The Role of Red Blood Cells in the Immune Response of Fish

The involvement of nucleated red blood cells (RBCs) as cellular mediators of the immune response in fish is a novel research ‘hotspot’ that has been under investigation by fish immunologists during the past decade. Fish are the most primitive vertebrates, possessing many of the cell types and molecules found in higher vertebrates. However, the immune system of fish is peculiarly dissimilar to that of higher vertebrates and it is poorly understood in most fish species, particularly in newly aquacultured species. This has limited the development of strategies to combat infectious disease thus far. For example, fish lack bone marrow and lymph nodes, and leukocyte–antigen interactions take place in immunological sites such as the kidneys, spleen, gills and intestine. Furthermore, fish possess phagocytic B lymphocytes and they do not share a similar repertoire of immunogloblins to that possessed by mammals. Also, the innate immune components of fish play more active roles in the immune response and exhibit increased diversity than comparable components of mammalian species and are of primary importance in combating infections. This is essentially due to the inherent inefficacy of the acquired immune response of fish which is linked to its evolutionary status and poikilothermic nature. This consequently results in a limited (i) antibody repertoire, (ii) affinity maturation and (iii) immunological memory capacity in the fish immune system. In teleost fish, a broad range of key immune cell types have been described to be involved in innate defences, including monocytes/macrophages, non-specific cytotoxic cells (NCC), NK-like cells and granulocytes (e.g. neutrophils), which may complement the inefficiency of the acquired immune response in fish.

Concerning RBCs, a set of biological processes relevant to immunity have been described for a diverse group of organisms, which include: (i) pathogen recognition; (ii) clearance of pathogens by means of binding microbial immune complexes and (iii) production of cytokines or specific signalling molecules in response to pathogens. In fish, RBCs are nucleated and contain organelles in their cytoplasm, enabling these cells to be transcriptionally active and able to respond to invading pathogens. Despite the importance of this topic, the involvement of nucleated RBCs as cellular mediators in the immune response of fish remains poorly understood. Recent findings indicate that RBCs can act as phagocytes, release cytokine-like factors such as interferon-gamma or interferon alpha and develop specific gene responses to different pathogen associated molecular patterns (PAMPs) at the level of the gene transcription. Taken together, these results are indicative of the capacity of fish RBCs to mediate immune responses.

This Research Topic aims to gather new research findings and advances on the role of RBCs in mediating the immune response in fish, with the hope that this forum will foster further collaborations in this emerging area of research. We welcome the submission of Original Research, Review and Method articles to this Research Topic.