The aim of the present study was to examine the effect of the overexpression of tyrosine hydroxylase(TH), a rate-limiting enzyme for the synthesis of catecholamines(CAs), in lymphocytes on the differentiation and function of Thelper(Th) cells. A recombinant TH overexpression plasmid (pEGFP-N1-TH) was constructed and transfected into mesenteric lymphocytes using nucleofection technology. These cells were stimulated with concanavalinA(ConA) for 48h and then examined for THexpression and CAcontent, as well as for the percentage of Th1 and Th2cells, cytokine concentrations and for the levels of signaling molecules. The lymphocytes overexpressing TH also expressed higher mRNA and protein levels of TH, and synthesized more CAs, including norepinephrine(NE), epinephrine(E) and dopamine(DA) than the mock-transfected control cells. THgene overexpression in the lymphocytes reduced the percentage of interferon-γ(IFN-γ)-producing CD4+cells and the ratio of CD4+IFN-γ+/CD4+IL-4+cells, as well as the percentages of CD4+CD26+ and CD4+CD30+cells and the ratio of CD4+CD26+/CD4+CD30+cells. THoverexpression also reduced the secretion of IFN-γ and tumor necrosis factor(TNF) from lymphocytes. Moreover, NE inhibited the ConA-induced lymphocyte proliferation and decreased both cyclic adenosine monophosphate(cAMP) levels and p38mitogen-activated protein kinase(MAPK) expression in the lymphocytes. Our findings thus indicate that THgene overexpression promotes the polarization and differentiation of CD4+cells towards Th2cells, and this effect is mediated by the cAMP and p38MAPK signaling pathways.