Test 3

Question 1

What cell type is the critical portal for lymphocytes to extravasate into lymph nodes?

Answer 1

Endothelial cells that line the blood vessels make it possible for lymphocytes to extravasate
into lymph nodes.

Question 2

List the four steps (and molecules) involved in leukocyte extravasation.

Answer 2

1. Rolling: Selectins expressed by endothelial cells allow for leuokocyte attachment to
the endothelium as they are expressed in an inflammatory response, binding to
carbohydrates on the leukocytes. This causes temporary binding, so the leukocyte
rolls along the endothelium.

2. Activation: As the leukocyte’s rate of rolling decreases, chemokines are detected by
the leukocytes on the endothelial surface. These chemokines help recruit
leukocytes to the site of infection and cause conformational changes on the surface
of the leukocyte so integrins start to gather together to begin adhesion.

3. Arrest and adhesion: the clustered integrins help the leukocyte to stop rolling and
adhere to the endothelial tissue. They bind to Ig-superfamily CAMs on vascular
endothelial cells, strengthening the leukocyte’s bond to the endothelial cells so it is
not as likely to be swept away by the sheer force of blood running through the blood
vessel.

4. Migration: the platelet-endothelial-cell adhesion molecule-1 (PECAM-1), expressed
on both the leukocyte and vascular endothelial cells, is used to allow the leukocyte
to squeeze between the endothelial cells and into the inflamed tissue. By both
binding to PECAM-1, as well as other integrins, the structural integrity of the blood
vessel is saved so no blood or anything else leaks out as they leave the blood vessel.

Question 3

Describe the different locations T and B cells traffic to upon entering the lymph nodes.

Answer 3

B cells enter the B-cell follicles in the cortex of the lymph node, while T cells enter the
paracortex (T-cell zone) of the lymph node.

Fibroblast reticular cells are found in the T cell-zone, upon which T lymphocytes browse for matching antigen. Follicular dendritic cells are found in the B-cell follicle, which B lymphocytes browse to bind matching antigen.

Question 4

Why are fibroblast reticular cells and follicular DCs important for immune responses?

Answer 4

Fibroblast reticular cells provide “roads” for lymphocytes to travel on when traversing the
lymph node, facilitating the possibility of these lymphocytes to find antigen.

Both B and T lymphocytes use fibroblast reticular cells, but B lymphocytes switch over to follicular DCs to provide “roads” for them. This is because T lymphocytes bear CCR7, which bind to CCL21 and CCL19 on fibroblastic reticular cells; B lymphocytes, in contrast, bear CXCR5 which binds to CDCL13 on follicular DCs.

Question 5

How does processed and unprocessed antigen get to T and B cells in the lymph nodes?

Answer 5

Processed antigen is picked up and processed by antigen-presenting cells, especially dendritic cells, which migrate to the closest lymph nodes to present the processed antigen to naïve T and B cells in the lymph node.

Unprocessed antigen can travel to the lymph node via blood if it is small and soluble enough; if larger, they can bind to APCs and be carried to the lymph node that way.

Question 6

What is the role of CD169+ macrophages in the lymph node?

Answer 6

CD169+ macrophages reside in the subcapsular sinus and are responsible for transferring
opsonized antigen to the interior of the lymph node. Nonantigen-specific B cells pick up the antigen from there by binding them to complement receptors, then deliver the antigen to follicular dendritic cells.

Question 7

After being activated, how do B cells traffic to find T cell help and how is help provided?

Answer 7

When B cells are activated by antigen, they need help from a T cell in order to be fully activated and differentiate. The T cell provides cytokines to help with B cell differentiation and interact with CD40 to further stimulate the B cell.

When B cells bind to antigen, CCR7 is upregulated which helps them move toward the border between the follicle and the T-cell zone. When the B cell finds its antigen-matching T cell, they bind, forming an immunological synapse between them. They migrate together, with the T cell following along while it gives cytokines to the B cell to stimulate its differentiation. CD40 interaction also occurs and helps the B cell further differentiate and proliferate.

Question 8

How do CD4 T cells provide help to CD8 T cells when they do not directly interact?

Answer 8

CD4+ T cells provide help by first interacting with DCs which presents both class I and class II MHC. Antigen on class I MHC is presented to CD8+ T cells, while the class II MHC
presents antigen to the CD4+ helper T cells.

