Abstract
Phagocytes are the principal component of the innate immune system, playing a key role in the clearance of foreign particles that include potential pathogens. In vertebrates, both neutrophils and mononuclear cells like monocytes, macrophages and dendritic cells are all professional phagocytes. In teleosts, B-lymphocytes also have potent phagocytic ability. We have isolated a population of small (<5 µm), mononuclear blood cells from Atlantic salmon (Salmo salar L.) not previously characterized. In order to identify them, we have performed morphological, gene expression, flow cytometry, cytochemical, ultrastructural and functional analyses. Interestingly, they highly express the gene encoding CD83, the most characteristic cell surface marker for dendritic cells in mammals, and MHC class II limited to professional antigen presenting cells. They did not express genes nor did they have cell markers for B-cells, T-cells, monocytes/macrophages or neutrophils as shown by qRT-PCR, flow cytometry and immunoblotting. A remarkable feature of these cells is their potent phagocytic capacity. Their oxygen-independent killing mechanism, as shown by intense acid phosphatase staining, is supported by lack of respiratory burst and myeloperoxidase activity and the acid phosphatase's sensitivity to tartrate. They show a high level of morphological plasticity, as, upon stimulation with mitogens, they change morphology and obtain branching protrusions similarly to dendritic cells. We suggest, based on our findings, that the small, round cells described here are progenitor cells with potential to differentiate into dendritic like cells, although we can not exclude the possibility that they represent a novel cell type.
Highlights
In fish, the phagocytic defense mechanism plays a pivotal role in non-specific immunity
Double immunostaining with anti-TO and monoclonal antibodies (MAbs) C4B6 revealed small, round C4B62 cells which are abundant among peripheral blood (PBL)
Double immunostaining of cytospin preparations of PBL with the polyclonal anti-TO antibody, staining all salmon leukocytes, and the MAb C4B6, reacting with different cell types like polymorphonuclear (PMN) leukocytes, B-cells and monocytes/ macrophages, revealed small, round cells that were C4B62 (Fig. 1)
Summary
The phagocytic defense mechanism plays a pivotal role in non-specific immunity. Fish Bcells are professional phagocytes with microbicidal abilities [1,2]. Phagocytosis by other professional phagocytic cells in fish like dendritic cells (DCs) have only recently been described [3,4]. There is currently limited knowledge about the cell specific markers, hematopoiesis and activation/maturation stages of cells, and identification of cell subsets in fish is often based on morphology. Gene expression analyses are a valuable tool for species for which the genome is sequenced, there is currently little knowledge of the specific gene expression pattern in the different cell subsets in fish
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