Autoantibodies detected by immunofluorescence, ELISA, and complement-fixation techniques have provided discriminatory markers for many human diseases. However, these commonly applied assays may fail to detect antibodies against antigenic sites which are either inaccessible or not displayed in recognizable cellular structures. Moreover, molecular identities of recognized antigen(s) are not determined with such methods. We have used Western blot analysis of cellular proteins derived from human renal microvascular endothelial cells (HRMEC) to identify autoantibodies in patients with pathological endothelial injury. Exploring the possibility that endothelial injury may expose cryptic endothelial antigens to immune recognition, we detected antibodies binding a number of distinct HRMEC proteins. Among these, antibodies recognizing specific HRMEC proteins of 43 kDa were commonly detected in plasmas from patients with thrombotic thrombocytopenic purpura (TTP) (13 of 14) and hemolytic uremic syndrome (HUS) (4 of 5) but were absent in 9 of 10 healthy subjects and 11 patients with a range of diseases not associated with endothelial injury or insult. Antibodies binding 43-kDa HRMEC antigens were detected in individual patients with systemic lupus erythematosus, anti-glomerular basement membrane nephropathy, and heparin-associated thrombocytopenia, as well as in one of three patients with immune thrombocytopenic purpura. Similar antibodies were detected in one hypercholesterolemic subject. Antibodies from four TTP patients were affinity purified and shown by two-dimensional analysis to recognize 43-kDa proteins having identical pl's (5.9, 6.0, and 6.1). Subcellular fractionation localized these antigens to cytosolic and nuclear compartments, sites presumably protected from immune recognition in the absence of endothelial injury.(ABSTRACT TRUNCATED AT 250 WORDS)