Lecture 19: Defense against Infection and Alternative Complement

Question 1

What are the direct pathogenic mechanisms of tissue damage?

Answer 1

Exotoxin release acts at cell surfaces.
Endotoxin release after phagocytic digestion results in cytokine secretion.
Pathogens can directly kill cells like viruses.

Question 2

What are the indirect pathogenic mechanisms of tissue damage?

Answer 2

Ab:Ag immune complexes can localize in joints, kidneys, blood vessels and cause damage.
Autoimmune conditions can occur when aby against pathogens have cross-specificity with self.
The presence of pathogens often induces an inflammatory response which can damage self cells (collateral damage)

Question 3

What host defences exist at what sites of infection?

Eg) Where would antibodies act and where would apoptosis occur?

Answer 3

Interstitial spaces, blood, lymph: antibodies, complement, phagocytosis

Epithelia: IgA, antimicrobial peptides

Cytoplasm: Cytotoxic T cells, NK cells

Vesicles: Activated macrophages

Question 4

How does the epithelium act as an immune barrier?

Answer 4

The epithelium is a continuous barrier.

There is mechanical protection: scleroproteins, tight junctions, cilial movement.

There is chemical protection of enzymes (lysozyme, pepsin, antibacterial defensins) and pH modulation.

There is microbiological protection: bacterial commensalism

Question 5

Detail the formation of the soluble C3 convertase iC3Bb of the alternative pathway.

Answer 5

C3 is produced by the liver and secreted into the blood in an inactive form.
C3 slowly, spontaneously, changes conformation to expose an active thioester bond which binds water to produce iC3.
Soluble iC3 binds inactive factor B making factor B more susceptible to cleavage.
Protease Factor D cleaves iC3 bound Factor B. Ba is released while Bb remains and has protease function. (iC3Bb)
soluble iC3Bb binds another C3 and cleaves it producing C3a (anaphylatoxin) and C3b.
Some reactive C3b, which has an exposed reactive thioester bond, binds the bacterial cell surface covalently.

Question 6

Detail the creation of and tagging of a microbial surface by the surface bound C3 convertase in the alternative pathway.

Answer 6

The soluble iC3Bb cleaves C3. Some C3b will be sufficiently close to and react with the microbial cell surface.
This bound C3b associates with factor B, which is cleaved by Factor D to produce C3bBb. This acts to cleave more C3 into C3b and C3a.
The C3b continues to bind to and tag a microbial surface.
C3a acts to recruit phagocytic cells which bind C3b by a CR1 receptor and engulf and destroy the target.

Question 7

What is the purpose of Factor P in the alternative pathway of complement activation?

Answer 7

Properdin (Factor P) stabilizes the membrane bound C3 Convertase C3bBb. This lengthens the half life of the protein; it increases the speed and power of complement activation by limiting degradation. by Factor H.

Question 8

How do self-cells prevent their destruction by alternative pathway complement activation?

Answer 8

Decay accelerating factor (DAF) is present on human cells and binds to C3bBb causing the dissociation of Bb.

Membrane Cofactor Protein (MCP) is present on human cells. It causes Bb to be dislodged. It also makes C3b more susceptible to cleavage by factor I.

Both DAF and MCB act to protect self-cells from complement mediated destruction.

Question 9

What receptors do macrophages have for pathogenic constituents?

Answer 9

Toll receptor: signalling
fungal glucan receptor
Scavenger, LPS, Mannose receptors for bacterial components.
CR3: binds C3b deposited on surface

Question 10

Detail the process of a toll -like receptor activation in macrophages.

Answer 10

LPS binds CD14.
CD14 associates with TLR-4 and MD2 forming a complex.
MyD88 is an adaptor between the TLR4 complex signal and activates a protein kinase cascade.
This cascade results in the release of nuclear transcription factor NFkB.
This activates transcription of genes for several inflammatory measures.

Question 11

What products does a macrophage secrete upon sensing microbial product and what are their local functions?

Answer 11

IL-1B: activates vascular endothelium

TNF-a: activates vascular endothelium and increases vascular permeability

IL6: lymphocyte activation, increased Aby, local up regulation of metabolism of fat and muscle cells

CXCL8: recruits neutrophils, others, to sites of infection.

IL-12: activation of NK cells, inducement of naive T cells to Th1 cells.

Question 12

What are the systemic effects of large-scale macrophage activation?

Answer 12

IL-6 acts on the liver hepatocytes to induce the acute-phase protein function.

IL1 and TNFa act on the hypothalamus to raise temperature (increased metabolism). Fever acts to hinder microbial replication.

Question 13

What is the acute phase response?

Answer 13

This describes a significant change in the concentration of plasma proteins in response to infection. There is an increased amount of proteins that act as opsonins (C-reactive protein CRP and mannose binding lectin MBL) and those that amplify the inflammatory response. This process occurs in the liver hepatocytes in response to IL6 secretion.

