Initiation of Extrinsic Coagulation via Complement
Authors: Freda Christopher Thor , Greene Vaughn , Yin Wei , Rubenstein David A. 
Published: June 2021

Thrombosis and inflammation are prevalent during and characteristic of vascular diseases. Understanding the interplay of these processes would likely allow us to treat vascular diseases more effectively. Activation of complement component 1 (C1) is an important contributor to innate immunity and ultimately drives the lysis of pathogens. Upon C1 activation, significant concentrations of circulating C1q are observed. C1q interacting with gC1qR, one of its cell‐surface receptors, initiates many pathological pathways. There is considerable evidence that C1q/gC1qR plays a role in inflammatory processes observed during atherosclerosis and in initiating intrinsic coagulation. While this link between inflammation and thrombosis is important, the intrinsic cascade plays a minor role in physiological and pathological thrombosis. The effects of C1 and the role of C1q on extrinsic coagulation, which is more physiologically relevant, has not been studied. Extrinsic coagulation initiates upon vessel damage, which exposes blood to sub‐endothelial constituents, especially tissue factor. Adventitial fibroblasts and vascular smooth muscle cells are the primary tissue factor bearing cells. We hypothesized that C1q binding to gC1qR initiates this pathway through the enhanced expression of sub‐endothelial tissue factor (TF) in these cells. The objective of this study was to elucidate the role of C1q‐mediated TF expression. Using a solid‐phase ELISA, TF expression was observed on aortic adventitial fibroblasts (HAAFs) and aortic smooth muscle cells (HASMCs) under various conditions. Cells were conditioned for one hour with either platelet poor plasma (PPP), purified human C1q, LPS, anti‐gC1qR (60.11), or a combination of 60.11 and purified human C1q. After the exposure conditions, cells were incubated for one hour with one of the following primary antibodies: anti‐human gC1qR (74.5.2), anti‐human TF, or anti‐human CD54 (ICAM‐1). Following primary antibody incubation, cells were incubated with an appropriate alkaline phosphatase conjugated secondary antibody for one hour. PNPP was added to the cells and allowed to develop for 30 minutes. Absorbance was documented at 405 nm. Our results suggest that HAAF and HASMC expression of TF increased significantly by approximately 35% after incubation with C1q as compared with unconditioned cells. We also observed increased ICAM expression, 20% above baseline, in HASMCs. Cells conditioned with gC1qR blocking antibodies exhibited a marked decrease in TF and ICAM expression (by approximately 10%) when compared to cells conditioned with C1q alone. Overall, this data illustrates a role for C1q in activation of extrinsic coagulation. Furthermore, our data suggests that gC1qR acts a mediator for both inflammatory and thrombotic reactions that may occur during vascular diseases. This work illustrates a step in identifying common inflammatory and thrombotic pathways that can be therapeutically targeted to mitigate vascular diseases and may provide new information regarding off‐target effects. Future work will determine if the expressed TF can support extrinsic coagulation activity and new therapeutics can be designed to mitigate vascular disease processes.

This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.