C. diphtheriae infections induce a DT-mediated disease of respiratory and cutaneous tissues known as diphtheria, which results in extensive necrosis of affected tissues stemming from inhibition of host protein synthesis and cell death. Using a 3D organotypic skin model of cutaneous diphtheria, Robinson et al. (2023) found that both DT and bacterial infection induce RSR and epidermal damage characteristic of diphtheria. Treatment with ZAKα and p38α/β inhibitors abrogated the extent of epidermal damage by DT and C. diphtheriae infection, implicating RSR and subsequent NLRP1 inflammasome activation as a driver of C. diphtheriae pathogenesis. However, CASP1 inhibition nominally impacted the extent of epidermal damage, suggesting that other caspases may contribute to NLRP1-dependent inflammation. Alternatively, ZAKα also mediates inflammasome-independent cell death pathways that contribute to tissue damage, and the relative impact of these responses versus pro-inflammatory cytokines and pyroptosis in the innate response to C. diphtheriae and diphtheria pathogenesis remains unclear.

P. aeruginosa is an opportunistic pathogen that can cause chronic, life-threatening infections, and is the primary bacterial pathogen associated with cystic fibrosis (CF). Pathogenic mutations in the CF transmembrane conductance regulator (CFTR) gene reduce CFTR expression, stability, and/or function, leading to defects in ion and fluid transport, aberrant mucus accumulation, and other dysfunctions that underlie CF pathogenesis. CFTR deficiency also induces p38α/β kinase activity (Bérubé et al., 2010). Pinilla et al. (2023) found that epithelial cells from people living with CF (PwCF) underwent a more rapid and robust cell death in response to ExoA treatment compared to healthy donor cells. In contrast, RSR-independent activation of NLRP1 did not cause heightened inflammasome responses in PwCF cells, implying that CFTR dysfunction specifically lowers the threshold for or otherwise licenses RSR-mediated NLRP1 inflammasome activation. Indeed, CFTR corrector therapy (i.e., TRIKAFTA) and chemical inhibition of ZAKα alleviated the hypersensitivity of NLRP1 to ExoA in PwCF donor epithelia. Although several aspects of the molecular basis linking CFTR, p38, and NLRP1 remain unknown, these findings support a model in which the increased sensitivity to RSR-triggered NLRP1 inflammasome activation contributes to CF pathogenesis.

Despite these exciting advances, the impact of NLRP1 inflammasome activation on bacterial burden, host immunity, and bacterial pathogenesis remains an open question at the organismal level, a particularly challenging pursuit given that the RSR does not activate murine orthologs of NLRP1.