BIOL 2022 - Introduction to the immune system and innate immunity

role of the immune system

• defence against infectious disease

• complex to deal with a range of organisms

• protection against some tumors

• vaccines offer new hopes for infection and cancer therapy

• evolves very fast

• needs to be regulated —> chronic immune response can cause disease

  • E,g: sepsis, autoimmunity, type 2 diabetes

• has 2 arms: the innate and the adaptive immune system

cells of the immune system

HEAMOPOETIC STEM CELL

• give rise to the cells of the immune system

• either lymphoid or myeloid lineage

• generated in the bone marrow

CYTOTOXIC T CELLS

• occur after the generation of active t cells

• interact with the antigen on surface of cells which signal that the cell is infected

• drives downstream signalling which causes the immune attack on infected cells

MACROPHAGES

• can recognise, digest, take up pathogens and present their antigens

B CELLS

• also recruited by dendritic cells but their role is to produce antibodies

COMPLEMENT

• not a cell but binds to pathogens which leads to direct lysis

• produced in the liver and excreted in the blood

• produced in response to threat

PART 1: INNATE IMMUNE SYSTEM

innate immune system

• quick and non specific response

• results in the release of chemical mediators which create inflammation which supports the immune response

• can lead to the direct removal of the infectious agent

• have receptors which recognise the pathogen and can distinguish between viruses and bacteria

• activation drives downstream signalling to produce mediators (E.g: cytokines and interferons)

• made up of the humoral and cellular immune system

  • HUMORAL: driven by cytokines, proteins, mediators

  • CELLULAR: phagocytes and NK cells involved in taking up or killing the cells (cellular components)

• the innate immune system is in control of detection of pathogens and the immediate response to them

• it can then recruit cells from the blood and send signals to promote the required response

• in order to activate the innate immune system it must breach one of the bodys natural barriers

bodys natural barriers

SKIN

• anatomical mediator

• very protective barrier

PHYSIOLOGICAL BARRIERS

• includes:

  • temperature

  • pH

  • chemical mediators

INFLAMMATORY BARRIERS

• leakage of serum proteins with antibacterial activity

• influx of phagocytic cells into the affected area

signs of inflammation

• swelling (tumor)

• redness (rubor)

• pain (dolor)

• heat (calor)

• and it was later added: functio laesia —> loss of function

when a barrier is peirced

1. histamine is produced as a response to the skin being peirced

2. dilation and increased leakiness of blood vessels due to the leakiness of tight junctions

3. phagocytes migrate to the area and consume the bacteria and cell debris so the tissue heals

toll like receptors

DISCOVERY

• 1995-1997

• believed that the immune system needed a danger system to be activated

• just prescence of foreign antibodies is not enough

• receptors on immune cells must respond to danger

• toll gene important in embryogenesis but later a toll knockout resulted in flies covered in fungus which proves that the toll gene has a role in immune defence

• a group of genes important in immune sensing are the toll like receptor genes

• the first proof of toll like receptors sensing microbial patterns was in toll like receptor 4

• mutation in Lps gene will selectively impede liposaccharide signal transduction and results in resistance to endotoxin as TLR4 proteins role is recognition of LPS

pathogen recognition receptors

• theres at least 12 types

• has a structure which recognises PAMPs

• LPS PAMP from gram negative bacteria initiates various innate responses

  • phagocytosis of pathogens

  • chemokines and cytokines to recruit cells

  • lysis of pathogens

• there are a whole range of pathogen recognition receptors but toll like receptors are a subclass

toll like receptors (TLR4)

• trigger signal cascades

• LPS interacts with toll like receptor 4 which causes a conformational change in the intracellular domain and can now interact with proteins

• one of the proteins that it interacts with is MyD88 which is an adaptor protein that leads to upregulation of:

  • cytokines

  • chemokines

  • endothelial adhesion molecules whih allow glucosides to bind to and allow leukocytes to move into cells from blood

• MyD88 also activates the costimulatory molecules which promote a t cell response

• toll like receptor 4 also activates the proetin TRIF which activates

  • interferon expression through TFs

  • (interferons are important in viral recognition but also prepare cells to be recognised by immune cells)

how does signalling of toll like receptors work

• their extracellular structures are leucine rich structure which interacts with LPS of a range of gram negative bacteria

• the intracellular region can then interact with MyD88 only when it undergoes conformational change

• MyD88 then interacts with a range of kinases (IRAK 1 and 4) which phosphorylate the complex

• phosphorylation generates a docking signal for TRAF6 which forms a dimer —> dissociates from the 1st part of the signalling cascade

• this dimer then complexes with TAK1 which causes kinase activation

  • the IKK complex is phosphorylated and Ikappa b degraded.

