We found that not only CD4+ effector T cell subsets, including Th1, Th2, Th17, and Tfh cells, but also naive and memory CD4+ T cells, have significantly increased levels of Fe2+ in patients with SLE compared with healthy donors (Supplemental Figure 1, A and B; supplemental material available online with this article; https://doi.org/10.1172/JCI152345DS1). Here, CD4 is linked to systemic lupus erythematosus.