SummaryThe glycosylphosphatidylinositol (GPI)-anchored membrane protein urokinase plasminogen activator-receptor (uPA-R; CD87) is one of the key molecules involved in migration of leukocytes and tumor cells, uPA bound to uPA-R provides the cell proteolytic potential used for degradation of extracellular matrix, uPA-R is also involved in induction of ceU adhesion and chemotaxis. Here, we provide a molecular explanation for these uPA-R-related cdlular events. By size ffactionation of monocyte lysate and af~nity isolation on its natural ligand uPA, we demonstrate uPA-R as a component of a receptor complex of relatively large size. Reprecipitation and immunoblotting techniques allowed us to detect the protein tyrosine kinases (PTKs) p60fY ", p53/56 lyn, p58/64 hck, and p59f~ as components of this "uPA-R complex". Activation of monocytes even with enzymatically inactivated uPA resulted in induction of tyrosine phosphorylation, suggesting modulation of uPA-R-associated PTKs upon ligand binding. In spite of their presence in large complexes, we did not find the GPI-linked proteins CD14, CD58, and CD59 in the uPA-R complex, which indicates the presence of different receptor domains containing GPI-linked proteins in monocytes. However, we identified the leukocyte integrins LFA-1 and CR3 as components of the uPA-R complex as indicated by coisolation of these molecules, as well as by cocapping and comodulation of uPA-R and leukocyte integrins on the monocyte surface. The assemblage of uPA-R, PTKs and membrane spanning 32-integrins in one receptor complex indicates functional cooperation. In regard to the involvement of these molecules in pericellular proteolysis, signal transduction, as weU as adhesion and chemotactic movement, we suggest uPA-R complex as a potential cellular device for call migration.

Transforming growth factor-beta1 (TGF-beta1) is a critical cytokine for cell proliferation and differentiation. It is secreted by many cells in a latent pro-form (LTGF-beta1) from which biologically active TGF-beta1 is released by an in vivo mechanism that is not known. Here we show that the mannose-6-phosphate/insulin-like growth factor II-receptor (M6P/IGFII-R), which binds LTGF-beta1, complexes with urokinase (plasminogen activator)-receptor (uPA-R) on the surface of human monocytes and directly binds plasminogen (Plg). Plasmin generated from Plg in the complex mediates release of TGF-beta1 when M6P/IGFII-R is associated with uPA-R. Thus, this interaction of M6P/IGFII-R and uPA-R suggests a potential mechanism for the generation of TGF-beta1 by cells.

SummaryCD31 is a 130-kD glycoprotein of the immunoglobulin (Ig) superfamily expressed on the surrice of endothelial cells, platelets, and several leukocyte subsets. Previous reports indicated that CD31 can mediate intercellular adhesion via both homophilic and heterophilic interaction mechanisms. Using a soluble recombinant CD31-Ig fusion protein (CD31 receptor globulin [Rg]), we demonstrate here that human CD31-T lymphocytes and CD4+CD31-T cell clones express a heterophilic CD31 ligand that is upregulated 18 h after activation. Interaction of CD31Kg with CD31-T helper cell (Th) clones was divalent cation independent but could be blocked by heparin, thus indicating that the CD31 counterreceptor on T cells can be distinguished from the ligands identified on other cell types. Moreover, a single chain protein of 120 kD was precipitated by CD31Rg from the lysates of CD31-Th clones. CD31Rg completely downregulated the proliferative response and cytokine production (interleukin-4, interferon-% and tumor necrosis factor-o 0 of CD31-Th clones when the cells were maximally stimulated via immobilized CD3 monoclonal antibody. These results suggest that interaction of CD31 with a heterophilic counterreceptor on T lymphocytes can interfere with a positive regulatory pathway of T cell activation, or directly signal T cells to downregulate immune function.

These data suggest that adhesion molecule expression in donor biopsies is not a predictor for early allograft rejection, but can be used as a marker for the development of postischemic acute renal allograft failure.

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