Immunoglobulin M response in different intestinal segments of largemouth bass (Micropterus salmoides) following largemouth bass virus (LMBV) infection.

In teleost fish, the gut functions not only as a primary site for nutrient digestion and absorption but also as a critical mucosal immune organ. Although the functional heterogeneity of nutrient absorption among different intestinal segments has been well characterized, the segment-specific features of intestinal immune responses remain poorly understood, particularly with regard to mucosal IgM responses. In this study, we established a virus infection model via intraperitoneal injection and demonstrated that the virus triggers robust innate and adaptive immune responses in the gut of largemouth bass (Micropterus salmoides), accompanied by significant upregulation of multiple immune-related genes. The secondary infection induced a stronger IgM response compared to primary infection, characterized by a significant increase in IgM+ B cell proportions and markedly elevated LMBV-specific IgM titers. In addition, the rise in LMBV-IgM titers may be closely associated with their specific neutralizing activity against the virus. Notably, intestinal IgM responses exhibited clear segmental heterogeneity, with the hindgut displaying substantially stronger IgM responses than the foregut and midgut. Collectively, this study highlights the pivotal role of the hindgut in antiviral intestinal immunity and provides a theoretical foundation for the development of mucosal vaccines in teleost fish.