622 11/2 L pt 3 Immune System and Innate Immunity

Cells of Innate Immunity

neutrophils (PMNs)

monocytes

macrophages

dendritic cells

mast cells

basophils

eosinophils

natural killer cells

Monocytes

immature circulating macrophages, phagocytosis, leave blood by extravasation when inflammation occurs, differentiate into macrophages or dendritic cells

Monocytes on Wright Stain

large indented or curved purple nucleus, pale blue cytoplasm

Macrophages

tissue resident, longer-lived than monocytes, patrol by crawling, phagocytosis

express PRRs, Fc receptors, and complement receptors

secrete cytokines that regulate inflammation and wound healing, antigen presentation to T cells

Classic Activation of Macrophages

M1 macrophages, induced by PRRs or the cytokine IFN-y, trigger inflammation

fight infections, destroy microbes by phagocytosis

Alternative Activation of Macrophages

M2 macrophages, induced by cytokines IL-4 and IL-13, suppress inflammation

tissue repair, eliminate debris by phagocytosis

Dendritic Cells (DCs)

have numerous cytoplasmic tendrils (dendrites), antigen presentation to lymphocytes, 3 types

3 Types of Dendritic Cells

convetional

phagocytoid

follicular

Conventional Dendritic Cells

most abundant, phagocytosis, antigen presentation to T cells, secrete cytokines that regulate differentiation of T cells

may migrate through lymphatic vessels to secondary lymphoid tissues

in blood, spleen, liver, thymus, MALT gernimal centers, skin (langerhans cells)

Plasmacytoid Dendritic Cells

rarest, secrete antiviral cytokines (type I interferons: IFN-a and IFN-b), poor phagocytosis, antigen presentation to T cells

in blood and secondary lymphoid tissues

Follicular Dendritic Cells

use Fc receptors and complement receptors to trap and display native antigens to B cells without internal processing, no phagocytosis

non-hemtopoietic origin

from B-cell zones of secondary lymphoid tissues (follicles)

Mast Cells

tissue resident (skin, MALT, CT), mediate early effector phase of allergic reactions for type i hypersensitivity, degranulation

immune sentinels, first-line defense, response to helminths

produce chemicals that mediate inflammation

express PRRs, Crs, and FcRs

Chemicals that Mediate Inflammation Released by Mast Cells

degranulation releases stored histamine, heparin, proteases

secrete lipid-derived mediators (platelet-activating factor, prostaglandins, leukotrines)

secrete cytokies and chemokines

Basophils

functionally and phenotypically similar to mast cells but reside in blood, leave blood by extravasion when inflammation occurs, <1% of circulating wbcs

involved in early effector phase of type i hypersensitivity

Basophils Wright Stain

dark, irregular nucleus with numerous dark blue granules

Eosinophils Wright Stain

dark irregular nucleus, numerous red granules (eosin dye)

Eosinophils

degranulation and phagocytosis, mediate late effector phase of allergic reactions (type i hypersensitivity), mediate inflammation

present in helminth infections

circulate briefly in blood, mainly found in tissues (skin, MALT)

Eosinophils and Helminths

response to helminths (→ degranulation of cytotoxic proteins that are different from other degranulators)

the problem is that these cytotoxic proteins contribute to tissue damage in chronic inflammation

Natural Killer Cells

kill virus infected cells, some tumor cells, and pathogens

lymphocytes of innate immunity, activation depends on balance between activating signals and inhibitory signals that allows them to determine what is and isn’t a threat

Lymphocytes (B, T, and NK) Cells Wright Stain

smallish cells with large, round, dark purple or blue nucleus and thin rim of pale blue or lavender cytoplasm

cannot differentiate between them on stain

Histology of Innate Immunity Cells

Natural Killer Cells Activating Signals

provided by infected cells, cancer cells, stressed/damaged cells, opsonins

engage activating receptors on NK cells, and trigger activating pathways via ITAMs (immunoreceptor tyrosine-based activation motifs)

Natural Killer Cells Inhibitory Signals

provided by healthy cells (MCH I on all healthy nucleated cells)

engage inhibitory receptors on NK cells, block activating pathways via ITIMs, and prevent NK cells from attacking normal host cell

4 Ways Activated NK Cells Kill Target Cells

secrete IFN-y which activates macrophage (M1) killing

degranulate, releasing perforin (creates holes in target cell’s membrane) and granzymes (enter target through holes and trigger caspase pathway → apoptosis of target)

engage Fas receptor on target cell via Fas ligand on Nk cell → trigger apoptosis of target via caspase pathway

engage antibody opsonin via FcRs → degranulation of NK cell → release of perforin and granzymes → apoptosis of target (antibody-mediated cell-mediated cytocoxiticity ADCC)

Capsase Pathway

culminates in apoptosis, present in nearly all normal nucleated cells

can be triggered by NK activating cells when they release perforin and granzymes or when they engage a Fas receptor

