American paddlefish Polyodon spathula leukocytes demonstrated cytoplasmic staining patterns very similar to mammalian leukocytes when stained with acid phosphatase, a-naphthyl butyrate esterase and b-glucuronidase. American paddlefish monocytes, lymphocytes and granulocytes stained positive for acid phosphatase. Monocytes stained positive for a-naphthyl butyrate esterase. Lymphocytes that stained positive for a-naphthyl butyrate esterase were designated type A. Lymphocytes that stained positive with antibodies to the L chain of white sturgeon Acipenser transmontanus immunoglobulin (Ig) were designated type B. Type A and type B lymphocytes stained positive for b-glucuronidase. All leukocytes observed were negative for Sudan Black B. Monocytes, lymphocytes and granulocytes were present in the renal haematopoietic tissue, spleen, thymus, pericardial myeloid tissue, lamina propria of the spiral valve, and in meningeal myeloid tissue located dorsal to the brain, at the base of the brain and around the notochord. Peyer's patches were present in the gut. Morphological characteristics of leukocytes stained with Wright's and haematoxylin and eosin and appeared very similar to those of other fish species. # 2005 The Fisheries Society of the British Isles

The temporal and spatial distribution of American paddlefish Polyodon spathula immune cell populations was determined using enzyme cytochemistry and immunohistochemistry. Monocytes and macrophages were present in the renal haematopoietic tissue, spleen, meningeal myeloid tissue, cardiac myeloid tissue and lamina propria of the spiral valve at 7 days post‐hatch (dph). Immature lymphocytes were present in the renal haematopoietic tissue, spleen, meningeal myeloid tissue, cardiac myeloid tissue, thymus and lamina propria of the spiral valve at 7 dph. Type A lymphocytes (T‐cell like) were demonstrated in the thymus by 21 dph. Type B immunoglobulin positive lymphocytes (B‐cell like) were present in the renal haematopoietic tissue, cardiac myeloid tissue and lamina propria of the spiral valve by 7 dph, the thymus at 21 dph, the spleen by 56 dph, and were not observed in the meningeal myeloid tissue of paddlefish aged 7–28 dph. Granulocytes were present in the renal haematopoietic tissue, thymus, spleen, meningeal myeloid tissue, cardiac myeloid tissue and lamina propria of the spiral valve by 7 dph. The spleens in 7–28 dph fish were predominately red pulp. Differentiation of leukocytic and erythrocytic compartments (white and red pulp, respectively) was not apparent in the spleen until 56 dph. Remarkable thymic cortical and medullary differentiation was not yet present at 28 dph, and the thymus was not sampled at 56, 84 or 112 dph. The cardiac myeloid tissue was not developed until 56 dph. Peyer’s patches were present in the lamina propria by 56 dph. Paddlefish lympho‐myeloid structures are therefore slow to develop, and vaccination procedures should be performed at 2–4 months post‐hatch.

Beta glucan exposure induced trained immunity in channel catfish that conferred long-term protection against Edwardsiella ictaluri and Edwardsiella piscicida infections one month post exposure. Flow cytometric analyses demonstrated that isolated macrophages and neutrophils phagocytosed higher amounts of E. ictaluri and E. piscicida. Beta glucan induced changes in the distribution of histone modifications in the monomethylation and trimethylation of H3K4 and modifications in the acetylation and trimethylation of H3K27. KEGG pathway analyses revealed that these modifications affected expressions of genes controlling phagocytosis, phagosome functions and enhanced immune cell signaling. These analyses correlate the histone modifications with gene functions and to the observed enhanced phagocytosis and to the increased survival following bacterial challenge in channel catfish. These data suggest the chromatin reconfiguration that directs trained immunity as demonstrated in mammals also occurs in channel catfish. Understanding the mechanisms underlying trained immunity can help us design prophylactic and non-antibiotic based therapies and develop broad-based vaccines to limit bacterial disease outbreaks in catfish production.