Lecture 11: White Blood Cells (1) - Introduction and The Innate Immune System

White Blood Cells

• Highly flexible cells → allows them to deal with different pathogens,

• Even though different cells are more suited to different pathogens, cooperation between cells types is almost always needed

  • more that one type of ___ cells involved in clearing an infection

Red Blood Cells

• Cells that are highly specialised for a single task → O2 transport

Types of WBCs

• Neutrophils

• Eosinophils

• Basophil

• Monocyte

• B-lymphocyte

• T-lymphocyte

• Natural (T) Killer

Neutrophil

• String sausage nuclei

• Typical granulation

  • ‘resting form’ – circular cytoplasm

  • Involved in killing small organisms

Eosinophils

• Cell has an affinity for eosin dye – red colour      

• Multilobed, but generally 2-lobed nucleus       ‘

• Contain dense granules that do not overlie the nucleus

  • When stained appear red → acid stains

• Involved in killing large organisations e.g.worms

Basophils

• Contains dense basophilic granules

  • Basophilic/ basic dyes stain granules → stained blue

• Specific features in allergy response and offer protection from worms etc

  • Involved in killing large organisms

• Resemble tissue mast cells - have a similar origin

Monocytes

• Helps to kill bacteria and small organisms  

• Helps activate B and T cells by becoming an antigen-present cell

  • Links the 2 systems

B-Lymphocytes

• Make Antibodies

• Involved in destroying toxins or blood pathogens

T-Lymphocytes

• Assist B-cells in their function      

  • Involved in fighting and killing viruses

Natural Killer Cells

• Evolutionary ancient cell that recognises anything as non-self and destroys it

  • Involved in killing viruses

Need for So Many WBCs

• To address the wide range of pathogens:

  • The very big: e.g. worms – 20-30x bigger than WBC, can’t ‘eat’ it to destroy

  • The big: e.g. bacteria – replicate rapidly

    • ‘eaten’ by WBC due to small size

  • The small and hidden: e.g. viruses

    • Hijack cell’s enzyme machinery to produce virions that are released; spend most time in the cell – difficult to detect

  • The small and not hidden: e.g. toxins, initial virus exposure
    (antibody removal)

    • Available in the blood – must be detected and destroyed

Function of WBCs

• Killing large organisms

• Killing small organisms

• Killing viruses

• Destroying toxins and pathogens

• Anibody response

WBCs: Killing of Large Organisms e.g. Worms

• Can’t be engulfed and digested

• Able to wriggle and move around, evading WBCs and going in and out of tissues 

• Cells need to cooperate and destroy them in the tissues while minimising the damage to normal cells

• Major roles of eosinophils and basophils → immobilise the large organism so that it stays in one place to then be destroyed

  • muse release toxic substances

WBCs: Killing of Small Organisms e.g. Bacteria

• Can be engulfed and digested safely by cells

• This must be done quickly and effectively before the bacteria can cause damage

• Bacteria massively outnumber the neutrophils due to rapid replication      

• Major roles of neutrophils and monocytes → must respond rapidly, safely and many times to ingest and remove bacteria      

• Neutrophils in a healthy individual in a relaxed state – must be activated to attach and ingest bacteria

  • It can be done by monocytes but at a slower rate – useful for slowly proliferating bacteria → can wall things in

WBCs: Killing Viruses

• They infect normal cells and replicate inside them.   

• They are therefore hidden from the immune system.

• The immune cells must therefore recognise and destroy infected cells while not affecting normal cells      

• Major roles of T-lymphocytes and natural killer cells → Recognition, Finding, Increase in Cell N.o, Destruction

Glandular Fever

• Virus infects B-lymphocytes → a B-lymphotropic virus that invades and hides in B-cells

• Activated T-cells recognise the B-cells as virally infected and briefly bind to signal it to die via apoptosis  - preventing virus present inside the B-cell from being released

  • ‘autodigests itself and any virus present inside

WBCs: Destroying Toxins or Blood Pathogens

• Specific proteins (e.g. toxins, bacterial surface proteins, or free virus) may identify an invading organism or have a major effect on the body.

