The pleiotropic role of vitamin A in regulating mucosal immunity

The effect of vitamin A on mucosal immunity has never been subjected to extensive studies until recently. We started to work in this area in the early 1970s when we observed that children with protein-calorie malnutrition (PCM) often had defective mucosal immunity, judging from the incidence of respiratory tract infections and diarrhea. We reported that these children had depressed secretory IgA (sIgA) levels in their nasal wash fluids. The IgA level in specimens collected from those superimposed with some degrees of vitamin A deficiency state appeared to be more severely affected. In order to better understand the underlying mechanism associated with this condition, we started to study more detail the deficiency state using experimental vitamin A-deficient rats. From a series of experiments using this animal model, we proposed that vitamin A was needed for transport and/or secretion of sIgA across the mucosa. This conclusion was based on the observation that the secretory component of sIgA synthesized by the epithelial cells of these vitamin A deficient animals was adversely affected as compared to the control animals. From that time onward, much progress has been made by several other groups showing that other mechanisms could also influence the integrity and immune function of the mucosa. For instance, recent studies demonstrated that retinoic acid which is a biologically active form of vitamin A has an essential role in mucosal homeostasis, controlling tolerance and immunity in these non-lymphoid tissues. Such a conclusion was made possible by the availability of sophisticated new molecular biology and genetic engineering techniques together with advances in the field of immunoregulation, e.g., the discovery of dendritic cells (DCs) and T helper cell subsets in 1980s, and the role of Toll-like receptors (TLRs) together with other innate immune regulators in controlling adaptive immune response in the early 1990s. These advances provided considerable new insights into the pleiotropic roles of vitamin A including educating mucosal DCs, differentiation of lymphocyte lineages and imprinting them with mucosal-homing properties as well as in regulating tolerance and immunity. The identification of a novel lymphocyte subpopulation, innate lymphoid cells (ILCs), at the beginning of this century has provided us with an additional insight into a new role of vitamin A in regulating homeostasis at the mucosal surface through influencing ILCs. Another new player that regulates intestinal homeostasis and mucosal immune response is microbiota whose composition is known to vary with vitamin A status. So it appears now that the role of vitamin A on mucosal immunity is far beyond regulating the adaptive Th1-Th2 cell response, but is highly pleiotropic and more complicating, e.g., polarizing the phenotype of mucosal DCs and macrophages, directing gut-homing migration of T and B cells, inducing differentiation of effector T cells and Treg subpopulation, balancing mucosal ILCs subpopulation and influencing the composition of microbiota. In this review, I will attempt to bring together these important advances to provide a comprehensive and contemporary perspective on the role of vitamin A in regulating mucosal immunity.