(A) Representative microphotographs of healthy EPCs cultured for 14 days in 20% patient or healthy control serum. Cells were labeled with Dil-Ac-LDL (red) and UEA-1-FITC (green). MATURE ECS coexpress both markers (yellow). Representative images from serum of n=5 control, n=5 JIA, and n=7 cSLE across n=4 PMBC donors (x10 magnification). (B) Serum from cSLE but not JIA significantly reduces survival of healthy EPCs at day 15. Columns show mean ± SE number of cells in random fields (1-way ANOVA with Dunnett’s multiple comparisons test, **p < 0.01). (C) Blockade of IFNα signaling restores EC differentiation in control cells exposed to SLE, but not control or JIA, serum. Columns show mean ±SE number of cells with n=5 control, n=4 JIA, n=7 cSLE (paired t test, *p < 0.05).