This brings them in close enough proximity that the CD4+ T cells can provide the cytokines that CD8+ T cells need. The DC attracts the CD8+ cell by producing CCL3, CCL4, and CCL5 upon contact with the CD4+ helper T cell, thereby recruiting the CD8+ T cell with CCR4 and CCR5 receptors.

Question 9

Describe how effector cells leave the lymph node and home to specific tissue sites.

Answer 9

Depending on the type of antibody they produce, B cells will migrate to different regions of
the body.

IgM typically stays in the medulla of the lymph node;
IgG goes to the bone
marrow;
and IgA goes to MALT in the gut.
CD8+ T cells follow chemokines released by
innate immune cells to specific sites of infection.
CD4+ T cells vary on their destination by subtype; many stay in the lymph node to stimulate differentiation of other cells or travel to infection sites to help phagocytes take care of opsonized pathogen.

The key for effector cell migration is their chemokine and adhesion receptors and the chemokines being secreted by other cells.

Question 10

What was learned using two photon imaging technology about the origin and function
of the APCs that cause immune rejection of MHC mismatched skin grafts?

Answer 10

The researchers learned that host DCs infiltrated the graft and process antigens from the graft and present them to CD8+ T cells, which initiates skin graft rejection.

Question 11

What causes the type I hypersensitivy reactions?

Answer 11

Type I hypersensivity reactions are mediated by IgE antibodies and include many of the most
common respiratory allergens such as pollen and dust mites.

Question 12

What causes the type II hypersensitivy reactions?

Answer 12

Type II hypersensivity reactions are caused by IgG or IgM antibodies binding to host cells
that are destroyed by complement or cell mediated mechanisms

Question 13

What causes the type III hypersensitivy reactions?

Answer 13

Type III hypersensivity reactions are due to antigen-antibody complexes depositing on host
cells and inducing complement fixation and inflammation

Question 14

What causes the type IV hypersensitivy reactions?

Answer 14

Type IV hypersensivity reactions (Delayed type hypersensitivity - DTH) result from
inappropriate T cell activation.

Question 15

How does IgE function to cause type I symptoms?

Answer 15

IgE antibodies act by cross-linking Fcε receptors on the surfaces of innate immune cells.

The binding of IgE to FceRs activates granulocytes and causes degranulation.

Granule contents released include histamine, heparin, proteases, leukotrienes, prostaglandins, and
chemokines which act on surrounding tissues/cells to cause symptoms.

Question 16

How do the high and low affinity Fc receptors on mast cells regulate the response?

Answer 16

Mast cells express both FcεR1 (activating) and FcγRIIB (inhibiting) Ig receptors.

If a cell binds IgE and IgG, the inhibiting signal induced by IgG binding wins out. This is partially why inducing IgG in atopic individuals (usually through “allergy shots”) helps treat their allergies.

Question 17

What is the purpose and mechanism for allergy skin testing?

Answer 17

Skin testing is commonly used and is an inexpensive and relatively safe way to screen a wide range of antigens at once.

Small quantities of known allergens are introduced by injection or applying to a scratch at specific skin sites on the forearm or back. 30 minutes later the sites are reexamined and swelling and redness (resulting from local mast cell degranulation) indicate allergic response.

Question 18

What is the mechanism underlying hyposensitization treatment for type I allergy?

Answer 18

Repeated exposures via ingestion or injection to increasing doses of allergen may induce an increase in regulatory T cells and their anti-inflammatory cytokines (IL-10 and TGF-b).

May also induce competitive IgG subtypes that block antigen binding or bind to inhibitory Fc receptors.

This is by far the most effective way to manage allergies. It can reduce or even eliminate symptoms for months or years after the desensitization course is complete.

Question 19

What are the 3 mechanisms for an antibody bound to a cell to induce death?

Answer 19

1. Antibody bound to a foreign cell can act as an opsonin and enable phagocytic cells with Fc or C3b receptors to bind and phagocytize the cell.
2. Activate complement system and create pores in the membrane of a foreign cell.
3. Antibody dependent cell mediated cytotoxicity (ADCC). Cytotoxic cells bearing Fc receptors bind the Ab Fc region and promote the cytotoxic mediated killing of the target cell.

