Chapter 3: BICD 140

Innate Induced Immunity: Inflammation, innate immunity, and myeloid cells

Myeloid Cells

• macrophages, neutrophils, dendritic cells

Cytokines

• produced by macrophages to induce inflammation

• small soluble proteins (pro/anti-inflammatory)

Can be:

• chemokines

• interferons

• interleukins

Altered-self

• mutated, cancer, damaged, viral-infected cells

• phagocytosed

Pattern recognition pathways

1. Receptor-mediated endocytosis: receptor recycling

2. Intracellular signaling pathway: PAMP binds to PRR

Receptor-mediated endocytosis

eg. Mannose receptor

1. mannose expressed on bacterial surface bound by mannose receptor on macrophage surface

2. macrophage ingests bacterium and degrades it within an endosome

3. endosome—>lysosome fusion—>phagolysosome

4. mannose receptor returned to cell surface

PAMPs

• Conserved molecular patterns expressed on bacterial/fungal cells (carbs, proteins, lipids, etc.)

• recognized by effector cells: macrophages, monocytes, dendritic cells, granulocytes, NKs, ILC’s

PRRs

• receptors on effector cells that recognize and bind PAMPs

• eg. LPS antigen found on gram(-) bacteria

TLRs - Toll-like receptors

• both intra and extracellular

• intra—>recognize viral pathogens

• extra—>recognize bacterial pathogens

TLR-4 Pathway

• extracellular PRRs with intracellular domains (TIR)—>intracellular signaling

1. LPS expressed on bacterium binds to TLR-4 along w/ MD2 and CD14

2. MyD88 (adapter protein) binds TLR-4—>activating IRAK4

3. IRAK4 p TRAF6—>IKK activated

4. IKK p IkB inducing degradation, allowing NFkB translocation to nucleus

5. NFkB (TF) induces inflammatory cytokine gene txn—>inflammatory response

6. NK cells activated, nearby cells alerted of infection

NEMO: IKK deficiency

• X-linked immunodeficiency where PTs lack IKK subunit

• no removal of inhibitory IkB from NFkB—>no translocation to nucleus

• no inflammatory cytokines produced

• recurrent, severe bacterial infections

Interferons (IFN)

• made by a cell in response to detecting a viral infection

• interfere with viral replication

• receptors: constitutively expressed on nucleated human cells

• Cytokine-like: tell nearby cells there’s an infection

Viral recognition by RIG1 (RLRS)

1. RIG-1-like receptors (RLRs) recognize and bind viral RNA in cytosol

2. Associates with MAVS (adaptor protein on mitochondria) CARD domain

3. MAVS initiates signal via TRAF6

4. TRAF6 activates IRF3 and IRF7 TFs—> nucleus

5. IRF3/7 induce txn of IFN-B and IFN-a (interferons)—> exit cell to change gene expression

Type-1 Interferons

• eg. IFN-B and IFN-a: antiviral cytokines released after RIG-1 recognition of RNA

• made by viral-infected cells

IFN-B

1. binds type I IFN receptors stimulating autocrine IFN-a response

2. binds type I IFN receptors on adjacent cells stimulating paracrine IFN-B response

• maintains activity of infected cells

• alerts nearby cells to begin antiviral response

Interferon Response Factor (IRF)

1. IFN-B and IFN-a are produced after viral infection

2. downstream changes to gene expression (more interferons, endoribonucleases)

3. activation of NK cells

4. virus-infected cells are more susceptible to NK cells

Plasmacytoid Dendritic cells (PDCs)

“Type I interferon producing cells”

• makes 100-fold more Type I IFNs than other cells

• both myeloid and lymphoid cells with a high ER content

Inflammasomes

• upon bacterial recognition, enables activated macrophages to produce IL-1B by cleaving stored pro-IL-1B

• cleaves large quantities of pro-IL-1B quickly—>allows a burst to be released

*can also be released in response to viral recognition

eg. NLRP3: Inflammasome cytoplasmic receptor (PRR) that forms the inflammasome to cleave IL-1B

Pyroptosis

1. bacteria engulfed by macrophage—>LPS recognition leads to gasdermin D pore formation

2. activated macrophage creates pro-IL-1B

3. Pro-IL-1B cleaved rapidly by inflammasomes leading to burst from pore

4. macrophage contents leak out, cell dies

5. upon IL-1B release, more macrophages are activated throughout the tissue

IL-1B

• cytokine released in bursts by macrophages after inflammasome cleavage

  • cell recruitment

  • drives inflammation and induces fever to clear infection

• cannot be secreted like normal proteins as it cannot translocate to ER for secretion

Autoinflammatory diseases

• INNATE IMMUNITY

• macrophages, neutrophils, monocytes

  • secrete cytokines (IL-1B, TNF-a, IL-6)

