The level of IL-2 in silica group reduced to the same level with saline control group at late stage, however, in Treg-depleted group, the level of both IL-2 and IFN-γ was still significantly higher than that of silica group, which suggested depletion of CD4+CD25+Foxp3+ regulatory T cells caused failure to modulate the Th1/Th2 balance via suppression of Th1 response at the later stage of lung fibrosis. This evidence concerns the gene CD4 and pulmonary fibrosis.