Activation of various inflammasomes converges on the cleavage of gasdermin D (GSDMD) by pro-inflammatory caspases, followed by oligomerization of the N-terminal domain (GSDMDNT) and the assembly of pores penetrating target membranes. Yet, it remained unclear what triggers the conformational changes that allow membrane insertion, as methods to study pore formation in living cells were limited. We raised nanobodies specific for human GSDMD and found two nanobodies that prevent pyroptosis and IL-1β release when expressed in the cytosol of human macrophages. Nanobody binding to GSDMDNTblocked its oligomerization, while inflammasome assembly and GSDMD processing itself were not affected. The nanobody-stabilized monomers of GSDMDNTpartitioned into the plasma membrane, suggesting that pore formation is initiated by insertion of monomers, followed by oligomerization in the target membrane. When GSDMD pore formation was inhibited, cells still underwent caspase-1-dependent apoptosis, likely due to the substantially augmented caspase-1 activity. This hints at a novel layer of regulation of caspase-1 activity by GSDMD pores. Moreover, we revealed the unexpected therapeutic potential of antagonistic GSDMD nanobodies, as recombinant nanobodies added to the medium prevented cell death by pyroptosis, likely by entering through GSDMD pores and curtailing the assembly of additional pores. GSDMD nanobodies may thus be suitable to treat the ever-growing list of diseases caused by activation of the (non-) canonical inflammasomes.

NLRP1 is an innate immune sensor protein that activates inflammasome-driven pyroptotic cell death. Recent work demonstrates that human NLRP1 has evolved to sense viral infections. Whether and how human NLRP1 responds to other infectious agents is unclear. Here, and in a companion manuscript, we report that human NLRP1, as an integral component of the ribotoxic stress response (RSR), is activated by bacterial exotoxins that target human ribosome elongation factors EEF1 and EEF2, including Diphtheria Toxin (DT) from Corynebacterium diphtheriae, exotoxin A from Pseudomonas aeruginosa and sidI from Legionella pneumophila. In human keratinocytes, DT activates RSR kinases ZAKalpha, p38 and JNKs, upregulates a set of signature RSR transcripts and triggers rapid NLRP1-dependent pyroptosis. Mechanistically, these processes require 1) DtxR-mediated de-repression of DT production in the bacteria, as well as 2) diphthamide synthesis and 3) ZAKalpha/p38-driven NLRP1 phosphorylation in the host. In 3D human skin cultures, Corynebacterium diphtheriae infection disrupts barrier function and induces IL-1 driven inflammation. Pharmacologic inhibition of p38 and ZAKalpha suppresses keratinocyte pyroptosis and rescues barrier integrity of Corynebacterium diphtheriae-treated organotypic skin. In summary, these findings implicate RSR and the NLRP1 inflammasome in antibacterial innate immunity and might explain certain aspects of diphtheria pathogenesis.