Understanding Innate Immunity

• Innate immunity is the first line of defense
• Elements of the complement system, complement activation pathways, regulation, and its role in diseases are key topics
• Physical and chemical barriers keep foreign invaders out of the body
• Innate immune cells target foreign objects and signal the adaptive immune system when needed

Innate Immunity

• Defends the host against harmful organisms and pathogens
• Acts quickly with an immediate direct response in 0-4 hours, and a rapid induced response in 4-96 hours
• It's always present and active, constitutively expressed, and nonspecific, not specifically directed against any particular infectious agent or antigen
• There is no "memory development"; it remains the same every time.

Origins of Immune Cells

• The immune system has a variety of effector cells and molecules
• Both innate and adaptive immune responses depend upon white blood cells, leukocytes, or lymphocytes
• All white blood cells arise from pluripotent hematopoietic stem cells in the bone marrow and mature into two main categories: myeloid and lymphoid lineages

Myeloid Lineage

• Involved in the innate immune response
• Includes granulocytes, monocytes, platelets, dendritic cells, natural killer cells, and macrophages

Lymphoid Lineage

• Involved in the adaptive response
• Includes T cells, B cells, and natural killer cells

Complements

• Cells are activated by local inflammation and phagocytosis of invading bacteria
• Activation induces a cascade of proteolytic reactions that coat microbes with fragments of specific proteins
• Specific complement receptors recognize these proteins to initiate phagocytosis and killing

Discovery of Complements

• Research on complement began in the 1890s
• Jules Bordet showed that sheep antiserum to the bacterium Vibrio cholerae caused lysis and heating destroyed its bacteriolytic activity
• Bordet discovered that you can restore antibacterial abilities by adding fresh serum
• Paul Ehrlich coined the term "complement", defining it as activity of blood serum that completes the action of antibody
• Jules Bordet was awarded the Nobel Prize in 1919 for studying antibodies and the complement system
• Bacteriolysis results from the combined action of two factors: complement, a non-specific heat-sensitive factor, and antibodies, a specific heat-stable factor

Components of the Complement System

• Complements are a collection of soluble proteins present in blood and other body fluids
• The complement system is composed of more than 30 different proteins
• In the absence of infection, complement components circulate in an inactive form as proenzymes or zymogens
• These components make up 5% of the serum globulin fraction.
• The complement-reaction sequence starts with an enzyme cascade, where proteolytic cleavage removes an inhibitory fragment and exposes the active site
• In the presence of pathogens, complement proteins are activated and interact to form complement activation pathways

Functions of the Complement System

• Killing pathogens for phagocytosis
• Inducing inflammatory responses
• Lysis of infected cells, bacteria, and viruses
• Opsonization (coating) promotes phagocytosis of particulate antigens
• Binding of specific complement receptors triggers specific immune functions
• Inflammation and secretion of immuno-regulatory molecules
• Immune clearance removes immune complexes from the circulation

Complement Activation Pathways

• Pathways include classical, alternative, and lectin pathways

Classical Pathway

• Initiated by the formation of soluble antigen-antibody complexes or with the binding of antibody to antigen such as a bacterial cell

Alternative Pathway

• Initiated by cell-surface constituents that are foreign to the host, e.g. cell-wall constituents of both gram-negative/positive bacteria

Lectin pathway

• Initiated when Mannose-binding lectin(MBL), a protein that binds to mannose residues on the surface of microorganisms
• After MBL binds to the cell surface of pathogen, MBL-associated serine proteases, MASP-1 and MASP-2, bind to MBL and activate the lectin complement pathway

Nomenclature of the Complement System

• All components of the classical complement pathway are designated by the letter C followed by a number (C1-C9)
• Components are numbered in the order of their discovery, not in sequence of reactions
• Peptide fragments formed by cleavage of a component protein are denoted by small letters
• The smaller fragment is designated "a" and the larger fragment designated "b" (e.g., C3a = smaller fragment, C3b=larger fragment)
• C2 is an exception where C2a is the larger cleavage fragment
• Larger fragments bind covalently to the pathogen near the site of activation
• Smaller fragments diffuse from the site to initiate localized inflammatory responses by binding to specific receptors
• Complement fragments interact with one another to form functional complexes.
• Complexes with enzymatic activity are designated by a bar over the number or symbol

Complement Activation - Classical Pathway - Step 1

• Antigen-antibody complex induces the Fc portion of IgG or IgM antibody
• This exposes a site to C1 component of the complement system
• Each C1 molecule must bind to at least two Fc sites of antibody on bacterial surface so to allow C1q to bind for stable interaction
• Binding of C1q portion to Fc binding sites induces conformational change in C1r that converts it to active serine protease enzyme, C1r
• This activation leads to cleavage of C1s to a active serine protease active enzyme, C1s

Complement Activation - Classical Pathway - Step 2

• C1s has two substrates; C4 and C2
• Once C1s activates, it cleaves C4 and C2 into small and large fragments (C4a, C4b and C2b and C2a)
• Then, the large C4b fragment attaches to the pathogen surface and induces C2a to bind to C4b
• The resulting C4b2a complex is called C3 convertase and converts the C3 into an active form

Complement Activation - Classical Pathway - Step 3

• Here the C3 convertase hydrolysis C3 to generate C3a and C3b in a single step
• Single C3 convertase molecule can generate over 200 C3b molecules, resulting in tremendous amplification at this step of the sequence
• Some of the C3b generated by C3 convertase activity does not associate with C4b2a and diffuses away where they coats immune complexes/functions as an opsonin
• C3b binds to to C3 convertase (C4b2a) to form a trimolecular C4b2a3b and called the C5 convertase

Complement Activation - Classical Pathway - Step 4

• Amplification has worked to produce C5 convertase
• The C3b component in the C5 convertase unit binds to C5
• This action induces cleavage of C5 into small and larger fragments C5a and C5b

Complement Activation - Classical Pathway - Step 5

• The C5b component is extremely unstable and becomes ineffective within two minutes unless C6 binds to it to stabilize the activity
• C5b attaches to C6 and then to C7. and this complex adheres to the cell surface and permitting C8 to bind
• Binding of C8 to membrane-bound C5b67 induces conformational change of C8 and expresses the hydrophobic region which helps the plasma membrane interact
• The C5b678 complex creates a small pore, 10 Å in diameter
• Poly C9 is used to initiate formation of the membrane attack complex (MAC)
• Final activation step involves binding and polymerization of the enzyme perforin like molecule C9 , to the C5b678 complex
• As many as 10–17 molecules of C9 can be bound and polymerized by a single C5b678 complex
• During polymerization, the C9 molecules insert into the membrane
• The resulting MAC, which has a tubular form/functional pore size of 70–100 Å, includes C5b678 complex surrounded by poly-C9 complex
• Because ions small molecules diffuse via the central channel the cell cannot regulate osmotic ability is destroyed via influx water loss electrolytes