Immunology - Unit 1: Innate Immunity

immunology

study of host’s rxn when foreign substance is introduced to the body

antigen

foreign substance

can be anything—> bacteria, dust, pollen

immunity

all physiologic mechanisms that enable host to recognize materials as foreign and to dispose of them with or without injury to its own tissues

where does immunology get involved?

• protection from bacteria, viruses, fungi, and parasites

• vaccine responses and immune memory

• allergy, autoimmunity, and transplant rejection

• tumor surveillance and immunotherapy

• inflammatory contributions to chronic disease

• immunochemistry and immunohematology

what are protective outcomes of immunity?

• remove pathogens

• neutralize toxins

• clear damaged cells

• build memory

what are pathologic outcomes of immunity?

• allergy

• autoimmunity

• chronic inflammation

• transplant rejection

is immunity always protective?

no, it can cause injury (pathologic outcomes)

what are the 2 main types of immunity?

innate and adaptive

what strategies does innate immunity use?

• recognition: germline-encoded patterns

• initial speed: minutes to hours

• diversity: limited, but broad (doesn’t differentiate between different antigens/pathogens)

• repeat exposure: typically no memory

• major players: barriers, phagocytes, NK, complement

what strategies does adaptive immunity use?

• recognition: somatically generated receptors

• initial speed: days (takes time to adapt to produce things like specific antibodies)

• diversity: extremely diverse

• repeat exposure: faster, stronger memory

  • major players: B cells, T cells, antibodies

innate immunity feature of recognition

germline-encoded patterns

innate immunity feature of initial speed

minutes to hours

innate immunity feature of diversity

limited, but broad

innate immunity feature of repeat exposure

typically, no memory

innate immunity feature of major players

barriers, phagocytes, NK, complement

adaptive immunity feature of recognition

somatically generated receptors

adaptive immunity feature of initial speed

days

adaptive immunity feature of diversity

extremely diverse

adaptive immunity feature of repeat exposure

faster, stronger memory

adaptive immunity feature of major players

B cells, T cells, antibodies

What are two kinds of responses for adaptive immunity?

humoral and cell mediated

Main features of Humoral response/ Humoral Mediated Immunity (HMI)

B cells —> plasma cells (produce antibodies, typically tag antigens for disposal)

antibody-mediated

main defense against extracellular pathogens

Features of Cell Mediated Response/ Cell-Mediated Immunity (CMI)

T cells

Direct cell-to-cell contact (check if cell is part of the body) or production of cytokines (tells cells what to do)

Main defense against intracellular pathogens and cancerous cells

how does innate immunity shape the adaptive response (flow chart)?

Detect danger → release cytokines → activate antigen-presenting cells → direct T-cell responses to produce cytokines → activate B cells to produce Abs

what is the first line of defense for innate immunity?

anatomical

• intact skin

• mucosal surfaces

• mechanical clearance

• antimicrobial secretions

what are the anatomical defenses of innate immunity?

• intact skin

• mucosal surfaces

  • cilia clearance

• mechanical clearance

  • mucous blanket expelled by coughing

  • urine flow

• antimicrobial secretions

  • saliva and tears contain lysozyme

  • low pH of stomach and vagina

what physiological factors can affect immunity?

body temperature

• microbes grow poorly at or above 37ºC

acute phase reactants

• proteins normally found at low levels, but rapidly increase during infection, trauma, or injury

• produced by hepatocytes (w/i 12-24 hrs)

What are major acute phase reactants (proteins that increase during infection, trauma, injury)

• C-reactive proteins (CRP)

• Serum amyloid A (SAA)

• Fibrinogen

• Hepcidin

• Ferritin

• Complement proteins

• Haptoglobin

• Albumin/ transferrin

Major acute phase reactant: C-reactive protein (CRP)

opsonization and complement activation; common clinical inflammation marker

-large increase during acute phase

Major acute phase reactant: Serum amyloid A (SAA)

Immune-cell recruitment and lipid transport; rises markedly w/ inflammation

-large increase during acute phase

Major acute phase reactant: Fibrinogen

coagulation and repair; contributes to an increase ESR

-moderate increase during acute phase

Major acute phase reactant: hepcidin

restricts iron availability by decreasing absorption and macrophage iron release

-small increase during acute phase

Major acute phase reactant: Ferritin

stores and sequesters iron; may rise substantially during inflammation

-small increase during acute phase

Major acute phase reactant: complement proteins

amplify opsonization, inflammation, and pathogen elimination

-small increase during acute phase

Major acute phase reactant: haptoglobin

binds free hemoglobin; limits oxidative damage and iron availability

-small increase during acute phase

Major acute phase reactant: albumin/transferrin

negative acute-phase reactants; concentrations decrease during inflammation

-small decrease during acute phase

in what ways do the microbiota reinforce barrier defense?

