Type IV immunopathology

Question 1

What is a type IV immunopathology?

What are some examples in where type IV immunopathology represents all or most of the mechanism?

Answer 1

Some examples where T ype IV represents all or most of the mechanism:

• Rejection of allografts
• Graft - vs. - host disease (G v HD) - the reverse of allograft rejection

-A positive tuberculin skin test

• Resistance to Mycobacterium tuberculosis
• Resistance to fungal infections
• Contact dermatitis, e.g., poison ivy
• Chronic beryllium disease

Many autoimmune diseases, e.g. multiple sclerosis

Tumor immunity

Question 2

Immunization and effector phases:

Initiation

Do you usually get a reaction to the offending agent?

Answer 2

Consider poison ivy, an example of contact dermatitis due to the oil of Toxicodendron ( formerly Rhus) radicans . It contains the compound urushiol 1 which can penetrate intact skin and become associated with MHC on dendritic cells (either by binding directly to MHC, or by binding peptides which then get presented on MHC).

• The dendritic cell travels to the draining lymph nodes, where it presents its MHC plus antigen to the appropriate Th0 precursors, which develop into Th1 and Th17 cells.
• These begin to divide in the usual way, but by the time increased numbers of them are in the circulation, the antigen has been washed or worn off the skin, and there is no reaction. ► So at the time you became immunized (older word: “sensitized”) you probably didn’t know it happened.

But you have memory cells, both circulating and resident in the tissue that started things off.

Question 3

Elicitation

Memory T cells secrete what lymphokine to attract and activate macrophages (M1) ?

Why is the reaction labeles as ‘delayed-hypersensitivity’?

Answer 3

-Now imagine that, some months later, you take another walk through the forest and again encounter poison ivy plants. The oil rubs off on your skin and urushiol again associates with MHC on antigen - presenting cells.

–> This time though, ► memory T cells from the expanded clones are throughout the body, and rapidly get activated in the area where the oil has been deposited. They secrete interferon - γ which attracts and activates a large number of macrophages.

–>The result is a firm red area of inflammation that, because of all the cellular events that need to take place, begins to be visible in 6 to 12 hours, and peaks at 24 to 48 hours, thus earning the label ► delayed - type hypersensitivity 2

Breakdown of the skin often leads to ► blistering

Question 4

What are memory T cells?

The fact that they have a lower activation treshold means that?

Answer 4

Memory T cells are persisting cells in a clone that was expanded by contact with antigen.

• The key thing is that there are more of them than in a naïve person.
• They also ► have a lower activation threshold, so that it takes less antigen for elicitation of a reaction than it did to immunize in the first place.

Question 5

Tuberculin skin test

What test is the most commonly used in the USA?

The antigen is processed by APCs and presented in what type of MHC?

What size of induration is always positive?

Why is the induration significant?

What kind of cells would predominate in the induration?

Answer 5

The Mantoux skin test is most commonly used in the USA. In it, 0.1 mL of PPD — purified protein derivative, a standardized preparation of M. tuberculosis antigens — is injected intradermally. (It is necessary to see a skin “bubble,” because if not the injection has gone subcutaneously, and will diffuse away before the reaction can get established.)

The antigen is taken up by local macrophages and dendritic cells, and presented on MHC Class II. If the subject has an expanded number of anti - tuberculosis Th1 memory cells, they will come by and get stimulated, produce IFNγ , and attract macrophages.

The test is read at 48 hours, and the diameter of the induration (firm raised part) is measured; 15 mm is always positive, and 10 or even 5 mm can be called positive under certain conditions, for example if a person is partly immunosuppressed.

The induration is significant, since it represents a _cellular linfiltrate**_

One activated Th1 can attract 1000 macrophages, so ► these, not Th1, would be the predominant cell you’d see if you biopsied the site at 48 hours.

Question 6

Is the dose used for PPD to elicitic a positive reaction in an immune person enough to provide immunization?

Answer 6

The TB skin test emphasizes what we just said about memory cells: The dose of PPD needed to elicit a positive reaction in an immune person is far lower that wou ld be required to im munize him or her.

Therefore, it turns out that the tiny doses of ► TB skin test s are not immunizing, and they can be repeated regularly without the subject becoming positive .

***Memory cells are long - lived, and after immunization with vac cine or by infection you may stay skin - test positive for years, though not necessarily forever.

Question 7

If we want to determine if a patient has normal T cell function, test to what antigents can be performed?

A negative panel to these antigens might suggest what?

Answer 7

Exposure to m any other environmental antigens can produce delayed-type hypersensitivity.

