New perspectives in mucosal immunity with emphasis on vaccine development.

In this review, we have purposely focused on five areas that are currently receiving extensive research attention and will be of major importance for development of mucosal and systemic immunity to oral vaccines. These five areas include the following: (1) helper T-cell (Th) subsets and cytokines for mucosal IgA responses; (2) Th1- and Th2-type subsets in regulation of mucosal IgA responses; (3) antigen uptake and presentation in the mucosal immune system; (4) the importance of memory in mucosal immunity to vaccines; and (5) the determination of whether oral immunization alone induces immunity in all mucosal effector tissues. It is now established that the mucosal immune system can be divided into discrete mucosal inductive sites where vaccines/antigens are encountered and taken up, processed, and presented to B and T cells, and separate areas where immune cells actually function (mucosal effector tissues). Further, through the help provided by Th cells and cytokines, the B cells respond to antigen and undergo expansion including memory cell formation. Following the migration of memory B cells to mucosal effector tissues, the cells rapidly develop into IgA plasma cells, and the prevalence of the latter cell type represents a major characteristic of mucosal effector tissues. It also appears that antigen-specific Th cells and perhaps even CD8+ cytotoxic T lymphocytes can make this circular journey (along with memory/activated B cells) from inductive to mucosal effector sites, and this is termed the common mucosal immune system (CMIS). The major implications of the CMIS for development of vaccines would include each of the five components that are discussed.