The complement system is a collection of blood and cell surface proteins that is the main defense and a primary clearance component of innate and adaptive immune reactions. More than 30 different complement proteins function sequentially to produce a broad range of activities, from cell lysis to augmentation of adaptive responses. The complement system has several major antimicrobial functions. A process used by the immunity whereby members of the complement factor family recognize antibody-coated targets and promote cell lysis to cell death.
-
Regulation of Complement Activation
It is known that complement system can be activated by three pathways, the lectin pathway, the classical pathway, and the alternative pathway. These routes all converge at the cleavage of complement factors C3, which is the most abundant complement protein factor. The distinct pathways generate complex proteases for cleaving C3. These C3 convertases cleave C3 into two fragments, complement factors C3a and C3b. The major product, C3b, covalently binds onto the surface of the complement activating particle and functions as an opsonin. C3b also covalently binds onto C3 convertases to form complement factors C5 convertases and thus initiate assembly of the membrane attack complex (MAC), composed of complement proteins C5b, complement factors C6, complement factors C7, complement factors C8, and complement factors C9. And MAC can insert into the lipid bilayers and cause lysis of target cells.
-
The Mechnism of Complement-mediated Lysis
Fig.1 The complement pathway. (Dunkelberger, 2010)
Regardless of the pathway, complement activation converges on the generation of several wide effector functions that serve to enable the complement to perform its physiological imperatives in host defense. Cleavage of C5 begins the lytic pathway which is common to all three activating pathways. However, it’s only relevant when the target surface holds an accessible lipid bilayer. C5 is cleaved to release C5a, a chemotactic factor, and C5b. C5b has a binding site for C6, which together can bind C7 which changes the conformation of the C5b67 complex. This complex produces a binding site for the phospholipid bilayer of the target cell membrane. C8 binds this C5b67 complex and undergoes a conformational change, allowing the insertion of its α-chain into the membrane. C5b-8 binds a molecule of C9 which is able to polymerize to form a stable pore (with a maximum diameter of 10 nm) in the targeted membrane. The C5b-9 complex complex or MAC can contain between one and eighteen C9 molecules. The insertion of the C5b-9 complex complex in the membrane causes the target to lose its ability to control its osmotic pressure inducing cell lysis.
This C5b-9 complex is the fully formed MAC and is effective in attracting cell lysis in various targets through a multi-hit process. MAC lytic activity is featured by a rapid increase in [Ca2+]i, followed by the loss of mitochondrial polarity and adenine nucleotide pools (e.g., ATP, ADP) . Whether cell death is apoptotic or necrotic may be a function of C5b-9 quantity, and fragmentation of DNA can be detectable in as little as 0.5h after treated by a lytic dosage of complement, indicating the destructive potential of the terminal pathway of complement activation.
-
Membrane Proteins-associated Lysis
Regulation of complement-dependent lysis of cells is attributed to certain membrane proteins. One of them is CD55, also known as decay-accelerating factor (DAF), a 70kD glycosylated protein with four complement control protein (CCP) domains that can protect host cells from damage by autologous complement. In one study, there was a hypothesis that blocking DAF function by a monoclonal antibody could augment complement-dependent lysis mediated by another antibody to a cell surface antigen present on leukemia cells. Therefore, the effects of the anti-DAF antibody on the ability of both rabbit and human complement were tested to lyse human leukemia cells by activation of a complement-fixing murine antibody.
The final results demonstrated that the anti-DAF antibody can significantly block the actions of DAF. In the presence of this antibody, the cytotoxic activity mediated by human complement was similar to that of rabbit complement. The conclusion is that increased tumor cell killing could be achieved by DAF blockade. This finding has particular relevance to clinical trials using a complement-fixing monoclonal antibody for clearing bone marrow of arcane tumor cells prior to autologous transplantation.
Creative Biolabs is a leading biological company that has established comprehensive one-stop services for Complement Therapeutics. If you’d like to know more information, please directly contact us and consult our technical supports online.
Reference
-
Dunkelberger, J.R.; et al. Complement and its role in innate and adaptive immune responses. Cell Research. 2010, 20: 34-50.
Related Product
For Research Use Only.
Related Sections:
