Saadi, Soheyla

Background —Interaction of complement with endothelial cells (ECs) underlies the development of inflammation and coagulation in disease. Assembly of the membrane attack complex (MAC) of complement on EC membrane, like stimulation with cytokines, upregulates tissue factor and cyclooxygenase-2 but does so via the intermediary action of IL-1α. We asked whether the MAC activates porcine aortic and microvascular ECs in a global manner by this mechanism and whether this mechanism is used by membrane pore-forming structures. Methods and Results —Exposure of ECs to complement caused upregulation of mRNAs for E-selectin, intracellular adhesion molecule-1, vascular cell adhesion molecule-1, Iκ-Bα, interleukin (IL)-1α, IL-1β, IL-8, and plasminogen activator inhibitor-1 over a period of 6 hours. The expression of these genes was not a primary response to stimulation, however, because IL-1 receptor antagonist inhibited expression of these genes. Activation of ECs by complement depended on the autocrine action of IL-1α, because complement-mediated EC activation was inhibited by anti–IL-1α antibodies. Melittin and mastoparan, amphiphilic pore-forming peptides like the MAC, induced E-selectin through intermediary action of IL-1. Conclusions —These findings suggest that transmembrane pore-forming proteins, as a class of molecules, activate ECs through the autocrine effects of IL-1α.

Activation of complement stimulates inflammation and provides an initial vigorous defense against infection. Insertion of the membrane attack complex in cell membranes of vascular endothelial cells induces changes in cell differentiation that promote coagulation, thrombosis, inflammation, and immunity. These changes are mediated by production of interleukin (IL)-1α by endothelial cells, which acts locally on endothelial cells to contain infection and promote healing of the affected site. In healthy tissues, however, promoting coagulation and inflammation would be dysphysiologic. Accordingly, endothelial cell activation by the membrane attack complex depends on both transcriptional regulation of IL-1α and availability of that cytokine to broadly modify endothelial cell physiology. Here, we report that the IL-1α gene contains a suppressor sequence that cooperates with histone modification to regulate production of IL-1α by endothelial cells. The suppressor sequence binds C/EBP (CCAAT enhancer–binding protein) family DNA-binding proteins isolated from the nucleus of quiescent endothelial cells. These results suggest constitutive suppression of IL-1α maintains quiescence of endothelium and that terminal complement complexes remove that suppression, allowing IL-1α transcription and, ultimately, activation of endothelium to proceed.

Activation of complement on endothelium triggers physiological changes that promote coagulation, thrombosis, and inflammation. Unlike agonists such as cytokines and endotoxin that induce these changes through transcription of many genes, complement, particularly the membrane attack complex, primarily induces release of IL-1α by the endothelial cells; the cytokine may then be removed by normal blood flow or may promote activation of the full range of endothelial cell responses in an autocrine or paracrine manner. We studied the intracellular signaling pathways used by complement to activate interleukin (IL)-1α transcription in cultured endothelial cells. The membrane attack complex and other pore-forming proteins stimulated calcineurin and activated selective transcription of the IL-1α gene. In contrast, the action of cytokines such as IL-1α was not selective and not dependent on calcineurin activity. Transcription of IL-1α, whether stimulated by complement and calcineurin or by “conventional agonists,” such as IL-1α independent of calcineurin, proceeded via binding of nuclear factor κB transcriptional activators to the IL-1α gene promoter. These findings define a molecular mechanism through which complement regulates IL-1α production by endothelial cells and explain how blood flow may determine the extent of complement-stimulated inflammation.

ABSTRACT Although immune responses are generally considered to be systemic, local events such as interaction of complement products with blood vessels and with inflammatory cells play a pivotal role in determining the nature and manifestations of immune responses. This paper will discuss how blood vessel physiology and immunity influence one another to reach homeostasis upon exposure to an infectious agent. We review new insights into the mechanisms by which the microenvironment of tissues protects against microbial invasion yet facilitates migration of leukocytes and ‘decides’ whether immunity or tolerance ensues and whether, in the face of immunity, protective responses or tissue injury ensues. These ‘decisions’ are made based on interaction of components of normal tissues such as proteoglycans and injured tissues such as cell‐associated cytokines with receptors on immune cells and blood vessels.—Saadi, S., Wrenshall, L. E., Platt, J. L. Regional manifestations and control of the immune system. FASEB J. 16, 849–856 (2002)