The initial response of CD4+ IL-4-producing cells

Naive CD4+ T cells were reported to produce small amounts of IL-4 in vitro, which are implicated to be sufficient to initiate Th2 response in vivo. However, IL-4-producing naive CD4 + T cells are difficult to study in vivo because they are present in low numbers shortly after the first antigen exposure. Here, we used IL-4/green fluorescence protein (GFP) reporter mice (G4 mice) to track the initial response of CD4+ IL-4-producing cells. We first established a flow cytometry method to estimate the number of GFP+ cells. We demonstrated the effectiveness of this method by showing that the responding CD4+GFP+ cells exhibited an activated phenotype, possessed the capacity to express IL-5 and IL-13, but not IFN-g mRNA, and showed enhanced levels of GATA3 and c-maf mRNA expression. More importantly, we showed that the cell lines derived from FACS-sorted CD4+GFP+ cells were antigen specific. By using this newly established method, we showed that the majority of responding GFP+ cells were CD4+ T cells. Our study provides direct ex vivo evidence to show that a small percent of CD4+ T cells that have no previous experience of antigenic stimulation might produce IL-4 to initiate Th2 response