Question 20

How are blood transfusion reactions are an example of type II hypersensitivity?

Answer 20

Several proteins and glycoproteins on the membrane of red blood cells are encoded by genes with several allelic forms. Blood group Ag are carbohydrates (A,B, or H) rather than proteins.

Note that all blood types have the H antigen. Adults possess antibodies to the blood antigen they do NOT have. If they receive a transfusion of the “wrong” type of blood, their antibodies will quickly attach to the donor blood cells and trigger complement proteins. The degraded RBC components can build to toxic levels. Many other cell types also have these antigens.

Question 21

Many diseases can result from immune complex mediated (Type III) hypersensitivity reactions, list the three main types (categories) of diseases.

Answer 21

1) Autoimmune: Lupus, arthritis, MS.

2) Drug reactions: allergies to penicillin and sulfonamides.

3) Infectious disease: nephritis, meningitis, hepatitis, mononucleosis, malaria.

Question 22

Describe how the initiation of delayed-type (type IV) hypersensitivity (DTH) response
involves sensitization by an antigen.

Answer 22

Initial exposure triggers production of a T-cell response via langerhans cells (skin DCs) or
other APCs. Takes 1–2 weeks of time. The effector phase of a classical DTH response is induced
by second exposure to a sensitizing Ag. Second exposure induces production of inflammatory cytokines that recruit and activate macrophages to produce destructive substances. Symptoms are noticeable 24 hours later and peak 48-72 hours after the 2nd exposure.

Question 23

What is the relationship between Type II diabetes, obesity, and inflammation?

Answer 23

Obesity is associated with chronic inflammation. Visceral adipocytes are secretors of proinflammatory cytokines (TNF-α, IL-6, 1L-1). Greater obesity linked to greater Type 2 diabetes.

Chronic inflammation can cause systemic disease and insulin resistance.

Aspirin can help improve insulin function. Patients with HIV or RA had worse insulin resistance that was improved with anti-inflammatory drugs. TNF-α/IL-6 induce signaling cascades that inhibit ability of insulin receptors to function. JNK blocks the signaling of IRS by phosphorylating a serine residue, resulting in deficiency in insulin function. Macrophages, mast cells, NKT cells, CD8 cells, and adipocytes release of proinflammatory cytokines in diabetes.

Question 24

What is the difference between central and peripheral tolerance?

Answer 24

Tolerance is prevention of an immune response against self.

Central tolerance involves
deletion of lymphocytes before they mature and occurs in the thymus and bone marrow.

Peripheral tolerance occurs outside of the bone marrow and thymus and can render
selfreactive lymphocytes nonresponsive or actively generate inhibiting lymphocytes (e.g. Tregs).

Question 25

How are T regulatory cells identified?

Answer 25

T regulatory cells have high levels of CD25 (IL2Ra) and FoxP3 (transcription factor) and can prevent autoimmune diseases such as diabetes.

CD4+ T regulatory cells can also help suppress responses against non-self antigens such as commensal microbes.

Mice that are deficient in T regulatory cells have dramatically higher levels of inflammatory bowel disease.

Question 26

How does the gut microbiome alter the development/function of the immune system?

Answer 26

There is significant cross talk between microbiome and immune system.

Germ free mice have significant defects in both their humoral and adaptive immune systems and there is a significant increase in the development and severity of autoimmune disease.

Intestinal epithelial cells (IECs) and mucosal dendritic cells appear to play a key role in communicating
between the microbiome and adaptive immune cells.

Even what we eat can effect the immune cells. Sodium chloride can drive the development of CD4+ T cells to the TH17 phenotype.

Question 27

How do T regulatory cells actually inhibit the immune response?

Answer 27

Dependent mechanisms: TREG cells express high levels of inhibitory CTLA-4 molecules. Can kill APCs or CD8 T cells via granzyme/perforin or cause APCs to have reduced costimulatory
molecules, pro-inflammatory cytokines, and increased anti-inflammatory molecules.

Independent mechanisms: Rely upon secretion of anti-inflammatory cytokines (IL-10, TGF-β, IL-35) into the surrounding area, shutting down nearby cells’ responses. They can also act as
a sponge to soak up IL-2 since they have lots of non-signaling IL-2Ra

Question 28

What is the cause of Hashimoto’s thyroiditis and myasthenia gravis?

