Innate Immune Responses (1/2)

immediate and induced

Two broad components of the innate immune response

barriers and soluble mediators (cytokines, complement)

Immediate response includes, broadly:

cells, inflammation and the acute phase response, interferon and NK cells

The induced response includes

Molecules present in body fluids that directly neutralize pathogens or recruit immune cells.

Soluble mediators are:

• Opsonize (coat) pathogens for easier phagocytosis.

• Lyse pathogens by forming the Membrane Attack Complex (MAC).

• Recruit inflammatory cells through chemotaxis.

Complement proteins have these 3 main functions:

Proteins are triggered by antibodies bound to antigens.

Plasma protein activation by the classical pathway involves

Recognizing carbohydrates on microbial surfaces

Plasma protein activation by the lectin pathway involves

direct activation by pathogen surfaces

Plasma protein activation by the alternative pathway involves

Produces bradykinin, causing vasodilation and increased vascular permeability, promoting inflammation.

Plasma proteins include the kinin system, which —

neutrophils, eosinophils, basophils

Granulocytes include these 3 cell types

MHC class I molecules

natural killer cells Recognize and kill virus-infected or cancerous cells by detecting the absence of —

cytokines

Acute Phase Proteins are produced by the liver in response to —.

• Produced by virus-infected cells.

• Induce an antiviral state in neighboring cells by:

  • Inhibiting viral replication.

  • Increasing MHC class I expression

How type I interferons work

NK cell, neutrophil, eosinophil, basophil, DC, mast cell, monocyte, macrophage

Cells of the innate immune system include (8):

macrophage, neutrophil, dendritic

Phagocytic cells include (3)

macrophage, dendritic

Antigen presenting cells include (2)

neutrophil, eosinophil, basophil

Granulocytic cells include (3)

monocytes, natural killer cells, neutrophils, eosinophils, basophils

Innate immune cells that circulate in blood (5)

macrophage, dendritic cell, mast cell

Innate immune cells present in tissues (3)

hepatocytes; some production by monocytes, macrophages, and mucosal epithelial cells occur

Complement proteins are made mostly in —

Unactivated (zymogen) form; proteolytic cleavage removes inhibitory fragment

Most complement proteins circulate in this form

Commonly designated by numbers and letters (C for complement 1-9)

Frequently the smaller cleavage product is a and the larger is b

Naming conventions of complement

–C2a is larger and forms complexes while C2b (being smaller) diffuses

C2 naming exception

alternative, lectin, classical

convery into one effector/terminal pathway

The 3 pathways that initiate complement occur in this order and converge into —

activated C3 binds pathogen surface

Main event of alternative pathway

mannose-binding lectin binds to pathogen surface

Main event of lectin pathway

C-reactive protein or antibody binds to pathogen surface

Main event of classical pathway

Cleave C3 in order to generate the C3 & C5 convertases to produce functional outcomes: Inflammation, opsonization, lysis

The goal of complement initiation pathways

C3

This protein is the link between different types of complement activation and the effector outcomes of inflammation, opsonization, and pathogen lysis

severe, recurrent infections

Clinical manifestation of C3 deficiency

C3 convertase,

C3a and C3b

C3 is cleaved by — into —

When C3b attaches to the surface of the pathogen, tagging it for destruction.

What is complement fixation?

chemoattractant, recruiting effector cells to the site of infection.

C3a acts as a —

factor B

The reactive thioester bond on newly cleaved C3 allows it to interact with —

the alternative pathway by the binding of C3b to the pathogen surface.

The environment around pathogens facilitates the cleavage of iC3 into C3a and C3b, which activates —

Alternative pathway is activated when C3 is hydrolyzed into C3(H2O).

Factor B binds C3(H2O) then is cleaved by factor D into Ba (no further function) and Bb (remains bound to C3). This C3(H2O)-Bb complex is C3bBb, which is the C3 convertase of the alternative pathway.

Process leading to the formation of C3bBb

cleaves C3 into C3a and C3b. C3b then binds nearby pathogens and targets them for opsonization.

What C3bBb aka C3 convertase does

C3Bb complex binds additional C3b molecule

How C5 convertase is formed

Cleaves C5 into C5a and C5b,

C5b initiates the membrane attack complex (MAC) that forms pores in the pathogens membrane, leading to cell lysis.

What C5 convertase and its resulting products do

Properdin (P)

Plasma protein that binds to bacterial surfaces and stabilizes the C3bBb convertase

Once formed, C3bBb can rapidly cleave more C3, leading to a positive feedback loop—this is what is meant by "runaway complement activation."

