Immunohistochemistry of the Gut-Associated Lymphoid Tissue (GALT) in African Bonytongue (Heterotis niloticus, Cuvier 1829)

Heterotis niloticus is a basal teleost, belonging to the Osteoglossidae family, which is
widespread in many parts of Africa. The digestive tract of H. niloticus presents similar
characteristics to those of higher vertebrates, exhibiting a gizzard-like stomach and lymphoid
aggregates in the intestinal lamina propria. The adaptive immune system of teleost fish is linked
with each of their mucosal body surfaces. In fish, the gut-associated lymphoid tissue (GALT) is
generally a diffuse immune system that represents an important line of defense against those
pathogens inhabiting the external environment that can enter through food. The GALT comprises
intraepithelial lymphocytes, which reside in the epithelial layer, and lamina propria leukocytes,
which consist of lymphocytes, macrophages, granulocytes, and dendritic-like cells. This study aims
to characterize, for the first time, the leukocytes present in the GALT of H. niloticus, by confocal
immuno- fluorescence techniques, using specific antibodies: toll-like receptor 2, major
histocompatibility complex class II, S100 protein, serotonin, CD4, langerin, and inducible nitric
oxide synthetase. Our results show massive aggregates of immune cells in the thickness of the
submucosa, arranged in circumscribed oval-shaped structures that are morphologically similar to
the isolated lymphoid follicles present in birds and mammals, thus expanding our knowledge
about the intestinal immunity shown by this fish.