We have assessed regulatory Th cell and cytokine responses in mice after oral immunization with recombinant Salmonella (BRD 847) expressing fragment C of tetanus toxoid, since little information is available to explain how these vectors induce mucosal IgA responses. A single dose of BRD 847 elicited serum IgG2a and mucosal IgA anti-tetanus toxoid Ab responses. To assess Th1-and Th2-type responses, CD4+ T cells from Peyer's patches and spleen were restimulated in vitro, and cytokine-specific ELISPOT, ELISA, and reverse transcriptase-PCR assays were used to assess cytokine patterns. CD4+ T cells produced IFN-gamma and IL-2 as well as IL-10, but not IL-4 or IL-5. Although IL-6 was elevated, further purification of cells from in vitro cultures into CD4+ Mac-1- T cells and Mac-1+ CD4- cells revealed that only the latter cell population had consistently elevated IL-6 gene expression, whereas both sorted populations exhibited increased IFN-gamma and IL-10 gene expression. Thus, orally administered recombinant Salmonella expressing fragment C of tetanus toxoid elicited dominant Ag-specific Th1-type responses together with Th2-type cells producing IL-10 in both mucosal and systemic tissues. Macrophages producing IL-6 were also evident. Our results are consistent with the suggestion that Ag-specific Th1 cells and their derived cytokines, IFN-gamma and IL-2, and Th2-derived IL-10 together with IL-6 produced by macrophages provide important signals for the development of mucosal IgA and serum IgG subclass responses in the absence of preferential expression of Th2 cytokines IL-4 and IL-5.