Abstract
Although the skin constitutes the first line of defense against waterborne pathogens, there is a great lack of information regarding the skin associated lymphoid tissue (SALT) and whether immune components of the skin are homogeneously distributed through the surface of the fish is still unknown. In the current work, we have analyzed the transcription of several immune genes throughout different rainbow trout (Oncorhynchus mykiss) skin areas. We found that immunoglobulin and chemokine gene transcription levels were higher in a skin area close to the gills. Furthermore, this skin area as well as other anterior sections also transcribed significantly higher levels of many different immune genes related to T cell immunity such as T cell receptor α (TCRα), TCRγ, CD3, CD4, CD8, perforin, GATA3, Tbet, FoxP3, interferon γ (IFNγ), CD40L and Eomes in comparison to posterior skin sections. In agreement with these results, immunohistochemical analysis revealed that anterior skin areas had a higher concentration of CD3+ T cells and flow cytometry analysis confirmed that the percentage of CD8+ T lymphocytes was also higher in anterior skin sections. These results demonstrate for the first time that T cells are not homogeneously distributed throughout the teleost skin. Additionally, we studied the transcriptional regulation of these and additional T cell markers in response to a bath infection with viral hemorrhagic septicemia virus (VHSV). We found that VHSV regulated the transcription of several of these T cell markers in both the skin and the spleen; with some differences between anterior and posterior skin sections. Altogether, our results point to skin T cells as major players of teleost skin immunity in response to waterborne viral infections.
Highlights
The skin is the envelope that separates fish from the environment; it is a mechanical barrier, but a metabolically active organ, the largest organ in the fish
Transcription of genes related to T cell immunity throughout the trout skin When we studied the transcription of several immune genes related to T cell immunity throughout the different skin sections, we observed that, again, the levels of transcription were significantly higher in section 3 when compared to section 7
This was true for all genes studied, namely, T cell receptor α (TCRα), TCRγ, CD3, CD4, CD8, perforin, GATA3, Tbet, FoxP3, interferon γ (IFNγ), CD40L and Eomes, the latter being a key molecule associated with the function and differentiation of CD8+ T cells [28]
Summary
The skin is the envelope that separates fish from the environment; it is a mechanical barrier, but a metabolically active organ, the largest organ in the fish. Teleost skin is composed of three layers: the mucous layer, the epidermis and the dermis. T Cells in Teleost Skin Respond to Virus Infection. Because teleost skin is not keratinized, live skin cells are in close contact with the water medium and can immediately react to pathogen exposure, being the skin (together with the gills) the first line of defense against waterborne pathogens. There is a great lack of information regarding a phenotypic and functional characterization of the skin associated lymphoid tissue (SALT). Fish lack major lymphoid accumulations in mucosal tissues; the presence of different types of immune cells has been reported as part of the skin in many fish species [2].
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