Abstract
In jawed vertebrates, adaptive immune responses are enabled by T cells. Two lineages were characterized based on their T cell receptor (TcR) heterodimers, namely αβ or γδ peptide chains, which display an Ig domain-type sequence that is somatically rearranged. γδ T cells have been less extensively characterized than αβ and teleost fish, in particular, suffer from a severe scarcity of data. In this paper, we worked on the well-known model, the European sea bass Dicentrarchus labrax, to broaden the understanding of teleost γδ-T cells. The T cell receptor chain (TR) γ transcript was expressed at a later developmental stage than TRβ, suggesting a layered appearance of fish immune cells, and the thymus displayed statistically-significant higher mRNA levels than any other organ or lymphoid tissue investigated. The polyclonal antibody developed against the TRγ allowed the localization of TRγ-expressing cells in lymphoid organs along the ontogeny. Cell positivity was investigated through flow cytometry and the highest percentage was found in peripheral blood leukocytes, followed by thymus, gut, gills, spleen and head kidney. Numerous TRγ-expressing cells were localized in the gut mucosa, and the immunogold labelling revealed ultrastructural features that are typical of T cells. At last, microalgae-based diet formulations significantly modulated the abundance of TRγ+ cells in the posterior intestine, hinting at a putative involvement in nutritional immunity. From a comparative immunological perspective, our results contribute to the comprehension of the diversity and functionalities of γδ T cells during the development of a commercially relevant marine teleost model.
Highlights
The effect of diet on intestinal T cells expressing the TRγ was evaluated in sea bass fed with fish meal and fish oil increasingly replaced by the marine microalgae Nannochloropsis sp
Four T cell receptor chain (TR) (α, β, γ and δ) were identified [33], and the genes encoding for these subunits were demonstrated to be highly conserved both in sequence and organization along gnathostome evolution [31,32]
Based on the composition of their TR heterodimers, T cells can be divided into two distinct lineages: αβ T cells and γδ T cells [37,38]
Summary
(iii) have a CD4− CD8+ phenotype and show phagocytic activity against soluble and particulate antigens; (iv) recognize unconventional antigens including phosphorylated molecules, metabolic intermediates and lipids; (v) may act as pattern recognition receptors and participate in the humoral adaptive response by initiating CD4+ T cell proliferation, inducing B cell activation and IgM production; and (vi) have a role in the production of the mucosal immunity-related fish-specific IgT/Igz isotype [12,14,19,20,21] Based on these findings, mammalian γδ T cells were proposed as a primordial lymphocyte population predating αβ T cells and B cells and located at the interface between innate and acquired immunities [22]. D. labrax is an important marine model species for immunobiological studies due to the availability of tools that have allowed the elucidation of specific physiological features of T lymphocytes from an evolutionary perspective This knowledge, in turn, has improved sea bass farming with positive commercial consequences. We expect our results to increase the comprehension of the diversity and functionalities of γδ T cells in teleosts, in view of a possible interdisciplinary exploitation as per cutting-edge research [30]
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