CD4 and systemic sclerosis: Recently, Interleukin-9 (IL-9) and IL-9-producing CD4+ T cells (T-helper 9 (Th9) cells) have been found in SSc, suggesting that the IL-9 axis might be involved in SSc pathogenesis.12, 13 Other studies have shown that IL-9 induces the differentiation of Th17 cells.14, 15 Nevertheless, none of the studies have specifically addressed whether Th17 and Th9 cells interact with each other to promote the functional activation of DVSMCs in the pathogenesis of SSc.