A Sequential Light Microscopic and Ultrastructural Study on the Uptake and Handling of Vibrio salmonicida in Phagocytes of the Head Kidney in Experimentally Infected Atlantic Salmon ( Salmo salar L.)

The uptake and handling of Vibrio salmonicida in phagocytes of the head kidney of Atlantic salmon (Salmo salar L.) were evaluated by light and electron microscopy, including in situ identification of the bacterium by immunolabeling at the light microscopical and the ultrastructural level. Fish were injected with live bacteria, and 4, 24, 48, and 72 hours after inoculation, samples were collected after perfusion fixation. Morphologically, the most prominent change in the course of the experiment was an increasing number of intrasinusoidal, endothelial cell-adherent phagocytes and the elevated number of interstitial melanomacrophages. Immunohistochemically, bacterial antigens were initially identified in intrasinusoidal phagocytes, and at 24 hours postinfection in endothelium-adherent phagocytes and intrasinusoidal melanomacrophages. Later, (48 and 72 hours postinfection), the interstitial melanomacrophages were also found to harbor bacterial antigen. Ultrastructurally, bacteria were identified in phagosomes in intrasinusoidal phagocytes, and morphological findings also indicated an increased cellular degradation, including autophagocytosis. Immunoelectron microscopy indicated that bacterial antigens were associated with melanomacrophages, specifically in their electron-dense cytoplasmic granules. These findings indicate that intrasinusoidal phagocytes and melanomacrophages participate in the rapid and active clearance of particulate material from the circulation, i.e., pathogenic microorganisms, and in the scavenging of cellular degradation products. The process of formation of melanomacrophages and their possible function is discussed.