Pyroptosis during Abortive HIV-1 Infection

SummaryProgression to AIDS is driven by CD4 T cell depletion, mostly involving pyroptosis elicited by abortive HIV infection of CD4 T cells in lymphoid tissues. Inefficient reverse transcription in these cells leads to cytoplasmic accumulation of viral DNAs that are detected by the DNA sensor IFI16, resulting in inflammasome assembly, caspase-1 activation, and pyroptosis. Unexpectedly, we found that peripheral blood-derived CD4 T cells naturally resist pyroptosis. This resistance is partly due to their deeper resting state, resulting in fewer HIV-1 reverse transcripts and lower IFI16 expression. However, when co-cultured with lymphoid-derived cells, blood-derived CD4 T cells become sensitized to pyroptosis, likely recapitulating interactions occurring within lymphoid tissues. Sensitization correlates with higher levels of activated NF-κB, IFI16 expression, and reverse transcription. Blood-derived lymphocytes purified from co-cultures lose sensitivity to pyroptosis. These differences highlight how the lymphoid tissue microenvironment encountered by trafficking CD4 T lymphocytes dynamically shapes their biological response to HIV

Enhancer I Predominance in Hepatitis B Virus Gene Expression

Previous studies of human hepatitis B virus (HBV) transcription revealed the requirement of two enhancer elements. Enhancer I (EnhI) is located upstream of the X promoter and is targeted by multiple activators, including basic leucine zipper proteins, and enhancer II (EnhII) is located upstream to the PreCore promoter and is targeted mainly by nuclear receptors (NRs). The mode of interplay between these enhancers and their unique contributions in regulating HBV transcription remained obscure. By using time course analysis we revealed that the HBV transcripts are categorized into early and late groups. Chang (CCL-13) cells are impaired in expression of the late transcripts. This could be corrected by overexpressing EnhII activators, such as hepatocyte nuclear factor 4α, the retinoid X receptor α, and the peroxisome proliferator-activated receptor α, suggesting that in Chang cells EnhI but not EnhII is active. Replacing the 5′-end EnhI sequence with a synthetic Gal4 response (UAS) DNA fragment ceased the production of the early transcripts. Under this condition NR overexpression poorly activated EnhII. However, activation of the UAS by Gal4-p53 restored both the expression of the early transcripts and the EnhII response to NRs. Thus, a functional EnhI is required for activation of EnhII. We found a major difference between Gal4-p53 and Gal4-VP16 behavior. Gal4-p53 activated the early transcripts, while Gal4-VP16 inhibited the early transcripts but activated the late transcripts. These findings indicate that the composition of the EnhI binding proteins may play a role in early to late switching. Our data provides strong evidence for the role of EnhI in regulating global and temporal HBV gene expression

Dissecting How CD4 T Cells Are Lost During HIV Infection

Although the replicative life cycle of HIV within CD4 T cells is understood in molecular detail, less is known about how this human retrovirus promotes the loss of CD4 T lymphocytes. It is this cell death process that drives clinical progression to acquired immune deficiency syndrome (AIDS). Recent studies have highlighted how abortive infection of resting and thus nonpermissive CD4 T cells in lymphoid tissues triggers a lethal innate immune response against the incomplete DNA products generated by inefficient viral reverse transcription in these cells. Sensing of these DNA fragments results in pyroptosis, a highly inflammatory form of programmed cell death, that potentially further perpetuates chronic inflammation and immune activation. As discussed here, these studies cast CD4 T cell death during HIV infection in a different light. Further, they identify drug targets that may be exploited to both block CD4 T cell demise and the chronic inflammatory response generated during pyroptosis

Blood-Derived CD4 T Cells Naturally Resist Pyroptosis during Abortive HIV-1 Infection

Progression to AIDS is driven by CD4 T cell depletion, mostly involving pyroptosis elicited by abortive HIV infection of CD4 T cells in lymphoid tissues. Inefficient reverse transcription in these cells leads to cytoplasmic accumulation of viral DNAs that are detected by the DNA sensor IFI16, resulting in inflammasome assembly, caspase-1 activation, and pyroptosis. Unexpectedly, we found that peripheral blood-derived CD4 T cells naturally resist pyroptosis. This resistance is partly due to their deeper resting state, resulting in fewer HIV-1 reverse transcripts and lower IFI16 expression. However, when co-cultured with lymphoid-derived cells, blood-derived CD4 T cells become sensitized to pyroptosis, likely recapitulating interactions occurring within lymphoid tissues. Sensitization correlates with higher levels of activated NF-κB, IFI16 expression, and reverse transcription. Blood-derived lymphocytes purified from co-cultures lose sensitivity to pyroptosis. These differences highlight how the lymphoid tissue microenvironment encountered by trafficking CD4 T lymphocytes dynamically shapes their biological response to HIV

HIV-2 Depletes CD4 T Cells through Pyroptosis despite Vpx-Dependent Degradation of SAMHD1

Human immunodeficiency virus type 2 (HIV-2) infection results in a milder course of disease and slower progression to AIDS than does HIV-1. We hypothesized that this difference may be due to degradation of the sterile alpha motif and HD domain 1 (SAMHD1) host restriction factor by the HIV-2 Vpx gene product, thereby diminishing abortive infection and pyroptotic cell death within bystander CD4 T cells. We have compared CD4 T cell death in tonsil-derived human lymphoid aggregate cultures (HLACs) infected with wild-type HIV-2, HIV-2 ΔVpx, or HIV-1. In contrast to our hypothesis, HIV-2, HIV-2 ΔVpx, and HIV-1 induced similar levels of bystander CD4 T cell death. In all cases, cell death was blocked by AMD3100, a CXCR4 entry inhibitor, but not by raltegravir, an integrase, indicating that only early life cycle events were required. Cell death was also blocked by a caspase-1 inhibitor, a key enzyme promoting pyroptosis, but not by a caspase-3 inhibitor, an important enzyme in apoptosis. HIV-1-induced abortive infection and pyroptotic cell death were also not reduced by forced encapsidation of HIV-2 Vpx into HIV-1 virions. Together, these findings indicate that HIV-2 and HIV-1 support similar levels of CD4 T cell depletion in vitro despite HIV-2 Vpx-mediated degradation of the SAMHD1 transcription factor. The milder disease course observed with HIV-2 infection likely stems from factors other than abortive infection and caspase-1-dependent pyroptosis in bystander CD4 T cells.IMPORTANCE CD4 T cell depletion during HIV-1 infection involves the demise of bystander CD4 T cells due to abortive infection, viral DNA sensing, inflammasome assembly, and death by caspase-1-dependent pyroptosis. HIV-2 infection is associated with milder disease and lower rates of CD4 T cell loss. We hypothesized that HIV-2 infection produces lower levels of pyroptosis due to the action of its Vpx gene product. Vpx degrades the SAMHD1 restriction factor, potentially reducing abortive forms of infection. However, in tonsil cell cultures, HIV-2, HIV-2 ΔVpx, and HIV-1 induced indistinguishable levels of pyroptosis. Forced encapsidation of Vpx into HIV-1 virions also did not reduce pyroptosis. Thus, SAMHD1 does not appear to play a key role in the induction of bystander cell pyroptosis. Additionally, the milder clinical course of HIV-2-induced disease is apparently not explained by a decrease in this inflammatory form of programmed cell death