Helper T cell subsets and switch factors for regulation of mucosal IgA responses

Two major areas of research, induction of CD4$\sp+$ T helper (Th) cell responses for the IgA response and the effect of the neuropeptide substance P (Sub P) and transforming growth factor-$\beta$ (TGF-$\beta$) on the regulation of IgA isotype switching, were conducted in an attempt to understand the cellular and molecular mechanisms involved in the regulation of predominant IgA responses that are seen in mucosa-associated tissues.

For the induction of antigen-specific Th1 and Th2 cell responses, mice were either orally- or systemically-immunized with the T cell dependent (TD) antigen, sheep red blood cells (SRBC), or vaccine related protein antigens, i.e., cholera toxin (CT), or tetanus toxoid (TT), in the presence of the mucosal adjuvant CT. Oral immunization of mice with various TD antigens induced high numbers of antigen-specific IL-4 and IL-5, producing Th2-type cells in PP and spleen as well as high levels of IL-4 and IL-5 messages. Furthermore, a high frequency of TD antigen-specific IL-5-producing Th2 clones were found in PP of orally-immunized mice. In contrast, systemic immunization resulted in predominantly SRBC-specific Th1 cells and equal frequency of CT-B- and TT-specific Th1 and Th2-type cells in spleen at both protein, mRNA, and clonal levels. Therefore, the results indicated that oral immunization selectively induced antigen-specific Th2-type cells in PP and spleen, while systemic immunization elicited Th1 or Th1 and Th2 cell responses in spleen.

The effect of Sub P and TGF-$\beta$ on the regulation of IgA isotype switching was studied in the transformed surface IgM positive (sIgM$\sp+$) B cell lymphoma CH12.LX.5F5 (5F5) and in normal mouse PP and splenic sIgA$\sp-$ B cells. TGF-$\beta$ alone induced $\alpha$-sterile transcripts and a switch to surface and cytoplasmic IgA expression in both 5F5 and PP sIgA$\sp-$ B cells. However, without additional stimulation factors (e.g. lipopolysaccharide, LPS), it did not elicit IgA secretion in 5F5 cells. Sub P did not induce IgA secretion or expression of surface or low level cytoplasmic IgA. It did elicit $\alpha$-sterile and $\alpha$-productive messages in 5F5 and $\alpha$-sterile transcripts in PP sIgA$\sp-$ B cells in the presence of LPS. Further, it enhanced TGF-$\beta$ and LPS induced switching at both protein and message levels. Therefore, the study with TGF-$\beta$ and Sub P indicated that both of these two factors can significantly influence the IgA isotype switching in transformed and normal PP and splenic B cells.