CD4 and systemic lupus erythematosus: This could offer an initial explanation for the overexpression of PD-1 on the surfaces of CD4+ T cells, like cTfh and Tph cells, especially in autoimmune diseases such as SLE, where there is constant and repetitive stimulation of T cells by self-antigens and proinflammatory cytokines like IL-6, IL-21, and IFNα [2].