The first functional characterization of ancient interleukin-15-like (IL-15L) reveals shared and distinct functions of the IL-2, -15 and -15L family cytokines

The ancient cytokine interleukin 15-like (IL-15L) was lost in humans and mice but not throughout mammals. This is the first study to describe IL-15L functions, namely in the fish rainbow trout. Fish have only one -chain receptor gene IL-15R, whereas in mammalian evolution this gene duplicated and evolved into IL-15R plus IL-2R. Trout IL-2, IL-15 and IL-15L all could bind IL-15R and were able to induce phosphorylation of transcription factor STAT5. Reminiscent of the mammalian situation, trout IL-15 was more dependent on \"in trans\" presentation by IL-15R than IL-2. However, whereas trout IL-15 could also function as a free cytokine as known for mammalian IL-15, trout IL-15L function showed a total dependency on in trans presentation by IL-15R. Trout lymphocytes from the mucosal tissues gill and intestine were sensitive to IL-15, but refractory to IL-2 and IL-15L, which is reminiscent of sensitivities to IL-15 in mammals. Distinguishing engagement of the IL-2R/IL-15R receptor chain may explain why IL-2 and IL-15 were selected in evolution as major growth factors for regulatory T cells and lymphocytes in mucosal tissues, respectively. Trout IL-15L efficiently induced expression of IL-4 and IL-13 homologues in CD4-CD8-IgM- splenocytes, and we speculate that the responsive cells within that population were type 2 innate lymphoid cells (ILC2). In contrast, trout IL-15 efficiently induced expression of interferon {gamma} and perforin in CD4-CD8-IgM- splenocytes, and we speculate that in this case the responsive cells were natural killer (NK) cells. In fish, in apparent absence of IL-25, IL-33 and TSLP, primitive IL-15L may have an important role early in the type 2 immunity cytokine cascade. Among trout thymocytes, only CD4-CD8- thymocytes were sensitive to IL-15L, and different than in mammals the CD4+CD8+ thymocytes were quite sensitive to IL-2. In addition, the present study provides (i) the first molecular evidence for inter-species cytokine with receptor chain interaction across fish-mammal borders, and (ii) suggestive evidence for a tendency of IL-2/15/15L cytokines to form homodimers as an ancient family trait. This is the first comprehensive study on IL-2/15/15L functions in fish and it provides important insights into the evolution of this cytokine family.