Among various cytokines, interleukin (IL)-12 family cytokines have very unique characteristics in that they are composed of two distinct subunits and these subunits are shared with each other. IL-23, one of the IL-12 family cytokines, consists of p19 and p40 subunits, is mainly produced by antigen-presenting cells, and plays a critical role in the expansion and maintenance of pathogenic helper CD4+ T (Th)17 cells. Since we initially found that p19 is secreted in the culture supernatant of activated CD4+ T cells, we have further investigated the role of p19. p19 was revealed to associate with CD5 antigen-like (CD5L), which is a repressor of Th17 pathogenicity and is highly expressed in non-pathogenic Th17 cells, to form a composite p19/CD5L. This p19/CD5L was shown to activate STAT5 and enhance the differentiation into granulocyte macrophage colony-stimulating factor (GM-CSF)-producing CD4+ T cells. Both CD4+ T cell-specific conditional p19-deficient mice and complete CD5L-deficient mice showed significantly alleviated experimental autoimmune encephalomyelitis (EAE) with reduced frequency of GM-CSF+CD4+ T cells. During the course of EAE, the serum level of p19/CD5L, but not CD5L, correlated highly with the clinical symptoms. Thus, the composite p19/CD5L is a possible novel heterodimeric cytokine that contributes to EAE development with GM-CSF up-regulation.