through direct and immune effects

how do the microbiota directly reinforce barrier defense?

compete for nutrients

occupy space, making it difficult for pathogens to colonize

produce bacteriocins to inhibit growth from competing microbes

how do the immune effects of microbiota reinforce barrier defense?

support epithelial barrier integrity

provide low level signals to maintain low innate immune activity

helps shape the development of immune responses

what is the “Hygiene Hypothesis”?

theory that reduced exposure to microorganisms early in childhood may lead to increased risk: allergies, other immune-mediated disease

microbial diversity matters

antibiotics, diet, environment, and genetics interact

what is the repeatable logic of the innate response?

Barrier breach → pattern detection (PAMPs and DAMPs) → local alarm (potential cytokine release) → effector response → resolution

what are pattern recognition receptors (PRRs) and what do they detect?

detect context—> recognize specific molecular patterns

• Pathogen-associated molecular patterns (PAMPs)

• Damage-associated molecular patterns (DAMPs)

what are the major PRR families?

Toll-like Receptors (TLR)

C-type lectin receptors (CLRs)

Nucleotide-binding oligomerization domain receptor (NODs)

Scavenger Receptors (SR)

Where are Toll-like Receptors (TLR) located and what are some examples of what they detect?

cell surface or endosome (inside the cell)

detect LPS, lipoproteins, nucleic acids

Where are C-type lectin receptors (CLRs) located and what are some examples of what they detect?

cell surface

detect microbial carbohydrates

Where are Nucleotide-binding oligomerization domain receptors (NODs) located and what are some examples of what they detect?

cytosol

detect stress, bacterial products

Where are Scavenger Receptors (SR) located and what are some examples of what they detect?

Cell surface

detect modified lipoproteins, components of damaged cells

PAMPs: what receptor detects LPS (lipopolysaccharide) and what is the interpretation?

TLR4

gram-negative bacterial product

PAMPs: what receptor detects flagellin and what is the interpretation?

TLR5

motile bacterial structure

PAMPs: what receptor detects mannose (sugar not used in human cells) and what is the interpretation?

CLRs

Repeat patterns on the surface of microbes

PAMPs: what receptor detects zymosan (product of yeast) and what is the interpretation?

TLR2

yeast present

PAMPs: what receptor detects Viral RNAs and what is the interpretation?

TLRs 3, 7, 8

Viruses present

How does PRR signaling produce distinct outputs (flowchart)?

Cell recruitment → transcription factor activation (activator protein-1/ AP-1, nuclear factor kb/ NF-kb, interferon regulatory factors/ IRFs) → cytokine or interferon release (enhance inflammation) → local and systemic effects

what are the immune cells of innate immunity?

Neutrophils

Eosinophils

Basophils and Mast cells

Monocytes/ Macrophages

Natural Killer (NK) cells

Neutrophils

rapid, expendable responders

predominate granulocyte—> 40-75% of WBCs in peripheral blood

Half life= 8-20 hours → 1-2 days of survival in tissues

protect against microbes via phagocytosis

Receptors:

• Fc portion of IgG

• Complement receptor

what receptors do neutrophils contain?

Fc portion of IgG

Complement receptor

What are the two functional pools that neutrophils occupy?

circulating pool and marginated pool

Neutrophils - circulating pool

Moves freely in the blood

measured directly in routine counts

available for recruitment

Neutrophils - marginated pool

found in spleen, liver, and bone marrow (lymphoid organs)

can rapidly enter circulation

influenced by stress hormones and inflammation

Eosinophils

Granulocyte → granules released when activated causing an effect on the body

1-3% of cirulating nucleated cells

easy to identify (red stain)

protects against parasitic infections

dampens allergic response

weak phagocytosis

what cell pool do basophils come from?

blood

what cell pool do mast cell come from?

tissue

Basophils and Mast cells

granulocyte

<1% of circulating cells

Not from the same precursors

• Mast cells found in thymus, spleen, lung, GI tract, bone marrow

Easy to identify (blue/purple)

Involved in allergies→ granules contain histamine

Receptors:

• Fc receptor for IgE heavy chain

what receptor do basophils and mast cells have?

