Dr. Hiba Wazeer Al Zou’bi Chemical Mediators 2 Dr. Hiba Wazeer Al Zou’bi

Complement Activation Pathways

Complement Numbered C1 to C9, present in plasma in inactive forms, many of them are activated by proteolysis to acquire their own proteolytic activity. The critical step in the generation of biologically active complement products is the activation of the third component, C3 . C3 cleavage (by formation of C3 Convertase(Cleave C3 to C3a and C3B)) occurs by three pathways: (1) Classical pathway, triggered by fixation of C1 to antigen-antibody complexes (2) Alternative pathway, triggered by bacterial polysaccharides (e.g., endotoxin), and involving a distinct set of plasma proteins including properdin and factors B and D

(3) Lectin pathway: - Plasma lectin binds to mannose residues on microbes and activates an early component of the classical pathway (but in the absence of antibodies). C3b deposits on the cell or microbial surface ,then binds to the C3 convertase complex to form C5 convertase; this complex cleaves C5 to generate C5a and C5b and initiate the final stages of assembly of C6 to C9.

Complement Role in Inflammation 1- Vascular effects: C3a and C5a increase vascular permeability and cause vasodilation by inducing mast cells to release histamine. These complement products are also called anaphylatoxins because their actions mimic those of mast cells, which are the main cellular effectors of the severe allergic reaction called anaphylaxis. - C5a activates the lipoxygenase pathway of AA metabolism in neutrophils and macrophages, causing release of more inflammatory mediators. 2- Leukocyte activation, adhesion, and chemotaxis: - C5a, and to lesser extent, C3a and C4a

3- Phagocytosis: - When fixed to a microbial surface, act as opsonins, augmenting phagocytosis by neutrophils and macrophages, which express receptors for these complement products. 4- Membrane attack complex MAC - Made up of multiple copies of the final component C9, kills some bacteria (especially thin-walled Neisseria) by creating pores that disrupt osmotic balance.

Complement Role in Inflammation

The activation of complement is tightly controlled by cell-associated and circulating regulatory proteins. The presence of those inhibitors in host cell membranes protects normal cells from inappropriate damage during protective reactions against microbes. Inherited deficiencies of these regulatory proteins lead to spontaneous complement activation: 1- C1 inhibitor : Deficiency cuases hereditary angioedema, in which excessive production of kinins secondary to complement activation results in edema in multiple tissues, including the larynx.

2- Decay-accelerating factor (DAF) normally limits the formation of C3 and C5 convertases. In paroxysmal nocturnal hemoglobinuria, there is an acquired deficiency of DAF that results in complement-mediated lysis of red cells (which are more sensitive to lysis than most nucleated cells) 3- Factor H is a plasma protein that limits convertase formation; its deficiency is associated with a kidney disease called the hemolytic uremic syndrome

Coagulation and Kinin Systems Some of the molecules activated during blood clotting are capable of triggering multiple aspects of the inflammatory response. Hageman factor (also known as factor XII of the intrinsic coagulation cascade) - A protein synthesized by the liver that circulates in an inactive form until it encounters collagen, basement membrane, or activated platelets (e.g., at a site of endothelial injury).

Activated Hageman factor (factor XIIa) initiates four systems that may contribute to the inflammatory response: (1) Kinin system, producing vasoactive kinins (2) Clotting system, inducing the activation of thrombin, fibrinopeptides, and factor X, all with inflammatory properties (3) Fibrinolytic system, producing plasmin and inactivating thrombin (4) Complement system, producing the anaphylatoxins C3a and C5a.

Intrinsic Pathway XII XIIa Extrinsic Pathway XI XIa VIIa TF VII IX IXa HMWK Prekallikerin Surface XII XIIa Extrinsic Pathway XI XIa VIIa TF VII IX IXa VIIIa X Xa Va Prothrombin Thrombin Fibrinogen Fibrin XIIIa Cross linked fibrin

Interaction between the four plama mediator systems XII Kallikerin HMWK Prekallikerin Surface XIIa Prekallikerin XI XIa HMWK Bradykinin IX IXa Plasminogen Plasmin VIIIa X Xa Va Prothrombin Thrombin C3 C3a Fibrinogen Fibrin Fibrinopeptides Fibrin split products

Kinin system Kinin system activation leads to the formation of bradykinin from its circulating precursor, high-molecular-weight kininogen (HMWK). Bradykinin causes increased vascular permeability, arteriolar dilation, and bronchial smooth muscle contraction. The actions of bradykinin are short-lived because it is rapidly degraded by kininases present in plasma and tissues. kallikrein, an intermediate in the kinin cascade with chemotactic activity, also is a potent activator of Hageman factor.

clotting system Clotting system : Factor Xa, an intermediate in the clotting cascade, causes increased vascular permeability and leukocyte emigration. Thrombin participates in inflammation by: 1- binding to protease-activated receptors that are expressed on platelets, endothelial cells, and many other cell types, leads to their activation and enhanced leukocyte adhesion. 2- generates fibrinopeptides (during fibrinogen cleavage) that increase vascular permeability and are chemotactic for leukocytes. 3- cleaves C5 to generate C5a.

As a rule, whenever clotting is initiated (e. g As a rule, whenever clotting is initiated (e.g., by activated Hageman factor), the fibrinolytic system is also activated concurrently. This mechanism serves to limit clotting by cleaving fibrin, thereby solubilizing the fibrin clot . Plasminogen activator (released from endothelium, leukocytes, and other tissues) and kallikrein cleave plasminogen, a plasma protein bound up in the evolving fibrin clot. The resulting product, plasmin, is a multifunctional protease that cleaves fibrin and is therefore important in lysing clots Fibrin degradation products increase vascular permeability. Plasmin cleaves the C3, resulting in production of C3a. Plasmin can also activate Hageman factor, thereby amplifying the entire set of responses

Anti-inflammatory Mechanisms Inflammatory reactions subside because many of the mediators are short-lived and are destroyed by degradative enzymes. There are several mechanisms that counteract inflammatory mediators and function to limit or terminate the inflammatory response such as lipoxins, and complement regulatory proteins. Activated macrophages and other cells secrete a cytokine, IL-10, whose major function is to down-regulate the responses of activated macrophages, thus providing a negative feedback loop Other anti-inflammatory cytokines include TGF-β, which is also a mediator of fibrosis in tissue repair after inflammation.