Immuno Flashcards
HIV transmission and epidemiology
> 37 M people living with HIV/AIDS (2018 Report).
• ~ 39 M people have died of AIDS.
• >5000 persons infected per day - >10% (600) of these are children.
• Most will die within 20 years if no access to treatment
• Transmission = sexual, infected blood, mother-to-child (vertical – breastfeeding, in utero,
intra partum)
HIV Pathogenesis
HIV targets CD4 cells. T helper, dendritic and macrophages all express CD4 and therefore can be infected by HIV.
• The virus binds via gp120 (initial binding) and gp41 (conformational change) – on CD4+ T cells
• Most strains use* CCR5 (mac/tcel/dendritic) and CXCR4 (t cell only) chemokine co-receptors, which HIV binds to via gp120 also
• Once entered into the cell, HIV is converted from RNA to DNA in the cytoplasm via reverse transcriptase. It then goes and incorporates itself into the host DNA.
When there is a trigger (e.g. infection) the host cell transcribes its DNA and HIV proteins are transcribed** and these form HIV particles which then leave the cell and infect other cells. (This is the normal HIV form and so therefore is retroviral).
When HIV invades a macrophage it can be taken into the LN where there lots of CD4+ cells in close proximity.
- this refers to viral tropism as in what receptors the virus prefers to bind to
- *this stage is particularly prone to error and can result in HIV particles having varying csf proteins i.e. strains
What is the immune response to HIV?
The Innate response
• Non-specific activation of Macrophages, NK cells and complement
• Stimulation of dendritic cells via TLR
• Release of cytokines and chemokines
Adaptive response
• Neutralising antibodies: anti-gp120 and anti-gp41
• Non-neutralising antibodies: anti-p24 gag IgG
• CD8+ T Cells can prevent HIV entry by producing chemokines MIP-1a, MIP-1b, and
RANTES which block co-receptors.
How does HIV impair / defeat the immune response?
HIV remains infectious even when Ab coated
• Activated infected CD4+ Th are killed by CD8+ T cells
• Activated infected CD4+ Th are anergised (disabled)
• CD4 T-cell memory lost & failure to activate memory CTL
• Monocytes and dendritic cells are therefore not @ by the CD4+ T cells and cannot prime naïve CD8+ CTL (due to impaired antigen presenting functions)
• Infected monocytes and dendritic cells are killed by virus or CTL
• Quasispecies are produced due to error-prone reverse transcriptase = these escape from the i/response
• Effective immunity requires antibodies to prevent infection and neutralize virus, and sufficient CTL to eliminate latently infected cells
What are the different types of ‘progressors’ in HIV pts?
- Median time from infection with HIV to development of AIDS is 8 - 10 years (These are the typical progressors, 85% of patients)
- Rapid progressors (10%) in 2 - 3 years.
- Long Term Non Progressors (<5%) stable CD4 counts and no symptoms after 10 years
- Initial viral burden (set
HIV investgations
Screening Test: Detects anti-HIV Ab via ELISA
Confirmation Test: Detects Ab via Western Blot
A positive test requires the patient to have SEROCONVERTED (i.e. started to produce Ab)
This happens after ~10 weeks incubation period
After Diagnosis:
Viral Load – PCR is used to detect viral RNA (very sensitive)
CD4 Count – via FACS (flow cytometry), used to assess course of disease. Onset of AIDS correlates with diminuation in number of CD4+ T cells. AIDS <200cells/µL blood.
Resistance Testing – resistance to antiretrovirals:
• Phenotypic: Viral replication is measured in cell cultures under selective pressure of increasing concentrations of antiretroviral drugs – compared to wild-type
• Genotypic: Mutations determined by direct sequencing of the amplified HIV genome
HIV Mx
BHIVA guidelines for patients with chronic infection: Commence immediately once diagnosis
confirmed
HAART (Highly Active Anti Retroviral Therapy) = 2NRTIs + PI (or NNRTI)
Example regimen: Emtricitabine + Tenofovir + Efavirenz
Available as 1 pill: Atripla
Pregnancy – Zidovudine:
Antepartum PO; For delivery IV, PO to newborn for 6/52 reduces transmission
HIV Drug classes + examples
1) Fusion inhibitors - Enfurviritide
2) Attachment inhibitors - Maraviroc
3) RTI - Nucleoside RTI (NRTI)- Zidovudine, Lamivudine, Emtricitabine
4) RTI - Nucleotide RTI - Tenofovir
5) RTI - Nucleoside RTI (NNRTI) - Efavirenz
6) Integration inhibitors - Raltegravir
7) Protease inhibitors - Indinavir
HIV Drug classes side effects
FI - Local reactions to injection (hypersensitivity 0.1%)
AI - unkwon
NRTI - Generally rare. (Zido - GI distrubance, fever, headache)
Tenofovir - Bone and renal toxicity
NNRTI - Efavirenz - CNS effects (others: rash and hepatitis)
II - unknown
PI - Hyperlipideemia, fat redistribution, diabetes
How does Th1/17 and Th2 response differ?
