About this Research Topic
The mucosal immune system is a localized and specific immune cell organization protecting the mucosal surfaces of the oropharyngeal cavity, gastrointestinal tract, respiratory tract and the urogenital tract. Mucosal surfaces, being continuously exposed to the external environment defend against a vast array of pathogens, yet limit responses to commensal microbes under healthy conditions. Constant host-microbe interactions in mucosal surfaces are regulated by an intricate network of immune cells that not only induce innate and acquired mucosal immune responses, but also control inadvertent inflammation. Other than innate cells such as epithelial cells, neutrophils and antigen presenting cells, lymphocytes are one of the dominant players in these processes. Mucosal lymphocytes include αβ T cell receptor (TCR)-expressing cells such as major histocompatibility class II (MHC II)-restricted CD4+ T cells (helper T cells), MHC I-restricted CD8+ T cells and regulatory T cells (Tregs). Tregs are mainly implicated in maintaining commensal tolerance and in prevention of inflammation. These T cells develop in the thymus and migrate into mucosal sites after encountering antigen stimuli in lymphoid tissues. Other unconventional CD8αα+ T cells expressing either αβ or γδ TCRs also exist within the epithelial layer. Additionally, natural Th17 cells, innate lymphocyte lineages such as NK cells and the recently identified innate lymphoid cells (ILCs) play crucial roles in orchestrating immune responses in the mucosa. During the last decade, our understanding of immunity and inflammation in mucosal surfaces has been profoundly transformed by the discovery of new classes of some of these lymphocytes and their novel functions. In this review, we will highlight our current understanding of differentiation, interactions and functions of these mucosal immune cells during infections and inflammation. We will emphasize on the processes of T cell differentiation, migration and signaling, and how these processes may contribute to mucosal immune reactions.
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