Fc receptor for IgE heavy chain

what’s the difference between monocytes and macrophages?

monocytes are found in blood; macrophages are either tissue resident or migrated to tissue

Monocytes/Macrophages

3-10% of WBCs in peripheral blood

• Multipotent→ give rise to many different types of macrophages

• macrophages found in tissue (tissue resident or migrated)

Functions: phagocytosis, antigen presentation (important for adaptive immune system)

Markers:

• Fc receptor for IgG

• Receptors for CRP and complement

• MHCII

What markers/receptors are found in monocytes/macrophages

Fc receptor for IgG

Receptors for CRP and complement

MHCII

how do macrophages balance defense and repair?

they have roles in protecting the body as well as maintaining homeostasis

how do macrophages defend the body?

recognize PAMPs and DAMPs

phagocytose microbes

release inflammatory mediators

how do macrophages maintain homeostasis?

clear apoptotic cells thru efferocytosis (a type of phagocytosis that allows the recycling of what’s eaten)

recycle cellular material and release anti-inflammatory cytokines

coordinates repairs

natural killer (NK) cells

3% of peripheral lymphocytes

• large granular lymphs

• NOT T or B cell

Mediate rapid lysis of virally-infected, tumor, or foreign graft cells

Prior immunization isn’t needed→ doesn’t need an antibody marker to kill cell (if cell doesn’t identify itself w/ MHCI signal, it’s killed)

Markers

• CD16: Fc receptor for IgG

• Antibody-dependent cell mediated cytolysis (ADCC)

what are markers/receptors of natural killer cells?

CD16: Fc receptor for IgG

Antibody-dependent cell mediated cytolysis (ADCC)

how do NK cells make a balance-of-signals decision?

Scan target → assess inhibitory MHC I signal (infected cells are unable to produce signals to let NK cells they’re infected) → integrate activating ligands → release perforin and granzymes

what does immune recruitment convert a blood cell to?

a tissue responder —> migration from blood to tissue→ chemotaxis (signals attract cell towards particles that need to be phagocytosed

What are the steps to cellular migration in innate immunity?

Recruitment converts blood cell to tissue responder

• migration from blood to tissue, helped thru chemotaxis that attract cells to particles that need to be phagocytosed

inflamed endothelium becomes a gateway

• microbial mediators, cytokines, and chemokines in tissue inflame the endothelium

• adhesion molecules expressed on endothelium

  • selectins on endothelium bind to cellular receptors

  • weak rolling attachment

• Cell will bind to integrins, which will capture it on the blood vessel wall → firm attachment

Diapedesis/ extravasion

• the leukocyte can squeeze thru the endothelium (diapedesis)

• cells leaves blood vessel for tissue (extravasation)

Migration

• cell can migrate fully into tissue to phagocytose invading pathogen

what happens during the migration step during cellular migration in innate immunity?

• cell can migrate fully into tissue to phagocytose invading pathogen

what happens during the diapedesis/extravasion step during cellular migration in innate immunity?

• the leukocyte can squeeze thru the endothelium

• cells leaves blood vessel for tissue

what happens during the capture step when the inflamed endothelium becomes a gateway during cellular migration in innate immunity?

• Cell will bind to integrins, which will capture it on the blood vessel wall → firm attachment

what happens during the rolling adhesion step when the inflamed endothelium becomes a gateway during cellular migration in innate immunity?

• microbial mediators, cytokines, and chemokines in tissue inflame the endothelium

• adhesion molecules expressed on endothelium

  • selectins on endothelium bind to cellular receptors

  • weak rolling attachment (rolling adhesion)

What are the steps to phagocytosis?

Neutrophils engulf the microbes

• Physical contact between WBC and foreign antigen

  • Opsonins (C3b, IgG, CRP) increase rate and amount of uptake

  • Opsonization coats the pathogen and allows the phagocyte to bind and ingest it

Formation of Phagosome

• Particle is ingested

  • binding occurs thru opsonins or pattern recognition patterns (like foreign lipid not on human cells)

• once inside cell, it forms the phagosome

Fusion of lysosome and phagosome

• lysosome contains hydrolytic enzymes that break down antigens

• forms phagolysosome

Digestion

• lysosome granules kill using superoxide ions, hypochlorite, and hydrogen peroxide

• myeloperoxidase (MPO) and H2O2 needed for killing

• Reactive oxygen particles (toxic) give respiratory burst to kill pathogen

• Release of debris

During phagocytosis, what happens during digestion?

