Jacob Davis

Introduction: Shigella is a genus of Gram-negative bacteria that causes dysentery which can be life-threatening in resource-limited settings.¹ No vaccines are currently available.¹ Recent investigations have demonstrated the importance of pyroptosis—a lytic and inflammatory form of cell death—in defense against Shigella infection. For instance, NAIP-NLRC4 (an inflammasome upstream of pyroptosis) explains why Shigella can infect humans but not mice.² Similarly, multiple Shigella effector proteins secreted into host cells can directly inactivate pyroptosis—Ipah7.8 is a ubiquitin ligase that tags gasdermin D (a mediator of pyroptotic cell death) for degradation by the host proteasome,^3,4 and OspC3 inhibits host caspase-4/11 (a sensor upstream of pyroptosis) through a novel post-translational modification.^5,6 Methods and Results: Mitchell et al. (2020) generated a Nlrc4^-/- strain of 129S1 mice, which were inoculated orally with Shigella flexneri (a route that does not lead to infection in wild-type mice).² The mice were sacrificed 48 hours post-inoculation and then analyzed for evidence of shigellosis. Upon examination, these mice recapitulated signs of human Shigella infection including bloody diarrhea, increased intestinal epithelial cell bacterial load, elevated IL-1β, IL-18, and CXCL1, and histologic evidence of edema, hyperplasia, and epithelial disruption in infected tissues. In separate experiments, Li et al. (2021) inoculated wild-type mice intraperitoneally with Shigella flexneri ΔospC3 at a sub-lethal dose.⁵ They then measured levels of anti-Shigella IgG 48 hours post-inoculation via ELISA and performed rechallenge experiments using a lethal dose. These experiments were repeated using current candidates for a live-attenuated Shigella vaccine (S. flexneri ΔicsA and ΔguaBA). S. flexneri ΔospC3 induced a higher level of anti-Shigella IgG than either candidate, and antibody production was further increased with ΔospC3 on an ΔicsA or ΔguaBA background. Similarly, knockouts of ospC3 on an ΔicsA or ΔguaBA background conferred better protection from WT S. flexneri rechallenge infection. Conclusion: Evasion of pyroptosis is important for Shigella infection, and this information is driving new translational advances. Particularly, Mitchell et al. (2020) have created the first mouse model of oral Shigella infection and Li et al. (2021) have shown that deletion of anti-pyroptotic virulence factors such as OspC3 may be a viable strategy for creating a live-attenuated Shigella vaccine. Because pyroptosis is also involved in many other diseases, including Familial Mediterranean Fever (FMF)⁷ and cryopyrin-associated periodic syndromes (CAPS)⁸, inflammatory bowel disease, cancer, and sepsis,⁹ study of these virulence mechanisms may also serve as inspiration for future interventions that alter this pathway in a therapeutic context.

1. Kotloff KL, Riddle MS, Platts-Mills JA, Pavlinac P, Zaidi AKM. Shigellosis. The Lancet. 2018;391(10122):801-812. doi:10.1016/S0140-6736(17)33296-8
2. Mitchell PS, Roncaioli JL, Turcotte EA, et al. NAIP–NLRC4-deficient mice are susceptible to shigellosis. eLife. 2020;9:e59022. doi:10.7554/eLife.59022
3. Luchetti G, Roncaioli JL, Chavez RA, et al. Shigella ubiquitin ligase IpaH7.8 targets gasdermin D for degradation to prevent pyroptosis and enable infection. Cell Host & Microbe. 2021;29(10):1521-1530.e10. doi:10.1016/j.chom.2021.08.010
4. Hansen JM, de Jong MF, Wu Q, et al. Pathogenic ubiquitination of GSDMB inhibits NK cell bactericidal functions. Cell. 2021;184(12):3178-3191.e18. doi:10.1016/j.cell.2021.04.036
5. Li Z, Liu W, Fu J, et al. Shigella evades pyroptosis by arginine ADP-riboxanation of caspase-11. Nature. 2021;599(7884):290-295. doi:10.1038/s41586-021-04020-1
6. Oh C, Verma A, Hafeez M, Hogland B, Aachoui Y. Shigella OspC3 suppresses murine cytosolic LPS sensing. iScience. 2021;24(8). doi:10.1016/j.isci.2021.102910
7. Kanneganti A, Malireddi RKS, Saavedra PHV, et al. GSDMD is critical for autoinflammatory pathology in a mouse model of Familial Mediterranean Fever. J Exp Med. 2018;215(6):1519. doi:10.1084/jem.20172060
8. Xiao J, Wang C, Yao JC, et al. Gasdermin D mediates the pathogenesis of neonatal-onset multisystem inflammatory disease in mice. PLoS Biology. 2018;16(11). doi:10.1371/journal.pbio.3000047
9. Yu P, Zhang X, Liu N, Tang L, Peng C, Chen X. Pyroptosis: mechanisms and diseases. Sig Transduct Target Ther. 2021;6(1):1-21. doi:10.1038/s41392-021-00507-5