Several inflammatory diseases, including asthma, arthritis and psoriasis are associated with the production of leukotrienes by neutrophils, mast cells and macrophages. The initial enzymatic step in the formation of leukotrienes is the oxidation of arachidonic acid by 5-lipoxygenase (5-LO) to leukotriene A4. Osteosarcoma cells transfected with 5-LO express active enzyme in broken cell preparations, but no leukotriene metabolites are produced by these cells when stimulated with the calcium ionophore A23187, indicating that an additional component is necessary for cellular 5-LO activity. A new class of indole leukotriene inhibitor has been described that inhibits the formation of cellular leukotrienes but has no direct inhibitory effect on soluble 5-LO activity. We have now used these potent agents to identify and isolate a novel membrane protein of relative molecular mass 18,000 which is necessary for cellular leukotriene synthesis.

It was originally thought that the critical role of the CD40 ligand (CD40L) in normal and inflammatory immune responses was mainly mediated through its interaction with the classic receptor, CD40. However, data from CD40L ؊/؊ and CD40؊/؊ mice suggest that the CD40L-induced inflammatory immune response involves at least one other receptor. This hypothesis is supported by the fact that CD40L stabilizes arterial thrombi through an ␣IIb␤3-dependent mechanism. Here we provide evidence that soluble CD40L (sCD40L) binds to cells of the undifferentiated human monocytic U937 cell line in a CD40-and ␣IIb␤3-independent manner. Binding of sCD40L to U937 cells was inhibited by anti-CD40L monoclonal antibody 5C8, anti-␣5␤1 monoclonal antibody P1D6, and soluble ␣5␤1. The direct binding of sCD40L to purified ␣5␤1 was confirmed in a solid phase binding assay. Binding of sCD40L to ␣5␤1 was modulated by the form of ␣5␤1 expressed on the cell surface as the activation of ␣5␤1 by Mn 2؉ or dithiothreitol resulted in the loss of sCD40L binding. Moreover, sCD40L induced the translocation of ␣5␤1 to the Triton X-100-insoluble fraction of U937 cells, the rapid activation of the MAPK pathways ERK1/2, and interleukin-8 gene expression. The binding of sCD40L to CD40 on BJAB cells, an ␣5␤1-negative B cell line, and the resulting activation of ERK1/2 was not inhibited by soluble ␣5␤1, suggesting that sCD40L can bind concomitantly to both receptors. These results document the existence of novel CD40L-dependent pathways of physiological relevance for cells expressing multiple receptors (CD40, ␣5␤1, and ␣IIb␤3) for CD40L. (2). The expression of CD154 is inducible, and its expression on T cells is triggered primarily by T cell receptor signaling and is regulated by CD28-dependent and independent pathways (3) CD40L is stored in platelets and a subpopulation of T cells and rapidly translocates to the cell membrane following T cell and platelet activation (4, 5). A soluble form of biologically active CD40L trimer (sCD40L) is present in the supernatant of activated T cells (6) and platelets (7) and results from the proteolytic cleavage of the homotrimeric CD40L by a metalloproteinase (7).It was originally thought that CD40L had only one receptor, CD40, which is a type I transmembrane protein that is a member of the TNF receptor superfamily. CD40 is expressed on the surface of many immune and non-immune cells, including B lymphocytes, monocytes/macrophages, and dendritic cells, as well as platelets, epithelial, and endothelial cells (8). Most biological functions of CD40L have been attributed to its direct interaction with CD40. However, studies using CD40L Ϫ/Ϫ and CD40 Ϫ/Ϫ mice have suggested that CD40L may also bind to one or more other receptors (9). In support of this hypothesis, it has been elegantly demonstrated that sCD40L interacts with ␣IIb␤3 (GPIIb/IIIa), an integrin expressed on platelets (10), triggering outside-in signaling and inducing platelet activation and spreading (11). CD40L Ϫ/Ϫ mice exhibit increased bleeding time (12) and reduce...