• As more C3b is generated, it can bind to more surfaces, forming additional C3bBb complexes, which then amplify the response even further.

• This could result in an overwhelming immune response that damages host tissues. To prevent this, strict regulation of the C3bBb complex is essential.

What does it mean that C3bBb convertase is capable of generating rapid and runaway complement reactions?

• Properdin stabilizes the C3bBb convertase on microbial surfaces, ensuring efficient complement activation when needed.

• It essentially protects and prolongs the action of the C3 convertase on pathogens but does not allow this amplification to occur on host cells.

Properdin function

• Factor H is a regulatory protein that binds to C3b on the surface of host cells, competing with Properdin and limiting the formation of C3 convertases.

• Factor I works in tandem with Factor H by cleaving C3b into iC3b, an inactive form that cannot generate more C3 convertase complexes.

• Together, Factor H and Factor I control the amount of active C3 convertase on pathogen surfaces, ensuring that C3 levels in the blood, extracellular fluid, and lymph do not become depleted due to unchecked activation.

Factor H and Factor I functions

membrane-bound proteins

In addition to plasma proteins like properdin, host cells have — that regulate the activity of the alternative complement pathway, ensuring that the complement system does not attack the body's own cells.

• DAF promotes the dissociation of the C3bBb convertase complex.

• By breaking up the C3 convertase, DAF prevents the formation of new C3bBb complexes, limiting the amplification of the complement cascade on the surface of human cells.

• This is a protective mechanism that ensures the complement system doesn't mistakenly attack healthy tissues.

Function of Decay-Accelerating Factor (DAF—CD55)

within the cell membrane

Decay-accelerating factor is located here

• Like DAF, it causes the dissociation of C3bBb from the surface of host cells.

• In addition, MCP promotes the cleavage of C3b by Factor I, converting it to iC3b, which is inactive and cannot form further C3 convertases.

Membrane co-factor protein 2 functions

C-reactive protein (CRP) or antibodies (IgM or IgG) bind to an antigen (Ag) on the surface of a pathogen or damaged cell.

Action that triggers the initiation of the classical pathway

the complement system

The binding of C-reactive protein to a damaged cell or a pathogen marks it for recognition by —

C1q, C1r, C1s

Subcomponents of C1 complex

C1q binds to the antibody (IgM or IgC) that is already bound to the antigen/pathogen’s surface.

Binding of C1q triggers conformational change that activates C1r.

C1r cleaves and activates C1s, which is a serine protease that goes on to cleave C4 and C2.

C1 subcomponents functions

• C4b binds to the surface of the pathogen or antigen, where it forms the C4b2a complex with C2a.

C1s cleaves C4 into C4a and C4b and C2 into C2a and C2b. What happens next?

It is a C3 convertase that cleaves C3 into C3a and C3b.

What does the C4b2a complex do?

It can either

bind pathogen surface to mark it for opsonization

or

join with C4b2a to form C5 convertase (C4b2a3b)

Once C3b is cleaved, it can do 2 things:

• MBL is a pattern recognition molecule that binds to specific carbohydrates on the surface of bacteria, viruses, fungi, and other pathogens.

  • It specifically recognizes mannose and fucose residues that are arranged in a pattern unique to microbial surfaces (and not found on human cells).

• Once MBL binds to these carbohydrates on the pathogen surface, it undergoes a conformational change that activates MASPs (MBL-associated serine proteases).

What is MBL?

Binding of MBL (mannose-binding lectin)

What initiates the lectin complement pathway?

MASPs are serine proteases that are structurally and functionally similar to the C1r and C1s proteases of the classical pathway.

What are MASPs?

Upon activation, MASP-2 cleaves C4 and C2, similar to how C1s functions in the classical pathway.

What does activation of MASP-2 do?

MASP-2 cleaves C4 into C4a and C4b, and C2 into C2a and C2b.

• C4b binds to the pathogen surface, and C2a attaches to it, forming the C4b2a complex.

Once MASP-2 cleaves C4 and C2, what happens?

C3

In some cases, MASPs, especially MASP-1, may directly cleave —

C5b

C5 cleavage to obtain — is required to activate the MAC

C5 convertase

The terminal pathway begins once a — is formed.

