Immunology 3

several genetic deficiencies in complement are noted in humans & animals; affected individuals usually present at an early age w/ recurring bacterial infections; what are the 2 most prominent examples of interest?

- C3 deficiency in Brittany Spaniels - C3 receptor deficiency in cattle (bovine leukocyte adhesion deficiency)

what is the MAC made up of?

C5 through C9

what is likely the biggest promoter of inflammation in the complement system?

C5a

which component of the MAC mediates inflammation & functions in anaphylatoxin & chemotaxin?

C5a

which component of the MAC initiates its own assembly?

C5b

TRUE/ FALSE: C3b on microbial surface binding to C3b receptor on a phagocyte is a stronger level of binding than antibody on phagocyte to microbial surface

FALSE - C3b binds stronger than antibody

TRUE/ FALSE: the role of C3/C5 convertase in the classical pthwy differs from its role in the alternative pathway

FALSE - role is the same: C3/C5 convertase can cleave many molecules of C3; (the naming is different acc'd to each pthwy: C4bC2a vs. C3bBb)

TRUE/ FALSE: the different complement pathways each generate a different C5 convertase

FALSE - the classical & lectin pathways generate a different C5 convertase than the alternative pathway

how do B cell receptors indicate IgM has bound antigen?

Ig-alpha & Ig-beta transmit signal; CD21/CD19 co-receptors also amplify the signal

which antibodies start the classical complement pathway?

IgM & IgG

the classical complement pathway initiates after an IgM or IgG molecule attaches to the surface of _______

a bacterium or virus (antigen)

what are ficolins?

a family of proteins that binds to carbohydrate structures on bacterial & fungal cells to activate the cascade; they are present at higher levels in serum; animals lacking an adaptive immune response have larger families of ficolins to provide broader recognition of pathogens

what is the complement system?

a part of the immune system that enhances (complements) the ability of antibodies & phagocytic cells to clear pathogens from an organism; part of the innate immune system, but can also be recruited by the adaptive immune system

when stimulated, _______ in the complement system cleave specific proteins to release _______ & initiate an amplifying cascade of further cleavages. The end result of this complement activation/ complement fixation cascade is massive _______ of the response and activation of the cell-killing _______

proteases; cytokines; amplification; membrane attack complex

what is the most important action of complement deposition onto bacterial surfaces?

opsonization (enhances uptake by phagocytes)

what are the substrates of the alternative complement pathway?

pathogen + immature C3 (w/ thioester hydrolysis)

what is the end result of the classical pathway?

peptide mediators of inflammation, phagocyte recruitment

the complement system is a series of ~20 heat labile ______ synthesized in the ______

plasma proteins; liver

what aspect of the complement system are many bacteria resistant to?

pore formation by C9 in the bacterial membrane

what is the role of the regulatory proteins of complement?

protect host cells from complement-mediated damage that occurs 2ndary to inflammation

how are complement proteins triggered to enter the alternative pathway?

spontaneous activation in the blood or bacterial surfaces

in the alternative pathway of complement activation, C3 can be _______ activated by bacterial cell surfaces or in plasma, forming C3b attached _______ to the surface

spontaneously; covalently

what is C4bC2a?

the C3/C5 convertase (in classical pthwy)

in the classical pthwy of complement activation, which components bind covalently to pathogens & opsonize it?

C4b

describe the steps of MAC formation:

- C5b binds C6 & C7 - C5bC6C7 complexes bind to membrane via C7 - C8 binds to the complex & inserts into the cell membrane - C9 molecules bind to the complex & polymerize - 1-16 molecules of C9 bind to form a pore in the membrane

the generation of complement cleavage products amplifies the inflammatory response; this is mediated via: - smooth muscle _______ - mast cell _______ - _______ of blood vessels - activation of _______ - establishment of a _______ to recruit immune cells

- contraction - degranulation - vasodilation - phagocytes - chemotactic gradient

what are the 4 different stages at which complement may be inhibited by the host through regulatory proteins?

- dissociation of C1r & C1s to inactivate the initiation complex - displacement of C2b from C3/C5 convertase - cleavage & inactivation of C3b in the C3/C5 convertase - inhibition of the MAC

how do complement components such as C5a promote inflammation?

- increase vascular permeability - trigger mast cells to release granules containing histamine, making vasculature leaky - increase adherence of phagocytes to endothelial cells

what is the result of the leaky vasculature/ local inflammation created by C5a fragments?

