Lecture 18 B cell activation and Classical Complement

Question 1

What are the main steps in B-2 cell activation?

Answer 1

Antigen crosslinks B cell receptors (signal 1)
Antigen internalized and presented on MHC2.
Helper Th2 cell binds B cell by TCRCD4:MHC2 and CD40L:CD40R. Th2 cell secreted cytokines too. This creates the second signal.
The combination of signals induces B cell proliferation and differentiation (somatic hypermutation) into Aby producing plasma and memory cells.

Question 2

Where does B cell activation occur?

Answer 2

B cell area / Germinal centres of lymph nodes.

Question 3

Which cytokines lead to differentiation of which kinds of B cells?

Answer 3

Il4 induces memory

Il-10 induces plasma

Question 4

What are the key surface proteins and properties present and absent on naive and plasma cells?

Answer 4

Naive / resting B cells produce surface Ig, MHC, have a low rate of Ig secretion. These are inducible to grow, switch isotypes and undergo somatic hypermutation?

Plasma cells do not express significant surface Ig or MHC2 but have a high rate of Ig secretion. In this state they are less inducible for growth, somatic hypermutation, and isotype switching.

Question 5

By what methods do Abys induce humoral immunity?

Answer 5

Neutralization
Opsonization
Complement activation

Question 6

How do Abys induce neutralization of pathogens / toxins?

Answer 6

Abys could bind viral proteins preventing receptor-mediated endocytosis of viruses or bacteria. Abys also can bind toxins and prevent their interaction with / damage to cells.

Question 7

How to antibodies act as opsonins?

Answer 7

Aby (usu IgG) binds pathogen/toxin coating it. Fc region binds to receptors (FcyRI)of phagocytotic cell. Ig coated pathogen is internalized and neutralized.
Ab binding to pathogen can also recruit complement proteins which further aid in pathogen destruction.

Question 8

What are some examples of Fc receptors and what are their corresponding functions?

Answer 8

IgG:ag bind FcyR1 mainly on phagocytotic cells inducing uptake and killing of pathogens.

IgG:ag also bind FcyR3 on NK cells inducing NK cell killing.

IgE bind FceRI receptors on mast cells, basophils, and eosinophils at a very high affinity inducing hypersensitivity responses (type 1) esp in allergies and the immune response to parasites. Granules are released. IgE bind the FceRI receptors in the absence of antigen.

Question 9

Describe Mast cell granules.

Answer 9

These are readily released by mast cells after FceRI bound IgE are cross linked by an antigen.
They contain:
histamine, inflammatory mediators to induce inflammation.

Question 10

Describe NK cells.

Answer 10

These are large, circulating lymphocytes that do not express TCR or Ig.
They express surface receptor FcyR3 which recognizes IgG1 and 3 and will secrete granzyme and trigger apoptosis in human cells coated with these aby.
These are involved in antibody-dependent cell-mediated cytotoxicity.

Question 11

What are complement proteins and where are they synthesized?

Answer 11

These are serum glycoproteins synthesized in the liver. They are activated during the innate immune system and induce cytotoxicity of organisms through cell and antibody-based mechanisms. (MAC, opsonization etc)

Question 12

What are the three main pathways of complement activation?

Answer 12

Classical (acquired immune response, require bound aby)
Alternative (innate immune response, no Aby required; pathogen surface by itself conducive to activation)
Lectin/Collectin (lectin binds mannose on pathogen)

Question 13

What features are key/similar between all three pathways?

Answer 13

All three pathways lead to C3b or C4b being covalently bound to the surface of a pathogen. This results in recruitment of inflammatory cells, opsonization uptake and phagocytosis, or formation of MAC.
The desired end result is death for the pathogen.

Question 14

Describe the early acting components of the classical pathway?

Answer 14

C1q binds to ag:ab complexes (IgM or IgG)
C1r, C1s and C2 act as serine proteases.
C4 and C3 covalently attach to pathogens and ag:ab complexes after activation.

Question 15

Describe the process by which Aby binding leads to C3b attachment.

Answer 15

One pentameric IgM or several monomeric IgG bind to the pathogen surface in a manner so that the C1 complex can bind.

C1s cleaves C4 exposing a reactive thioester bond which will create a soluble C4b and pathogen bound C4b. C4a is an anaphylotoxin

C1s also cleaves C2. C2A associates with bound C4b to create the classical C3 convertase: C4b2a.

C4b2a cleaves C3 into C3a (anaphylotoxin) and reactive C3b which binds the pathogen surface.

Question 16

Describe the terminal components of the complement system involved in creation of the MAC.

Answer 16

C5: is activated to C5b to initiation assembly of MAC in solution
C6: Stabilizes C5b; forms binding site for C7
C7: Binds C5b6 and exposes a hydrophobic region permitting attachment to cell membranes.
C8: Binds C5b67. Hydrophobic region exposed and inserts into membrane.
C9: Polymerizes on the C5b678 complex to from a membrane spanning channel (MAC)

Question 17

Describe the formation of the classical C5 convertase from the C3 convertase.

Answer 17

The C4b2a classical C3 convertase cleaves C3. Some C3b will be soluble and will associate with C4a2a creating the classical C5 convertase C4b2a3b. This binds C5 inducing its cleavage into C5a (anaphylatoxin) and C5b (MAC formation)

Question 18

Describe the formation of the MAC complex from C5b.

Answer 18

C5b associates with 6 and 7 allowing association with the pm. 8 associates with this and inserts into the pm. This C5b678 complex recruits many c9 which insert and form a pore within the cell membrane of the target cell forming the MAC.

Question 19

Describe the role that complement anaphylatoxins have.

Answer 19

Cleavage of C3,4,5 result in C3a, 4a, 5a, which are anaphylatoxins. These induce vascular permeability enhancing the immune response. Aby, complement, macrophages, neutrophils etc have easier access to the infected area

Question 20

What roles for cellular receptors for complement components and fragments exist?

Answer 20

Complement receptors bind bound C3 to stimulate phagocytosis.
These are also important for the binding and clearing of immune complexes