2-4: Microbiology/infectious Diseases - 3/17: Humoral Immune System

Front 1

Mature naïve B cells express surface Ig___

Back 1

IgM and IgD

Front 2

What cell-cell interaction is crucial for optimal Ab production?

Back 2

B:T Cell

Front 3

What is class switching? When does it occur?

Back 3

Occurs following B cell activation so that IgG, IgA and IgE may be produced

Front 4

Why is there a size difference between naïve B cells and plasma cells?

Back 4

Plasma cells are 2.5 times larger to produce tons of Ab

Front 5

Describe the three steps required to generate a humoral immune response.

Back 5

1. Hematopoiesis
2. Activation and Differentiation (process by which naïve B cells are act’d by foreign Ag and selectively expanded)
3. Antigen Elimination (process by which foreign antigen and pathogen that produced it is removed from body

Front 6

Describe the cells required and produced by hematopoiesis.

Back 6

MHSC in BM (self-renewing) gives rise to:
Lymphoid Progenitor Cell: gives rise to B cells, T cells, NKC’s

Myeloid Progenitor Cell: RBC’s, granulocytes, platelets (via megakaryocytes), macs, DC’s

Front 7

Via what process and in what cell type does heavy chain arrangement occur?

Back 7

VDJ Rearrangement in Pre-B cells (Pre-B is between Progenitor-B and Immature B)

Front 8

Via what process and in what cell type does light chain arrangement occur?

Back 8

Rearrangement of lambda or kappa mRNA in immature B cell (between pre-B and mature B)

Front 9

Briefly describe what happens in VDJ rearrangement. What does this allow for?

Back 9

D is joined to J
V is joined to DJ
Transcription!

Generation of millions of Ab’s from relatively small amount of DNA

Front 10

Where does foreign antigen-independent lymphocyte development occur? Describe the major events of this stage.

Back 10

Primary Lymphoid Organs (BM, Thymus)

BM: Ab gene rearrangement, pos/neg selection

Thymus: migration of ProT from BM to thymic cortex
TCR gene rearrangement
pos/neg selection

Front 11

Where does foreign antigen-dependent lymphocyte development occur? Describe the major events of this stage.

Back 11

Peripheral Lymphoid Organs: Spleen, LN's, MALT, LV's

LN, Spleen: Ag exposure: lymphoid follicles-->germinal Centers

MALT: MORE PLASMA CELLS THAN IN ANY OTHER TISSUE COMBINED

Front 12

Describe the path of immune cells from the BM.

Back 12

BM-->Blood-->Spleen, MALT, LN's-->Blood

BM-->Thymus-->Blood-->LN's, Spleen, MALT-->Blood

Back and forth means surveillance!

Lymphocytes are never in one place for very long

Front 13

Thymus-Dependent Antigens vs Thymus-Independent Antigens

Back 13

Thymus-Dependent Ag’s :
Require B:T cell interaction for Ab production (most Ag’s fall in this category)
Undergo Isotype Switching
Undergo Affinity Maturation
Have secondary response via memory B cells
T-dependent = PROTEINS

Thymus-independent Ag’s can induce Ab responses in absence of T cells;
function as mitogens (agent stimulating mitosis), activating cells vis TLR’s regardless of Ig specificity (LPS), may cross-link membrane Ig (e.g. flagellin)
T-independent = NON-PROTEINS

Thymus independent antigens do NOT involve isotype stwitching, affinity maturation, or memory (demonstrates importance of T cells in humoral immunity)

Front 14

Primary vs Secondary Immune Responses:
Lag after immunization
Peak response (antibody number)
Antibody isotype
Antibody affinity

Back 14

Primary:
5-10 day lag time
Smaller peak response (antibody number)
IgM>IgG
Lower avg Ag affinity, more variable

Secondary:
1-3 day lag time
Larger peak response (antibody number)
Relative increase in IgG, IgA or IgE depending on situation (class switching)
Higher average affinity (affinity maturation)

Front 15

Describe the molecular events that follow a microbe binding a B cell surface immunoglobulin.

