Innate Immunity: Immediate Response

Mechanical, chemical and microbiolgical barriers to infection

for skin gut lungs eyes/nose/oral-mechanical : epithelial cells joined by tight junctions

chemical: antimicrobial peptides

microbiological : normal microbiota

pulmonary sufactanct is a detergent like lipoprotein produced by alveolar type II epithelial

Different compartments of the body are exploited by pathogens

Different innate immune system components are used for defense for extracellular pathogens( (bacteria/viruses/fungi/parasites) and intracellular pathogens -viral replicaton and assembly

Complement Components

are plasma proteins, made in the liver and more than 30 proteins, some circulate as enzyme zymogens

-a cascade that results in covalent binding of protein fragments to the pathogen’s surface

-phagocytes have receptors for these fragments

two major effector mechanisms of complement: complement mediates the uptake and destruction of pathogens by macrophages and neurophils, complement forms a complex that makes holes in teh cell membrane of pathogens

Three Complement Activation pathways are triggered in different ways

1) alternative: triggered by the failure of a surface to inhibit spontaneous C3 activation

2) lectin: triggered by the binding of mannose-binding lectin to the surface of microbes

3) classical: triggered by C-reactive protein or antibodies bound to the pathogen-longer!

all converge in the same reaction:

-C3 cleavage into C3b and C3a:

C3b: covalent binding of C3b to the surface of pathogesn is called complement fixation. : tags pathogesn for destruction by phagocytes (phagocytosis) and starts cascade that damages the pathogen’s membrane (makes holes)

C3a: soluble C3a recruitss inflammatory cells like neutrophils

Complement Activation

protelytic cleavage of complement component C3: large C3b fragment covalently attaches to pathogen and labels pathogen as dangerous while C3a attracts phagocytic cells

-cleavage of C3 exposes a reactive thiosester bond that covalently attaches the C3b frragment to a pathogen’s surface

C3 cleavage

covalent binding to hydroxly and amino groups on the hydroxyl and amino groups on the surface of the pathogen

Chart of all Pathways and it’s Effector mechanism

main recruitment of inflammatory cells, opsonization of pathogens:faciliating uptake and killing by phagocytes and lastly perforation of pathogen cell membranes

overall: elimination of the infecting pathogen

Initiation of Alternative Pathway of Alternative Pathway of Complement Activation

C3 hydrolyses near microbe, activated C3 binds Factor B, , Factor D cleaves Factor B: producing the soluble convertase iC3Bb, this converstase cleaves C3 into C3b and C3a. C3b binds to microbe and C3a attracts phagocytes

Part 2 : Amplification

C3b binds Factor B,

Factor D cleaves Factor B to make C3Bb which then is cleaved by C3 to make more C3a-attracts phagocytes and C3b-which binds to microbes

C3bBb-the alternative C3 convertase

Bb fragment of Factor B cleaves C3 (it has enzymatic activity) while C3b fragment of C3 holds the enzyme at the surface of the pathogen

Complement Control Proteins

On the pathogen surface : soluble proteins

-C3Bb is stablizied by Properdin (Factor P) on pathogen surface , Factor H binding induces a conformational change of C3b and Factor I cleaves C3b to eventually produce c3d which is bound by CR2 (complement receptor 2) facililating phagocytois - this is the inactivation of C3b

On the human cell (membrane proteins)

-DAF (decay accelerating factor): binds and inactivaes C3Bb

-MCP (membrane cofactor protein): binds C3bBb and makes it susceptible to cleavage and incactivaion by Factor i-degrade to not make C3d

Phagoyctes have complement receptor that bind to C3b and mediate the uptake of C3b-coated pathogens

Steps:

1) complement activation deposits C3b on bacterial surface which has CR1 on macrophage

2) Macrophage CR1 binds C3b on bacterium

3) Macrophage endocytoses the bacterium

4) Macrophage membranes fuse to create a phagosome

5) Lysosomes fuse with the phagosome to form a phagolysoosome- lysosomes are full of toxins and enzymes that kill

C4: most common immune protein deficiency in humans

two types C4A and C4B : 30% of the human population is deficient in one of the two forms

-C4A deficiency is associated with Systemic Lupus Erythematosus -an autoimmune disease

-C4B deficinecy is associated with lowered resistance to infections

Complememnt Receptors:

C3b on the surface of pathogesn is bound by complement receptors on cell surface

C3b stimulates a cellular response to the pathogen

four types of complement receptors :

-CR1 in macrophages, neutrophils, and RBC : complement opsonization of pathogens

-CR2 in B cells and follicular dendritic cells : part of B cell co-receptor and EBV receptor

-CR3 and CR4 : are integrins ( cell adhesion molecules)

Complement Receptors remove immune complexes from the criculation

IgG in immune complex binds C1

C1 enzymes that cleave C3 are activated , complex becomes tagged with C3b and is bound by cells that express CR1 (on RBCs). RBCs takes complex to liver and spleen where it is detached and taken up by macrophage . tissue macrophages in the liver and spleen remove and degrade immune complexes , RBCS are not altered , if immune complexes accumualte then it causes SLE (systemic lupus)

Membrane-Attack Complex

C5bb initates the formation of themembrane-attack complex -makes hoels int he bacterial and eukaryotic cells

C5b binds C6, C7 binds to C5b6 an the hydrophobic site of C7 is exposed and attached to the membrane / C8 binds to C5b67 and the hydrophobic site of C8 is inserted into the membrane

Poymerization of C 9forms the transmembrane pore

Complement lyses pathogens by forming membrane pores (holes)-Chart

C5b framgent formation by the C5 convertase

C3b binds to the C3 convertase: C3Bb to produce the C5 convertase

Alternative C5 convertase is C3b2Bb -consits of two C3b fragments and one Bb fragment -

Strucure of membrane-attack complex

CD59 is a protein that prevents assembly of C9 on human cells

on the cells of pathogens, complement components C5-C9 assemble a complex that perforates the cell membrane while in human cells, CD59 binds to the C5b678 complex and prevents recruitment of C9 to form the pore

HRF (homologoy restriction fator or CD46) prevents pore formation by C9 in the same way

Complement activation induces inflammation

C3a and C5a are chemotactic agents and inflammatory mediators : C5a is the most stable and potent

-Phagocytes and endotheilial cells have mast cells have receptors for C3a and C5a

they induce degranulation of mast cells and basophils, releasing histamine that induces: smooth muscle contraction, increased capillary premeability, increased blood flow

-these changes : help antibodies, complement and cells leave blood vessels and enter infected tissues . help antigen-presenting cells (APC) go to draining lymph nodes for initiation of B and T cell responses

C3a an C5a are anaphylaxtoxins that augment the inflammatory response to infection

how: increasing adherence of neutrophils and monocytes to cells walls, directing cell migration towards antigen, increasing phagoyctic capacity of macrophages and neutrophils , increasing surface expression of CR1 and CR3

Plasma regulatory proteins alos limit complement activation

C1 inhibiotr (C1INH) -inhibits the C1r and C1s enzymes at the active site of C1

C4-binding protein (C4BP)- makes C4b suscepible toi inactivation by Factor 1

these regulatory proteins block complement fixation on human cells and ensure that most complement is fixed to pathogen surfaces and very litttle fixed to human cell surfaces