–Forms large channel through membrane of target cell

–Disrupts osmotic balance

What the membrane attack complex does

On activation by cleavage, the soluble C5b fragment initiates assembly of the membrane-attack complex in solution

Functions of terminal pathway proteins: C5

Binds to and stabilizes C5b, thereby forming a binding site for C7

Functions of terminal pathway proteins: C6

binds to C5b6 and exposes a hydrophobic region that permits attachment to the cell membrane

Functions of terminal pathway proteins: C7

binds to C5b67, exposing a hydrophobic region that inserts into the cell membrane

Functions of terminal pathway proteins: C8

Polymerization of C9 on the C5b678 complex forms a membrane spanning channel that disrupts the cell’s integrity and can cause cell death

Functions of terminal pathway proteins: C9

CD59, also known as Protectin, is a membrane-bound regulatory protein found on the surface of human cells. CD59 binds to the C5b678 complex before C9 can be recruited, preventing pore formation and subsequent cell lysis.

How host cells aren’t affected by C5b-9 formation

C3b covalently binds to the surface of pathogens. CR1 (complement receptor 1) is found on the surface of phagocytes and binds to C3b, triggering phagocytosis. The pathogen is internalized into a phagosome, which fuses with a lysosome to form a phagolysosome where the pathogen is digested and destroyed.

How phagocytosis via C3b works

• Complement proteins C3b and iC3b coat the viral surface, interfering with the virus’s ability to attach to its target cell receptors.

How complement addresses viral infection

anaphylatoxins

Initiation of inflammation by smaller, soluble cleavage products called –

acute phase

The lectin and classical pathways occur following the — response

Pattern Recognition Receptors are receptors located on the surface or inside of certain innate immune cells that detect signs of infection by binding to Pathogen-Associated Molecular Patterns (PAMPs).

What are PRRs and what do they do?

PAMPs (Pathogen-Associated Molecular Patterns) are conserved molecular structures that are commonly found in microbes but absent in host cells.

What are PAMPs?

LPS

Classic example of a PAMP on Gram negative bacteria

innate

Innate or adaptive receptor? specificity inherited in the genome

adaptive

Innate or adaptive receptor? encoded in multiple gene segments

innate

Innate or adaptive receptor? triggers immediate response

adaptive

Innate or adaptive receptor? requires gene rearrangement

innate

Innate or adaptive receptor? recognizes broad classes of pathogens

adaptive (clonal expression refers to rapid B cell maturation or the process of cell division that creates genetically identical cells)

Innate or adaptive receptor? clonal expression present

both

Innate or adaptive receptor? able to discriminate between even closely related molecular structures

TLRs are a type of PRR that is found on the cell surface or in endosomes that recognize a broad range of PAMPs

Toll-Like Receptors do this

the production of pro-inflammatory cytokines (through changes in gene expression)

TLRs are PRRs that are potent stimulators of —

communication within the immune system

Cytokines are primarily involved in —

antiviral effects

Action of interferon family of cytokines

They direct cell migration, adhesion, and activation

Action of chemokine family of cytokines

regulate inflammatory and immune responses

Action of tumor necrosis factor family of cytokines

Largely communication, but variety of actions depending upon the type of interleukin and cell

Action of interleukin family of cytokines

promote cell proliferation and differentiation

Action of hematopoietin family of cytokines

regulation of immune cells

Action of transforming growth factor B family of cytokines

dimerize (does not have to be 2 of the exact same kind)

Most TLRs have to — in order to elicit an intracellular signal

JAK/STAT

Cytokines Exert Effects thru — Signaling Pathway

Cytokine receptors consist of at least 2 chains.

Cytokine binding dimerizes the receptor and brings the intracellular JAKs (connected to the receptors) close together, where they phosphorylate and activate each other.

Activated (phosphorylated) JAKs are bound by STATs (transcription factors), and the STATs are then phosphorylated by the JAKs.

Phosphorylated STATs form dimers that enter the nucleus and initiate transcription.

JAK/STAT signaling pathway summarized

NLRs complement the function of TLRs; they are intracellular receptors that bind degraded pathogen components.

They induce signaling pathways that cause changes in gene expression and enhance macrophage activation & cytokine secretion

What are NOD-like receptors and what do they do?

macrophages

Upon pathogen encounter, activation and signaling through TLRs/NLRs induce — to secrete cytokines & chemokines

polymorphonuclear

Neutrophils are referred to as — cells because of the appearance of their nuclei

the contents of preformed granules

Pathogens engulfed by neutrophils are killed by —

degradative enzymes, oxygen radicals

Neutrophils kill bacteria using — and —