1. IMMUN 441 AC Quiz Section Week 7
B Cell Immunity
Mucosal immunity

2. Exam #1
Exam #2
Final Exam

3. Review: Skin Graft Experiment
MHCa
MHCa/b
Review: Skin Graft Experiment
Negative Selection
MHCb IR IR
MHCa
MHCa
Skin Graft
delete any T-cell that is strongly reactive during neg selection, thus eliminating any possible self reactive T-cells
Graft Rejected
Graft Tolerated

4. B cell Immunity

5. B cell antigens differ in their requirement for T cell help
T-cell dependent (TD)
T-independent (TI)
'activation' signal
but not mitogenic
BcR signal
Mitogensis
Mitogenesis
Differentiation
Class switch Recombination somatic Hypermutation
Non-proteins/highly repetitive structure antigens
Today we’ll be talking about how b cells are activated and contribute to the adaptive immune response, as we’ve learned, there are 2 main types of b cell antigens, t cell dependent and t cell independent, the difference here being whether or not the b cell specific for the antigen requires help from cd4 t cells in order to become activated, much like t cell activation, the activation of b cells necessitates the integration of multiple signals in order to be productive, thus in the example on the left with a t cell dependent antigen (or td), the b cell binding antigen provides an activation signal, and then when the b and t cell interact via the b cell presenting peptides from the antigen to the t cell, this provides a second signal, this second signal is mitogenic which means it induces cell division, in addition b cell activation also parallels t cell activation in that both can integrate signals from cytokines in order to tailor their effector function to the specific type of infection, b cells do this thru CSR and SHM which occur in a GC reaction as we’ll talk about later, finally, one of the reasons that these two types of b cell antigens exist is because t cells can only provide help to b cells that recognize an antigen that contains protein because chopped up protein AKA peptide, is what the t cell is specific for, so many of the t cell independent or TI antigens, are non-proteins that t cells could never recognize, so in the case of TI antigens, these must provide both the BCR activation signal as well as the mitogenic signal
Protein antigens

6. Different classes of TI antigens
Within TI antigens, there are two types which differ in terms of how they activate the b cell, TI-1 antigens have inherent mitogenic properties which allows them to induce polyclonal b cell activation, the activation of b cells with a wide variety of antigen specificities when there is a high concentration of the antigen, when there is a low concentration though, only b cells that are actually specific for the TI-1 antigen will be able to get activated, how do these antigens work? Well they mainly consist of TLR ligands like LPS and bacterial DNA, since as we’ve learned TLRs recognize conserved PAMPs this helps to explain how when there is a large amount of LPS that B cells come into contact with, this could induce polyclonal activation, the other type of TI antigens TI-2 mainly consist of molecules with repetitive structures, if b cells are specific for part of this structure, their binding could induce very strong bcr crosslinking, which can activate the b cell, when this occurs, as shown in the middle panel, the b cell can then secrete antibody but only IgM, it cannot switch isotypes, however, there is some evidence for the scenario at right where TI-2 antigen activation is accomanied by signals from activated DCs like BAFF, which could allow for this b cell to class switch and produce IgG, finally its important to remember
Highly repetitive structures
Typically occurs in marginal zone B cells
Mainly TLR ligands
Can occur in all types of B cells

7. T-dependent responses
Antigen DC
T cell
T cell
B cell
plasma cells
Which T helper cell subset is important for T dependent B cell responses?
Dendritic Cell (DC)
internalizes antigen (Ag), processes into peptides, presents peptides together with MHC molecules to T cells
B cell binds Ag via surface Ig, transmits BCR signals and presents peptides to T cells, receives
T cell help (growth and differentiation factors)
Secretes Antibody (Ab)

