Naturally occurring CD4(+)CD25(+) regulatory T cells (nTregs) play a pivotal role in the maintenance of self-tolerance and immune homeostasis. To gain insight into the mechanism of action of nTregs in pathological and physiological immune responses, it is important to analyze bioactive molecules that modulate the maintenance and function of nTregs. From a library of bioactive lipids, we obtained lysophosphatidylcholine (LPC) as a molecule that enhanced the Foxp3 expression and suppressive function of human nTregs significantly in comparison with those of DMSO-treated nTregs (control). The expression levels of TGF-beta 1 mRNA and protein in LPC-treated nTregs were significantly higher than those in control nTregs. After treatment with anti-TGF-beta 1 antibody, the increases in Foxp3 expression and the suppressive properties of LPC-treated nTregs returned to the levels observed in control nTregs. These findings indicate that LPC enhances Foxp3 expression and the suppressive function of nTregs through TGF-beta 1 produced by nTregs themselves. Experimental knockdown of G2A and GPR4 showed that this LPC-induced TGF-beta 1 expression in nTregs was due to G2A signaling, and did not involve GPR4. Moreover, JNK was a major contributor to LPC-induced TGF-beta 1 expression in nTregs, although LPC activated MAPKs including ERK1/2, p38 MAPK, and JNK via G2A. LPC is a bioactive lysolipid highly abundant in the circulation. Therefore, LPC may contribute to the maintenance and function of human nTregs in vivo. (C) 2011 Elsevier Inc. All rights reserved.