Once in the tissue, specificity occurs

o

If are specific to an antigen in the tissue, you will bind and
stay there

o

If you are
not
specific to an antigen, you will return to the circulation

o

Activation of integrin receptors will cause the cell to stay put



EXAMPLE: CMI to Listeria

A.

Viable Listeria
cells increase, but if give the mouse passive immunity, you will not get the increase in Listeria cells

o

Clearly, the T
-
cells are helping

B.

If you try to transfer the serum (where there could be antibodies), there is no effect

o

This is b/c we don’t make antibo
dies against Listeria (b/c intracellular)

o

Clearly, Abs are not giving protection

C.

Looking at % killing (increase means better protection)

o

Just have immune T cells


no protection

o

Just have resting macrophages


no protection

o

Transfer activated macrophages


they can kill the listeria

o

Macrophages that are the “real” effector cells to get rid of the bacteria



These are activated by Listera
-
specific CD4 T
H
1 cells

o

T
-
cells are part of the CMI, but they themselves do not do the killing



Get a better eradication if
also

have CD8+ T cells

o

This is b/c the Listeria might get into the cytoplasm


DTH: Delayed Type Hypersensitivity



Excessive immune response


tissue trauma



CD4 T cells activate macrophages and induce inflammation

o

Caused by innate TNF and IL
-
1 cytokines



24
-
48 hours by a skin test





Overview of
Activation of Macrophages by T cells



IL
-
12 activates T cells


Effector T cells activate macrophages

o

IFN
-
γ

receptor is important in feedback

o

CD40 also is important as the anti
-
apoptotic receptor



Effector helper T

cell secretes
IFN
-
γ

and also has the anti
-
apoptotic factor



Macrophage can now kill phagocytosed microbes with ROI, NO


o

Activation of phagocytosis

o

Secretes TNF, IL
-
1 as inflammatory cytokines

o

IFN
-
γ

will upregulate the entire MHC locus

o

Also increase in B7 mo
lecule production


Balance between TH1 and TH2



If make too many TH2 cells, make lots of inhibitory cytokines and interfere with macrophage activation

o

IL
-
10, IL
-
4, and IL
-
13 are inhibitory cytokines



In leprae: a small group of people get defective TH1
response and a dominant TH2 response

o

Infectious and bad

o

Bacteria activates complement


lysis of cells


your tissues die





Killing of Infected cells by CD8 CTLs



Need to make a “zipper” out of hundreds of interactions all over the surface of the cells



Enzymes are released at a high concentration in a very confined space, trying to get the target cell as strong as
possible



A number of proteins made:

o

Granzyme



Pre
-
made in vesicles



Protease enzyme that induces apoptosis



Need to get the granzymes into the t
arget cell

o

Performin



Monomer of a protein that can go together and make a “tube” that will be inserted into the
membrane of the target cell



Granzymes go through this “tube” and get into the cell



Might also have some osmotic blow out



Cooperation between C
D4 and CD8 T cells



Viable bacteria in the cytoplasm as well as phagocytosed microbes in vesicles



Need both CD4 and CD8 cells to eradicate the infection






Resistance to CMI: pathogenic microbes



How do pathogens develop resistance to this type of
cell
-
mediated immunity




CHAPTER SEVEN: Humoral Immune Response

Phases of Humoral Response



Antigen recognition:

o

Naïve B cells do not recognize antigens yet

o

They might find an antigen, but if there is no second signal, the cell will kill itself

o

Helper
-
T c
ells and other signals are necessary to become more than a circulating B cell



Activation of B Lymphocytes

o

Helper T cells are
necessary

because they donate IL
-
2

o

IL
-
2 causes clonal expansion

o

This is a t
-
cell dependent response



We need to differentiate into s
ecreting antibodies instead of membrane
-
bound antibodies (receptors)

o

They become terminally differentiated plasma cells that just make antibody

o

IgG cells can differentiate into other isotypes “class switching”



Determined by mixture of cytokines that are th
ere

o

Yet another differentiation pathway that helps in parallel with the others



Induce form of mutation



Leads to selection of B
-
cells with
higher affinity

paratopes

o

Cytokines also determine whether a B cell becomes a memory cell



ANTIGEN SPECIFICITY REMAINS
UNCHANGED (paratope stays the same)




T
-
independent B cell Response



CD4 cells are necessary with a
protein

antigen

o

Can get class switching, affinity maturation, and memory cell production




Protein
-
free antigen

can have a t
-
cell independent response

o

Polyme
ric antigens (polysaccharides, glycolipids, nucleic acids)

o

Need 50
-
200ish BcRs being activated at the same time

o

No class switching, no affinity maturation, no memory B cells formed



Therefore every response is a primary response



Secretion of “natural” antib
odies (IgM)



Primary and Secondary Response



In primary, see only IgM, but in secondary, they have switched



Keep the same specificity for antigen/epitope



BcR Antigen Signaling



Ca++
-
enzymes, PKC, and ERK, JNK all lead to transcription factor changes



Transcription factors include Myc, NFAT, NF
-
kappa
-
B, AP
-
1



This is not the only signaling cascade…there are other secondary signals



Difference with T
-
cells:

o

No easy way to show tyrosine kinases in the picture

o

Must have a group of them because they do not wo
rk on each other…they work on adjacent molecules


Synergy



Lots of antigens have only single epitopes…how do you make an immune response to a single
-
epitope antigen?



