CD4 and systemic lupus erythematosus: A decrease in CD25 expression in Treg and a diminished proliferation ratio between Treg and Tcon, which are indicators of Treg exhaustion due to IL-2 deficiency, could be observed earliest at the onset stage of disease, when IL-2 production by CD4+ T cells was also found to be significantly impaired (16), indicating that IL-2 deficiency is an acquired and potentially reversible phenomenon in SLE.