{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"institution":[{"name":"bioRxiv"}],"indexed":{"date-parts":[[2026,10,23]],"date-time":"2026-10-23T21:18:39Z","timestamp":1792790319639,"version":"4.5.0"},"posted":{"date-parts":[[2026,9,25]]},"group-title":"Microbiology","reference-count":75,"publisher":"openRxiv","license":[{"start":{"date-parts":[[2026,9,25]],"date-time":"2026-09-25T00:00:00Z","timestamp":1790294400000},"content-version":"vor","delay-in-days":0,"URL":"http:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"National Institute of Allergy and Infectious Diseases","award":["5R01AI177842"],"award-info":[{"award-number":["5R01AI177842"]}],"id":[{"id":"https:\/\/ror.org\/043z4tv69","id-type":"ROR","asserted-by":"crossref"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":[],"accepted":{"date-parts":[[2026,9,25]]},"abstract":"<jats:title>Abstract<\/jats:title>\n                <jats:p>\n                  <jats:italic>Clostridioides difficile<\/jats:italic>\n                  infection (CDI) is characterized by toxin-mediated epithelial injury, intestinal inflammation, and a high frequency of disease recurrence. Although complement components are produced locally within the gastrointestinal tract, their contribution to\n                  <jats:italic>C. difficile<\/jats:italic>\n                  pathogenesis remains poorly understood. Here, we investigated the interaction of the complement proteins C1q and C3 with\n                  <jats:italic>C. difficile<\/jats:italic>\n                  spores and their contribution to spore\u2013host interactions and disease outcome. We found that C1q and C3 are accessible within the healthy ileal mucosa and that TcdB intoxication differentially remodels their spatial availability, decreasing accessible C1q while increasing accessible C3 at higher toxin concentrations.\n                  <jats:italic>C. difficile<\/jats:italic>\n                  spores associated with both complement components in vivo and directly interacted with purified human C1q and C3 in vitro, with immunogold electron microscopy localizing these interactions to the outer exosporium. Although the collagen-like BclA proteins modulated complement binding under some conditions, they were not required for C1q or C3 association. Far-Western analysis coupled to mass spectrometry identified the exosporium proteins CotE and CdeM as candidate complement-interacting proteins, and purified CdeM inclusion bodies directly associated with both C1q and C3. Exposure of spores to human serum promoted their interaction with intestinal epithelial cells, whereas depletion of either C1q or C3 markedly reduced spore internalization and reconstitution with the corresponding purified protein partially restored entry. C3-derived species, including C3dg, were selectively associated with spores but not vegetative cells, without reducing spore viability. Finally, C1q deficiency did not alter the initial onset of CDI but accelerated recovery and markedly attenuated recurrent disease following vancomycin treatment. Together, these findings identify intestinal complement as a previously unrecognized component of the\n                  <jats:italic>C. difficile<\/jats:italic>\n                  spore\u2013host interface and reveal C1q and C3 as modulators of spore epithelial entry, persistence, and recurrent disease.\n                <\/jats:p>\n                <jats:sec>\n                  <jats:title>Author summary<\/jats:title>\n                  <jats:p>\n                    <jats:italic>Clostridioides difficile<\/jats:italic>\n                    is a major cause of antibiotic-associated diarrhea, and recurrent infections remain difficult to prevent. Disease recurrence depends in part on highly resistant spores that persist in the intestine and can interact with the intestinal lining. Here, we show that two components of the complement immune system, C1q and C3, are present and accessible in the intestinal mucosa and can associate directly with\n                    <jats:italic>C. difficile<\/jats:italic>\n                    spores. We identified the spore surface protein CdeM as one molecular component capable of interacting with both C1q and C3. These complement proteins also enhanced the ability of spores to enter intestinal epithelial cells, suggesting that a host defense pathway can be exploited by the pathogen to promote interactions with the intestinal barrier. C3 was deposited and processed on spores but did not reduce their viability. Importantly, mice lacking C1q recovered more rapidly from infection and developed markedly less recurrent disease after antibiotic treatment. These findings reveal an unexpected role for intestinal complement in\n                    <jats:italic>C. difficile<\/jats:italic>\n                    pathogenesis and suggest that complement\u2013spore interactions may contribute to persistence and recurrence.\n                  <\/jats:p>\n                <\/jats:sec>","DOI":"10.64898\/2026.09.24.753972","type":"posted-content","created":{"date-parts":[[2026,9,26]],"date-time":"2026-09-26T04:00:13Z","timestamp":1790395213000},"source":"Crossref","is-referenced-by-count":0,"title":["Gut complement C1q and C3 interact with\n                  <i>Clostridioides difficile<\/i>\n                  spores via CdeM and contribute to pathogenesis"],"prefix":"10.64898","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-8089-5398","authenticated-orcid":false,"given":"Marjorie","family":"Pizarro-Guajardo","sequence":"first","affiliation":[{"name":"Department of Biology, Texas A&M University, College Station, Texas, USA;"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0000-7758-8584","authenticated-orcid":false,"given":"Christian","family":"Brito-Silva","sequence":"additional","affiliation":[{"name":"ANID \u2013 Millennium Science Initiative Program - Millennium Nucleus in the Biology of the Intestinal Microbiota, Santiago, Chile"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3370-4878","authenticated-orcid":false,"given":"Pablo","family":"Castro-C\u00f3rdova","sequence":"additional","affiliation":[{"name":"IMPACT, Center of Interventional Medicine for Precision and Advanced Cellular Therapy, Santiago, Chile"},{"name":"Laboratory of Nano-Regenerative Medicine, Centro de Investigaci\u00f3n e Innovaci\u00f3n Biom\u00e9dica (CiiB), Faculty of Medicine, Universidad de los Andes, Santiago, 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