• chronic, recurrent inflammation

• inflammasomes are constantly ON

• fevers without infection

Autoimmune disease

• ADAPTIVE IMMUNITY

• T/B cells (plasma cells)

  • secrete CD8+, antibodies

• attacks specific proteins/organs

• detectible via blood tests

Systemic acute phase response

*macrophage activated by IL-1B secrete cytokines
*ALL INDUCE INFLAMMATION (redness, heat, swelling, pain)

• TNF-a; permeable blood vessels, enables cells to enter infected tissue

• IL-6; increases temperature via fat/muscle cell metabolize

• CXCL8; recruits neutrophils to site

• CCL2; recruits monocytes to site

• IL-12; recruits and activates NK cells to site

Neutrophils

• stored in bone marrow

• lethal, fast-acting, short-lived

  • bind and engulf bacteria

  • destroy with toxic granules

• can only extravasate when tissue is inflammed

• death leaves behind pus

Neutrophil recruitment

1. TNF-a released by macrophages—> induces EC expression of adhesion molecules

2. neutrophils mobilized from bone marrow and travel via blood to tissue

3. rolling adhesion and interaction btw adhesion molecules on ECs and neutrophils—>strong binding

4. neutrophils follow chemokine (CXCL8) concentration and extravasate out of blood into tissue

CXCL8

• chemokine released by activated macrophages (performs chemotaxis)

• binds to CXCL8 receptors on mobilized neutrophils

• higher concentration in tissues recruits neutrophils to extravasate into tissues via:

  • increasing membrane permeability (vasodilation)

Pyogenic bacterial infections

• pus-forming infections caused by bacteria

• pus signifies dead neutrophils

Neutrophil degranulation

• bacterium is engulfed by neutrophil

• phagolysosome forms to degrade bacterium

• neutrophil dies by apoptosis and is engulfed by macrophage

have NETosis function: can still kill pathogens even after death

Respiratory burst reactions

• NADPH oxidase catalyzes release of —> superoxide (2O2-) causing collateral damage to nearby cells

• very acidic

Acute phase proteins

• cytokines released by activated macrophages induce liver to secrete proteins

• IL-6 secretion induces synthesis of proteins:

  • CRP; used diagnostically, high levels indicate infection, inflammation,

  • MBL; binds to mannose on pathogen surfaces

CRP (C-reactive protein)

• a PRR that binds directly to bacteria, fungi, some protozoa

• functions as an opsonin

• triggers CLASSICAL COMPLEMENT PATHWAY

MBL (Mannose- binding- lectin)

• a PRR protein that binds to mannose found on pathogen surfaces

• cleaves MASPs (15-18 binding sites) that bind to bacterium

• opsonin activity

• activates LECTIN COMPLEMENT PATHWAY

NK Cells

• circulating cytotoxic innate INDUCED lymphoid cells (ILCs)

• NOT PHAGOCYTIC

• intracellular: kills infected cells directly

• extracellular: secretes cytokines causing phagocytosis

• differ from CD8* by:

  • not having a T cell receptor

  • part of INNATE, not adaptive immunity

NK deficiency

• lacking NK cells, cannot clear viruses

• must be treated with antiviral drugs (endoribonucleases)

NK Cell Killing

• must make CONTACT with target cell—> SYNAPSE

  • involves multiple receptors and ligands

  • healthy cells express inhibitory receptors to prevent contact

• NK cytotoxic granules travel via microtubule tracks to synapse to kill cell

• killing relies on multiple signals

*kills viral infected cells, NO CONTACT WITH BACTERIAL (too small)

Macrophage x NK cell activation

1. macrophages activated by viral infection and release CXCL8 and IL-12

2. IL-12 recruits NK cells to conjugate—>synapse formation

3. IL-15 delivered via synapse works with IL-12 to activate NK cell

4. NKs proliferate and differentiate into effector NK cells

5. Secrete IFN-y to bind macrophages, increasing phagocytosis and cytokine secretion

*POSITIVE FEEDBACK LOOP

IFN-y

• pro-inflammatory type II cytokines

• released by NK cells to activate macrophages

• increases magrophage phagocytotic ability

macrophages—>IL-12—>IL-15—>NK cells—>IFN-y

Dendritic cells (DCs)

3 types:

1. plasmacytoid (makes type I interferons)

2. myeloid (search for infection, brings antigens to NKs)

3. follicular (directs B cells to primary follicle)

Myeloid Dendritic Cells

• sentinel cells: live inside tissue, first responders to infection

• drives the activation, proliferation, and differentiation of NK cells

• function in both innate AND adaptive

• can phagocytose pathogens and be infected by pathogen

NKs v DCs

• loser signals other to go on

• NK wins: kill dendritic cells, virus is terminated and no adaptive needed

• DCs win: NKs signal them to mature and initiate adaptive immune response

  • adaptive: travel to secondary lymphoid tissues to recruit B cells to lymph node