The Complement System

A group of > 30 plasma proteins which comprise the primary soluble component of innate immunity.

Rapidly activated in response to infection, without induction or recall of adaptive immunity.

The Complement System

However, in the presence of an adaptive immune response, complement proteins interact with both its soluble and cellular components (antibodies, lymphocytes, activated macrophages, dendritic cells).

Complement proteins circulate in plasma as inactive precursors.

The Complement System

Infection results in activation of complement proteins via a series of proteolytic reactions that yield biologically active fragments.

These coupled proteolytic reactions result in an amplification cascade, in which limited stimulation of proximal complement components results in massive activation of distal complement proteins.

Complement System: Overview

Complement proteins opsonize microbes and promote their phagocytosis.

Complement receptors on the surface of leukocytes bind complement proteins

Phagocytic receptors: CR1, CR3, CR4 - present on neutrophils, monocytes, macrophages, dendritic cells.

- Bind C3b / C3bi that are attached to microbial surfaces (as opsonins) phagocytosis

Complement receptors on the surface of leukocytes bind complement proteins

CR2: surface of B cells and dendritic cells. Binding of activated complement fragments to CR2 amplifies antigen-induced cellular activation - provides a link between the innate and acquired immune systems.

The Complement System kills microbes via the Membrane Attack Complex (MAC).

Activation of the Complement System

2 activation pathways are components of innate immunity : Alternative pathway, Lectin pathway

The Classical pathway of complement activation is part of the adaptive immune response - dependent on Abs.

Activation of the Complement System

All 3 activation pathways result in generation of an enzyme complex which can cleave C3, called the C3 convertase.

C3 C3a + C3b

- Integration point of the complement system

(Innate and Adaptive immunity)

- Major amplification point

Activation of the Classical Pathway

Requires Antigen - Antibody complexes

Complement component C1 binds to the Fc region of the Ab (IgM- most potent activator)

C1 is a protease that cleaves C2 and C4 to form the C3 convertase of the classical path.

Activation of the Alternative Pathway

Soluble proteins of the Alternative Pathway (B,D,P) bind to repetitive structures on microbial surfaces, such as components of the cell wall.

The complex of B, D and P forms the C3 convertase of the Alternative Pathway.

Recognition is selective for microbes, but not highly specific (pattern recognition).

Activation of the Lectin Pathway

The soluble plasma protein, Mannan-Binding Protein (MBP or MBL) binds to the sugar mannan which is restricted to the surface of certain microbes (not on vertebrate cells)

This leads to attachment of other complement proteins (C4,C2) to form the C3 convertase of the Lectin Pathway.

Regulation of Complement System

Due to its potential for rapid amplification, and the ability of activated complement proteins to damage host tissue as well as microbes, strict regulation is needed.

Soluble Inhibitors of Complement

C1 inhibitor (C1 INH, C1 esterase inhibitor) - binds and inactivates C1 inhibition of Classical Pathway.

Lack of C1 INH (hereditary angioneurotic edema) - spontaneous activation of complement and kinin systems, inflammation of the skin and critical organs.

- Symptoms are completely reversed by

administration of C1 INH.

Cellular Inhibitors of Complement

Decay accelerating factor (DAF) and CD59 are proteins present on mammalian cells, inhibit the assembly of the MAC on host cells.

Deficiencies of DAF and CD59 occur in the disease Paroxysmal Nocturnal Hemoglobinuria (PNH) - intermittent hemolysis of RBCs due to unregulated deposition of activated complement components on the cell surface.

Complement Deficiencies

Congenital: Deficiencies of C7, C8, C9 - “terminal complement components”. Markedly increased incidence of Neisserial infections.

- N. meningitidis: meningitis, bacteremia

- N. gonorrhoeae: STDs, bacteremia

Acquired: consumption of complement proteins during extensive activation of the complement system.

Complement System - Summary

Plasma proteins + Complement Receptors on leukocytes + Regulatory proteins

Integral to innate immunity, also functions with the adaptive immune system.

Amplification cascade that proceeds via coupled proteolytic reactions.

Complement System - Summary

Functions:

Recruit inflammatory cells: C3a, C5a

Opsonizes pathogens: C3b, C3bi

Directly microbicidal: MAC

General Biomedical PrinciplesIn addition to its intrinsic biochemical beauty and physiologic importance, the complement system illustrates several principles which are fundamental to the biomedical sciences.

1. Amplification - coagulation, fibrinolytic, kinin systems

2. Multiple levels of compensatory regulation

3. Receptor - ligand interactions determine specificity

4. Inflammation - both beneficial / deleterious

Cytokines

Cytokines are essential soluble mediators of both the innate and adaptive immune systems. In fact, cytokines are one of the major integrators which bridge these complementary systems.

1. recruitment of leukocytes (chemokines)

2. activation of phagocytes augment innate immunity

3. absolutely required for the maturation and activation of lymphocytes

Activated Macrophages Secrete Cytokines that Stimulate both Innate and Acquired Immunity

Cytokines Exhibit Both

Immunoprotective and Tissue-

Damaging Effects

Integrators of Immunity

Components which are responsible for the interdependence of innate and acquired immunity

1. Complement system

2. Cytokines/chemokines

3. Macrophages

4. Dendritic cells

5. Antibodies

The Integrated Immune Response