So when we want to determine if a patient has normal T cell function, we perform skin tests just like the Mantoux test, using a panel of common antigens, which may include:

tetanus toxoid, Candida (yeast) extract, mumps antigen, PPD, streptococcal proteins, and Trichophytin (from a common skin fungus).

-Studies have shown that over 95 % of adults will have a positive DTH response to at least one of these, so a negative panel suggests “ anergy ” and requires follow - up investigation.

Question 8

Why do I test positive to TB?

Answer 8

Immunization to TB antigens normally happens during a primary infection, which is usually unappare nt to the patient, so a positive routine skin test usually comes as a surprise. Exposure to other species of Mycobacteria can occasionally produce a false - positive skin test. In many countries, Bacille Calmette-Guérin (BCG) vaccine — it is attenuated bovine tuberculosis bacteria — is given to newborns, and most people so immunized have positive PPD skin tests due to cross - reaction .

Question 9

The QuantiFERON test

It is preferred to skin testing when subject has had __________ immunization.

What type of proteins are used?

What is measured by an ELISA assay?

In people vaccinated with BCG it gives a_________ test.

Answer 9

The QuantiFERON - TB Gold test is new, very nice, and is preferred to skin testing when the subject has had BCG immunization.

*Purified M. tuberculosis proteins (only human-specific , not bovine, epitopes ) are added to a sample of whole blood, and after incubation, interferon - γ is measured in the medium by a capture ELISA assay.

Unlike the skin test, it remains negative in people vaccinated with BCG, (why? the human epitopes do not cross-react with BCG,) allowing you to distinguish infection from previous immunization.

Question 10

CYTOTOXIC T LYMPHOCYTES IN DTH

Why have CTLs been studied less that Th1 T cells?

Answer 10

These have been much less studied than Th1 in T cell - me diated immunity, because there is no in vivo test for them.

They take part in most manifestations of T cel-mediated immunity, and are quite important in many autoimmune diseases , tumor immunity, and transplant rejection.

To demonstrate their presence, we need a suitable target cell (for example, an antigen-presenting cell exposed to the antigen, or any cell infected by it, if that is possible; sometime, normal cells can be soaked in an epitope-_sized peptide which associates directly with MHC without having to be processed_.) These are then mixed with the patient’s T cells (or purified CD8 cells) and after several hours, target cell death is measured, usually by the release of intracellular contents.

Question 11

Contact dermatitis

Why is the term contact allergy not used?

What is the main requirement to cause contact dermatitis?

Which MHC do they associate with?

Answer 11

This condition is also called contact hypersensitivity or contact sensitivity or, incorrectly, contact allergy; ‘allergy’ should be reserved for IgE - mediated events.

The classic example of this is poison ivy, but many other chemicals can cause it; the *main requirements are that they pass through intact skin to reach antigen - presenting cells, and they associate with MHC Class II.

Metals like *nickel (used in plated goods, including jewelry, watch straps, garters); chemicals like paraphenylenediamine, the only permanent hair dye ; latex in gloves; topical antibiotics like neomycin and bacitracin; plants, including poison oak and poison sumac; soaps, detergents and industrial chemicals. How do you treat these?

–>Avoidance, and topical steroid creams or ointments.

Question 12

HLA and drug reaction:

Give an example:

People who develop “abacavir hypersensitivity syndrome” are HLA _____.

This HLA is class______, which means it activated ______.

How does abacavir causes this?

How do you stop this hypersensitivity?

Answer 12

Up to 8% of people who are given abacavir, a nucleoside reverse transcriptase inhibitor, for HIV, develop “abacavir hypersensitivity syndrome” which is quite awful and difficult to diagnose correctly. Nearly all people with the syndrome are HLA - B* 5701. We now test for this allele befor e offering the drug, a good example of “personalized medicine.”

Note that HLA - B* 5701 is Class I, not the Class II which is recognized by Th1. This is predominantly a CTL problem.

–>Work by P.T. Illing et al. in 2012 showed that abacavir changes the structu re of HLA-B*5701 so that it binds certain self - peptides that are not, of course, normally presented; the syndrome is actually a drug-induced autoimmune reaction!

Fortunately it’s self-limited: stop the drug, and you stop the hypersensitivity.

Better still: first test for HLA - B*5701 .

Question 13

HLA and drug hypersensitivity:

The strongest association between HLA and drug-induced hypersensitivity has been detected for __________ in the Han Chinese population.

What allele does it involve?

What cell is probably the main cause of the diseases?

Answer 13

The strongest association (OR > 1,000) between HLA alleles and drug -induced hypersensitivity has been detected for carbamazepine in the Han Chinese population.