Answer 28

Hashimoto’s thyroiditis: Auto-ab and sensitized TH1 cells specific for thyroid Ag are produced. Ab interfere with iodine uptake. Decreases thyroid function leads togoiter & hypothyroidism.

Myasthenia gravis: Auto-ab bind acetylcholine receptors on motor end plates of muscles.
Block the normal binding of acetylcholine, induce complement-mediated lysis of cells. Result is a progressive weakening of the skeletal muscles and can effect the ability to move and eat.

Question 29

Why are women more susceptible than men to autoimmunity?

Answer 29

Evidence suggests that women mount a more robust humoral and cellular immune response
than men.

They have higher CD4 T cell levels, higher antibody titers in primary and secondary responses, and increased levels of graph rejection. Males are more prone to infections than women. It is believed that estrogen enhances immunity whereas testosterone decreases it. Females are more likely to mount a proinflammatory TH1 response than men.

Question 30

Why is autoimmunity induced in some genetically similar individuals and not in others?

Answer 30

Several possible mechanisms have been proposed. Induction may be multifactorial. Combining
a series of triggering events that cross an individual’s systems of tolerance over a threshold.
Infections and molecular mimicry may activate cross reactive lymphocytes.

Damage/stress events may expose sequestered Ag.

Foods that alter gut microbial balance, promoting chronic inflammation and hypersensitivity reactions.

Question 31

Describe the differences between an autograft and allograft transplant. What is are the
three main steps for screening for a histocompatible organ donor?

Answer 31

Autograft = self tissue grafted to another self area (skin grafts, blood vessels).

Allograft = tissue transferred between genetically different members of the same species (majority of transplant cases).

1) Test for blood group match (blood typing)

2) Find full or partial HLA match (tissue typing)

3) Look for anti-HLA antibodies in serum (cross matching)

Question 32

What is the difference between the sensitization and effector stage of graph rejection?
Describe the characteristics of hyperacute, acute, and chronic rejection.

Answer 32

The sensitization stage of graft rejection is where CD4+ and CD8+ T cells recognize alloantigens expressed on foreign graft cells (MHC only or pepMHC). The effector stage of graft rejection is where the cell mediated response infiltrates and rejects the graft tissues.

Hyperacute rejection by preexisting antibodies occurs before grafted tissue revascularizes in as few as 24 hours.

Acute rejection is mediated by T-cell responses. Begin 7–10 days posttransplantation. Induce massive infiltration of lymphocytes and macrophages.

Chronic rejection phase develops months or years after acute rejection reactions have subsided.

Question 33

Antigen specific immunotherapies show great promise for aiding in long term success after organ transplantation, describe how these therapies work.

Answer 33

Monoclonal antibodies can achieve some of this desired effect. mAb to CD3 (OKT3) depletes T cells prior to transplant.

Can also use anti-CD20 antibodies to deplete B cells or antibodies against cytokines to slow down the immune response.

Soluble CTLA-4 fusion proteins
(belatacept) can induce T-cell anergy by blocking co-stimulation and are showing promise.

Question 34

Primary immune deficiencies occur in a number of cell types, a deficiency in what cell
type is the most damaging and why?

Answer 34

A deficiency in T cells is the most damaging because they direct most of the immune
response, affecting both the humoral and cell-mediated branches of the immune response.

Question 35

What are the main causes of SCID and what is the effect on the individual?

Answer 35

1. Mutations in genes regulating cytokine signaling (defective cytokines, receptors or
regulatory molecules) cause defective cytokine signaling in T-cell precursors.
Typically a mutation in the gamma chain of the IL-2 receptor.
Effect: Defects in B cell, T cell, and NK cell development

2. Defects in the purine metabolism pathway result in buildup of metabolites toxic to
the body which cause lymphoid-lineage cell death prematurely
Effect: No development of T, B, or NK cells

3. Mutations in RAG1/RAG2 or other rearrangement genes (such as Artemis) produce
defective V(D)J recombination.
Effect: Absence of functional T and B cells that rely on recombination events for
development, but NK cells still remain.