• lysosome granules kill using superoxide ions, hypochlorite, and hydrogen peroxide

• myeloperoxidase (MPO) and H2O2 needed for killing

• Reactive oxygen particles (toxic) give respiratory burst to kill pathogen

• Release of debris

During phagocytosis, how does the phagolysosome form?

fusion of phagosome w/ lysosome

During phagocytosis, what happens during the formation of the phagosome?

• Particle is ingested

  • binding occurs thru opsonins or pattern recognition patterns (like foreign lipid not on human cells)

• once inside cell, it forms the phagosome

During phagocytosis, what happens when neutrophils engulf microbes?

• Physical contact between WBC and foreign antigen

  • Opsonins (C3b, IgG, CRP) increase rate and amount of uptake

  • Opsonization coats the pathogen and allows the phagocyte to bind and ingest it

How do opsonins like C3b, IgG, and CRP increase the rate and amount of uptake of microbes/antigens during phagocytosis?

they are antibodies that can be bound to bacteria → make targets easier to recognize

direct interaction btwn antibody and receptor on neutrophil allow engulfing

what happens during Chronic Granulomatous Disease (CGD)?

Absence of NADPH oxidase enzyme leads to decreased H2O2 → unable to generate respiratory burst and kill pathogen

Granulomas form where phagocytes can’t kill pathogen

Recurrent pus-producing infection

previously fatal in childhood, but can now be treated w/ granulocyte transfusion or BMT as treatment

what is the process of inflammation creating a controlled access zone (flowchart)?

Local alarm (w/i hrs) - cells release mediators; Mast cells release histamine; Injured cells, other immune cells release cytokines

Vasodilation (days)

—Cellular recruitment. Cells attach to wall, follow chemokine gradient, and migrate into tissue

Increased Vascular permeability (days)

Effector response (days/weeks)

—neuts, macros, etc. use PPRs to recognize pathogen and start phagocytosis. Systematic and pro-resolving mediators expressed. Macros promote tissue repair.

Resolution (days weeks)

What are the inflammatory mediators that are small soluble proteins?

cytokines, specificially: IL-1, IL-6, TNF-a

what are cytokines?

small soluble proteins that regulate the immune system; inflammatory mediators

Proinflammatory: IL-1, IL-6, TNF-a

function:

-increase adhesion and diapedesis (travel through tissue)

-chemoattractants

-cell activation and differentiation

-physical symptoms

what are the functions of cytokines as pro-inflammatory mediators

-increase adhesion and diapedesis (travel through tissue)

-chemoattractants

-cell activation and differentiation and differentiation

-physical symptoms

Cytokines: IL-1

Induced expression by microbes (LPS)

Produced by monos/macros (can also be produced by other immune cells to a lesser extent)

Cellular effects: Induces IL-6, IL-2 secretion in T helper cells

Systematic effects: fever, acute phase reactants (like CRP), and activation of immune cells

what is the cytokine IL-1 produced by?

monos/macros

(can also be produced by other immune cells to a lesser extent)

what are the cellular effects of IL-1?

induces IL-6

IL-2 secretion in T helper cells

what are the systematic effects of IL-1?

fever

acute phase reactants (like CRP)

activation of immune cells

what is the pro-inflammatory cytokine IL-6 produced by and what are its functions?

produced by innate and lymphoid cells

Function: acute phase reactants; stimulates B cells to proliferate and differentiate into plasma cells (which can then produce antibodies)

what is the pro-inflammatory cytokine TNF-a triggered by and produced by? What are its functions?

Expression triggered by LPS

Secreted by monos/macros

Function: enhances antigen presentation, causes apoptosis of tumor cells

what are inflammatory mediators other than cytokines?

Vasoactive amines (ex. Histamine)→ causes rapid permeability changes

Lipid mediators (ex. prostaglandins, leukotrines) → Hormone-like, trigger vasodilation, swelling, pain

Plasma cascades (Ex. complement, bradykinin)→ causes opsonization and lysis; amplification and vessel permeability

What is an example of a vasoactive amine and what does it do?

Histamine

Inflammatory mediator → causes rapid permeability changes

What are examples of lipid mediators and what do they do?

prostaglandins, leukotrines

Inflammatory mediator → Hormone-like, trigger vasodilation, swelling, pain

What are examples of plasma cascades and what do they do?

complement, bradykinin

Inflammatory mediator→ causes opsonization and lysis; amplification and vessel permeability