Potential of nasopharynx-associated lymphoid tissue for vaccine responses in the airways

Nasopharynx-associated lymphoid tissue (NALT), constituting Waldeyer's ring in humans, is a unique inductive site for B-cell responses and plasma cell generation. This makes the nasal route of vaccine administration interesting for induction of mucosal and systemic antibodies. The unpaired nasopharyngeal tonsil (adenoids) and the paired palatine tonsils are prominent NALT structures, functionally similar to the paired rodent NALT structures located dorsal to the cartilaginous soft palate. Human NALT is more strategically located, however, because its elements are exposed to both airborne and alimentary antigens and have antigen-retaining crypts. It also shows similarities with lymph nodes and participates both in systemic- and secretory-type mucosal immunity. Primary follicles occur at 16 weeks of gestation, which is similar to Peyer's patches but different from rodent NALT whose organogenesis begins at birth. The formation of germinal centers reflecting B-cell activation does not take place until shortly after birth, and terminal differentiation of plasma cell can be seen about 2 weeks postnatally. Germinal centers arise in T cell-dependent B-cell responses and are associated with somatic hypermutation of Ig V-region genes. Downstream switching to various Ig isotypes also takes place, with or without concurrent expression of the J-chain gene. The J chain is a crucial part of dimeric IgA and pentameric IgM, making these Ig polymers able to interact with the epithelial polymeric Ig receptor. This interaction is central in the formation of secretory IgA and secretory IgM. Accumulating evidence suggests a major role for NALT in antibody immunity of the respiratory tract and associated glands.