This is where a second signal system is directly linked to generating the signal



CR2, CD19
, CD81 is a second signal receptor system that recognizes innate molecules on microbes

o

CR2 binds a complement protein fragment

o

The complex not only has a substrate site where the kinases from the BcR work on, it also brings in a
kinase that works on the Ig

α

and
β


o

Very strong B cell activation

o

This will reduced the amount of antigen epitopes needed

to induce a response




B cell activation changes cell differentiation





Types of differentiations that can occur



Cross
-
linking of BcR is the best way to get
this process going

o

This type of cross
-
linking leads very quickly to B cell activation



To enter into mitosis, there needs to be the help of IL
-
2 from Helper T cells (not shown in the picture)



B cell may also differentiate in a different way

o

Starts exposing
second signals for the T
-
cell (B7)

o

B7 is a ligand for T cells and therefore is more able to interact with the T
-
cell

o

Also expression of cytokine receptors (up
-
regulation of IL
-
2, IL
-
4, etc.)



Up
-
regulation of CD
-
40 system (receptor that prevents apoptosis f
rom occurring)

o

Make sure this proliferation is accompanied by this up
-
regulation, or all the cells that were made will die



Center of follicles


migration to surface (because needs to start interacting with T cells surrounding the
follicles)

o

This is due to

chemotactic responses


Activation and Migration of Helper T cells



T cell initially sees the dendritic cell (only APC available in a primary response)



T cells differentiate and become activated



B cells start taking up microbe and express on MHC
-
II molecule
s so that helper
-
T cells can interact with them



At the follicle
-
parafollicular cortex, there is a direct interaction between helper T cells and B cells

o

Cytokines are there (they will determine which isotype should be made)



Ag Presentation by APC B cells



Uptake of microbe by BcR endocytosis



MHC
-
II response is upregulated, esp if there is IFN
-
γ



B7 signal is introduced




Helper T cells interact by MHC
-
II, CD28 complex




CD40 Ligand



Seen in previous chapter



This is a secondary immune response



T cell gives
signal to the B
-
cells to further proliferate (IL
-
2)



CD40 ligand provided by T cell (anti
-
apoptotic)

o

Also secretion of cytokines

o

By
-
stander B cells may get enough cytokines to proliferate, but they will die because they don’t have the
anti
-
apoptotic recepto
r
-
ligand interaction



6hr doubling


this is a very fast rate



Concurrent

forms of B cell differentiation:

o

Secretion of BcR as antibodies (turn on splicing machinery that will not have the TM domain)



Seen in T
-
cell
independent

responses if there is strong en
ough signal

o

Induction of DNA recombination to create isotype switching

o

Induction of somatic mutation to create affinity maturation (specifically in immunoglobulin genes)



Selective DNA damaging response and repair mechanism

o

Memory cell formation (don’t know

mechanism)





Class Switching



No signals from helper T cells


mu gene translation

o

Get a C
-
mu chain


IgM antibodies



Cytokines or CD40 ligands will selectively open up chromatin areas called “switch regions”

o

Form of somatic recombination (similar to VDJ
recombination)

o

Cytokine receptors stimulated at B cell surface


specific transcription factors turned on


specific
switch regions activated


different constant regions transcribed

o

Join up the variable domain with a different set of constant domains



I.E.

IL
-
4 leads to production of IgE



Our VDJ recombination was antigen independent, unlike this recombination

o

Different mechanisms, but are similar outcomes kinda



In all these cases, the VDJ is unchanged, meaning the paratope is unchanged

o

Light chain is not af
fected at all

o

Heavy chain is the one that changes



IFN
-
γ

makes switch to IgG1 or IgG3 (typical forms in phagocytic responses)



TH2 cells produce IL
-
4


IgE antibodies



Some of the IgG antibodies will go to the placenta if there is a fetus



Class switching is
also defined by environment…mucosal tissues have TGF
-
β



IgA production





Affinity Maturation



Induction of mutation mechanism specifically in variable domains (not exclusively specific to light or heavy sites
only)



Day 7 primary cells have only a few

poi
nt

mutations in the germ line



Day 14 primary cells have more

variation

b/c of
j
unctional diversity

o

Affinity has increased by a factor of 10



Secondary exposure to antigen

o

Have quite a number of B cells that will respond, but only given a limited amount of a
ntigen

o

Those B cells with the highest affinity will bind the antigen and proliferate

o

Therefore these “outlive” the B cells that have less affinity for the antigen



This occurs only in the variable regions of the genes



Most of these variations are actually
BAD, but there are some that are “good” and get a slightly better binding


o

Maybe make a “better fit” in the paratope for the epitope



Selection of Highest Affinity B Cells



Affinity can be lost because of random mutation, so no visibility of antigen

o

Cell
will die because will not bind antigen



Affinity can be increased

o

If antigen becomes
limiting,

the B cells with the highest affinity will win and get the antigen


proliferation


these make the memory B cells



Have increased affinity over the course of the
immune response




Anatomy of Humoral Immune Response



Primary follicle where B cell sees antigen


moves to outside boundry



T cells with macrophages


meet up with B cells



Proliferation


antibodies secreted go into the blood



Plasma cells will typically no
t start circulating



Activated B cells will migrate out of the follicles, but not into circulation



Germinal centers are the sites of memory B cell formation

o

Don’t know exactly how this works



A limited number of plasma cells move into bone marrow and become
long lived and secrete AB without Ag
stimuli

o

Bone marrow provides signals to overcome default apoptosis

o

No BcR, so not responsive to antigen

o

Just secrete a continuous low level of antibody, whether or not the antigen is present

o

“baseline” amount





Feedba
ck Control



One of the Fc receptors that we know binds IgG is on the b
-
cell



Why does a B cell need an IgG receptor? It isn’t a phagocyte?

o

It has a phosphatase that takes phosphates off tyrosines, thus inhibits Ig signaling at the BcR



Normally, if have anti
gen, activate BcR



However, if already have antigen
-
Ig complexes, there are clearly plenty of Igs available


tells the BcR to “slow
down”



The MAJOR control is default apoptosis because there is not antigen left to bind to Igs