Question 14

What is the difference between a CC and CXC chemokine?

Answer 14

A CC chemokine has two cysteine residues side by side. These activate mainly monocytes, then NK and T cells, basophils, dendritic cells.

A CXC chemokine has an aa in-between two cysetines. These activate mainly neutrophils then naive or resting T cells B cells, and NK cells.

Question 15

Distinguish a local from systemic infection?

Answer 15

A local infection is confined to an area. A systemic infection results from bacteria or infection of the blood. Liver macrophages are activated by LPS in the blood. There is systemic secretion of pro-inflamatory cytokines (especially TNF-a) causing widespread coagulation and organ failure.

Question 16

What are the two main dedicated phagocytes and what are their characteristics?

Answer 16

Macrophages are long lived, tissue resident and secrete cytokines. They are ready to attack invading pathogens on demand.

Neutrophils have a short life span, are more abundant, but must be first recruited by cytokines like CXCL8 released at sites of infection by cells (macrophages).

Question 17

Detail neutrophil recruitment to sites of infections.

Answer 17

Neutrophils travel through the blood quickly but in areas of inflammation, there is a greater change that they may interact with surface adhesion molecules.

Neutrophils are directed to sites of infection through interactions with adhesion molecules.

Endothelial selections hook silly-lewisx glycoproteins on neutrophils inducing rolling.

Endothelial iCAMS bind cellular integrin (LFA1)

Neutrophils migrate into tissues under guidance of CXCL8 receptors CXCR1 and 2

Neutrophils secrete proteases and other enzymes to break through the basement membrane (collagen and laminin) as they follow the CXCL8 gradient.

Question 18

What are the neutrophil effector functions?

Answer 18

Receptors for microbial constituents. Binding induces phagocytosis and killing.

Phagosomes fuse with preformed neutrophil granules and lysosomes.

Neutrophils use respiratory bursts and myeloperoxidase too.

Question 19

What are some mechanisms of a neutrophil killing?

Answer 19

Acidification 3.5-4
Oxygen radials including:
superoxide
hydrogen peroxide
hypo halite
Nitrogen oxides
defensives and other antimicrobials
NADPH-dependent oxidases produce toxic oxygen species (respiratory burst)
Lysozyme etc
Lactoferrin, other chelators.

Question 20

What happens to the short lived neutrophils?

Answer 20

Once all neutrophil granules are used up, they die by apoptosis or netosis. DNA NETs. Mess cleaned up by macrophages.

Question 21

What are DNA NETs?

Answer 21

These are neutrophil extracellular traps, which trap and kill pathogens at the end of a neutrophil life cycle.
The nucleus of a neutrophil swells and bursts. Chromatin dissolves and becomes extruded from the cell in a network of decondensed DNAand histones.
These NETs and the trapped pathogens are cleaned up by macrophages.

Question 22

Which cytokines are induced in response to viral infection?

Answer 22

All human cells can produce and have receptors for Type 1 interferons a and B.

Type 1 interferons are secreted in response to viral infection and interfere with viral replication as well as warn adjacent cells.
(increased viral resistance to viral replication, more MHC1 and antigen presentation, activation of NK cells).

Question 23

Detail the interferon response due to viral infection.

Answer 23

Virus infects cell.
IRF3 induced to move to nucleus along with other txn factors inducing production of IFNB. Production of IFNB signals in autocrine fashion for the cell to secrete IFNa. It also signals in paracrine fashion to other cells to induce the interferon response.
Type 1 interferons recruit NK cells which manage virally infected cells while killer T cells are being produced.

Question 24

What are B-1 cells?

Answer 24

These cells have more general affinity for bacterial antigens and can response quickly to a primary infection.

Question 25

What are y:d T cells?

Answer 25

These secrete inflammatory cytokines and develop cytotoxic function. These are more abundant in tissues than in circulation, share properties with NK cells, have less specific T cell receptor.

Question 26

Detail an immunological memory response from first infection to clearance of second infection.

Answer 26

In the primary adaptive immune response acts to subdue infection outrunning innate immunity. There is production of pathogen specific T and B cells creates. There are many short lived effector cells and fewer long lived memory cells of matching isotype.
Low levels of Ab are maintained by long-lived plasma cells in the bone marrow.
A second infection by the same pathogen overwhelms the innate immune system (Ab insufficient), However, the memory response of B and T cells acts to quickly produce highly specific, isotype switched antibodies. Thus the memory response acts to prevent damage by a pathogen by overwhelming it with antibodies (or cytotoxic cells) to quickly clear the infection.

Question 27

What is the original antigenic sin?

Answer 27

The first strain of a virus constrains the future response to other strains of the same virus. This is common in highly mutagenic viruses like influenza. The immune system fights what it remembers prioritizing original ab production over novel production. Only when all epitopes of the original response are gone can another primary response be elicited.