Ontogeny of the Immune System and T Cells

Question 1

Please define stem cell.

Answer 1

Stem cells are undifferentiated cells which, when they divide, â–ºgive rise on average to another stem cell and a daughter committed to differentiation.

Question 2

Please define B cell and pre-B cell.

Answer 2

B cells are lymphocytes that play a large role in the humoral immune response. They make antibodies and become memory B cells.A Pre-B cell is a precursor to the development of B cells. A cell with cytoplasmic IgM but no surface IgM is called a pre-B cell.

Question 3

Please define T cell and pre-T cell.

Answer 3

T cells or T lymphocytes belong to a group of white blood cells known as lymphocytes, and play a central role in cell-mediated immunity. They have t cell receptors and mature in the Thymus.In the bone marrow one finds pre-T cells, which do not yet have the characteristic surface markers that distinguish T cells from other cells (differentiation antigens), but are committed to expressing them in the right environment.

Question 4

Please define self tolerance.

Answer 4

the process by which the body does not mount an immune response to self antigens

Question 5

Draw an outline diagram which shows bone marrow, thymus and lymph node. Indicate the development and movement of cells of the B and T lines, starting with the hematopoietic stem cell and ending with mature T and B cells.

Answer 5

T cells: They carry out their development in three different locations: the bone marrow, then the Thymus, and finally the peripheral lymphoid organs. In the bone marrow one finds pre-T cells, which do not yet have the characteristic surface markers that distinguish T cells from other cells (differentiation antigens), but are committed to expressing them in the right environment. These go to the thymus, where they rearrange their receptor genes and then are selected for their responsiveness to “self plus antigen.”

Question 6

Define the Bursa of Fabricius, and discuss where its functions take place in mammals.

Answer 6

The abbreviation B”

Question 7

Describe the sequence of appearance of cytoplasmic and surface immunoglobulins in developing B cells. Using these data, derive a model that could explain self-tolerance at the B cell level (“clonal abortion”).

Answer 7

If an immature B cell (sIgM but no sIgD) is exposed to antigen, this signal causes the cell to try receptor editing; if that fails it activates a suicide program (apoptosis), and dies.This deletion mechanism is called clonal abortion, and it partially explains why we do not make antibody to self: in the bone marrow pre-B cells are differentiating into immature B cells; you can imagine that any cell whose receptors happen to be anti-self will almost surely encounter self in the environment of the bone marrow, and either make a new receptor, or be aborted.If it does not encounter antigen (because its receptors are not against self) then it will mature further so that it expresses both sIgM and sIgD. Then, when it meets antigen, it will be stimulated, not aborted.

Question 8

Draw a graph showing the antibody response to a typical antigen in a primary and in a secondary response. Show both IgM and IgG antibody levels.

Answer 8

During primary (initial exposure) B cell responses to antigen, IgM is secreted first, then for most antigens, helper T cells get involved and there is a switch to IgG, or possibly to IgA or IgE. The helper T cells in the gut and lung preferentially drive the M to A switch. The ‘switch helper’ mechanism indicates that B cells in general do what T cells tell them to. As we’ll see later, an inappropriate antibody response may often be the T cell’s fault.

Question 9

Draw a graph which shows relative IgG and IgM levels in a normal infant from conception to one year of age. Distinguish maternal from infant’s antibodies.

Answer 9

The fetus makes IgM before birth, but only acquires the capacity to make IgG about 3-6 months postnatally. However, at birth the baby has as much IgG in its blood as does an adult; this IgG is maternal, because IgG crosses the placenta, by active transport, from mother to fetus (no other class of immunoglobulin does). The half life of IgG is about 3 weeks, so in 7 half lives = 21 weeks after birth there is less than 1% of the starting amount of maternal IgG left; fortunately, the infant should be making reasonable amounts of its own IgG from about 12 weeks. IgA also starts about 2-3 months.

Question 10

Given a newborn’s antibody titer, interpret its significance if the antibody is IgG, or IgM. If IgG, calculate what the titer will be at 4 months of age, and state the assumptions you made when you did the calculation.

Answer 10

If it is IgG, the baby has the mother’s antibodies. If it is IgM, then they have been making it in utero, which is normal. The maternal IgG half life is 3 weeks, so in four months, it will have undergone about 5 half lives, so it will be down to 3%, but the baby will have just started making its own at 12 weeks, so it will be slightly more then 3% of adult levels.

Question 11

Discuss the decrease in diversity seen in the immune repertoire of older people.

Answer 11

People can completely reconstitute their T cell numbers and diversity up to about 40 years of age, then diversity becomes increasingly limited, and more and more cells show a ‘memory’ phenotype while fewer are naïve; old people have fewer but larger clones than do the young. A similar change takes place in B cells, too, possibly a decade or two later.â–ºSo older folks generally make good responses to antigens they saw in their youth, but fail to respond well to new antigens.

Question 12

Please tell me about Th0 cells.

