Generation and characterization of a novel MHC-II tetramer for tracking and characterization of toxin B-specific CD4+ T cell responses.

The gastrointestinal pathogen Clostridioides difficile is a major burden for health systems due to high rates of recurrence. C. difficile pathogenesis is mediated by two virulence factors, toxin A (TcdA) and toxin B (TcdB). Antibodies specific for TcdA and TcdB are correlated with protection from symptomatic recurrence; however, the role for CD4+ T cells is poorly understood in part due to the lack of tools to study the toxin-specific CD4+ T cell response. Our group recently demonstrated the antibody and CD4+ T cell response to C. difficile toxins is impaired via the glucosyltransferase activity of the toxins; however, tools do not exist to study the protective capacity and the phenotype of toxin-specific CD4+ T cells. Therefore, we developed a major histocompatibility complex class II (MHC-II) tetramer to identify TcdB-specific CD4+ T cells via flow cytometry. Herein, we identified an immunodominant epitope (TcdB1961-1975) in the CROPs region of TcdB and optimized an MHC-II tetramer for use in tracking and phenotyping TcdB-specific CD4+ T cell responses following multiple different immunization strategies in mice. Utilizing the tetramer, TcdB-specific T follicular helper cells were detected following TcdB-CROPs messenger RNA lipid nanoparticle vaccination validating the advantage of the tetramer. Furthermore, using a modular messenger RNA vector expressing the TcdB1961 peptide covalently bound to the beta chain of MHC-II, we were able to generate a robust population of TcdB-specific CD4+ T cells. These data outline the generation of new tools for the C. difficile field and lay the groundwork for future studies of toxin-specific CD4+ T cell responses.