The role of the macrophage in progressive glomerulosclerosis.

Glomerulosclerosis is the final outcome of a number of causes of glomerular injury during which the structures of the glomerulus are obliterated by extracellular matrix. Accumulating evidence has focused on macrophages as playing a pivotal role in the pathogenesis of this process. This thesis was undertaken to delineate the role played by macrophages at both the cellular and molecular levels on the initiation of the sclerotic process, to characterise the factor(s), and to elucidate the potential interactions involved. Macrophages were shown to promote potent pro-sclerotic effects in mesangial cells, by inducing them to express the protein and message for the matrix protein fibronectin. In addition, the genes for the matrix proteins laminin and collagen IV, the tissue inhibitor of matrix metalloproteinases TIMP-1, the pro-fibrotic growth factors TGFbeta and PDGF, and the specific macrophage chemoattractant MCP-1 were also upregulated. TGFbeta1, PDGF, TNFalpha and IL-1beta individually, could not reproduce the effects of macrophage conditioned medium (MPCM) on fibronectin production. However, in combination, as may be found in vivo, they were able to synergistically upregulate the production of this matrix protein to levels comparable with those seen with MPCM. Although these cytokines could not be detected in MPCM per se, constitutive secretion of TGF? and PDGF was upregulated in mesangial cells in response to MPCM. However, experiments with neutralising antibodies and a ligand: receptor binding antagonist demonstrated that these growth factors only play a minor role in macrophage stimulated rat mesangial cell fibronectin production. Characterisation studies suggested that the macrophage-derived factor(s) responsible for the accumulation of fibronectin was a protein of the order of 12.5 kD, heat stable and susceptible to some degree of proteolytic digestion. The data presented in this thesis provides the most compelling evidence to date for a direct role of macrophages in the pathogenesis of glomerulosclerosis.