HLA: Immunology Basics, Transplantation, Autoimmunity

Question 1

Define MHC.

Answer 1

Major histocompatibility complex.
This is the cluster of genes that encode molecules involved in antigen presentation. Nearly every mammal has one in its genome.

Question 2

Define HLA.

Answer 2

Human Leukocyte antigen.
It is used to refer to genes & to the molecules that they encode. The HLA region is the human MHC.
The loci are class I and class 2.

Question 3

What are the class I loci? (3)

Answer 3

HLA-A, HLA-B & HLA-C

Question 4

What are the class 2 loci? (3)

Answer 4

HLA-DP, HLA-DQ & HLA DR

Question 5

Talk about alpha and beta chains in class II.

Answer 5

DQA, DPA, DRA encode alpha chains, & DPB, DQB, & DRB encode beta chains.

Question 6

What do the HLA molecules do?

Answer 6

They present peptide antigen to T-lymphocytes.

Question 7

Endogenous antigen is presented on what class?

What T lymphocytes do they present antigens to?

Answer 7

Class I HLA molecules
CD8 T cells
C locus seems to be more concerned with NK cells.

Think.. I ‘ate (8) killing in the END (endogenous/CD8/class I).

Question 8

Exogenous antigen is presented on what class?

What T lymphocytes do they present antigens to?

Answer 8

Class II HLA molecules
CD4 T cells

Think… EXtra help 4 you 2 (exogenous/CD4/class II).

Question 9

What upregulates expression of HLA molecules?

Answer 9

Cytokines, especially Interferon gamma.

Question 10

What is meant by a haplotype?

Answer 10

The team of alleles encoded on a short section of one
chromosome. A single haplotype will have been inherited from one parent, & will usually be passed on intact to offspring, if the segment of chromosome is short enough. The longer the segment of chromosome considered, the more likely it is that crossovers
will occur in the formation of the gametes, & the haplotype. passed on to the child will be a mixture of one parent’s two haplotypes.

Question 11

You would expect no association between particular alleles at one locus and particular alleles at a neighbouring locus if the alleles were randomly distributed.
However, there is a common haplotype.
What is this phenomenon called?

Answer 11

A1, B8, DR3, DQ2

Linkage disequilibrium

Question 12

What are two explanations for the linkage disequilibrium phenomenon?

Answer 12

1. Founder effect

2. Advantageous combination

Question 13

Compare no. of different specificities on one cell in innate vs adaptive immunity.

Answer 13

Innate - A variety. 20-30? Toll, lectins.

Adaptive - One specificity

Question 14

Comparing innate vs adaptive immunity - cells different from one another?

Answer 14

Innate - no, all cells of one kind/same

Adaptive - yes, diversity of specificities

Question 15

Comparing innate vs adaptive immunity - is antigen receptor directly encoded?

Answer 15

Innate - yes

Adaptive - no, each cell randomly mutates gene

Question 16

Comparing innate vs adaptive immunity - clonal? Mediate memory?

Answer 16

Innate - no

Adaptive - yes, clonal expansion

Question 17

Comparing innate vs adaptive immunity - danger of autoimmunity?

Answer 17

Innate - no

Adaptive - yes, clonal deletion required

Question 18

Comparing innate vs adaptive immunity - cells.

Answer 18

Innate - neutrophils, monocytes, eosinophils, mast cells

Adaptive - B and T lymphocytes

Question 19

Where do T cells mature?

Answer 19

Thymus

Question 20

Antigen receptor is antibody for T or B cells?

Answer 20

B cells

Question 21

Recognition of antigen for B cells?

For T cells?

Answer 21

B cells - ANYWHERE
T cells - ONLY WHEN PRESENTED ON SURFACE OF
ANOTHER CELL, i.e. so antigen needs to presented by another cell

Question 22

Explain clonal deletion of B lymphocytes.

Answer 22

A pool of lymphocytes in bone marrow are sensitive to clonal deletion at this stage. If they encounter a component of self, the lymphocyte is deleted. If they don’t, they survive and mature and move out of the bone marrow and into the circulation. They are no longer sensitive to clonal deletion. If they encounter an antigen that fits their receptor now, they will become activated and DIVIDE!!

Also, this can occur in lymph node cortex and germinal centre, for T-dependent B cells, if they fail to get help from CD4 T cells.

Question 23

Explain clonal expansion.

Answer 23

A pool of lymphocytes in the peripheral lymphoid tissues. If they encounter an antigen, then a few days later, those with specificity to that antigen are greater in number than before and easier to activate (memory cells).

Question 24

Class I molecules encoded at A, B and C loci.

How many locus alleles can any individual have?