What changes does the B cell undergo in response to antigen binding?

Back 15

**Ig’s cross link on B cell
**ITAMs on cytoplasmic side get Pi’d
Downstream signaling leads to actvn of transcriptional factors (NFAT, NF-kappaB, AP-1)
Upon Ag binding, B cells:
**Enter cell cycle (mitosis) to allow for clonal expansion

**Increase expression of B7 costimulators (helps activate Th cells)

Increase expression of cytokine receptors (now able to respond to cytokines produced by Th cells)

**Migrate out of lymphoid follicles and into parafollicular cortex AKA T Cell zone (to interact with Th cells; B cells serve as APC’s)

Secrete low levels of IgM (early phase of humoral immune response)

Front 16

Describe the molecular events that follow microbe associated with C3d binding a B cell surface immunoglobulin.

Back 16

C3d is a breakdown product of complement factor C3(b)
Microbe will bind surface Ig (dimerization etc)
C3d will bind CR2 (B cell co-recepgtor)

Signals from Ig and CR2 complex both lead to B cell activation

Front 17

Describe the molecular events that proceed B cell recognition of native protein antigens and result in T cell recognition of the antigen and B cell proliferation.

Back 17

B cell recognition leads to receptor mediated endocytosis of antigen

Undergoes Ag processing and presentation via MHC II

Th Cell binds to MHC II AND B7 (via CD28 RECEPTOR)

Activated Th cell expresses CD40L and releases cytokines

B cells activated by CD40 engagement AND cytokines

B cells proliferate and differentiate

Front 18

What is isotype switching and what is required for it to occur?

Describe the process by which isotype switching occurs.

Does this affect antigen specificity?

Back 18

Isotype switching is process through which CONSTANT regions of H CHAIN GENES are rearranged and lead to variety of act’d B cells expressing a new Ig isotype (IgG, IgA, IgE); DOES NOT AFFECT V REGIONS (variable regions); this is NOT VDJ REARRANGEMENT

Th cell cytokines required for isotype switching (result of isotype switching depends on cytokine to which it’s exposed)

(some isotype switched act’d B cells enter recirculating pool and seek Ag in tissue, can produce Ab there)


Isotype switching is the result of splicing out constant regions that proceed CDJ region

DOES NOT AFFECT ANTIGEN SPECIFICITY OF ANTIBODY

Front 19

What is affinity maturation and how does it occur? Purpose?

Back 19

Affitinity maturation is the result of somatic hypermutation which generates point mutations at a high rate in V regions of H and L genes of act’d cells
(Mutation occurs in “hypervariable’ regions: CDR1,2,3; CDR = complementarity determining region)

A few of these mutant B cells have higher affinity for Ag and are selected for add’l proliferation (most undergo apoptosis bc of lower affinity)

Result is avg affinity of Ab’s increases and thus affinity is said to “mature”

Front 20

How is B cell regulation achieved?

Back 20

An antibody that is NOT secreted by the B cell to be regulated binds Ig AND Fc receptor
Results in inhibition of B cell response

(“There’s enough Ab in blood; hold off production)

Front 21

Agglutination vs Neutralization vs Opsonization

What do these all require?

Back 21

Agglutination:
Bacteria or viruses clumped together and immobilized

Neutralization:
Prevent pathogen (or toxin) from binding

Opsonization:
Coating of Ags or target cells with Abs and complement proteins to enhance phagocytosis

ALL REQUIRE AB's

Front 22

Antibody-Dependent Cell-Mediated Cytotoxicity vs Complement-Dependent Lysis

Back 22

ADCC: Ab's bind to offending/infected cells and direct non-specific cells to it via Fc portion of bound Ab's

Complement-dependent lysis: acts against cells or enveloped viruses via assembly of Membrane Attack Complex

Front 23

Which Ig isotypes are involved in ADCC?

