Identification of immunodominant T cell epitopes from enterotoxigenic E. coli colonization factor antigen I (CFA/I) responsible for T helper cell cytokines

Abstract

Enterotoxigenic Escherichia coli (ETEC) are a major cause of diarrheal disease contracted by consuming contaminated food or water. ETEC is able to adhere to the small intestine by utilizing pili or fimbriae, one of which is the fimbriae Colonization Factor Antigen/I (CFA/I). The extracellular portion of CFA/I fimbriae is comprised of two fimbrial subunits, cfaB and cfaE. Expression of CFA/I fimbriae on the surface of an attenuated Salmonella vaccine vector, Salmonella-CFA/I, results in a biphasic T cell response in immunized mice. This response is characterized by the initial production of Th2-type cytokines, including IL-4 and IL-5, followed by a shift after 4 weeks toward an IFN-gamma-associated, Th1 response. Restimulation of CD4 + T cells from Salmonella-CFA/Iimmunized mice with CFA/I fimbriae also generates the anti-inflammatory cytokine, IL-10. Salmonella-CFA/I is able to generate antigen-independent Foxp3 + regulatory T cells, which are able to reduce symptoms of Experimental Autoimmune Encephalomyelitis in immunized SJL mice and Collagen Induced Arthritis in DBA/I and C57BL/6 mice, via production of IL-10 and TGF-beta by phenotypically distinct regulatory T cell subsets. The following research describes the contribution of cfaB and cfaE to the observed therapeutic and immunological responses. This was measured by independently expressing recombinant cfaB and cfaE proteins and evaluating the associated cytokine responses from the co-culture of these proteins with CD4 + T cells from immunized mice. Major Histocompatibility Complex II-restricted immunodominant regions were also mapped for both cfab and cfae proteins using cytokine ELISAs, ELISPOTs, Proliferation Assays, and flow cytometry. We mapped an IFN-gamma-producing peptide from cfaB and an IL-4-producing peptide from cfaE. We further determined that co-culture with peptides from both fimbrial proteins is able to generate regulatory T cell-associated cytokines including IL-10 and TGF-beta as well as the newly described suppressive cytokine, IL-35. These results show that the immune responses to cfaB and cfaE are mediated by multiple immunodominant regions within each protein.