Answer 24

Any individual can have 2 A locus alleles, 2 B locus alleles, & 2 C locus alleles, and all of these can be expressed on each cell that expresses class I (most cells in the body).

Question 25

Class II molecules encoded at DR, DQ, and DP loci. | How many locus alleles can any individual have?

Answer 25

``` Any individual can have 2 DR locus alleles, 2 DQ locus alleles, & 2 DP locus alleles, and all of these can be expressed on each cell that expresses class II (mostly antigen presenting cells: dendritic cells, macrophages, monocytes, B-lymphocytes, though class II expression can be induced on other cells by cytokines). ```

Question 26

In a full allelic name, there are 8 digits after the locus letter, e.g. A*02:01:02:11. What do these digits mean?

Answer 26

The first pair of digits defines the “cross-reactive group”. All alleles that have the same first two digits have similar shapes on the exposed parts, & “look” the same to antibodies. They represent the alleles as they were originally defined by serology (antibodies). The second pair of digits represent differences in the amino-acid sequence that cannot be detected by antibodies: these are revealed by molecular biology. The polymorphisms may be within the groove, where they cannot be detected by antibodies, but can effect peptide selection for presentation, and therefore function. The third pair of digits represent silent polymorphisms within the exons: where the amino-acid sequence is the same, but the DNA base sequence differs. The fourth pair of digits represents polymorphisms in the introns.

Question 27

Why is the nomenclature more complicated for class II alleles encoded at the DR, DQ, and DP loci?

Answer 27

Both chains are polymorphic. For example, the DPA locus encodes the DP alpha chain, and the DPB locus encodes the DP beta chain. Alleles at each of these loci are named with the locus first, followed by eight digits with the same significance as in the class I loci above. Eg DPB*01:04:10:05

Question 28

To survive selection process in the thymus, T lymphocytes must be ...? They must not be...?

Answer 28

Positively selected (i.e must be attracted to some of the “HLA + peptide” complexes displayed in the thymus) Negatively selected (ie they must not be too strongly attracted to any “HLA + peptide” complex in the thymus).

Question 29

When might T lymphocytes undergo clonal deletion?

Answer 29

1. In the thymus, if the receptors on a developing T-lymphocyte show a “passionate interest” in any HLA + peptide being expressed on the thymic epithelial cells or dendritic cells. 2. After leaving the thymus, if the T-lymphocyte recognises an HLA + peptide being expressed on a dendritic cell, but does not get the second signal via CD28 engaging with its CD80 & CD86.

Question 30

Define autograft.

Answer 30

A graft of tissue given back to the same person: eg | skin in burns victims, or bone marrow stem cells.

Question 31

Define isograft.

Answer 31

Graft between two identical (twin etc) siblings.

Question 32

Define allograft.

Answer 32

Graft between two members of the same species who | are not identical. May be related.

Question 33

Define heterograft/xenograft.

Answer 33

Graft between two members of different species, eg pigs heart to human, pigs heart valves etc.

Question 34

Kidney transplants are usually what type of graft? | How can they be rejected?

Answer 34

Usually allografts. Can be rejected by antibodies (against ABO blood group, against foreign class I HLA molecules) or by T-lymphocytes.

Question 35

What is hyperacute rejection?

Answer 35

Rejection immediately, within minutes, hours or up to about 4 days. Antibody mediated with complement activation.

Question 36

What is acute rejection?

Answer 36

Rejection from 5 days to 3 months. | Antibody or T lymphocyte mediated.

Question 37

What is chronic rejection?

Answer 37

Rejection after 3 months | Antibody or T lymphocyte mediated.

Question 38

What type of kidney donor transplants do better - live or cadaveric?

Answer 38

Live

Question 39

How to avoid rejection?

Answer 39

1. Do not do the transplant if the recipient has antibodies against the donor HLA molecules (i.e. if sensitised through blood transfusion, pregnancy etc). Screen for anti-HLA antibodies. Perform a cross-match between donor cells and recipient serum. Use ABO compatible donors. 2. Match the donor & recipient for HLA alleles - try to match for A, B and DR loci (the broad-antigen groups, i.e. the first 2 digits). 3. Use antirejection therapy.

Question 40

What antirejection drugs are used?

Answer 40

Glucocorticosteroids eg prednisolone Immunosuppressives eg azathioprine & mycophenolate. Immunosuppressives such as ciclosporine & tacrolimus, which interrupt the signalling within T-lymphocytes. Monoclonal & polyclonal antibodies which deplete T-lymphocytes.

Question 41

In bone marrow transplants, what are all rejection problems mediated by?

Answer 41

T lymphocyte mediated.

Question 42

What is graft versus host disease?

Answer 42

Donor T-lymphocytes react against host “HLA+ peptide” complexes. --> severe skin rash, multi-organ involvement and failure, death

Question 43

How can HvG and GvH be prevented? (3)

Answer 43

Autografts are performed, eg in malignancy, using the patient’s purified stem cells with the malignant cells removed. In allografts, the T-lymphocytes may be depleted from the donated marrow, or purified stem cells used. Antirejection therapy is used as in kidney transplantation, but it can be stopped.

Question 44

What is meant by hypersensitivity?

Answer 44

Damage to the body by the body’s own immune system.

Question 45

What is type 1 hypersensitivity?

Answer 45

IgE-mediated release of histamine from mast cells/basophils. E.g. Hayfever, asthma, anaphylactic shock. Always allergic. Some people genetically predisposed to making IgE to common environmental antigens = atopy.

Question 46

What is type 2 hypersensitivity?

Answer 46

IgG & IgM mediated destruction by complement &/or phagocytosis. Eg. Antibody mediated haemolytic anaemia, thrombocytopenia, myasthenia gravis, Goodpastures, pemphigus & pemphigoid. Almost always autoimmune, occasionally “allergic”.

Question 47

What is type 3 hypersensitivity?

Answer 47

Immune complexes causing pathology either where they are formed (eg extrinsic allergic alveolitis) or when deposited from circulation (vasculitis, glomerulonephritis, arthritis). May be autoimmune, “allergic” or antimicrobial activity

Question 48

What is type 4 hypersensitivity?

Answer 48

Mediated by activated T lymphocytes. Broadly two patterns: with granulomas, involving CD4 T-lymphocytes & macrophages, & without granulomas, probably involving CD8 T-lymphocytes. May be antimicrobial activity: tuberculosis (granulomatous), Hepatitis B (non-granulomatous), “allergy” eg nickel sensitivity, Coeliac, or autoimmune: possibly sarcoid (granulomatous) & all the organ-specific endocrine autoimmune diseases (non-granulomatous).

Question 49

What is type 5 hypersensitivity?

Answer 49

Mediated by antibodies which stimulate rather than destroying their target. Classical example is Graves’ disease. Autoimmune.

Question 50

What are the following conditions associated with? | Ankylosing spondylitis, Reactive arthritis, colitic arthritis, psoriatic arthritis

Answer 50

HLA B27

Question 51

What HLA is coeliac disease AND T1DM associated with?

Answer 51

DQ2 and DQ8

Question 52

What HLA is psoriasis associated with?

Answer 52

Cw6

Question 53

What HLA is MS associated with?

Answer 53

DQ6

Question 54

What HLA is rheumatoid arthritis associated with?

Answer 54

DR4

Question 55

What is the pathophys of Graves' disease?

Answer 55

