Antibacterial responses

Question 1

Some examples of pathogenic bacteria

Answer 1

S.aureus causes infection of skin, lungs, soft tissue, toxic shock.

• Mechanism: pore forming toxins, causes acute inflammation and can produce toxins that act as super antigens.

V.choleare produces diarrhoea.

• Has potent cholera toxin, ribsoylation of G protein subunits - increase cAMP.

M.tuberculosis causes TB

• induces dysregulated activation of macrophages, formation of granulomas and tissue destruction.

Rickettsia causes thyphus by causing endothelial infection and dysfunction

N.meningitidis causes meningitis.

• Mechanism by causing acute inflammation, tissue damage and sepsis due to potent endotoxin.

Question 2

Features of a bacterial infection

Answer 2

Bacterial pathogens live and replicate in extracellular spaces

Several of the most acute and dangerous bacterial diseases are caused by the toxins bacteria produce

Key steps in infection:

• entry
• invasion and colonisation of host tissue
• evasion of immunity
• tissue damage

Question 3

Does all bactria cause disease?

What is the microbiota?

Answer 3

No

The intestine in a healthy adult contains about 10¹⁴ essential bacteria

With about 10¹² on skin

Microbiota is a mechanism of protection to infection both ecological and immunological – because we have good bacteria, pathogenic bacteria find it difficult to find issue and colonise.

• but because we have microbiota that is diverse, immune system samples it, gets used to it and avoids incorrect immune responses

Question 4

First line of defence of innate immunity is barriers

What does the mechanical barriers include?

Answer 4

Skin, flow of fluid, perspiration, slough off of skin

GI tract, flow of fluid, mucus, food, saliva

Respiratory tract, flow of fluid and mucus

Urogenital tract, flow of fluid, urine, mucus

Eyes, flow of tears

Question 5

What do chemical barriers include?Next

Answer 5

Skin - sebum (fatty acids, lactic acid, lysozyme)

GIT - acidity, enzymes (proteases)

RT - lysozyme in nasal secretions

UT - acidity in vaginal secretions, spermine and zinc in semen

Eyes - lysozymes in tears

Within those secretions there are chemical molecules which will be harsh for bacteria to live around and colonise

• Like enzymes
• Low pH / acidity
• Fatty acids
• Lysozymes

Question 6

What are the microbial barriers?

Answer 6

Normal flora of skin, GIT, RT, UT and eyes

protects us from pathogenic bacteria finding a niche or unwanted immune responses

Question 7

What are anti-bacterial peptides (defensins) capable of?

Answer 7

Killing bacteria by penetrating microbial membranes and thus disrupting their integrity

active against bacteria, fungi and enveloped and non-enveloped viruses.

Question 8

What are the two types of defensins?

Answer 8

Alpha defensins secreted mainly by neutrophils and by Paneth cells

Beta defensins are secreted by a broad range epithelial cells, in particular, those in the respiratory tract, skin and urogenital tract

Question 9

If these initial barriers are crossed what happens?

Answer 9

First element of innate response is complement

Complement consists of many complex molecules, some can detect PAMPs

Complements are PRRs

• complement can recognise peptidoglycan or LPS and activate alternative pathway of complement
• bacteria with mannose sugars on cell walls detected by mannose binding lectin, activates lectine pathway

Question 10

What is the complement system and its three pathways?

Answer 10

Key effector function of the humoral response

Serum and cell surface proteins that interact with one another to generate products that eliminate extracellular bacteria

Three pathways

• Classical pathway
• Lectin pathway
• Alternative pathway

Question 11

What are the three pathways activated by?

Answer 11

Classical pathway activated by antigen;antibody immune complexes

Lectin pathway activated by PAMP recognition by lectins – MEL and other carbohydrates

Alternative pathway activated by spontaneous hydrolysis/pathogenic surfaces – specific pamps on surface of bacterial cells

Question 12

What molecules are involved in the complement system pathways

Answer 12

The complement has molecules to start the cascade of reactions and interactions between many proteins e.g

• C1q for classical pathway
• C3 and C3B for other pathways

Pathways converge onto C3 and C5 convertase and then branches out and has specific indirect and direct effector functions

Indirect – induces inflammation, recruit inflammatory cells. Can act like an antibody and enhance phagocytosis by opsonisation.

