CD28 and systemic lupus erythematosus: On the contrary, anti-CD3/CD28-stimulated T-cell proliferation was promoted by antagonistic anti-CD200R1 in a concentration-dependent manner in SLE patients but not HCs, suggesting that anti-CD200R1 may block the endogenous signal provided by increased expression of CD200 and, thereby, permit increased CD4+ T-cell proliferation.