TNF Signaling Dictates Myeloid and Non-Myeloid Cell Crosstalk to Execute MCMV-Induced Extrinsic Apoptosis

Abstract

Cytomegaloviruses all encode the viral inhibitor of caspase-8-induced apoptosis (vICA). After binding to this initiator caspase, vICA blocks caspase-8 proteolytic activity and ability to activate caspase-3 and/or caspase-7. In this manner, vICA has long been known to prevent apoptosis triggered via tumor necrosis factor (TNF) family death receptor-dependent extrinsic signaling. Here, we employ fully wild-type murine cytomegalovirus (MCMV) and vICA-deficient MCMV (∆M36) to investigate the contribution of TNF signaling to apoptosis during infection of different cell types. ∆M36 shows the expected ability to kill mouse splenic hematopoietic cells, bone marrow-derived macrophages (BMDM), and dendritic cells (BMDC). Antibody blockade or genetic elimination of TNF protects myeloid cells from death, and caspase-8 activation accompanies cell death. Interferons, necroptosis, and pyroptotic gasdermin D (GSDMD) do not contribute to myeloid cell death. Human and murine fibroblasts or murine endothelial cells (SVEC4-10) normally insensitive to TNF become sensitized to ∆M36-induced apoptosis when treated with TNF or TNF-containing BMDM-conditioned medium. We demonstrate that myeloid cells are the natural source of TNF that triggers apoptosis in either myeloid (autocrine) or non-myeloid cells (paracrine) during ∆M36 infection of mice. Caspase-8 suppression by vICA emerges as key to subverting innate immune elimination of a wide variety of infected cell types.

Multiple Autonomous Cell Death Suppression Strategies Ensure Cytomegalovirus Fitness

“…Necroptotic RIPK3 [30], pyroptotic mediators (CASP1 and CASP11) [96,97], pyroptosis executioner gasdermin D [98], MCMV-associated inflammasome components (adaptor ASC, nucleic acid sensor AIM2) [99], as well type I and type II interferon signaling (via interferon alpha/beta receptor 1 [IFNAR1] or interferon gamma receptor [IFNGR] respectively) were all dispensable for the BAX/BAK-dependent death induced by the vMIA/vIBO-deficient virus (Figures 3D and S3A). The behavior of ∆M36 affirmed previous findings [29] showing that vICA was necessary to suppress TNFR1-dependent cell death (Figure 3A). M45mutRHIM-infected WT or Tnfr1 −/− cells exhibited similar susceptibility to cell death that remained dependent on necroptosis mediator RIPK3, but not on RIPK1, CASP8, nucleic acid sensor ZBP1, or BAX and BAK (Figures 3B-D and S3B).…”

“…To determine the impact of combined BAX/BAK suppression by M38.5-encoded vMIA and M41.1-encoded vIBO, we compared the replication of mutant viruses to parental K181 (Figure 1A-C). The genomic integrity of double mutants and control viruses were confirmed by direct evaluation (Figure S1A), as previously shown for single mutant viruses [29,30,38]. MCMV was shed in saliva mediates horizontal transmission [88], so viral load in these glands may therefore be considered an indicator of viral fitness.…”

“…RIPK1 engagement has been observed downstream to both extrinsic TNF-dependent and intrinsic BAX/BAK-mediated signaling [110][111][112]. MCMV infection engages RIPK1, CASP8, and RIPK3 in BMDM [29,30], but how these proteins play out in the context of vMIA/vIBO-dependent signaling remains unresolved. RIPK3 and ZBP1 may contribute to ripoptosome function.…”

Insights into the Role of Defective Apoptosis in Cancer Pathogenesis and Therapy

“…TNF family contains cysteine-rich extracellular domains and a cytoplasmic domain of about 80 amino acids called the "death domain". This death domain plays a critical role in transmitting the death signal from the cell surface to the intracellular signaling pathways [13][14][15].…”

Special Issue: “Innate Immune Sensing of Viruses and Viral Evasion”

“…These publications exemplify the fact that viruses have evolved multiple ways to dampen the host IFN response by interfering, disrupting, or evading specific host regulators, both up-and downstream of IFN induction. The diverse viral escape pathways are described in the following reviews [7,8,[10][11][12][13] and one original article on herpes-viruses [14]. A recent discovery stems from the observation that DNA sensing by the cGAS/STING pathway can be triggered by certain RNA viruses.…”