Back 23

ADCC: IgG, IgE

Front 24

Which Ig isotypes are involved in agglutination?

Back 24

IgM (first responder and involved in classical pw of complement)

Front 25

Which Ig isotype exists as a dimer? As a pentamer?

Back 25

Dimer: IgA (held by J chain)

Pentamer; IgM (lots of Ag binding sites)

Front 26

How are intestinal pathogens neutralized in the gut?

Back 26

Multiple IgA's binds pathogen which prevents entry into epithelium

(Note: mucosal epithelial cells express polyIg receptor to allow transport of IgA through epithelium; must be proteolytically cleaved before exit)

Front 27

Briefly describe the events of opsonization beginning with IgG.

Is there another molecule that can trigger this? How?

Back 27

IgG antibody binds microbe

Opsonized microbe binds phagocytic Fc receptor

Fc receptor signal activates phagocyte

Microbe is phagocytosed

C3b can also activate opsonization! It does so by binding CR1 receptor on phagocyte

Front 28

Describe the steps of antibody dependent cell-mediated cytotoxicity beginning with IgG and IgE.

Discuss receptor affinity.

Back 28

IgG binds surface antigen and coats cell

IgG coated cell binds LOW AFFINITY Fc receptor on NKC

NKC kills

IgE binds surface Ag on parasite
IgE binds HIGH AFFINITY Fc receptor on eosinphil and results in degranulation

Front 29

Describe the three complement activating pathways. Which complement proteins are important for each?

Back 29

Alternative:
C3b binds surface of microbe and serves as opsonin
Also requires Factors B, D, Properdin

Classical:
IgG antibody binds microbe
C1 binds Fc portion of IgG
Also requires C2, C4

Lectin Pathway:
Lectin binds Mannose on microbe
Requires C2, C4

Front 30

What do all complement pathways result in?

In what order do the proteins involved assemble?

Back 30

Formation of Membrane Attack Complex

(C4 convertase: C5-->C5b, C6-C5b-C7-C8)
Cell Lysis

C5,6,7,8,9

Front 31

What are the three functions of complement?

How is the complement system regulated?

Back 31

Three fns:
1) Opsonization and phagocytosis (by a macrophage via C3b-CR1 interaction)
2) Complement-mediated cytolysis (by MAC complex via C3b)
3) Stimulation of inflammatory response (C3a, 4a, 5a recruit and activate leukocytes)

REgulation by:
1) Decay-Accerlerating Factor (DAF): interferes with formation of C3 convertase by displacing Bb from C3b (blocks ALTERNATIVE AND CLASSICAL)
2) Membrane Cofactor Protein (MCP) destroys any C3b formed (Factor I mediated cleavage of C3B)
3) C1 Inhibitor (C1-INH) prevents assembly of C1 complex (blocks CLASSICAL)

Front 32

What are three ways microbes evade humoral immunity? Provide examples of microbes.

Back 32

1) Antigenic Variation (viruses: HIV, flu)

2) Inhibition of complement activation (bacteria)

3) Resistance to phagocytosis (pneumococcus)

Front 33

Would an immune response to capsular polysaccharides require T cells?

How does this relate to H. influenzae B vaccines and children? What's the solution?

What antibodies are involved?

Back 33

No T cells required

Vaccine antigens = polysacchs and won't result in memory

Children mount poor T-independent Ab responses

Solution: Conjugate bacterial polysacchs to protein carrier to improve immunogenicity

THUS makes it a T-dependent response and resulting in HIGH levels of IgG

T cell independent vaccines would produce little IgG

Front 34

What is passive immunization? When is its use indicated?

Back 34

Transfer of preformed ab's to recipient, conveying instant immunity

Transient protection, no memory induced

(naturally occurs with maternal Ab's across placenta--IgG--and through colostrum--sIgA)

Indications for Use:
Immunodeficiency affecting Ab production
Immunoprophylaxis in at-risk populations
Rapid immunity needed (travel, recent exposure)