Answer 12

Helpers begin as an undecided precursor: we’ll call it Th0 (zero). When their correct antigen is brought to them by dendritic cells (DC), they begin to divide and differentiate, becoming either Th1, Th17, Th2, Tfh, or Treg cell

Question 13

Please tell me about Th1 cells.

Answer 13

TH1: hypersensitivity T cell: The most important lymphokine secreted by Th1 is interferon gamma (IFNγ) which is pro-inflammatory, being chemotactic for blood monocytes and tissue macrophages.

Question 14

Please tell me about Th17 cells.

Answer 14

Th17 CELLS. There is a newly described, intensely researched Th subset called Th17 because it makes the inflammatory lymphokine IL-17 among others. It resembles the Th1 in that its main job seems to be causing inflammation; not surprisingly, then, it has been implicated in several autoimmune diseases, as has the Th1. It must do something useful for a living, of course; maybe that is resistance to particularly difficult pathogens.

Question 15

Please tell me about Th2 cells.

Answer 15

Th2 CELLS. Activated Th2 cells derived from Th0 leave the lymph node as do Th1, and circulate through blood and lymph until they encounter their antigen again in the tissues.Here the IL-4 and IL-13 they make have other actions: attracting and activating macrophages, but differently than IFNγ; such macrophages are called alternatively activated or M2, and are more involved in healing (debris removal, scar formation, walling off pathogens that angry macrophages have failed to kill).IL-4 is also chemotactic for eosinophils, cells specialized for killing parasites like protozoans and worms.

Question 16

Please tell me about Tfh cells.

Answer 16

Tfh. Soon after the arrival of antigen-presenting DC in the lymph node, some activated T cells can be seen migrating into the follicles of the cortex, where B cells are abundant.These are referred to as follicular helpers (Tfh), â–ºand their role is to help B cells that have recognized antigen become activated and differentiate into antibody-secreting plasma cells. They also direct the B cells to switch from secreting IgM, to IgG, IgA, or IgE. They tend to be heterogeneous; the Tfh in the gut, for example, switch B cells preferentially to IgA; those in spleen switch B cells to IgG.â–ºTfh that order a switch to IgE are rather Th2-like.

Question 17

Please tell me about Treg cells.

Answer 17

Treg. A small population of cells (about 5% of all Th cells) has been identified â–ºwhose main job is to suppress the activation and function of all other Th cells; they are our 5th helper T cell type. Most regulatory T cells have the phenotype CD4+/CD25+. Surface CD4 puts them in the helper family. â–ºThey produce TGFβ and IL-10. They are very potent; one can suppress 1000 Th cells.

Question 18

Please tell me about cytotoxic T cells.

Answer 18

Cytotoxic Killer T Cells: CTL: Kills infected cells. In the few minutes of CTL-target contact, the killer gives the target the ‘kiss of death’ or lethal hit. â–ºIt has signaled the target to commit suicide by activating a physiological cell death process (called apoptosis) that leads to rapid DNA fragmentation and nuclear collapse (this would be useful in preventing virus replication).

Question 19

Describe the surface markers that can be used to distinguish between T and B cells in humans.

Answer 19

T cells have CD3,4,8 surface markersB cells have antibody surface markers. B cells are readily distinguished using antibodies to immunoglobulins or their chains, or to the surface marker CD20.

Question 20

Describe markers that helper and killer T cell subpopulations in humans have on their surfaces.

Answer 20

The most useful molecules on T cells are CD3, CD4, and CD8. CD stands for “cluster designation”, though I am not sure what that means.â–ºCD3 is on the surface of virtually all T cells.CD4 is on Th2, Th1, and Treg.CD8 is on CTL.There are no reliable antibodies yet to distinguish Th1 from Th2; you have to look at the lymphokines they make. All these molecules play a role in T cell recognition or activation

Question 21

Describe an activity of interferon-gamma (IFNγ).

Answer 21

The most important lymphokine secreted by Th1 is interferon gamma (IFNγ) which is pro-inflammatory, being chemotactic for blood monocytes and tissue macrophages. These cells move in large numbers into the area where the Th1 is recognizing antigen.The macrophages are also activated by IFNγ, becoming â–ºclassically activated M1 or ‘angry’ macrophages which avidly ingest and kill bacteria or other foreign invaders.

Question 22

Define mitogen, and name two T cell mitogens. Name a mitogen that stimulates both B and T cells in humans.

Answer 22

Mitogen: stimulates T cells to divideT cell mitogens: Phytohemagglutinin A (a plant lectin, binds CD3), concanavilin A, Con AT and B: Pokeweed mitogen, PWN

Question 23

Distinguish between the effects of a mitogen and an antigen, when added to normal blood lymphocytes.

Answer 23

Antigens are specific, mitogens are nonspecificMitogen: doesn’t actually bind to antigen-binding site on T cell, like an antigen does. What it does bind to is the CD3 domain that controls signal transduction from the antigen-binding chains. This is like bypassing the light switch in your house- the lights are going to always stay on because there’s no longer a mechanism for turning them off.

