Pharyngeal Immunity in Early Vertebrates Provides Functional and Evolutionary Insight into Mucosal Homeostasis

Abstract

The pharyngeal organ is located at the crossroad of the respiratory and digestive tracts in vertebrate, and it is continuously challenged by varying Ags during breathing and feeding. In mammals, the pharyngeal mucosa (PM) is a critical first line of defense. However, the evolutionary origins and ancient roles of immune defense and microbiota homeostasis of PM are still unknown. In this study, to our knowledge, we are the first to find that diffuse MALT is present in PM of rainbow trout, an early vertebrate. Importantly, following parasitic infection, we detect that strong parasite-specific mucosal IgT and dominant proliferation of IgT + B cell immune responses occurs in trout PM, providing, to our knowledge, the first demonstration of local mucosal Ig responses against pathogens in pharyngeal organ of a nonmammal species. Moreover, we show that the trout PM microbiota is prevalently coated with secretory IgT and, to a much lesser degree, by IgM and IgD, suggesting the key role of mucosal Igs in the immune exclusion of teleost pharyngeal bacteria. Overall, to our knowledge, our findings provide the first evidence that pharyngeal mucosal immunity appear earlier than tetrapods.

Immunoglobulins, Mucosal Immunity and Vaccination in Teleost Fish

“…However, the immunoprotective role of IgD in teleosts remains largely unknown. Although some reports have hypothesized the role of IgD in teleost gills ( 26 ), this antibody is known to be uninvolved in specific immunity in the gills and PM of rainbow trout during parasitic infection ( 4 , 6 ). Additionally, teleost sIgD can coat a subset of commensal microbiota in mucosal tissues, including the gut mucosa, gill mucosa, BM, and PM.…”

“…Additionally, teleost sIgD can coat a subset of commensal microbiota in mucosal tissues, including the gut mucosa, gill mucosa, BM, and PM. Nonetheless, mucosal bacteria coating by sIgD occurs significantly less than sIgT coating ( 4 – 6 ). These results suggest that teleost sIgD may also be involved in mucosal homeostasis.…”

“…Unlike the PC of mammals, which possesses both mucus-secreting cells and mucus glands to secrete mucus into the mucus layer ( 62 ), aquatic animals lack mucus glands and instead possess numerous mucus-secreting cells in the PM. Similar to the buccal MALT, IgT + B-cells are chiefly located in the pharyngeal epithelium (PE), where their numbers increase significantly upon pathogenic invasion ( 6 ).…”

“…B cells and Ig are present in the PM of teleost fish ( 6 ). In fish challenged with Ich for 28 days, IgT + B-cells increased dramatically in the PE and were found to secrete IgT.…”

“…So far, three main Ig isotypes have been identified in teleosts including IgM, IgD, and IgT/Z ( 8 , 9 ), of which IgT/Z is thought to be specialized in mucosal immunity. The predominant roles of IgT/Z antibodies and IgT + B-cells have been elucidated in six different teleost MALTs, including GALT ( 10 ), SALT ( 11 ), GIALT ( 4 ), NALT ( 12 ), and buccal ( 5 ) and pharyngeal MALT ( 6 ).…”