Immune responses to novel Escherichia coli and Salmonella typhimurium vectors that express colonization factor antigen I (CFA/I) of enterotoxigenic E. coli in the Absence of the CFA/I positive regulator cfaR

An asd-stabilized plasmid carrying enterotoxigenic Escherichia coli cfaABCE genes was constructed and called pJGX15C-asd⁺. Expression of colonization factor antigen I (CFA/I) by this plasmid occurs independently of the cfaABCE positive regulator cfaR in attenuated Salmonella Δaro Δasd strain H683 and nonpathogenic laboratory E. coli asd strain χ6212. Oral immunization of mice with nonpathogenic E. coli χ6212 (pJGX15C-asd⁺) does not elicit significant serum or mucosal responses against CFA/I. In contrast, oral immunization with a single dose of attenuated S. typhimurium H683(pJGX15C-asd⁺) elicits a 10⁵-fold increase in CFA/I-specific serum immunoglobulin G and significant elevation of CFA/I-specific immunoglobulin A-secreting B cells in the lamina propria, mesenteric lymph nodes, and spleen. Thus, only the Salmonella-CFA/I construct effectively delivered CFA/I to the inductive sites of the gut-associated and systemic lymphoid tissues.