PDCD1 and systemic lupus erythematosus: Considering their ability to produce IL-21 and to aid in B cell differentiation without expression of the classical Tfh marker CXCR5, the ICOS+PD-1+ effector CD4+ T cells enriched in the inflamed lung of aged mice resemble the recently reported human CXCL13+CXCR5– age-associated helper T (ThA) cells that were found to accumulate during aging and in patients with systemic lupus erythematosus (39).