Mitochondrial NLRP3 protein induces reactive oxygen species to promote Smad protein signaling and fibrosis independent from the inflammasome.

Nucleotide-binding domain and leucine-rich repeat containing PYD-3 (NLRP3) is a pattern recognition receptor that is implicated in the pathogenesis of inflammation and chronic diseases. Although much is known regarding the NLRP3 inflammasome that regulates proinflammatory cytokine production in innate immune cells, the role of NLRP3 in non-professional immune cells is unclear. Here we report that NLRP3 is expressed in cardiac fibroblasts and increased during TGFß stimulation. NLRP3-deficient cardiac fibroblasts displayed impaired differentiation and R-Smad activation in response to TGFß. Only the central nucleotide binding domain of NLRP3 was required to augment R-Smad signaling because the N-terminal Pyrin or C-terminal leucine-rich repeat domains were dispensable. Interestingly, NLRP3 regulation of myofibroblast differentiation proceeded independently from the inflammasome, IL-1ß/IL-18, or caspase 1. Instead, mitochondrially localized NLRP3 potentiated reactive oxygen species to augment R-Smad activation. In vivo, NLRP3-deficient mice were protected against angiotensin II-induced cardiac fibrosis with preserved cardiac architecture and reduced collagen 1. Together, these results support a distinct role for NLRP3 in non-professional immune cells independent from the inflammasome to regulate differential aspects of wound healing and chronic disease.

Inflammasome-independent NLRP3 augments TGF-ß signaling in kidney epithelium.

Tubulointerstitial inflammation and fibrosis are strongly associated with the outcome of chronic kidney disease. We recently demonstrated that the NOD-like receptor, pyrin domain containing-3 (NLRP3) contributes to renal inflammation, injury, and fibrosis following unilateral ureteric obstruction in mice. NLRP3 expression in renal tubular epithelial cells (TECs) was found to be an important component of experimental disease pathogenesis, although the biology of NLRP3 in epithelial cells is unknown. In human and mouse primary renal TECs, NLRP3 expression was increased in response to TGF-ß1 stimulation and associated with epithelial-mesenchymal transition (EMT) and the expression of α-smooth muscle actin (αSMA) and matrix metalloproteinase (MMP) 9. TGF-ß1-induced EMT and the induction of MMP-9 and αSMA were significantly decreased in mouse Nlrp3(-/-) renal TECs, suggesting a role for Nlrp3 in TGF-ß-dependent signaling. Although apoptosis-associated speck-like protein containing a CARD domain(-/-) TECs demonstrated a phenotype similar to that of Nlrp3(-/-) cells in response to TGF-ß1, the effect of Nlrp3 on MMP-9 and αSMA expression was inflammasome independent, as IL-1ß, IL-18, MyD88, and caspase-1 were dispensable. Smad2 and Smad3 phosphorylation in response to TGF-ß1 was attenuated in Nlrp3(-/-) and apoptosis-associated speck-like protein containing a CARD domain(-/-) cells, accounting for the dampened EMT and TGF-ß1 responsiveness in these cells. Consistent with these findings, overexpression of NLRP3 in 293T cells resulted in increased Smad3 phosphorylation and activity. Taken together, these data support a novel and direct role for NLRP3 in promoting TGF-ß signaling and R-Smad activation in epithelial cells independent of the inflammasome.

The Nlrp3 inflammasome promotes myocardial dysfunction in structural cardiomyopathy through interleukin-1ß.

Heart failure is associated with a low-grade and chronic cardiac inflammation that impairs function; however, the mechanisms by which this sterile inflammation occurs in structural heart disease remain poorly defined. Cardiac-specific heterozygous overexpression of the calcineurin transgene (CNTg) in mice results in cardiac hypertrophy, inflammation, apoptosis and ventricular dilatation. We hypothesized that activation of the Nlrp3 inflammasome, an intracellular danger-sensing pathway required for processing the pro-inflammatory cytokine interleukin-1ß (IL-1ß), may contribute to myocardial dysfunction and disease progression. Here we report that Nlrp3 mRNA was increased in CNTg mice compared with wild-type. Consistent with inflammasome activation, CNTg animals had increased conversion of pro-caspase-1 to cleaved and activated forms, as well as markedly increased serum IL-1ß. Blockade of IL-1ß signalling via chronic IL-1 receptor antagonist therapy reduced cardiac inflammation and myocyte pathology in CNTg mice, resulting in improved systolic performance. Furthermore, genetic ablation of Nlrp3 in CNTg mice reduced pro-inflammatory cytokine maturation and cardiac inflammation, as well as improving systolic performance. These findings indicate that activation of the Nlrp3 inflammasome in CNTg mice promotes myocardial inflammation and systolic dysfunction through the production of pro-inflammatory IL-1ß. Blockade of IL-1ß signalling with the IL-1 receptor antagonist reverses these phenotypes and offers a possible therapeutic approach in the management of heart failure.