• The association is also in Thai, Malay, and Indian populations, but _not in Caucasian_s.
• The allele is HLA- B*1502 and the correlation is specifically with a nasty condition called Stephens-Johnson Syndrome, or similar though nastier Toxic Epidermal Necrolysis, both of which are probably CTL-dominated forms of Type IV immunopathology of skin

Question 14

Graft rejection:

Answer 14

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Question 15

Hyperacute or white graft reactions:

Why do they get rejected so fast?

Most common in what type of grafts?

Answer 15

If you keep putting A grafts onto B, eventually they will rejected even before they heal in, that is, they stay white and bloodless. This is due to the development of antibodies to histocompatibility antigens.

Hyperacute rejection is common when xenografts (from another species) are attempted. It’s usually because of pre-existing antibody to ubiquitous carbohydrate epitopes which are present in the foreign species but not in the human. People are going so far as to try to breed transgenic pigs that lack these carbohydrates, as potential organ donors for human patients.

Question 16

Autoimmune DIseases

MS treatment responds to both ___ and _____ cells medications.

Is the brain antigenic? Immunogenic?

Answer 16

Many conditions are clearly autoimmune, and T cells are involved in the pathogenesis. Some of these conditions also involve autoantibodies, and thus there is both Type II and Type IV immunopathology. Which comes first, or is most important? That is still controversial.

For example, MULTIPLE SCLEROSIS , the demyelinating disease in which T cell reactivity to an autoantigen (myelin basic protein) was first shown, responds to therapies directed at T cells, such as the humanized monoclonal antibody natalizumab. But it also responds to the B cell - depleting monoclonal rituximab.

How can one make an immune response against one’s own brain? This is a critical question. ► The brain is, in fact, antigenic in its owner, but not immunogenic. So as long as you keep it in your head, you should not have a problem. Even if T cells find their way into the normal brain, they will not be stimulated, because that would require (at least) professional antigen-presenting cells, an innate response, and cell damage; common in skin but not in the well - defended brain.

Question 17

Can your brain become imunogenic under certain conditions?

Answer 17

Question 18

Molecular mimicry and MS

Answer 18

Several groups have studied the way myelin basic protein peptides sit in MHC Class II, and analyzed the distribution of positive and negative charges over the surface that would contact a T cell receptor. Using the information to scan a database of microbial proteins they have found several cases where a viral peptide, whose sequence is not necessarily the same as the MBP sequence, but which has close structural similarity, that is, distribution of charges and hydrophobicity, acts as a strong stimulator of clones of T cells derived from MS patients.

► Could a prior infection with such a virus produce activated T cells that could then enter the brain and attack the myelin there? It would appear to be so.

Question 19

Sjogren Syndrome

What are its clinical presentations?

Answer 19

Question 20

Type 1 Diabetes

What cells are dominat cells in the pathogenesis of T1D?

What HLAs are strongly associated with T1D?

Which HLA genes seems to be the problem? Why?

Answer 20

Autoimmunity is strongly implicated in Type 1, or juvenile, or insulin - dependent diabetes mellitus (T1D). Depending on the te chnique used, antibody to beta - cells (the islet cells that produce insulin) can be detected in the serum of over 90% of patients at the time of diagnosis (normals, fewer than 2%).

Tissue obtained at autopsy from an occasional patient early in the disease show antibody and typical inflammatory responses. Although antibody is a useful marker, and B cells may play a role as APCs to the harmful T cells , T cells are dominant in the pathogenesis of T1D.

It brings up many interesting questions; for example, should antibody - positive people be treated even before diabetes develops? Studies have tried to induce oral tolerance by feeding such kids small doses of insulin; it is thought to induce insulin - specific Treg in the gut, which, it’s hoped, could travel to the pancreas .

There is a strong HLA association between T1D and HLA - DR3 or HLA - DR4.

–>These are in linkage disequilibrium with HLA - DQ2 and HLA - DQ8, respectively.

The DQ genes are thought to be the problem; they have unusual amino acids placements in the antigen-binding groove that allows ready presentation of islet cell-associated peptides.

The best animal model, the NOD mouse, has an unusual Class II MHC molecule (H2 I - A g7 ) which is very similar to DQ8.

Question 21

Reumatoid Arthritis

Affects more_____ than ______.

Usually attacks the ________, specially those of the fingers.

Agglutination RF/IgG happens due to the reaction with______.

Answer 21

This is probably the most common autoimmune disease, affecting more than 1 in 100 Americans.

It is the ‘inflammatory arthritis.’ (Osteoarthritis, the ‘degenerative arthritis,’ where the joints wear out, is even more common.)

