Mucosal vaccination: Opportunities and challenges

Abstract

Mucosal vaccination has been the common generic name attributed to the oral, intranasal, pulmonary, rectal, and vaginal routes of vaccine administration. Mucosal surfaces, with a combined surface area of about 400 m 2 [1], are undoubtedly the major site of entry for most pathogens. Therefore, these vulnerable surfaces are associated with a large and highly specialized innate and adaptive mucosal immune system that protects the surfaces and the body against potential destructive agents and harmless substances from the environment. In a healthy human adult, this local immune system contributes almost 80 % of all immune cells [2]. These immune cells accumulate in a particular mucosa or circulate between various mucosa-associated lymphoid tissues (MALT), which together form the largest mammalian lymphoid organ system [1]. In theory, mucosal surfaces seem to be the more accessible lymphoid organ for the induction of an immune response such as that required for immunization. Nevertheless, one of the more important reasons for the development of mucosal vaccines is the increasing evidence that local mucosal immune responses are important for protection against disease, principally for diseases which start on mucosal surfaces such as the respiratory, gastrointestinal, or urogenital mucosae. On the other hand, mucosal immune responses are most efficiently induced by the administration of vaccines onto mucosal surfaces, while injected vaccines are generally poor inducers of mucosal immunity and are therefore less effective against infection at mucosal surfaces. However, even with the many attractive features of mucosal vaccination described, it has often proven difficult in practice to stimulate strong sIgA immune responses and protection by mucosal antigen administration [2]. As a consequence, no more than half a dozen mucosal vaccines are currently approved for human use and no subunit vaccines are listed among those approved.