CD8 Tregs SQa-1shing transplant rejection.

MHC-E restricted CD8+ regulatory T cells have a restricted TCR repertoire and eliminate pathogenic CD4 T cells to mediate immune responses.

Separation anxiety: Splitting PD-L1 from CD80 suppresses autoimmunity.

A monoclonal antibody targeting CD80 on antigen presenting cells disrupts cis-interactions with PD-L1, reviving T cell inhibitory checkpoint signaling to suppress autoimmunity.

KIR+CD8+ T cells suppress pathogenic T cells and are active in autoimmune diseases and COVID-19.

In this work, we find that CD8+ T cells expressing inhibitory killer cell immunoglobulin-like receptors (KIRs) are the human equivalent of Ly49+CD8+ regulatory T cells in mice and are increased in the blood and inflamed tissues of patients with a variety of autoimmune diseases. Moreover, these CD8+ T cells efficiently eliminated pathogenic gliadin-specific CD4+ T cells from the leukocytes of celiac disease patients in vitro. We also find elevated levels of KIR+CD8+ T cells, but not CD4+ regulatory T cells, in COVID-19 patients, correlating with disease severity and vasculitis. Selective ablation of Ly49+CD8+ T cells in virus-infected mice led to autoimmunity after infection. Our results indicate that in both species, these regulatory CD8+ T cells act specifically to suppress pathogenic T cells in autoimmune and infectious diseases.

Tissue-selective alternate promoters guide NLRP6 expression.

The pryin domain (PYD) domain is involved in protein interactions that lead to assembly of immune-sensing complexes such as inflammasomes. The repertoire of PYD-containing genes expressed by a cell type arms tissues with responses against a range of stimuli. The transcriptional regulation of the PYD gene family however is incompletely understood. Alternative promoter utilization was identified as a mechanism regulating the tissue distribution of human PYD gene family members, including NLRP6 that is translationally silenced outside of intestinal tissue. Results show that alternative transcriptional promoters mediate NLRP6 silencing in mice and humans, despite no upstream genomic synteny. Human NLRP6 contains an internal alternative promoter within exon 2 of the PYD, resulting in a truncated mRNA in nonintestinal tissue. In mice, a proximal promoter was used that expanded the 5' leader sequence restricting nuclear export and abolishing translational efficiency. Nlrp6 was dispensable in disease models targeting the kidney, which expresses noncanonical isoforms. Thus, alternative promoter use is a critical mechanism not just for isoform modulation but for determining expression profile and function of PYD family members.

Hierarchical regulation of wound healing by NOD-like receptors in cardiovascular disease.

SIGNIFICANCE: Persistent nonmicrobial tissue injury leads to the nonlinear activation of integrated wound-healing pathways. In chronic cardiovascular diseases, local tissue undergoes dynamic remodeling involving both structural cells and professional innate immune cells in attempts to limit burden of injury. While the final effector mechanisms by which these different cellular populations participate in wound healing are functionally distinct, their upstream molecular signaling pathways can often be shared. RECENT ADVANCES: The NOD-like receptors (NLRs) are intracellular pattern recognition receptors that have been well characterized as key regulators of pro-inflammatory cytokine production in innate immune cells. However, recent evidence has shown that some NLR proteins are additionally expressed by resident structural cells despite negligible cytokine production. These results indicate the potential for noncanonical routes of innate immune signaling by NLRs within solid organ systems. CRITICAL ISSUES: Here, we review the emerging functions of NLR proteins in professional immune and tissue-resident cells, and discuss the implications in wound healing during chronic cardiovascular diseases. Emphasis is placed on NLRP3 and its regulation of cardiac structure and function in response to injury. Specific cellular and subcellular signaling paradigms are also discussed. FUTURE DIRECTIONS: The characterization of how NLRs participate in homeostasis during cellular injury is essential to develop their potential utility for therapeutic intervention in cardiovascular disease.