Direct - can lead onto formation of a complex that punches holes in the membranes of bacteria, inducing lysis

Question 13

One of the complement functions is opsonisation and phagocytosis

Answer 13

Here was have a microbe that has been detected by molecule C3b

which decorates microbe via C3BR and then targets microbe for a more efficient phagocytosis via opsonisation.

C3b tags microbe for more efficient phagocytosis.

One way complement stops and attacks extracellular bacteria.

Question 14

Another function of complement is stimulation of inflammatory reactions

Answer 14

Stimulation of inflammatory reactions:

Tagging of specific PAMPs on surface of microbe by complement molecules like C3b can via C3a and C5a contribute to inflammatory response.

• C3a and C5a released

Brings in antibacterial neutrophils to destroy the pathogen.

Question 15

Functions of complement system are opsonisation and phagocytosis, stimulation of inflammatory response and complement-mediated cytolysis

Describe complement-mediated cytolysis

Answer 15

Implementation of an effector function specific to complement.

Polymerisation of membrane attack complex.

These are groups of proteins coming together to form cylinders, punches holes in membranes of bacteria, lysising the bacteria.

Complement itself destroys bacteria.

Question 16

What is the role of complement receptors in phagocytosis?

Answer 16

Complement interacts with phagocytes

Macrophages have high levels of expression of complement receptors on surface e.g CR3, CR4, CR1

Complement molecules on bacteria detected by complement receptor, internalised more efficiently for phagocytosis

• leads onto production and start of inflammatory response

Question 17

What are the three pathogenic recognition receptors?

Answer 17

• TLRs
• Nod like receptors
• Rig like receptors

receptors that are membrane bound are not soluble e.g TLRs

imporant in allowing cell to detect infection

Question 18

There are many TLR in humans and mice

Answer 18

TLR1 to TLR10

Surface TLRs deect elements outside the cell like extracellular bactera

Intracellular TLRs detect intracellular bacterial infection

• TLR 9 which detects bacterial CpG DNA
• TLR8 detects ssRNA
• TLR7 detects ssRNA
• TLR3 detects dsRNA
• TLR2 detects peptidoglycan
• TLR4 detects LPS

Question 19

PRR and PAMPs interactions leads to phagocytosis and cytokines

What are the consequences of detection of PAMPs by PRR?

Answer 19

Phagocytosis and cytokine production.

Some molecules that tag pathogens will give signals to macrophages to phagocytose and engulf them.

PAMPs provide signals to macrophages that an infection exists for example TLRs.

Phagocyte recognises and finds target cell that is tagged for phagocytosis , it carries out phagocytosis efficiently.

At the same time if it detects presence of infection through TLR’s, it gets activated and starts producing pro-inflammatory cytokines and chemokines, leading onto inflammatory response.

Question 20

Describe the process of inflammation

Answer 20

Multi-process, multi-location and multicellular, process

Initiated by detection of infection by a resident cell in the tissue, which gets activated and produce inflammatory cytokines and chemokines

• IL-1, IL-6, TNF-a

Exerts effect on endothelium and beyond, changing characteristics of endothelium. Allows for circulating leukocytes to go to site of damage/infection.

• Neutrophils go first
• Followed by infiltration of monocytes
• And specific lymphocytes

Some local cells can also contribute to process e.g mast cells releasing histamine, prostaglandins which have local and systemic effects.

Inflammation is a response that can be beneficial if in equilibrium but also injurious, can cause damage.

Infections that cause severe/dysregulated inflammation is damaging.

Question 21

How do neutrophils kill bacteria?

Answer 21

Phagocytosis and degranulation of granules – intracellular killing of bacteria

Phagocytosis and oxidative burst kills the bacteria

Neutrophils can also kill bacteria phagocytosing bacteria – neutrophils extracellular traps (NETS)

Early on recruited via chemo attractants like IL-8

Question 22

Humoral response: function of antibodies

Answer 22

Antibodies are good at neutralising bacterial toxins, good at stopping mechanisms of pathogenesis

Trigger classical complement pathway by binding of IgM to the bacterial cell surface

Good at inducing opsonisation; coating bacteria with antibody thereby aiding phagocytosis

Question 23

What to toxins cause?

How do antibodies stop this?

Answer 23

Bacteria secrete toxin that binds to cellular receptors, causing internalisation and alteration of metabolic pathway or of an essential activity of the cell.

Leading onto cell death.

Antibodies can block the cytokine in the process of neutralisation, stopping the toxin from binding to its receptor.

