The spleen is a separate organ of the teleost, playing an essential role in immune reactions. The morphology of the spleen is different from the fish species. Little knowledge about the spleen structure and the blood splenic barrier (BSB) in Nile tilapia has been reported. To address this issue, we studied the histology of the spleen and the BSB in healthy Nile tilapia. The morphology of the spleen was observed, then H&E staining, modified Jame’s staining, and ultrastructural techniques were performed to portion the spleen into three subregions and analyze the location of components and fibers. Thereafter, vital staining of Nile tilapia with Trypan blue was conducted to elucidate the composition and function of BSB. Histologically, the spleen could be divided into three subregions (inner, middle, and outer). The venules, clumps of lymphocytes, and vessels were separately characterized features of the outer, middle, and inner layers. Post injection, Trypan blue was intercepted in the endotheliocytes of ellipsoids in the middle layer (i.p.) or was deposited to the reticular fibers surrounding the ellipsoids (i.v.). Additionally, the amount of Trypan blue was shown to be positively correlated to that of the Acid phosphatase expressed. In conclusion, the spleen could be portioned into three subregions, and the BSB lay in the middle layer, composed of the cuboidal-shaped endotheliocytes and the surrounding reticular fibers of the ellipsoid capillaries. The present study enriched the research of immune tissues and system in tilapia and provided reference for the study of spleen in other fish species.

The liver of fish is an essential metabolic organ that also serves an immune regulatory role. In this study, we constructed a model of largemouth bass (Micropterus salmoides) infected with Nocardia seriolae by injection to explore the immune and antioxidant functions of the liver. The results showed that N. seriolae infection caused severe pathological changes in the liver, including cell necrosis, granuloma formation, and leukocyte infiltration. The level of mRNA expression of immune-related genes in the liver was significantly increased 2 days post-infection. Moreover, the combined analysis of transcriptome and metabolome showed that N. seriolae infection markedly affected liver metabolism, including glutathione metabolism, arginine and proline metabolism, arachidonic acid metabolism, as well as starch and sucrose metabolism. Additionally, multiple key biomarkers were identified as involved in regulating responses to N. seriolae infection, including arginine, glutathione, gpx, GST, PLA2G, GAA, and PYG. To further elucidate the regulatory effects of arginine on the immune and antioxidant processes in the liver, primary hepatocytes were isolated and cultured. The results demonstrated that arginine supplementation significantly reduced the expression of LPS-induced apoptosis-related genes (bax, cas3, cas8, and cas9) by up to 50% while increasing the expression of antioxidant genes (gpx, GST) by up to 700% at 24 h. Through the analysis of metabolic changes and immune responses in the liver following N. seriolae infection, combined with in-vitro experiments, this study elucidated the anti-apoptotic and antioxidant effects of arginine, revealing the immune response mechanisms in fish liver and laying the groundwork for using nutritional strategies to improve fish health.

Acute phase response related molecules are a class of proteins whose concentration changes rapidly and significantly in response to abnormal conditions such as injury, infection, and inflammation. The levels of acute phase response related molecules can reflect abnormalities in the body, but related studies in teleost fish are still relatively limited. In this study, Cp, Hap, Hep, Hx, IL-1β, and IL-6 of largemouth bass (Micropterus salmoides) were identified, the spatiotemporal dynamics of these six acute phase response related molecules in blood, liver, spleen, and brain tissues of largemouth bass before and after infection with Streptococcus agalactiae were analyzed, and they were compared and analyzed for their potential use as indicators to detect S. agalactiae infection. The results showed that in healthy largemouth bass, the distribution of acute phase response related molecules (except IL-6) was highest in the liver. All the examined genes exhibited significant changes in their transcription levels across the tested tissues after infection. Furthermore, most genes exhibit higher expression levels in the spleen compared with other tissues. Hep and IL-1β genes in spleen tissue and Hx and IL-6 in brain tissue showed significant increase from 0 to 72 h post infection (hpi) compared with the control group. Among these, IL-1β is the only acute phase response related molecule whose expression levels were more than 150-fold, reaching 165-fold to 270-fold from 6 to 24 h post infection in the spleen. This study explores the temporal correlation between several acute phase response related molecules and streptococcal infection in largemouth bass, providing insights and references for subsequent research on early infection response proteins.