Intercellular adhesion molecule-1 (ICAM-1) is a cell-surface protein regulating interactions among immune cells. To determine whether altered expression of ICAM-1 occurs in autoimmune lupus nephritis, we studied ICAM-1 expression in kidneys of normal and autoimmune MRL-lpr and (NZBX NZW)F1 (NZB/W) mice. By immunoperoxidase staining, ICAM-1 is constitutively expressed at low levels in proximal tubules (PT), endothelium and interstitial cells in normal C3H/FeJ mice. In nephritic MRL-lpr and NZB/W kidneys, staining for ICAM-1 is increased in the PT, particularly in the brush border, and is prominent in the glomerular mesangium and the endothelium of large vessels. By Western blot analysis, ICAM-1 is not detected in the urine of normal BALB/c and C3H/FeJ or autoimmune MRL-lpr. By Northern blot analysis, nephritic MRL-lpr and NZB/W have a two- to fivefold increase in steady state levels of ICAM-1 transcripts in the kidney as compared with normal or prenephritic mice. This is paralleled by an increase in MHC class II transcripts. In cultured PT cells, ICAM-1 is expressed at basal levels in PT and is increased by the cytokines interferon-gamma, IL-1 alpha, and TNF-alpha. Thus cytokine-mediated upregulation of ICAM-1 in lupus nephritis may promote interaction of immune cells with renal tissue. The predominant apical expression of ICAM-1 opposite to the basolateral Ia expression suggests a novel role for this adhesion molecule in PT.