Immune cells in the spiral intestine of Blackmouth catshark, Galeus melastomus: New insights into ancient gut defenders

This study aimed to describe the morphology and to characterize the immunohistochemistry of the gut-associated lymphoid tissue (GALT) in the spiral intestine of the Blackmouth catshark Galeus melastomus. In the catshark, the lympho-myeloid aggregate of the GALT was mainly confined to the central region of the spiral valve. A panel of eight immune molecular markers was applied to histological sections to show the cell types of the GALT using immunohistochemical and -fluorescence methods. Two main reactive cell types were identified: (i) a mast cell lineage positive to histamine, serotonin, and immunoglobulin E-like receptor antibodies, and (ii) a macrophage lineage positive to CD4, interleukin-6, lysozyme, toll-like receptor-2, and tumor necrosis factor-α antibodies. Confocal microscopy revealed which immune markers were co-localized in the same cell type. The presence of cells positive to the tested antibodies confirmed the defensive role against pathogens of the GALT in the spiral intestine of the catshark. Given the limited knowledge of the elasmobranch immune system, the current study provides new insights into the features of ancient defender cells of the spiral intestine.