We have developed a unique model for studying the role of the kidney in the systemic acute phase response and have studied the role of the kidney in the clearance of serum amyloid A (SAA) from the circulation and the mechanisms by which the liver "clears" the acute phase proteins from the circulation. 1. We have completed several studies on the effect of SAA on cultured mesangial cells and have characterized these cells with immunofluorescence. The glomerular mesangium has been proposed to be the source of SAA in the circulation and a possible target for the action of this acute phase protein. Studies in these glomerular cells demonstrated that human SAA can increase the secretion of human PDGF and can stimulate the secretion of PAI-1. This suggested that SAA can influence mesangial cell growth and extracellular matrix production through a variety of biochemical pathways. We have also completed studies showing that SAA may inhibit plasminogen activation in glomerular cells and that SAA can bind to glomerular mesangial cells. 2. The role of SAA in inflammation-induced renal pathology. One of the most important features of the acute phase response is the increase in plasma levels of SAA, an acute phase reactant produced by the liver. During bacterial infection or chronic tissue injury, SAA serum levels can rise several hundred fold. We have studied the effect of the endogenous acute phase protein on immune complex deposition and tissue injury in the kidney and the liver in the well characterized murine model of human immune complex glomerulonephritis (ICGN) induced with cationic bovine serum albumin (CBSA). Treatment with SAA induced marked deposition of immune complexes in the kidney and increased mesangial cell injury, glomerular capillary damage and interstitial inflammation. This increase in inflammation and immune complex deposition was associated with increases in serum cytokine levels and circulating IL-6. These data show that SAA can modulate the expression of inflammatory and repair processes in the kidney and implicates it as a potential contributing factor in mediating the tissue pathology associated with inflammatory disorders. 3. The role of SAA in human renal diseases. Two recent studies have characterized a novel isoform of acute phase protein produced by the human kidney in response to inflammatory stimuli and not by the liver. This "renal" SAA can be induced by IL-1, TNF and bacterial lipopolysaccharide (LPS). This "renal" SAA is localized in the cytoplasm of tubular epithelial cells in the proximal and distal nephron and in the interstitium of the kidney. We have also studied the role of this isoform of acute phase protein in human renal disease. Serum levels of renal SAA are markedly elevated in patients with IgAN and Cogan's syndrome, but not in other types of glomerulonephritis (GN). In normal human kidney, the renal SAA isoforms were primarily expressed in tubular epithelial cells. In the absence of glomerular injury, we did not detect glomerular staining for renal SAA. These data demonstrate a correlation between renal SAA production and inflammatory renal diseases. 4. Proposed mechanism of SAA production in the kidney and release into the circulation. We hypothesize that the source of SAA in the kidney in normal individuals is the proximal tubular epithelium. Upon stimulation with inflammatory cytokines, these cells undergo transdifferentiation to a mesenchymal phenotype and migrate from the site of injury to the interstitium of the kidney. We have shown that SAA mRNA can be detected in normal human kidney and that increased SAA production in the kidney is associated with a variety of inflammatory renal diseases. We propose that during inflammation, SAA is synthesized and secreted in increased amounts by tubular epithelial cells and that SAA is then released into the circulation and is a major component of the acute phase response. Future studies in patients with inflammation-induced acute kidney injury will address the question of the role of SAA in this disease process. 5. Study of SAA post-translational modifications by the renal cell and the role of proteolytic enzymes on SAA clearance. We have characterized a major renal isoform of acute phase protein that is not recognized by an anti-SAA antibody directed against the amino terminus of the protein. The biochemical mechanisms of post-translational processing of acute phase proteins are poorly understood and may depend on the cell type. We have studied the intracellular processing and release of this isoform from renal tubular cells in response to inflammatory cytokines and compared this with the processing and release of SAA in response to cytokine treatment in monocytes. We have demonstrated that the renal isoform of SAA is completely resistant to proteolysis by a variety of proteinases. Furthermore, IL-1 and TNF treatment causes an increase in SAA production but not in the release of the major renal isoform of SAA. These data suggest that there are biochemical mechanisms in renal cells that selectively process the various isoforms of acute phase proteins by a distinct pathway. We have proposed that different isoforms of acute phase proteins are produced under different conditions and that each may have distinct biological activities and potential functions in the body. We are currently exploring the hypothesis that SAA in the circulation may be cleared by a unique mechanism mediated by the kidney. We have recently shown that the kidney removes SAA from the circulation in response to albumin-induced inflammation by a unique mechanism that involves degradation of SAA in the glomerulus and processing of SAA to a very small molecular weight protein in the renal tubular cells. These data may provide insight into the clearance of this potent cytokine that has been shown to have pro-inflammatory and pro-fibrotic effects on the kidney. 6. Effect of SAA on mesangial cell growth and extracellular matrix production. Studies in our laboratory have shown that SAA can stimulate the secretion of angiotensin II, tissue plasminogen activator (tPA) and other mediators by cultured mesangial cells. These observations suggest that SAA may be important in mediating the events that occur in glomerular cells during the repair process. A rat model with high levels of SAA (SAA high) in the blood of rats has been developed and studied. We have shown that the SAA-high rats have significantly higher glomerular matrix deposition