• Spasms caused by the toxin released by bacteria (tetanus) → binds to muscle receptors and activates it

• The body must recognise and respond to these small protein antigens and ideally have memory if the antigen is reencountered:

• Major role of antibody-secreting B-lymphocytes

  • Must produce antibodies to remove the toxin itself – slow process

Role of WBCs in the Antibody Response to COVID 19

• Exposure to the virus was detected by specific antibodies in the blood

• The neutralising Ab was one mechanism by which immunisation was effective against the virus

  • Abs only present in the blood for 3-6 months after which no protection is offered

• Antibodies can prevent viruses from entering cells and mark them for destruction by the immune system

• T-cell response to virus is a major component to the immune response - important in providing ongoing and longer-term protection

Vaccine Design

• Aims to stimulate such antibodies and are one measure of success

‘History’ of the Innate Immune System

• Ancient evolutionary origin.

  • Present since multi-cellular origin.      

• Its cells appear to act in similar ways to free-living organism

Amoeba

• Free living microorganism moves freely:

  • It recognises and ingests prey (non-self)

  • It recognises but does not ingest other amoeba as non-prey (self)

Why are the Cells of The Innate Immune Sytems Compared to Amoeba

• Cells such as neutrophils, eosinophils, basophils and monocytes patrol blood and tissues, ignoring the normal cells (“self”) while recognising and destroying bacteria or worms (“non-self”)

Mechanism of Pathogen Recognition and Response

• Neutrophils, eosinophils, basophils and monocytes recognise common proteins associated with infection.      

• Infected organisms have different proteins not encountered by the body (non-self)

  • Either the products of damage to self or the products of bacterial infection

• These are called DAMPS or PAMPS and are recognised by receptors on the neutrophil surface and activate the cell so that is primed and ready to destroy

Pathogen Associated Molecular Patterns (PAMPs)

• e.g. bacterial cell wall - lipopolysaccharide

• Recognised as foreign

• Recognised by and activates neutrophils, which then destroys/ eliminates them

Damage Associated Molecular Patterns

• e.g. released bacterial components → bacterial BAM and intracellular components

• Recognised as foreign

• Recognised by and activates neutrophils, which then destroys/ eliminates them

Cytokines

• Molecules that are released once a pathogen is recognised to help with the increase formation, (early) release and activation of white blood cells from the marrow and location tissues

• E.g.

  • G-CSF

  • GM-CSF (granulocyte/monocyte colony-stimulating factor)

  • M-CSF(monocyte colony-stimulating factor)

    • together enhance the production of monocytes and granulocytes.

  • Prime the immune system – activate neutrophils and monocytes

G-CSF

• Cytokine that enhances the production and release of neutrophils

  • Activation causes them to become more motile, more sticky and more likely to engulf things

• It also increases neutrophil adhesion, granulation and responsiveness → increases the rate at which the proliferative compartment will mature and increases survival  

  • increased protein synthesis

  • increased granular content,

  • activated cytoskeleton

  • more capable of wriggling  and have increased phagocytic abilities → more likely to attack

Chemokines

• Small peptide hormone molecules are released to help recruit and attract WBCs to the site of infection

• Released due to local inflammation responses caused by DAMPs and PAMPS → cause white cells to leave the circulation and enter areas of inflammation/ damage

  • E.g. CXCL8 or IL8

• Attraction of WBCs is dependent on the cause of inflammation

CXCL8 and IL8

• Chemokines that attract neutrophils to inflammed tissues/sites where the pathogen is concerned

• Can also cause a similar activation of increasing neutrophil, adhesion, granulation and responsiveness

What happens when the cause of an infection is eliminated in relation to cytokine and chemokine production?

• The production of cytokines in the marrow and chemokines in tissues decreases, as the presence of pathogens (PAMPs and DAMPS) that stimulate their production is no longer present.

  • consequence of self-harm by the innate immune responses

Effects of reduced cytokines and chemokines after pathogen removal

• Reduced white cell production: Proliferative compartment dies.

• Reduced entry into infected areas: No chemokines present; if cells enter, they are not activated.

• Reduced activation: Prevents tissue damage.

Why May Complete Limitation of Self Harm Not Be Possible

• White cells use enzymes and other destructive molecules to destroy the organism

• The combination of enzymes of different types allows cells to kill all pathogens irrespective of type.

  • Toxic enzymes are non-selective when in contact with the body

How May Damage By Enzymes be Limited

• By targeting them to components in the bacteria e.g. iron-binding protein, or to something that can assist the entry of WBCs e.g. elastase

  • Aims to limit the WBC response and limit self harm

How May The WBCs Response Be Limited

• Killing by WBCs can destroy normal cells, and so there must be a balance between mechanisms to avoid damaging tissues → different cells have different methods to do this

How May The Neutrophil Response Be Limited

• WBCs kill the bacteria present within themselves –

• No bacteria  is released

• When the cells undergo apoptosis, they remove granules and enzymes to prevent any damage – localisation in the area of function

Mechanism of Killing Bacteria By Monocytes

• Due to their small size, bacteria can be engulfed by these WBCs and digested

• Engulf 50-100 bacteria to become full of granules

• Granules then fuse with a vacuole containing the bacteria  and release them locally – bacteria killed in the vacuoles, which are then apoptosis and disappear

  • bacteria are destroyed within neutrophils allowing the killing to occur without damage to tissues.