  • this frees up and activated NFkappaB

  • activation of NFkappa B which is important for MAPK pathways —> causes transcription of TNFa and IL6

NFkappa B

• in most mammalian cells, usually present in the cytoplasm associated with I kappa B

• teh TAK1 is able to phosphorylate I kappa B causing dissociation

• it then becomes ubiquitinated and degraded in the proteosome

• NFkappa B can now move into the nucleus and initiate transcription of cytokines

• eventually NFkappa B will move into the nucleus where it will reassociate ith Ikappa B

toll like receptors on macrophage/dendritic cells

• both have toll like receptors but respond in different ways

• both have external TLRs on the CSM which are useful for fungi parasites and bacteria

• has internal TLRs in endosomes which can detect viruses

MACROPHAGES:

• gene transcription from the activation of TLRs causes cytokine synthesis and oxidative burst

• OXIDATIVE BURST:

  • increased oxygen radicals

• various TLRs and various cytokines to differentiate between virus and bacteria

DENDRITIC CELLS:

• the outcome is different

• still results in cytokine production but also results in production of costimulatory molecules which is key for efficient antigen presentation

• helps to activate t cells

• various TLRs and various cytokines to differentiate between virus and bacteria

types of TLRs

• TLR4 is the only TLR which responds to the TRIF pathway

• TLR 1, 2, 5, 6 also can respond to PAMPs but they might be gram negative or bacteria with flagella

• extracellular TLRs dont use the interferon pathway, they use the MyD88 pathway

• endosomal receptors interact with ss or ds RNA from the viruses

• TLR 3 is the only TLR which uses the interferon from TRF pathway and recognises ds RNA

cell types

NK CELLS

• not technically part of the innate immune system but important for immune surveilance

DENDRITIC CELLS

• produce a number of cytokines which allows other cells to be activated and drive specific immune response themselves

MACROPHAGES

• phagocytosis and cytokine production

NEUTROPHIL

• can phagocytose and produce potent enzymes

cytokines and interferons

• cytokines are local mediators but can also act systemically if secreted in the blood

• they also have their own receptors which act through the jaks and stats signalling pathway

LOCAL FUNCTION

• they can signal to cells to put up barriers —> increased expression of TLRs

• this might result in direct cell death

• will recruit WBCs to the site of infection

JAKS AND STATS

• the janus kinase has 2 faces: one which interacts with the receptor, the other that initiates downstream signalling pathway

• the janus kinase binds to receptor and phosphorylation of the tyrosine kinase creates a binding domain for SH2 domain of stats (signal transduction and activation of transcription)

• stats activates a TF that binds DNA sequence specifically and promotes transcription from GAS element in response to cytokine stimulation

• a combination of different jaks and stats allows immune cells to respond differently to different cytokines

• not just the cytokine itself that signals danger but also the response to the cytokine

local vs systemic effects of cytokines

LOCAL

• increased permeability

• increased adhesion molecules

• decreased flow rate

• increased chemokine expression

• increased activation degranulation

• all of which leads to increased recruitment of immune cells

SYSTEMIC

• acute phase proteins

• hypothalamus: fever which acts as phsiological defence

• mobilises cells from the bone marrow

interleukins

IL 1beta

• activates vascular endothelium

• produced by macrophages

• promotes local tissue destruction

• increased access of effector cells

IL 6

• activates B cells

IL 8

• recruits neutrophils

• chemokines

TNF alpha

• causes complement activation

• increased permeability

• generates fever

cells needed for viruses

• not recognised well by the innate immune system

• macrophage needs to produce cytokines which activate T cells

• IL12 is a good at this - it differentiates TH1 cells which produce LT and interferon gamma

• IL4, 4, 6, 10, 13 differentiate TH2 cells to B cells

complement activation

• complement system important for immediate response

• hemolysis of red blood cells is mediated by antibody involved complement

• has roles in innate and teh adaptive immune system

• not only activated by antibodies, can be activated by other parts of innate IS

• mammalian complement has 3 pathways:

  • classic activation

  • mannose binding lectin

  • alternative actiavtion

alternative activation

• activation pathway used by pathogens

• C3 in the blood is usually active, it undergoes small ammounts of spontaneous hydrolysis in water

• in the prescence of the pathogen hydrolysis increases and can interact with B, D, P factors

• interaction with these factors allows it to undergo a conformational change and be cleaved

• the C3 protein is the central protein of all 3 pathways

• theC3 confetase acts as an enzyme to cleave C3 protein in the blood (unhydrolysed)

• cleaved C3 forms C3a, C3b and lymphocyte activation

downstream effects of C3

ANAPHLATOXINS (C3a)

• highly attractive for immune cells

OPSINS (C3b)

• can bind and incorporate into cell membranes

• forms a conformational change forming C5 confetase

• C5 gets cleaved into C5a (an anaphylatoxin)

• C5 can also bind to C6, C7, C8 to form poly C9 - a membrane attack complex which forms a pore in the membrane of bacteria and leads to the lysis of the membrane

mannose binding lectin pathway

• complement pathway activates by sugar proteins, rather than structures on the bacteria

• mannose sugars activates MASP1 and MASP2 which recruits C4 and causes C2 binding

• this complex forms the C3 conferase which cleaves the central protein

• C3a and C3b pathway then the same

classical pathway

• mediated by antibodies

• antobodies bound to the membrane acts as a platform for Clq binding

• Clq looks like 6 tulips and interacts with the fc compartment of an antibody

• a small conformational change which allows binding of Clr and Cls to be cleaved

• C4 and C2 can then bind to the antibody Clq complex and converted to C3 confetase

• follows the same C3a and C3b pathway

key role of complement

• produces opsonins

• produces anaphylatoxins

• direct killing of organisms

• enhance antigen specific immune response

• maintain homeostasis

importance of complement

• importance of MC which causes lysis

• opsonisation —> phagocytes have receptors for opsonised bacteria and so enhances recognition

• complement receptors are also on the neurophils which helps with extravasion

• might cause degradation of mast cells

• also potent in clearance of the immune complex

• can recognise apoptitic cells

• forms a bridge between innate and adaptive immune system

neutrophils

• mechanism that facilitate inflammation

• short lived - a few hours

• produced in the bone marrow

• attracted by chemokines IL8 and C5a

• can detect and phagocytose pathogens

• effector mechanisms to kill pathogen

• results in pus

• macophage/mast cells produce histamine which can signal to cells in the blood

• cells in the blood when signalled to will extravate into the tissues

leukocyte extravasion

• chemoattraction of leukocytes —> they tether and start rolling on the endothelium which slows them down

• mediated through selectin interations

• similar in lymphocytes

• the final step is migration between tight junctions

MECHANISM

1. tethering and rolling

• siayl lewis X on neutrophils binds to E&P selectins on endothelial cells

• the integrin LFA1 binds to E and P selectins on endothelial cells

• slowing down and transient adhesion of neutrophils

2. ARREST (driven by chemokines)

• cchemokines activate neutrophils

• activation of endothelial cells increases the expression of adhesion molecules

• a conformational change of LFA1 allows tight binding to ICAM1 on endothelial cells

• firm adhesion and arrest of neutrophil

3. EXTRAVASION

• neutrophil squeezes between endothelial cells into tissues attracted by chemokines at the site of attraction

• phagocytosis and effector mechanisms directed at the pathogen (still dependent on chemokines)

• t cells, monocytes, dendritic cells all help in recruitment of chemokines

• chemokines expressed on immune cells cal allow viruses to enter the cells

macrophages

• recognise particles, bacteria, dyes etc

• many diff types with homeostatic functions to ensure normal physiology

  • liver - kupffer cell

  • brain - microglia

  • bone - osteoblast

  • lung - alveolarmacrophage

• monocyte derived macrophages important in immune response

• activated macrophage has different function to homeostatic macrophage

macrophage mechanism of uptake

• recognised by pseudopods and taken up

• forms a phagosome which fuses with the lysososme

• the phagolysosome partially digests the microbe

• sometimes the antigens are presented on the surface which links to the adaptive immune system

• till like receptors, mannose, glucan receptors, scavenger receptors all halp with the uptake of cells

• there are also different methods of pathogen destruction

  • acidification (3.5 - 4.0 pH)

  • toxic O2 derived products which are highly reactive oxygen radicals

  • antimicrobial peptides

  • enymes - lysozmyes

  • competitors