8. T cell-dependent antigens: Linked antigen recognition
B cells that capture linked antigen are the ones that can get T cell help I
Only T cells that recognize epitopes that are physically linked to the BCR epitope can provide costimulation signal Y I
Internalized
B cell
B cell
B cell
CD40
Alright so that’s how b cells can be activated independent of t cells, but most of the b cell response is mediated by t cell dependent antigens where the t cell interacts directly with the b cell to provide help, this interaction relies on the concept of linked recognition, so here you have a b cell that is specific for this green molecule on the outside of this virus, the binding of the b cell to its antigen allows for the whole virus to be endocytosed into the b cell, following endocytosis, the b cell can then present the broken down parts of the virus via the MHC class II pathway, at this point the b cell can meet up with a t cell that is specific for the viral peptide presented by the b cell which leads to b cell activation and antibody production, it’s important to remember here, that the specific part of the virus recognized by the b cell is not necessarily the same part that is recognized by the t cell, in this example the b cell is specific for the green molecule but presents to a t cell which is specific for the red peptide which is inside of the virus, so this concept of linked recognition really relies on the fact that b cell antigen presentation, the processing of peptides and presentation via MHC II, is almost entirely dependent on what the b cells BCR binds, unlike DCs which can endocytose and otherwise internalize extracellular molecules in wide variety of ways, b cells are only good at endocytosis via binding to their bcr, thus this ensures that when a b cell presents a mhc ii peptide to a t cell, that peptide came from the molecule that the bcr binds, this concept also serves to limit autoimmunity as the chance of a self reactive b cell causing damage to the host necessitates the creation of both a self reactive b cell and a self reactive t cell, and then the two of these cells would have to find each other
Linked recognition helps ensure self-tolerance: autoimmune response occur only if both a self reactive T/B cell meet
Linked recognition: important for immunization of infants → chemically link polysaccharide antigens with proteins that activate helper T cells. THM: B cells make antibody to polysaccharides thereby protecting the infant who cannot effectively mount an immune response to these polysaccharides Y I Y I
cytokines
CD40L Y I
Antigen- primed
T cell
Activated
B cell
Epitope BCR recognizes is not necessarily the same peptide the TCR recognizes

9. Linked antigen recognition
T cell
B cell
B cell
A way to increase this rare encounter is for the T cell to be able to recognize any processed epitope from same complex that was originally recognized by the B cell.
T cell
Germinal centers are each their own isolated event i.e. one germinal center formed because of a bacterial infection and the GC next to it could be from a helminth infection.
HOWEVER, the FDC is a source of all antigens ie can be from both bacterial and helminth
FDCs trap immune complexes and present to high affinity B cell (typically in the light zone).
These B cells have the ability to “grab” the antigen off the FDC and as the ag source gets depleted, the specificity of the B cell and the antibody gets more specific.
T cell

10. Chemokine-mediated migration facilitates T:B interactions and germinal center formation

11. Germinal center & follicular DCs
Each germinal center is an isolated event based on what the B cell encountered i.e. bacterial, viral, or parasite protein
Migrate to the dark zone where affinity maturation can occur (SHM and CSR)
CXCL13 Y
B cell Y
FO DC
CXCR5 Y
B cell
B cell
T FH
Follicular DC (light zone)
Can trap antigen complexes at cell surface and holds all antigens
Present to T cell for survival/differentiation signal
Destination: 1) Back to dark zone
2) Memory B cell or a long-lived plasma cell

12. Germinal center & follicular DCs
Each germinal center is an isolated event based on what the B cell encountered i.e. bacterial, viral, or parasite protein

13. FDCs GC B cells Naive B cells
Immunofluorescence of a mouse lymph-node germinal center 7 days after secondary immunization. Red: FDCs (CD35); green: GC B cells (10% of which express GFP); blue: naive B cells (IgD)
FDCs GC B cells Naive B cells

14. After appropriate activation the B cell differentiates into an antibody secreting cell, also known as a Plasma Cell
B cell
Plasma Cell
membrane Ig
Master
Transcription
Factors:
secretory Ig
Blimp1
XBP1

15. B cell antibody response - clonal replication enters
into a higher order upon plasma cell differentation
3 days
12 divisions
1 day
differentiation
1 day
103 Ab/cell/sec
naive
B cell
activated B cells
plasma cells
antibodies
>1012 1
212 = 4,096
4,096

16. Antibody Affinity Maturation
Germinal Center
Low affinity B cell
High affinity B cell
mutation
selection VH VL Ag VH VL VH VL Ag VH VL