``` Autoantibody against TSH receptor stimulates thyroid into hypertrophy (Goitre) & increased secretion of T3 & T4: hyperthyroidism. Also causes inflammation of extrinsic eye muscles (diplopia) & growth of tissue behind eyes (proptosis). Associated with DR3. ```

Question 56

What is the pathophys of myasthenia gravis?

Answer 56

Autoantibody to acetylcholine receptor causes muscle weakness. Weakness, diplopia, respiratory paralysis, maybe death if untreated. Association with thyoma or thymic hyperplasia. Association with DR3.

Question 57

What is the pathophys of Eaton-Lambert syndrome?

Answer 57

Autoantibody to pre-synaptic Ca++ channels in neuromuscular junction. Causes weakness (myasthenia). Associated with carcinoma of the lung (especially small cell).

Question 58

Auto-immune haemolytic anaemia pathophys.

Answer 58

Autoantibodies to red cells, causing destruction by complement system &/or phagocytes. Some antibodies act in the warm, some in the cold. Causes: Idiopathic, Infections (syphilis, mycoplasma, EBV & other viral infections), Neoplasms (especially CLL, lymphoma), Other autoimmune disease (SLE, RA, PSS, UC), Drugs (penicillin, alpha methyl dopa, these may be more allergic than autoimmune). Coombes test.

Question 59

Autoimmune thrombocytopenia - pathophys.

