CD28 and systemic lupus erythematosus: To determine whether Tfh cell-derived IL-21 from SLE patients induces IL-10 production during the differentiation of Breg cells, CD4+CXCR5+PD-1+ Tfh cells were first sorted from PBMCs of active SLE patients and healthy controls and stimulated with anti-CD3 and anti-CD28 for 48 hours.