• RA affects women more than men, and usually attacks the smaller joints, especially those of the fingers, first.
• The initial evidence that it was autoimmune came with the discovery of rheumatoid factor (RF), which can be detected by adding the patient’s serum to microscopic beads coated with normal human IgG. RF makes the beads agglutinate; ►it is IgM anti-IgG! It is a useful biomarker, but may not actually cause much joint damage. It’s been difficult to identify a pathogenic antibody (unlike the case in lupus), nevertheless RA can respond extremely well to rituximab, a monoclonal antibody against the CD20 on the surface of B cells, which effectively depletes them from the body . The antibodies appear much earlier than the T cells, which correlate better with disease activity, as they do in T1D.

Several groups have conducted genome - wide single nucleotide polymorphism screens of RA patients, and the loci identified are interesting: HLA - DRB1 (one of the β chain genes of HLA - DR, associated of course with antigen presentation, in this case maybe autoantigen;) PTPN22 (a tyrosine phosphatase involved in T cell signaling;) C5 (the 5 th component of complement;) TRAF1 (a modifier of signal transduct ion through proinflammatory TNF receptors;) and PADI4 (a deiminase that converts arginine in proteins to citrulline.) This last is intriguing, since antibodies to citrullinated peptides seem to be absolutely specific to RA, though their role in pathology i s not known. Most of the pathogenesis of RA seems to be due to ***T cells.

Air pollution, and especially smoking, is an important RA risk factor. It is known to increase the citrullination of proteins in the lung.

Question 22

Graft vs. Host reactions

For G vs. H reactions to occur, what three conditions must be met?

Answer 22

If a non-identic al graft contains T cells (and, except for corneas, they usually do, because they contain blood or have tissue spaces where leukocytes can be hiding ) there is a perfectly good possibility that these cells will recognize HLA antigens of the recipient (host) as foreign, and so the graft will try to reject the host. Normally, the host has a lot more T cells than the graft, and will usually reject the grafted lymphocytes before they can begin to mount a serious reaction.

So: for graft - versus - host disease (GvHD) to result, ► the following three conditions must be met:

1. The graft must contain immunocompetent T cells (even bone marrow has mature T cells in it).
2. There must be at least one antigen in the host which the graft’s T cells can recognize (so, no worries with identical twins . )
3. The host must be relatively immunoincompetent or unable for genetic reasons to recognize the graft’s MHC antigens, otherwise the graft would be rejected too rapidly.

Question 23

Acute Graft vs. Host disease

Answer 23

This develops in 2 to 10 weeks after bone marrow transplantation in humans.

The symptoms include a nasty maculopapular skin rash; diarrhea; hepatic inflammation with jaundice; and infections (probably due to immunosuppression , as Tregs try to control the raging immune activation.) The treatment is with anti-infl ammatory drugs like corticosteroids and, paradoxically, with immunosuppressives.

Question 24

Chronic Graft vs. Host disease

Answer 24

This develops in months to years, even in patients with a perfect HLA match; therefore it is probably against minor histocompatibility antigens. With a lot of chronically activated T cells pouring out cytokines, regulation is compromised and autoimmunity can become an issue.

Question 25

In bone marrow transplant is removing T cells decrease the risk of Graft-vs-Host disease?

Why?

Answer 25

In bone marrow transplantation, removing the T cells from the bone marrow may decrease the risk of acute GvH disease.

Oddly, this usually results in a poorer engraftment of the bone marrow’s stem cells.

It’s possible that a few activated T cells make hematopoietic -stimulating growth factors that improve graft success.

Question 26

Th2 cells in immunopathology

In response to leprosy, how do the responses of the body based on Th1 or Th2 differ?

Answer 26

Th2 cells are found in the periphery in certain inflammatory and infectious states,

► especially asthma and chronic worm infestation.

They activate macrophages (alternatively activated or M2) which produce fibrosis under these chronic conditions; and also attract eosinophils which, in excess, make inflammation more intense. So although this raises the question of whether allergy and asthma might really be T cell diseases, for now most people refer to asthma as a Type I condition because of the role played by IgE. But as more studies show the central role of Th2 cells, and minor contributions of IgE in established asthma, the classification is due for a change.

Mycobacterium leprae, the causative organism of leprosy, has strange effects on dendritic cells, with the result that some people make strong Th1 responses against M. leprae , and others make Th2 responses. Both get immunopathology as the organism is extremely difficult to clear from the body .

► Those with Th1 responses get tuberculoid leprosy with large skin and nerve lesions, but they control the infection.

► If the response is dominated by Th2 the initially - uncontrolled infection is widely disseminated in many small granulomas (lepromatous leprosy).