Neutrophil: Resting State

• Multilobed nuclei (3-5 lobes)

• Finely granulated cytoplasm – ‘killing’ enzymes stored within

• Round regular shape

• Highly motile cell, which can enter tissues at sites of inflammation

• Survives 12-24 hours in blood

  • Limited lifespan – don’t want half-dead neutrophils; may die by necrosis and release granules - further damage

• Further 24-48 hours in tissues

• Most neutrophils simply live out their lives and then die without needing to perform infection.

• The key therefore is to do nothing unless stimulated this avoids damaging the body.

Neutrophil: Activated State

• WBCs are released from marrow early – do not have a lobular nucleus

• Dohle bodies present – synthesis of proteins to increase the n.o granules present that will be highly active

  • Highly activated cell – senses surrounding environment – sensitisation – regular outline of cell ready to fight infection

• Cytokines and chemokines that are released during inflammation.

• These enhance neutrophil function → Enhanced adhesion and movement

Neutrophil Killing Mechanism:

• Adhesion to bacteria and vacuole formation

• Vacuole and granular fusion to phagosome

• Extracellular Traps

Neutrophil Killing Mechanism of Bacteria: Adhesion

• Cells bind to the bacteria using adhesion receptors that recognise PAMPS and engulf them into a vacuole

• Vacuole then fuses with the phagosome to destroy the bacteria

Neutrophil Killing Mechanism of Bacteria: Role of Granules

• They fuse with phagosomes to destroy bacteria through:

  • Microbiocidal proteins: Myeloperoxidase and lysozyme bind to and destroy proteins (not lipids).

  • Acid hydrolases: Function only in acidic environments; enzymes become inactive if they escape the vacuole → don’t destroy tissue

  • Iron binding (Lactoferrin): Binds free iron, preventing bacterial replication, movement, and energy production.

Neutrophil Killing Mechanism of Bacteria: Extracellular Traps

• DNA net – final measure if neutrophils haven’t cured infection

• As it apoptoses, cells break down their membrane and release its DNA

  • DNA tangles up and bacteria become tangled and trapped – netosis

How Do Neutrophils Prevent Tissue Damage When KIlling Bacteria

• Phagocytoses cells - destroyed internally  

• Enzyme contents are relatively safe if released: as they depend on low pH or oxidising power which are only found within the cell

• Following cell killing neutrophils die by apoptosis avoiding tissue damage 

Structure of Monocyte

• Kidney shaped nucleus, grey cytoplasms, with fine granules

• Large cells

• Phagocytic cells with a range of functions

  • Main function is within tissues - may be regarded as continuing the effects of neutrophils at a later stage – deal within things not dealt with by neutrophils

• Generally, irregular shape that is dynamically active contacting other cells or migrating

• Spend 17 hours in blood before entering tissues where they become “tissue macrophages”

  • Can persist for a long time and form a ring/ sphere around any pathogen and wall it off

Function of Monocytes

• Phagocytose bacteria

• Can “wall in” pathogens (forming a granuloma)

• Removes dead cells and promote wound healing

  • ‘Act before the stage of scaring’

Monocyte/ Macrophage Response to Tuberculosis

• Celss ‘wall in’ tissue infections

• Bacteria can escape death by neutrophils

• These cells wall it off to prevent it from moving - seen in lung CT scans - walled off scars seen

• Not a permanent solution

Why Isnt the Walling off of TB by monocytes not a permanent solution

• As in immunosuppressed individuals, TB can be reactivated

  • the protective ring of monocytes can break down in response to an insult

Eosinophil and Basophil Mechanism For Killing Larger Organisms

• Cells contain numerous granules that fill the cytoplasm and stain strongly with particular acid dyes – appearing red (eosinophil) or blue (basophil)

• GRANULES ARE RELEASED IN TISSUES TO DESTROY PARASITES

  • Must be regulated between killing organisms and preventing excess damage

Granular Contents of Eosinophils

• Histamine

• Active proteins

  • Nucleases: breaks down DNA and RNA,

    • specific, rarely damaging the host

  • Lipases - break down fat

    • Not too problematic as they damage fat tissue - worms are fatty

• Major basic protein

• Contents may potentially cause tissue toxicity as these proteins may damage tissue