17. Affinity Maturation Selection Model

18. B cell activation leads to production of antibodies with diverse functions
Alright so we’ve just talked a lot about how b cells get activated to produce antibodies, the rest of class will be spent on how the antibodies function, their mechanism of action can be broadly categorized into neutralization, opsonization and complement activation, neutralizing antibodies act by binding to their antigen, which prevents the pathogen from infecting cells, opsonizing antibodies bind to their antigen and act as a tag for other immune cells to bind and phagocytose the pathogen, finally some antibodies are important for inducing the activation of the complement pathway

19. Antibody isotypes facilitate diverse types of immunity
So those are the broad ways that antibodies can function, but as we’ve learned there are multiple classes of antibodies, and these different classes vary with regard to their ability to function in each of these catagories, so IgM is really good at activating complement but not as good at neutralization and opsonization, on the other hand IgG is really good at opsonization, part of the different abilities of the antibody isotypes has to do with their structure, most antibodies exist as monomers but IgA can be a dimer and IgM is a pentamer, we’ll go over how these different structures work in the upcoming slides
Monomer: IgD, IgG, IgE, IgA
Dimer: IgA
Pentamer: IgM

20. Neutralizing antibodies can block viral/bacterial entry
So now that we’ve talked about the different isotypes, we’ll look at how they actually provide protection, in the case of neutralization, the binding of antibodies to a virus, shown in the left panel, directly inhibit the ability of the virus to infect cells, this can also occur for intracellular bacterial infections, whereby antibody binding to the bacteria prevents it from interacting with the host cell surface receptors it normally uses to enter and infect the cell

21. Memory B cells Generated during the primary response
best characterized for T-dependent responses involving GC but some evidence exists for memory to TI antigens
Small, recirculating cells
Typically isotype switched (e.g. IgG+ or IgA+)
Typically have higher affinity for the inducing Ag
Longer lived than naïve B cells
Persistence of memory B cells after an immune response ensures that we have increased numbers of B cells specific for the antigen and ready to respond on re-encounter
May have intrinsic differences that promote greater clonal expansion and more rapid differentiation to plasma cells
differences in cytoplasmic domains of IgG vs IgM/D
upregulation of TLRs

22. Mucosal Immunity

23. The mucosal immune system: overview
Small Intestine:
bacteria per ml
Large Intestine:
441 Lecture #18 Slide 23 of 28
Savan 11/09/2016

24. Organization of the mucosal immune system
M Cells

26. T cell priming and redistribution in the MALT
T cells activated in the MALT
are imprinted with adhesion
molecules that mediate
distribution throughout the MALT
Naïve T cells enter
PP the same way they
enter LN: through HEV

28. Mucosal IgA neutralizes antigens

29. IELs live in between epithelial cells
Lamina propria lymphocytes
Lamina Propria: the “space” under the epithelial cell layer: rich in connective tissue
and contains abundant innate and adaptive immune cells
Lymphatic vessels collect fluids, antigens, pathogens
Epithelial layer contains specialized immune cells that live “between” epithelial cells
Direct contact with epithelial cells allows for close communication and rapid
effector responses

30. Two types of IELs kill infected epithelial cells

31. Commensals directly compete with pathogens
Niche-filling function prevents overgrowth of pathogenic bacteria
Another reason why antibiotics should be used sparingly!

32. Epithelial cells express key PRRs to detect invasion
DCs that reach into lumen also contribute to innate immune detection in the gut 1 3 2
Coincident detection of multiple PAMPs in distinct
cellular locations may also allow the innate immune
system to distinguish pathogens from commensals:
BOTH express PAMPs (MAMPs?)
ONLY pathogens invade
Commensals stimulate 1
Pathogens breach the barriers and stimulate 1, 2, 3

33. Questions?

34. Acronyms
BALT-bronchial associated lymphoid tissue
GALT-gut associated lymphoid tissue
NALT-nasal associated lymphoid tissue
MALT- mucosa associated lymphoid tissue (all)
IEL-intra epithelial lymphocytes
M cells- microfold cells
PIGR- poly-Ig receptor
PP- Peyer’s Patch