Answer 59

``` Idiopathic (Idiopathic thrombocytopenic purpura, ITP) but also associated with other autoimmune disease (SLE), haematological malignancies (CLL, lymphoma) & drugs (lots, including antibiotics eg co-trimoxazole). ```

Question 60

Pemphigus pathophys.

Answer 60

Skin disease with autoantibodies to desmosomes. Bullae form within epidermis forming sores on the skin, & also mouth ulcers. Death can occur from fluid loss & infection. Various subdivisions, including a paraneoplastic form 2ary to lymphoma which involves oesophagus & bronchioles.

Question 61

Pemphigoid pathophys.

Answer 61

Skin disease with autoantibodies to hemidesmosomes. Stable bullae that are less likely to break down than in pemphigus. Various subdivisions, can affect mucous membranes.

Question 62

Goodpasture’s syndrome pathophys.

Answer 62

Autoantibodies to Type IV collagen in basement | membranes, causing glomerulonephritis (haematuria, renal failure & alveolitis (haemoptysis, respiratory failure).

Question 63

Systemic Lupus erythematosus (SLE) pathophys.

Answer 63

Fevers, malaise, lymphadenopathy, arthralgia, arthritis, vasculitis, serositis, fibrosing alveolitis, glomerulonephritis, photosensitive skin rash. Various autoantibodies causing type II & type III hypersensitivity: anti-red cell, anti-platelet, anti-cardiolipin & anti-clotting factors. Anti-nuclear antibodies. Circulating immune complexes (type III hypersensitivity) cause much of the pathology. T-lymphocytes are also involved in the pathology, but the antibodies play an important role. Tests: Anti-nuclear antibody test (ANA) indirect immunoflurescence, anti-DNA antibodies, & a variety of “Extractable nuclear antigen” tests (ENA).

Question 64

Type 1 Diabetes mellitus pathophys.

Answer 64

Infiltration of islets with lymphocytes, including CD8 Tlymphocytes: “insulitis”. Specifically kills beta cells. There are auto-antibodies: anti-islet cell antibodies, anti-insulin antibodies, but not in all patients, & probably a byproduct of the damage, though anti-insulin antibodies may cause insulin resistance. Can get autoantibodies against replacement insulin giving insulin resistance. Autoantibodies not routinely used in diagnosis.

Question 65

Hashimoto’s thyroiditis pathophys.

Answer 65

Florid lymphocytic infiltration of the thyroid. Goitre & reduction in thyroid function (hypothyroidism, mysoedema). Many Blymphocytes & autoantibodies including anti thyroid peroxidase (TPO) & antithyroglobulin, but most damage probably done by T-lymphocytes. Anti-TPO is the usual test, together with thyroid function tests.

Question 66

Pernicious anaemia (autoimmune gastritis) pathophys.

Answer 66

Lymphocytic infiltrate in lamina propria. Autoantibodies against parietal cells (anti-parietal cell antibody, useful diagnostic test), also anti intrinsic factor antibodies in gastric juice. Prevents secretion of intrinsic factor, so no absorption of B12. Also achlorhydria, & predisposition to stomach malignancy.

Question 67

Primary sclerosing cholangitis pathophys.

Answer 67

Associated with UC. Often ANCA positive.

Question 68

Primary biliary cirrhosis

Answer 68

. Immune attack against biliary tree, culminating in cirrhosis & liver failure if untreated. Anti-micochondrial antibodies are the diagnostic test. ANA may also be positive.

Question 69

Chronic autoimmune hepatitis

Answer 69

Classically affects young women. Untreated, ends in liver failure. Various subtypes, partly distinguished by autoantibodies: ANA positive, Anti-smooth muscle antibodies (SMA) Anti-liver/kidney microsome, Antibodies to Soluble liver antigen.

Question 70

Coeliac Disease

Answer 70

Basically an “allergy” to gluten (gliadin), but autoantibodies occur: antitransglutaminase (an IgA antibody) & anti-endomysial. Also anti-gliadin antibodies. Increased intraepithelial lymphocytes, lymphocytic infiltration in lamina propria, shortening of villi, deepening of crypts, malabsorption with steatorrhoea, dermatitis herpetiformis, predisposes to bowel malignancy especially MALT lymphoma. Association with DQ2 & DQ8.

Question 71

Rheumatoid arthritis

Answer 71

Inflammatory multisystem disease. Synovium forms inflammatory pannus, which attacks & destroys cartilage: destructive arthritis. Also vasculitis, fibrosing alveolitis. 80% of cases are Rheumatoid factor positive, autoantibody against antibodies! Difficult to see how this can cause pathology, but useful diagnostic test. Also (more specific anti citrullinate protein antibodies. Associated with DR4.

Question 72

Vasculitis

Answer 72

PAN & Wegener’s ANCA is useful diagnostic test.

Question 73

Seronegative arthritis, ankylosing spondylitis.

Answer 73

``` Inflammation at “enthesis” where capsule & ligaments are attached to joint. Also iritis, and aortitis. No autoantibodies (hence “seronegative”). Strongly associated with HLA B27. ```

Question 74

Psoriasis

Answer 74

Patches of red flakey skin, which shows increased cell division.Microabscesses of Monro. No autoantibodies. Associated Cw6_