4. Mutations of genes involved in TCR signaling (tyrosine kinases, adapter molecules,
messengers, or transcription factors) result in faulty pre-TCR or TCR signaling
Effect: No T cells

Question 36

What are the advantages of the the three main immunodeficiency mouse models described in the chapter?

Answer 36

1. Nude (Athymic) mice – Due to the mutated transcription factor FOXN1, these mice
are severely immunocompromised. This is advantageous in studying transplants (since they will tolerate allografts and xenografts) and cancer (i.e. growth of
hybridomas, tumors, pharmacological treatment, and tumor imaging).

2. SCID mice – These mice have a mutation in DNA activated, catalytic polypeptide that does not allow lymphocytes to mature and differentiate by impairing the DNA break-repair pathway needed for recombination; therefore, these mice are lacking
an adaptive immune response. This mutation is said to be leaky, meaning that some mice still response to and reject allografts; however, they can accept xenografts. They have been important in developing humanized mice as well as make a good model for HIV infection since HIV cannot infect mouse cells.

3. RAG knockout mice – these mice have specific mutations in RAG1/RAG2 so they
cannot rearrange antigen-specific receptors and develop properly, so both B and T cells are absent. They can be used as an alternative model for the other two models described previously in performing cancer or infectious disease experiments, or specifically investigate the role of immune genes. These mice can be used for the development of transgenic mice that carry specific T or B cell receptor genes.

Question 37

What are the main causes of secondary immunodeficiencies?

Answer 37

Secondary immunodeficiencies cause immune loss of function through various means,
usually through infection, drug treatment, metabolic disease, or malnutrition.

Question 38

What is the structure of the HIV retrovirus?

Answer 38

The HIV retrovirus consists of two ssRNA genome copies, 2 reverse transcriptase molecules (p64), a protease (p10), and an integrase (p32) surrounded by the p24
nucleocapsid, which is surrounded by the p17 matrix, with the host-derived viral envelope on the outside; the viral envelope is embedded with 72 glycoproteins composed of gp120
and gp41, as well as host proteins such as MHC Class I and II.

Question 39

What is unique about the replication of HIV?

Answer 39

HIV uses reverse transcriptase to convert its RNA genome into DNA, uses an integrase to integrate its genome into the host cell DNA genome, and uses a protease to cleave viral proteins for replication of new virions.

Question 40

Outline the 3 main stages in the typical course of an HIV infection.

Answer 40

1. Acute phase – no detectable anti-HIV antibodies but patient may experience flu-like symptoms 2-4 weeks after infection. The HIV-1 virus begins to spread and viral load
significantly increases and CD4+ T cells begin to decrease.

2. Asymptomatic phase – this phase is marked by seroconversion, where anti-HIV
antibodies are detected in the blood. CD4+ T cells continue to decline but the patient has no symptoms of the disease because the virus is kept in check by antibodies and CD8+ T cells. This phase has the most variable time frame.

3. AIDS – this phase is marked by the arising of opportunistic infections such as Candida albicans or P. carinii which produce thrush/yeast infection or hacking cough respectively. This phase is characterized by a significant rise in circulating HIV-1 and decreasing CD4+ T cell numbers.

Question 41

What are long term non progressors and why are they of interest to researchers?

Answer 41

Long term non progressors are individuals infected with HIV-1 but remain asymptomatic for long periods of time without treatment.

This is very interesting because somehow their immune response keeps HIV-1 in check and may be the key to developing preventative treatments or therapies.

Question 42

What stages in HIV replication cycle provide targets for therapeutic antiretroviral drugs?

Answer 42

1. Reverse transcription – drugs inhibit reverse transcriptase, which is necessary for
converting the RNA genome into DNA

2. Protease – drugs inhibit the viral protease so precursor proteins can’t be cleaved to
make new virions

3. Integrase – drugs inhibit viral integrase so viral genome cannot be integrated into the host genome

4. Fusion – drugs inhibit gp41 so the virus cannot fuse its membrane with that of the
host and gain entry into the cell

5. Attachment – drugs compete for access to the CCR5 chemokine coreceptor, preventing entry of the virus into the cell

Question 43

Why is it so difficult to make an HIV vaccine?

Answer 43

HIV mutates very rapidly, creating many variants that are difficult to target all at once (much like influenza).