Lymphocyte apoptosis in septicaemic sandbar sharks (Carcharhinus plumbeus, Nardo 1827)

Introduction: Lymphocyte apoptosis has been confirmed in human pathology as an important mechanism in the pathogenesis of sepsis, by impairing the immune response. This study describes extensive lymphocytes apoptosis in post mortem examination of sandbar sharks (Carcharhinus plumbeus) with sepsis. Methodology: Histological, immunohistochemical and TUNEL analysis were carried out from various organs of six sandbar sharks died with sepsis. Results: Histological findings consisted with multifocal haemorrhages in gills, spleens, livers and meninges with associated severe vascular microthrombosis. Other significant microscopic lesions observed were: severe lymphohistiocytic meningoencephalitis with neuronal necrosis, severe diffuse acute tubular renal necrosis, multifocal moderate interstitial lymphohistiocytic gastro-enteritis, moderate diffuse lymphohistiocityc uveitis, multifocal lymphocytic epicarditis with necrosis and inflammation of rectal glands and gonads. Various clusters of Gram-negative bacteria were often observed within these microscopic lesions. All the sharks exhibited significant lymphocytes apoptosis within the inflamed organs and also in the epigonal glands. The morphological changes of apoptotic lymphocytes consisted with nuclear fragmentation, membrane blebbing, condensed nuclear chromatin, cellular shrinkage and presence of apoptotic bodies. TUNEL analysis confirmed the fragmentation of DNA in the apoptotic lymphocytes. Immunohistochemical staining showed significant loss of CD3 T lymphocytes. Conclusions: Studies of sepsis in murine and fish experimental models and in human patients confirmed that the immune response follows a biphasic pattern, with an initial hyperinflammatory period and a secondary extremely dangerous period characterized by apoptosis of immune cells, lymphocytes in particular. Thus our findings suggest that extensive lymphocytes apoptosis appear in elasmobranchs suffering from severe sepsis also as a prominent morphological features, possibly responsible for a decreased response of the immune cells to inflammatory stimuli