- leakage of antibodies & other factors, such as complement proteins, into the site of the immune challenge - migration of cells of the innate immune system into inflammatory lesions - activation of phagocytes entering the lesion

complement functions in a cascade to attack extracellular pathogens; they do this via:

- lysing bacteria - opsonizing bacteria - promoting inflammation

in the lectin pathway of complement activation, there are 2 major host-derived serum proteins that bind to bacterial surfaces to start the cascade; what are they?

- mannose binding lectin (MBL) - ficolins

what is the sequence of events involving C5a, starting with neutrophil activation?

- neutrophil adhesion - neutrophil emigration & chemotaxis - monocyte activation - mast cell degranulation - smooth muscle contraction & increased vascular permeability

which complement factor is essential for the coagulation pathway?

C5 convertase (thrombin)

what is the active form of C3 in the alternative pthwy? what is its function?

C3b; (opsonin) - binds to bacterium & binds B for cleavage by D

what are the substrates of the classical complement pathway?

Ab + Ag --> C1 complex (w/ serine protease)

what factor does C3b bind to in the alternative complement pathway?

B - (forming C3bB)

which cell type has CR2 receptors to stimulate phagocytosis?

B cells

what component of the alternative pthwy of complement activation is the active enzyme of the C3/C5 convertase?

Bb

what is the active form of factor B in the alternative pthwy? what is its function?

Bb; active enzyme of C3/C5 convertase that cleaves C3 & C5

in the classical pthwy of complement activation, which components bind directly to pathogen surfaces or indirectly to antibody bound to pathogens, thus allowing further cleavages?

C1 (q, r, s)

in the classical pathway, which complement proteins are involved in the initial cascade?

C1 --> C4 --> C2 --> C3

in the classical & lectin pathways, which deficiencies make an individual susceptible to immune complex diseases (Lupus- (SLE) like)?

C1, C4, C2

in the classical pathway, C4b binds to ______, making it susceptible to cleavage by C1; this yields _______ on the surface of the bacterium

C2; C4bC2a (C3/C5 convertase)

in the classical pthwy of complement activation, which component activates enzymes of C3/C5 convertase?

C2a

regarding the C5 convertase, C5 binds to the C3b component, allowing cleavage by ______ (classical/ lectin) or ______ (alternative)

C2b; Bb

what component of the alternative pthwy of complement activation binds to pathogen surface, & binds B for cleavage by D

C3

in all 3 complement pathways, which deficiency makes an individual susceptible to severe pyogenic infections (particularly Strep pneumoniae, Neisseria meningitidis, Hemophilus influenzae)?

C3 (factors H & I)

in the classical pathway, which complement proteins are involved in the opsonin?

C3 --> C5 --> C6

at what step do the 3 different complement activation pathways converge?

C3 convertase - where C3 & C5 proteins are being cleaved (activated)

what is C3bBb?

C3/C5 convertase (in alternative pathway)

all pathways form a _______, which cleaves & activates ______ & ______; after this, the pathways overlap & use the same proteins

C3/C5 convertase; C3; C5

in the classical pathway, C4bC2a can cleave many molecules of _____, yielding ______

C3; C3a & C3b

many blood cells have receptors for certain activated complement proteins, the most important being _____ on ______ & ______

C3R; macrophages; neutrophils

which complement pathway components play an important role in anaphylaxes & allergies?

C3a & C5a (a ~ anaphylaxes/ allergies)

in the classical pthwy of complement activation, which component has many molecules of itself that bind pathogen surface & act as opsonins; initiate amplification via the alternative pathway; & bind C5 for cleavage by C2b?

C3b

the C5 converses can be inactivated by displacing ______ & cleaving it by Factor I or displacing ______ by Factor H

C3b

what gives a greater degree of binding than C3b receptor to C3b on the pathogen?

C3b + antibody (get very strong binding by combining complement AND antibody)

what is the end result of the MB-lectin pathway?

C3b binds to complement receptors on phagocytes --> opsonization of pathogens --> removal of immune complexes

in the classical pathway, activated C1 can cleave ______, yielding ______, which has no proteolytic activity, & ______, which binds covalently to the microbial surface near C1

C4; C4a; C4b

in the classical pathway, which complement proteins are involved in the membrane attack complex?

C6 --> C7 --> C8 --> lots of C9

what is CD18? what does loss of CD18 do?

CD18 = CR3 (which binds iC3b opsonin); it is an integrin; severely impairs migration of neutrophils & macrophages from the blood into sites of inflammation

the MAC can be disassembled by binding of ______

CD59

what factor of the alternative pthwy of complement activation is a plasma serine protease, that cleaves B, when it is bound to C3b, to Ba & Bb

D

what is the active form of factor D in the alternative pthwy? what is its function?

