Clostridioides difficile toxin A and toxin B inhibit toxin-specific adaptive immune responses through glucosyltransferase-dependent activity.

Clostridioides difficile colonizes the gastrointestinal tract and secretes two virulence factors: toxin A (TcdA) and toxin B (TcdB). Protective immunity against C. difficile infection is limited as patients are susceptible to multiple rounds of recurrent infections. The factors determining whether immunity to TcdA and TcdB is generated remain incompletely defined. We determined that C. difficile-infected mice generate antibody and IL-17A-producing CD4⁺ T cell responses to TcdA but not TcdB. To determine the mechanism of the failed anti-TcdB immunity, C. difficile mutant strains expressing glucosyltransferase inactive (GTX) TcdA, and/or glucosyltransferase inactive TcdB were used. Infection with TcdB_GTX or dual mutant (TcdA_GTX TcdB_GTX) restored TcdB-specific antibody responses, while infection with TcdA_GTX or TcdA_GTX TcdB_GTX led to an earlier induction of TcdA-specific antibodies. Finally, infection with the dual GTX mutant enhanced TcdA and TcdB-specific CD4⁺ T cell responses. These data demonstrate that the glucosyltransferase activity of TcdA and TcdB hinders the antigen-specific adaptive immune response to itself and may be a mechanism that underlies high recurrence rates following C. difficile infection in patients.