Recently, it was reported that the expression of Runt-related transcription 
factor 3 (Runx3) is up-regulated in CD4(+) helper T cells during Th1 cell 
differentiation, and that Runx3 functions in a positive feed-forward manner with 
the T-box family transcription factor, T-bet, which is a master regulator of Th1 
cell differentiation. The relative expression levels of IFN-gamma and IL-4 are 
also regulated by the Th2-associated transcription factor, GATA3. Here, we 
demonstrate that Runx3 was induced in Th2 as well as Th1 cells and that Runx3 
interacted with GATA3 and attenuated GATA3 transcriptional activity. Ectopic 
expression of Runx3 in vitro in cultured cells or transgenic expression of Runx3 
in mice accelerated CD4(+) cells to a Th1-biased population or down-modulated 
Th2 responses, in part by neutralizing GATA3. Our results suggest that the 
balance of Runx3 and GATA3 is one factor that influences the manifestation of 
CD4(+) cells as the Th1 or Th2 phenotypes.