Question 24

Compare and contrast the antigen receptors of T and B cells.

Answer 24

T-Cells: That is, they do not “see” antigen alone, but only antigen presented to them on the surface of a genetically-identical cell.B-Cells: Bind antigen directly with surface antibodiesâ–ºBecause T cells see antigen only when it is complexed with cell-surface MHC molecules, T cells focus their attention on cell surfaces, and do not interact with free antigen; that is a job for the B cell and its antibodies.

Question 25

Discuss the structures recognized by T cell receptors. Distinguish between what is recognized by helper and cytotoxic T cells. Explain the special role of dendritic cells in this process.

Answer 25

â–ºAntigen-presenting cells, APC, chop up antigenic proteins and display them on their surface. Dendritic cells are the best at this. It’s this MHC-antigen complex that is presented to, and recognized by, the receptor of an appropriate T cell. When antigen is endocytosed and presented by a dendritic cell (DC) it associates primarily with Class II MHC molecules in the endocytic vesicle, and these complexes are what the DC presents to the T cell. Th1, Th17, Tfh, Treg, and Th2 are programmed to recognize peptides on Class II molecules.Class I MHC molecules associate best with peptides that are sampled from proteins synthesized within the cell itself, not taken up by endocytosis. Most peptides would be from normal ‘self’ proteins, but antigens would include abnormal (mutated) molecules and especially internal pathogens such as virus-encoded molecules. CTL are programmed to see antigen in association with MHC Class I molecules.

Question 26

Discuss what is meant by “MHC-restriction”. Name the classes of MHC molecules by which CTL and helper T cells are restricted.

Answer 26

MHC restriction, refers to the fact that a given T cell will recognize a peptide antigen only when it is bound to a particular MHC molecule T cells are MHC-restricted.MHC antigens are very variable, that is, there are many alleles in any population. The chances of yours being exactly the same as mine (or of A’s being the same as B’s) are extremely small, so my T cells almost certainly won’t work in you.

Question 27

Describe the role of T cells in ridding the body of a viral infection.

Answer 27

CTL’s will see infected cells and kill themHelper T cells will see B-cells, dendritic cells, and other APCs and launch an immune response

Question 28

Describe the characteristics of T-independent antigens.

Answer 28

T-independent antigens: They tend to be molecules with the same epitope repeated over and over; rare in proteins but â–ºcommon in complex carbohydrates. The response to T-independent antigens is almost all IgM; T cell help is needed to switch over to IgG, IgA, or IgE. This is important because it means that even if people are extremely deficient in T cells they will be able to make some antibody to carbohydrates, such as occur in bacterial cell walls and capsules.

Question 29

Outline an experiment that shows that an antibody response can be “T-dependent”.

Answer 29

I’m not sure if he has a particular thing in mind here, but I would test two leukocyte populations’ ability to make antibodies to the same antigen– one population with the full complement of T and B cells, the other with the T cells killed off by radiation (which selectively destroys T cells).

Question 30

Diagram the mechanism by which Tfh cells help B cells.

Answer 30

The B cell binds the epitope on a foreign molecule that its receptor is specific for. It then endocytoses the bound molecule, and breaks it down in the endocytic vesicle. Peptide fragments bind to MHC Class II molecules brought in by other vesicles that fuse with the endosome, and then move to the surface; the B cell now displays antigen + Class II. (Sound familiar?) Eventually, along comes the correct Tfh and sees its epitope + Class II on the B cell’s surface. It binds and focuses surface interactions and helper lymphokines on the B cell. Note that the epitope that the T cell sees does not have to be the same as the one the B cell saw, and it hardly ever is.

Question 31

Describe the cellular and molecular events following intradermal injection of tuberculin antigen into a person who has cell-mediated immunity to it. Justify calling the process “delayed hypersensitivity”. Characterize the cells that would be seen in a 48-hour biopsy of the site with regard to whether T cells or macrophages predominate.

Answer 31

TB antigen comes in; it gets picked up by dendritic cells and broken up, then displayed; circulating Th1 cells bind to dendritic cell surfaces, causing their activation; they begin to release IL-2, which activates any nearby Th1 or Th2 cells that can bind to TB as well, and also release IFN-gamma, that cause the capillaries to open up and macrophages to be attracted to the site over 48 hours or so. Given that 1 activated T cell can attract 1000 macrophages with its chemotactic signals, I think it’s a fair bet that the macrophages would outnumber the T cells.The process is called delayed - I think - because the first time you’re exposed to the antigen, it doesn’t happen. This has to do with T cell timing– if you’re exposed for the first time, your dendritic cells have to bring antigen fragments to the lymph nodes, you have to make antibodies, etc. If you’re exposed the second time, you already have circulating T antibodies that recognize those antigen fragments at the site of infection, and can prompt a quick, local, and painful reaction.