CD4 and synovial sarcoma: Since the majority of the T cells in the salivary glands of primary SS patients are CD4+ T cells, and the IL-21 produced by activated CD4+ T cells is known to up-regulate their expression of CXCR5 [16], IL-21 possibly promotes the co-localization of T cells and B cells in the functional ectopic germinal centers in the salivary glands of SS patients, which in turn elevates autoantibody production.