D; plasma serine protease which cleaves B when it is bound to C3b

what can inhibit C3 convertase?

DAF

TRUE/ FALSE: C5b binds surfaces covalently

FALSE

TRUE/ FALSE: when complement proteins are cleaved during activation, the larger fragment is designated "a" & the smaller fragment is "b"

FALSE - "a" is smaller, "b" is larger

TRUE/ FALSE: the complement cascade is highly specific

FALSE - (highly regulated; not specific)

what is the end result of the alternative pathway?

MAC, lysis of certain pathogens & cells

once C3b is attached, the ______ can form; this process is the same regardless of the initiation pathway; the outcome is ______

MAC; cell lysis

what is the end product of the complement pathway & what does it do?

MAC; it coats the pathogen for opsonization & introduces transmembrane channels --> lysis

in the classical & lectin pathways, which deficiency makes an individual susceptible to increased infections, particularly in life?

MBL (mannose-binding lectin)

what are the 2 substrates of the lectin complement pathway?

MBL (mannose-binding lectin) & CHO (usually mannose)

what factor of the alternative pthwy of complement activation is a plasma protein that stabilizes the C3bBb convertase on bacterial cells?

P (properdin)

which 3 cell types have CR1 receptors to stimulate phagocytosis?

RBCs, macrophages, B cells

TRUE/ FALSE: C5b initiates formation of the MAC

TRUE

TRUE/ FALSE: MBL & ficolins both bind serine proteases that cleave C4 & C2

TRUE

TRUE/ FALSE: the complement cascade is highly regulated

TRUE

TRUE/ FALSE: complement proteins are present in inactive forms in the serum

TRUE - (they're then triggered by 1 of the 3 mechanisms)

TRUE/ FALSE: the end result of the classical pathway & alternative pathway is the same

TRUE - they are initiated differently, but end result is the same

how are complement proteins triggered to enter the classical pathway?

activated by IgM or IgG binding to their surface

how are complement proteins triggered to enter the lectin pathway?

activated by opsonins binding to bacterial cell surface

definition of opsonization:

altering the surface of a pathogen to make it more efficiently engulfed by phagocytes

which complement activation pathway begins with a naive/immature C3?

alternative

which complement activation pathway involves innate immunity?

alternative

several complement cleavage fragments, esp. C5a are ______ that promote inflammation locally

anaphylatoxins

what are the 2 mechanisms by which complement regulatory proteins are used?

associated with host cell surface, or secreted in serum

what is MBL (mannose binding lectin)?

binds to mannose residues & several other sugars present on many pathogens, but not on mammalian cells; it is also produced by the liver in low quantities that are increased during an acute phase response

small complement cleavage products act on _______ to increase _______, allowing fluid leakage & extravasation of components into sites of _______

blood vessels; vascular permeability; inflammation

what is the general role of the MAC ?

cell lysis

regarding the MAC, C3b binds to C5 & permits cleavage by C4bC2b (or C3bBb in the alternative pthwy); the resulting C5a is _______ for inflammatory cells; C5b deposits on _______

chemotactic; bacterial membrane

which complement activation pathway uses antibodies?

classical

in the classical pathway, does the C3b (lots of it) bind covalently or non-covalently to the bacterial surface?

covalently

in the alternative pthwy, the complement component C3 can be cleaved spontaneously in plasma to form C3(H2O) which in turn binds ______; this now allows ______ to cleave ______, generating a C3 convertase; C3b then binds to membranes or _______

factor B; factor D; factor B; rapidly decays

after binding factor B, C3bB may be cleaved by ______, yielding ______ on the surface of the cell

factor D; C3bBb (C3/C5 convertase)

in the alternative pathway, which deficiencies make an individual susceptible to severe infections with Neisseria?

factors B, D, & properdin

in forming the MAC, C5bC6C7C8 binds to C9; what does C9 do?

forms a ring structure, forming a pore in the bacterial membrane

which cell type has CR3 receptors to stimulate phagocytosis & activate macrophages?

macrophages, monocytes, PMNs (granulocytes: neutrophils, eosinophils, basophils, mast cells)

many bacteria have outer structures that can: - block ______ Ab formation - prevent ______ by MAC - inhibit ______

neutralizing; lysis; phagocytosis

what part of the complement cascade is going on when there is a lot of C3b stuck to the surface of a microbe?

opsonization