Major Basic Protein

• Found in the granular contents of eosinophils

• A sem-specific substance that attacks an organism substrate and will cause damage to the host when released

Granular Contents of Basophils

• They contribute to inflammation

  • Histamine – dilate blood vessels

  • Serotonin – dilates blood vessels

  • Heparin – prevents clotting to allow the entry of immune cells

    • Keeps work accessible

  • Enzymes (elastase) break down tissue matrix – vessels more ‘floppy’

  • IL4 – stimulates immune reactions especially IgE – allows innate immune system to communicate with T-cells

Histamine

• Found in the granular contents of eosinophils and basophils

• Dialtaes blood vessels, allowing more blood cells to arrive and cause swelling to trap invading organisms

  • Vessels first constrict tightly, followed by dilation to release fluids into tissue which results in localised swelling → causes tissue to become tight and immobilises the organism

    • Tissue fluid pressure traps the organism

Inter-Relationships Between The Innate System and The Organism

• The response to danger signals produced by organisms allow responses to be proportionate to the level of threat (amount of danger signal) and time (duration of danger signal)

Inter-Relationships Between The Innate System and The Body

• Locally active factors such as cytokines and chemokines can be secreted by cells in a range of tissues - this links the immune system to overall health but also allows responses to be localised to sites of threat.

Inter-Relationships Between The Innate System and The Adaptive System

• Although arising later in evolutionary terms, the adaptive immune system produces factors that influence the behaviour of the innate system providing extra control

Chediak-Hihashi Syndrome

• Low and abnormal granules and nucleus shape of neutrophils

  • Seen in children – unable to fight infection

  • Require a bone marrow transplant

• includes albinism, recurrent infections, a mild bleeding disorder and peripheral neuropathy.

• There are giant granules in the neutrophils, eosinophils, monocytes and lymphocytes, accompanied by neutropenia, thrombocytopenia and marked hepatosplenomegaly.

• due to mutations in the CHS1 (LYST) gene, which encodes a lysosomal trafficking regulator.

Immune Neutropenia

• An inherited disorder that causes chronic infection due to a low number or defect in the function of neutrophils

  • Due to an immune attack on these cells

• Antibodies directed against Fc gamma receptor IIIb (FcγRIIIb), aka CD16b, found on the surface of neutrophils.

  • antibodies interfere with phagocytosis, making it difficult for neutrophils to destroy pathogens and leading to an increased infection risk

Rhematiod Artiritis

• An inappropriate maintenance of joint inflammation due to high numbers of neutrophils - contributes to damage in inflammatory disease

  • Drift of fingers due to destruction

Hypereosinophilic Syndrome

• A condition characterised by an excess of eosinophils (WBCs), which become over-activated.

• This can involve low-level cancer of the eosinophils, leading to excessive histamine release

• Eosinophil count elevated above 1.5 × 109/L for over 6 months and associated with tissue damage

  • heart valves, central nervous system (CNS), skin and lungs may be affected

• Mepolizumab a humanised antibody to IL‐5, is a promising new treatment

Consequences of a High Eosinophil Number and Excess Granule Release

• It can be harmful, resultin in heart valve damage - ;

• Valves deteriorate due to the release of toxic granules in the wrong place

Asthma and Allergy

• Caused by an inappropriate number or incorrect circumstance of basophil granule release

  • Harmful

Hypersensitivity of Basophil in Lung Tissue

• Once granule content is released, it causes the swelling of vessels and the compression of small airways - not enough air in the lung

  • Histamine can cause the airways to become inflamed and narrow, causing wheezing

How May Allergic Responses Be Prevented

• By using drugs to stabilise basophils

  • Suppression of the immune system via steroids

  • Stabilisation of mast cells e.g. Sodium cromoglicate

Cytokine Storm

• The immune system response becomes uncontrolled with cytokine and chemokine production causing excessive activation of inflammatory cells, causing the destruction of normal tissues that further increases cytokine release

• This provokes increasing uncontrolled inflammation.

Cytokine Storm in COVID-19 Patients

• Seen in severely affected individuals, where the normal response to a viral infection by B and T lymphocytes became uncontrolled, with increasing cytokine release that provokes inflammation due to inappropriate activation of the innate immune system.

Dexamethasone

• An example of a steroid that can be used at an early stage to suppress inflammation and prevent the progression of cytokine storms

  • Had major impacts on COVID-19 disease progression