To examine the role of complement components as regulators of the expression of endothelial adhesive molecules in response to immune complexes (ICs), we determined whether ICs stimulate both endothelial adhesiveness for leukocytes and expression of E-selectin and intercellular and vascular cell adhesion molecules 1 (ICAM-1 and VCAM-1). We found that ICs [bovine serum albumin (BSA)-anti-BSA] stimulated endothelial cell adhesiveness for added leukocytes in the presence of complement-sufficient normal human serum (NHS) but not in the presence of heatinactivated serum (HIS) or in tissue culture medium alone. Depletion of complement component C3 or C8 from serum did not prevent enhanced endothelial adhesiveness stimulated by ICs. In contrast, depletion of complement component Clq markedly inhibited IC-stimulated endothelial adhesiveness for leukocytes. When the heat-labile complement component Clq was added to HIS, the capacity of ICs to stimulate endothelial adhesiveness for leukocytes was completely restored. Further evidence for the possible role of Clq in mediating the effect of ICs on endothelial cells was the discovery of the presence of the 100-to 126-kDa Clq-binding protein on the surface of endothelial cells (by cytofluorography) and of message for the 33-kDa Clq receptor in resting endothelial cells (by reverse transcription-PCR). Inhibition of protein synthesis by cycloheximide blocked endothelial adhesiveness for leukocytes stimulated by either interleukin 1 or ICs in the presence of NHS. After stimulation with ICs in the presence of NHS, endothelial cells expressed increased numbers of adhesion molecules (E-selectin, ICAM-1, and VCAM-1). Endothelial expression of adhesion molecules mediated, at least in part, endothelial adhesiveness for leukocytes, since leukocyte adhesion was blocked by monoclonal antibodies directed against E-selectin. These studies show that ICs stimulate endothelial cells to express adhesive proteins for leukocytes in the presence of a heat-labile serum factor. That factor appears to be Clq.Vascular endothelium plays a central role in inflammation by expressing specific adhesion molecules including P-selectin, E-selectin, and intercellular and vascular cell adhesion molecules 1 (ICAM-1 and VCAM-1) that attract and localize leukocytes to inflamed sites (reviewed in ref. 1). The recent observation that endothelial expression of E-selectin is required for development of inflammatory injury after intravascular injection of immune complexes (ICs) suggests an active role for endothelium in IC-mediated vasculitis (2). However,

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In the absence of antibody, the alternative pathway of the complement system can function as a first barrier in preventing infection. However, certain microorganisms have developed mechanisms to escape attack and survive in the host's bloodstream (1). Trypanosoma cruzi, the causative agent of Chagas' disease, is a protozoan parasite that cycles between invertebrate insect vectors and mammalian hosts. During its development in the insect, T. cruzi assumes various forms and the infectivity of the parasite for the mammalian host is associated with the aquisition of resistance to lysis by complement (2-5). The epimastigote is the noninfective multiplicative form found in the gut of the insect . Epimastigotes transform into metacyclic trypomastigotes, which can invade cells of the mammalian host. While both epimastigotes and metacyclics activate the complement cascade, only epimastigotes are lysed. Metacyclics are not lysed because they have developed mechanism(s) to prevent the assembly of C3 convertase, a key amplifying enyzme of the complement system (6-9).When trypomastigotes enter the host cells, they transform into amastigotes; the amastigotes then mulitply, transform again into trypomastigotes, and are released into the bloodstream to continue the cycle. Contrary to the conventional view that amastigotes are exclusively the intracellular multiplicative stage of the parasite, recent studies demonstrate that amastigotes can be found in circulation during the acute stages ofthe infection and can enter and develop in cells (10). In vitro infection of monocytes by amastigotes occurs in the presence of fresh human serum, indicating that they also avoid destruction by complement . These studies, however, did not discriminate between lack ofcomplement activation and protection from attack .Here, we address this question and show that amastigotes are extremely efficient activators of the cascade and that they bind terminal components . Functional channels, however, are not formed and the parasites are not destroyed. Volume 169 March 1989 881-891 Materials and MethodsParasites . Strain Y of T, cruzi was maintained in monolayers of LLC-MK2 cells in DMEM containing 5% FCS at 37 0C in a 5°Jo C02 atmosphere. Amastigotes were obtained as follows. Trypomastigotes were collected from a 5-d culture of infected cells within 24 h after changing the medium . As determined by light microscopy, most trypomastigotes transformed into