Antigen presentation of mucosal pathogens: the players and the rules

A vast number of infectious pathogens gain entry into the host through mucosal surfaces, which have a much greater total surface area than the skin. Since the mucosa is continuously exposed to those pathogens, the development of an effective local immune response is of utmost importance. An obligatory step in the development of most immune responses is the presentation of antigens by specialized accessory cells, termed antigen-presenting cells (APC) to T lymphocytes. The recognition of antigens by T cells is largely determined by how the antigens are handled by the APC. Complex antigen-processing events generate a selected set of peptides which ultimately become associated with MHC molecules. The type of MHC molecules that bind the peptides in turn determine what T lymphocyte subset recognizes the peptides. Thus, an understanding of the molecular and cellular processes preceding the T cell recognition event is a prerequisite for understanding how mucosal immune responses develop, as well as for investigating alternative approaches to vaccine development and therapeutic strategies to control autoimmune diseases. This review discusses the cell biology of antigen processing and how various APC populations may participate in mucosal responses.