IL-18 Stimulates the Proliferation and IFN-γ Release of CD4+ T Cells in the Chicken: Conservation of a Th1-Like System in a Nonmammalian Species

The phylogeny of Th1 and Th2 subsets has not been characterized mainly due to the limited information regarding cytokines in nonmammalian vertebrates. In this study, we characterize a Th1-like regulatory system focusing on the IL-18-regulated IFN-γ secretion. Stimulation of splenocytes with chicken IL-18 induced high levels of IFN-γ secretion. Depletion of either macrophages or CD4+ T cells from the splenocyte cultures caused unresponsiveness to IL-18. In contrast, PBL were unresponsive to IL-18 in the presence or absence of macrophages, but IFN-γ secretion was stimulated by suboptimal anti-TCR cross-linking combined with IL-18. Splenocytes from five different chicken lines responded equally well to the IL-18 treatment. LSL chicken splenocytes, however, responded only to IL-18 when stimulated either with optimal TCR cross-linking alone or suboptimal TCR cross-linking combined with IL-18. IL-18 not only induced IFN-γ secretion, but also stimulated splenocyte proliferation. This IL-18-induced proliferation was compared with the effects observed with IL-2. Both cytokines activated the splenocytes as demonstrated by increased size and MHC class II Ag up-regulation in the case of IL-18. Phenotypic analyses following 6 days of culture revealed that IL-2 mainly affected the proliferation of CD8+ cells, whereas IL-18 had an opposite effect and stimulated the proliferation of CD4+ cells. Taken together, these results demonstrate the conservation of Th1-like proinflammatory responses in the chicken; they characterize IL-18 as a major growth factor of CD4+ T cells and identify two distinct mechanisms of IL-18-induced IFN-γ secretion.