Antibodies can block the causes of disease and allow for continued normal function of host cells.

Question 24

How do antibodies carry out opsonisation?

Answer 24

Antibodies enhance neutralisation by interacting with Fc receptors on surface of pathogen, blocks toxin from binding to antigen, aiding destruction by phagocytosis

• Through intercation with FcR, antibodies coat entire bacterial cell
• More efficiently phagocytosed because they’re opsonised
• Antibodies can also aid complement and induce robust phagocytosis through complement activation and antibody opsonisation

Question 25

Different classes of immunoglobulins have different anti-bacterial qualities

Answer 25

IgG and IgA good at neutralising toxin

Osonisation IgG1/3 are good at it, others aren’t like IgG2/4

Sensitisation for killing by NK cells is efficient by IgG1/IgG3

Sensitisation of mast cells specifically performed by IgE

Activation of complement efficiently performed by IgM and IgG3

Properties indirectly linked to infection:

• IgA can cross epithelium and be useful by being present in area with high levels of toxins
• Can cross placenta, provide protection like IgG1/3/4
• Diffusion into extravascular sites, IgG

Question 26

What is the humoral response against extracellular bacteria (adaptive)

Answer 26

Bacterial infection

Detection by B cells

Processing of APC

T cells help to become plasma cells that produce antibodies that can neutralise, opsonise through Fc receptor mediated phagocytosis

Or through complement mediated phagocytosis

Can also activate and induce the same types of phagocytosis as well as inflammation and lysis of microbe

Question 27

What is the cellular adaptive immune response to bacteria?

Answer 27

Infection

Dendritic cell processing antigen from bacteria

Presented in context of MHC class 2 molecule to a T helper cell

Could produce TNF, IL-71 and other cytokines, leading onto inflammation

Anti-bacterial processes mediated by cytokine IFN-y, further activates macrophages to destroy intracellular bacteria by phagocytosis

Various cytokines could support antibody response

• link between cellular responses and humoral is through antibody production

Question 28

How are antigens presented by DC?

Answer 28

Extracellular bacterial will rapidly detected by dendritic cells locally present in tissues, detect PAMPs through TLRs.

Takes up antigen, quickly migrates into lymph node to present to T cells

Activate specific T cells in presence of polarising cytokines, can send T cells into other phenotypes e.g T helper 1 and induce a cellular response

Question 29

Describe the time course of infection vs number of viable bacteria

Answer 29

Pathogen load increases, initial immune responses will kick in, start controlling the pathogen

Starts tailing off towards control of infection

Innate immune responses important to control bacteria, eliminating any potential intracellular pathogen that hasn’t reached space, and triggering inflammation

Inflammation in environment rich in IL-12 further activate other innate immune cells such as NK cells to produce IFN-y and support elimination of intracellular bacteria

Innate immune response only controls infection, not enough to clear

Once this breeches onto adaptive immune response through antigen presentation and activation of T cells

T cells differentiate in an environment rich in IL-12 and TFN-y are likely to differentiate into Th1 cell that could produce more IFN-y to help macrophages

and provide other signals through CD40L to activate macrophages

Eradicates infection

Question 30

MHC class 2 presents bacterial antigens derived from extracellular antigens to helper CD4 T cells

Answer 30

Helper T cells enhance phagocytosis of extracellular pathogens by activating macrophages

macrophages: intracellular infections need to start an immune response through processing and presentation of antigens (in context of MHC 2)

Bacteria phagocytosed by macrophages, peptides available to MHC II molecules in lysosomes

bound peptides transported by MHC II exported to cell surface

Helper T cell recognises complex of peptide antigen with MHC class 2, activates macrophage

In right cytokine (IL-12 + TFN-y) environment the T cell will help macrophage through production of IFN-y.

For organisms like M.tuberculosis this is not enough and may need extra layer of cooperation by other T cells.

Question 31

How can CD8+ T cells cooperate during intracellular infections?

Answer 31

CD8 t cells can cooperate to fight intracellular infections.

Here you have macrophage that is infected, Th1 cell that can produce IFN-y and can help killing the bacteria inside the cell.

if that fails, proteins of the bacteria could be present in the phagolysosomes and also in cytoplasm of cell.

Potential for those proteins to be mounted and presented ontop of MHC I and then presented to bacterial specific cytotoxic T cell.

Kills infected cell.