Complement

about 50 proteins produced mainly by hepatocytes but also monocytes, macrophages, and epithelial cells, and fibroblasts complement (or assist) the antimicrobial activity of antibodies

activation involves proteolytic cleavage

Complement Circulation and Activation

proteins circulate in blood and lymphatic fluid in inactive form

acitvation involved proteolytic cleavage, activated complement proteins trigger a chain reaction (complement cascade) amplifying the response

3 Pathways of Complement Activation

classical

alternative

lectin

Classical Pathway of Complement Activation

activated by antigen/antibody complexes

Alternative Pathway of Complement Activation

activated by molecules on microbial surfaces that lack the complement regulatory proteins normally present on host cells

Lectin Pathway of Complement Activation

activated by binding of lectins to carbs on microbial surfaces

4 Basic Functions of Activated Complement

opsonization (enhance phagocytosis)

promote inflammation and chemotaxis of leukocytes

destroy microbes by osmotic lysis (MAC/membrane attack complex)

immune clearance

Immune Clearance

function of activated complement, removal of antigen/antibody complexes and microbes from blood by macrophages in liver and spleen that express matching complement receptors

What do all 3 pathways of complement activation generate and culminate in the formation of?

generate C3 convertase and C5 convertase

culminate in formation of MAC

C3 Convertase (C4b2a or C3bBb)

generated by all 3 complement activation pathways, cleaves C3 into fragments of C3a and C3b

C3a

product of fragmentation of C3 by C3 convertase in complement activation, chemoattractant for neutrophils and monocytes and triggers mast cell degranulation leading to inflammation

C5 Convertase (C4b2a3b or C3bBb3b)

generated by all 3 complement activation pathways, cleaves C5 into C5a and C5b

C3b

product of fragmentation of C3 by C3 convertase in complement activation, an opsonin which enhances phagocytosis (binds to CR1 receptors on macrophages and neutrophils)

involved in immune clearance

C5a

product of fragmentation of C5 by C5 convertase in complement activation, chemoattractant for neutrophils and monocytes and triggers mast cell degranulation leading to inflammation

C5b

product of fragmentation of C5 by C5 convertase in complement activation, initiates formation of membrane attack complex (MAC)

Parts of Membrane Attack Complex

the culmination of all three pathways of complement activation

C5b + C6 + C7 + C8 + C9_12-18 = C5b6789_12-18

Membrane Attack Complex Function

inserts onto target’s membrane, creating holes

target dies by osmotic lysis (water in cell makes it explode)

most effective against gram - bacteria (esp neisseria), enveloped viruses, and some parasites

Complement Regulatory Proteins

protect our own cells from complement-mediated damage

most activated complement proteins have a short half-life, preventing activity beyond initial site of activation

3 Complement Regulatory Proteins

c1 inhibitor

decay accelerating factor

cd59

C1 Inhibitor (C1 INH)

complement regulatory protein, soluble protein that inhibits activation of C1 during classical pathway

also separately inhibits production of bradykinin

Bradykinin

powerful trigger of inflammation, production is inhibited by action of C1 inhibitor

Decay Accelerating Factor (DAF)

complement regulatory protein, surface protein that inhibits formation of C3 convertase in all three pathways of complement activation

CD59 (Protectin)

complement regulatory protein, surface protein that inhibits formation of MAC by blocking C9 polymerization

3 Complement Deficiencies

C3 deficiency

hereditary angioedema

paroxysmal nocturnal hemoglobinuria

C3 Deficeincy

complement deficiency, rare autosomal recessive, very low or absent C3

severe recurrent infections, especially encapsulated gram - bacteria (Neisseria and Streptococcus) because MAC struggles to form

C3 Deficeincy Clinical Presentation

causes symptoms resembling systemic lupus erythematosus (SLE) due to failure of immune clearance and deposition of antigen/antibody complexes in blood vessels and kidneys

C3 Deficeincy Prevention

vaccinations and antibiotics reduce risk

Hereditary Angioedema

complement deficiency, rare usually autosomal dominant, C1 INH deficiency leads to elevated bradykinin levels (bc C1 INH usually inhibits bradykinin)

Hereditary Angioedema Clinical Presentation

severe recurrent inflammation of face, limbs, intestines, and airways

Hereditary Angioedema Treatment

bradykinin receptor antagonists, infusions of recombinant C1-INH

Paroxysmal Nocturnal Hemoglobinuria (PNH)

complement deficiency, rare, usually due to spontaneous mutation, defective DAF and CD59

complement-mediated destruction of rbcs by overactive MAC

Paroxysmal Nocturnal Hemoglobinuria Clinical Presentation

dark urine (especially in morning) due to presence of hemoglobin, anemia

Paroxysmal Nocturnal Hemoglobinuria Treatment

infusions of monoclonal antibodies against C5