Immunology - Ch. 5 - Innate Immunity II

Lecture 5!! Last one for the exam!! WOOO

Hans Buchner

observed that serum could kill bacteria, even if from non-immunized animals that had no previous antibodies

Jules Bordet

discovered heat-sensitive component of serum that could lyse bacteria. also showed heat insensitive component (antibodies) that could enhance this activity.

Lysis complemented the immunity granted by antibodies

Complement System

integrated innate immune mechanism for detection and destruction of microbes, also critical factor for initiation of adaptive immune response

C3 protein

alpha-beta heterodimer linked by disulfide bond, highly reactive thioester bond,

initiates the sponstaneous C’ cascade, which involves enzymes to defend against pathogens

Step 1 - C’ cascade

C3 spontaneously hydrolyzes to C3b-O-R (unstable), via C3 convertase

Step 2 - C’ cascade

factor B binds C3b

factor D cleaves B to generate C3bBb

Step 3 - C’ cascade

properdin stabilizes C3bBb complex, (from neutrophils),

Step 4 - C’ cascade

active C3bBb converts more C3 to active C3b

Step 5 - C’ cascade

If no inactivation occurs, results in downstream activation of complement cascade and membrane attack complex (MAC), via C5 convertase

Step 1 - Lectin Cascade

Mannose binding lectin (MBL) recognizes carbohydrates on microbial surfaces

Step 2 - Lectin Cascade

MASP2 converts C4 and C2 to C4bC2a, via C3 convertase

Step 3 - Lectin Cascade

C4bC2A converts C3 to C3b, creating C4bC2bC3b

Step 4 - Lectin Cascade

C4bC2bC3b converts to C5b-9 (MAC), via C5 convertase

All same regulatory components displace C2b and inactivate C3b

Natural antibodies

low affinity, poly reactive antibodies that are spontaneously produced by B1 B cells

primarily develop from fetal liver

-recognizes phospholipids, glycoproteins, nucleic acids, some proteins

-may be either self or microbial products

-roles in scavenging and innate recognition

Antibody deficiency

defective innate responses, because of no natural levels (death)

Amplification of Complement Cascade

Lectin (MBL) pathway does million-fold amplification, whereas spontaneous pathway does thousand-fold amplification

MAC Mechanism of Action

Hypotonic lysis of microbes (influx of water molecules)

Loss of membrane potential shuts down ATP/H+ ion pump

Shuts down Na+/K+ transporter to disrupt cell signaling

Anaphylatoxins

Potency:

C5a > C3a » C4a > C2b

(activating) —> (inhibitory)

C5a & C3a bind to receptors on smooth muscle (myalgia) and endothelia (vascular permeability)

activation of other innate immune cells (inflammation)

Host proteins (complement proteins)

Innate cells (mostly macrophages) express complement receptors, that recognize host proteins

CR1, CR2 most important complement receptors

-Both bind complement products, activate macrophages and granulocytes, aiding in transport of antigen to lymph nodes

-CR2 also acts as a coreceptor for initial B cell activation in lymph nodes

CR1

binds primarily C3b on macrophages, leads to formation of MACC

C5a

activates macrophages to phagocytose CR1 bound to C3b, leads to phagocytosis of microbe

CR2

binds primarily to iC3b, favors transport of dead microbes by macrophages to lymph nodes and B cell activation

Bb

displaced by Factor H, DAF, MCP, CR1

Factor I

cleaves C3b into iC3b (inactive)

C3b:Ag

complexes that are brought to lymph nodes by CR2 on macrophages, “handed off” to follicular dendritic cells (FDC) that present to B cells

Initial B cell activation

BCR + CR2 on B cells is crucial to initiate B cell activation

1/3 - Functions of Complement System

-Aid in killing microbes (MAC & phagocytosis) 1/3

2/3 - Functions of Complement System

-transport of antigens to lymph nodes (iC3b, CR2) 2/3

3/3 - Functions of Complement System

-initiate inflammation (anaphylotoxins) 3/3

1/3 - Mechanisms of Complement Cascade Activation

-spontaneous C3 hydrolysis (spontaneous pathway) 1/3

2/3 - Mechanisms of Complement Cascade Activation

-MBL binding (MBL pathway) 2/3

3/3 - Mechanisms of Complement Cascade Activation

-C1q binding (classical pathway) 3/3

Natural killer (NK) cells

survey host cells for changes, mostly in protein expression to check for pathogens

Host Cells (for NKs)

express ligands for activating and inhibitory receptors,

inhibitory receptors are dominant over activating, preventing NK killing under normal conditions, binding to MHC proteins needed for adaptive responses

Infection (for NKs)

during infection, pathogens use host ribosomes leading to loss of inhibitory ligand expression, dominance of activation signal & NK killing host

NK Mechanism

Caspase-3 activation, DNA fragmentation, membrane blebbing (filling with fluid)

Step 1 - NKS

NKs recognize stressed host cells

Step 2 - NKs

Loss of ligands for inhibitory receptors, stimulation of activating receptors and killing of host cell

Step 3 - NKs

NK cells lyse target cells by two mechanisms

1) Perforin → Granzyme B → BID → Caspase 9 → Caspase 3

2) TNFR → FADD → Caspase 8 → Caspase 3

Step 4 - NKs

Caspase 3 → results in DNA fragmentation and apoptosis