Abstract. Human, but not murine, renal peritubular and glomerular capillaries constitutively express class II major histocompatibility (MHC) proteins at high levels in normal human kidney. Expression of class II proteins on renal microvascular endothelial cells (RMEC) makes it available to circulating lymphocytes and imparts a surveillance capacity to RMEC for controlling inflammatory responses. In this report, the coexpression of HLA-DR and the endothelial marker CD31 are used to identify RMEC as a distinct population of cells within a standard renal biopsy using flow cytometry. A three-laser, multicolor flow cytometry analysis using Alexa dyes, developed for characterizing the expression of cell surface antigens, identifies RMEC as a population separate from HLA-DRexpressing leukocytes. HLA-DR RMEC co-express HLA-DP and HLA-DQ. RMEC also express the T cell costimulatory factor CD58 but not CD80, CD86, or CD40. On the basis of high HLA-DR expression, RMEC are isolated for culture using fluorescence-activated cell sorting and magnetic beads. Cultured RMEC require normal basal physiologic concentrations of gamma interferon (␥IFN) to maintain HLA protein expression. This expression is regulated by CIITA, the MHC class II-specific transcription factor. Four tissue-specific promoters have been described for CIITA. In freshly isolated RMEC, RT-PCR and hybridization using specific oligonucleotide probes to CIITA promoter sequences identify only the statinsensitive ␥IFN-induced promoter IV of CIITA. Therefore, the constitutive expression of HLA-DR on RMEC in normal human kidney is located in a position for immune surveillance, depends on basal physiologic concentrations of ␥IFN, and may be amenable to regulation with statins.MHC proteins are of two classes distinguished on the basis of structure and function. Class I molecules, composed of a polymorphic subunit complexed with ␤-2-microglobulin, are found on all nucleated cells and present antigenic peptides to CD8ϩ T lymphocytes; class II molecules, composed of polymorphic ␣ and ␤ chains, are constitutively expressed on a limited number of cell types (dendritic cells, macrophages, B lymphocytes) and present antigenic peptides to CD4 ϩ T lymphocytes. We recently described an unusual expression of MHC class II proteins in normal human kidneys that is not found in murine kidneys (1). The human MHC class II protein HLA-DR is abundantly expressed on peritubular and glomerular capillary endothelial cells but not on endothelial cells of larger blood vessels of normal kidney. Antibodies to HLA-DR and CD31, a protein highly expressed on endothelial cells, co-localize on peritubular and glomerular cells within sections of kidney tissue, indicating capillary endothelial cell location of HLA-DR. HLA-DR has also been identified on rare scattered circulating leukocytes found within the kidney, but over 98% of the DR identified by immunofluorescence microscopy in kidney cortex is located on capillary endothelial cells (1). We refer to these cells co-expressing HLA-DR and CD31 as rena...