Th1/Th17 are triggered by foreign structures i.e. microbial PAMPS.
Multicellular organisms don’t necessarily have conserved structures that the immune system can recognise. Therefore we recognise them by the damage they cause to our epithelial barriers. Worms/ Allergens/Venoms stress our epithelium and cause it to releases Cytokines e.g. TSLP*. These act on Th2, Th9 ILC2 cells and promote the secretion of IL4, IL5 and IL13. These trigger eosinophils /basophils to expel the allergen/worm. IL 4 then actually stimulates B cells to make IgE and IgG4 (bridges the innate and adaptive system).
How does the mast cell act as a sensor?
There are several ways the immune system ‘senses’ pathogens. In Th2 response, the epithelium is the sensor. Mast cells can also be the sensor when an allergen causes IgE to cross link on the mast cell surface membrane. This results in the production of histamines, leukotrienes and prostaglandins.
What is the mechanism behind oral tolerance to allergens?
Oral exposure to allergen promotes immune tolerance compared to skin/resp induces sensitisation. This is because the GI tract tends to immune suppress as the T reg cells play a role in dampening immune activity.
Oral tolerance to food antigen requires induction of CD4 T reg immune response within GI mucosa. They inhibit secretion of IL10 and IL35. They inhibit IgE plasma cell cytoloysis and dendritic APC function. They are associated with the resolution of food allergies.
NB tolerance is only built orally. (oral exposure promotes immune tolerance whereas skin and respiratory exposure induces IgE sensitisation)
Which allergic diseases are common at which ages?
Infants o Atopic dermatitis o Food allergy (milk, egg, nuts) (8%) Childhood o Asthma (house dustmite, pets) o Allergic rhinitis Adults o Allergic Rhinitis (most common apparently) o Drug allergy o Bee allergy o Oral allergy syndrome o Occupational allergy (Food is 5% in adults)
Theories for the rise in incidence in allergy
- Hygiene hypothesis (lack of childhood exposure to infectious agents/city living/ small family sizes increase suspectibility to allergic disease by suppressing the natural development of the immune system)
- Lack of Vit D in infancy is a rf for developing food allergy
- Reduced omega and fatty acids
- Rise in food allergy might be associated with high concentration of dietary advance glycation end products and pro-glycating sugars which immune system mistakenly detects as causing tissue damage and responds by creating IgE. This can be found in fast foods and soda drinks.
What are some common drug targets in allergic disease?
IgE
IL-13
histamine
IL-5
Presentation of an allergic response
Criteria: Within minutes – hours, usually 2 organs involved, reproducible, Sx can be triggered by cofactors (OHs, exs, NSAID, viral inf in childhood)
- Skin (angioedema, urticarial, flushing itch
- Resp (wheeze, SOB, discharge)
- GI tract D&V
- Vascular – Hypotension Sx (faint, dizzy)
- Sense of impending doom
List the allergy tests
Elective Investigations
1. Skin prick and intradermal tests (Skin prick and blood tests are used to detect the presence/absence of IgE antibody against external proteins. A positive IgE test only demonstrates sensitisation NOT clinical allergy. Diagnosis requires clinical hx, blood tests, skin prick etc. Stop antihistamines 48hrs beforehand)
- Laboratory measurement of allergen-specific IgE (Allergen is bound to a sponge and the specific IgE (if present) will bind to the allergens.This is washed over with anti-IgE antibody which is tagged with a fluorescent label. Expensive. Good for people who can’t stop anthistamines, skin is damaged from allergic itch (dermatographism), extensive eczema, history of anaphylaxis or borderline skin prick test results)
- Component-resolved diagnostics
(A newer blood test to detect IgE to single protein components - instead of whole allergen. Useful for peanut and hazelnut allergy (may reduce need for food challenges)) - Basophil activation test - expose basophils to allergen extract. Not used clinically yet.
- Challenge test (supervised exposure to the antigen) (Gold standard)
Patients are given increasing dosage of allergen. Has to occur under close supervision.
During Acute Episode
Evidence of mast cell degranulation:
• Serial mast cell tryptase
Mast cell tryptase is a biomarker for anaphylaxis and can be useful to measure when it’s unclear whether anaphylaxis has occurred e.g. if the pt can’t otherwise communicate it such as being under anaesthetic
• Blood and/or urine histamine
Oral Allergy Syndrome
Is a Type 1 Hypersenstiivty (IgE mediated) food-allergy syndrome in patients who have hay fever. Existing IgE antibodies against pollen cross react with other structurally similar proteins found in botanically related plants. Symptoms occur within minutes of eating the food (tend to be raw food- allergens are heat labile). Symptoms are limited to oral cavity, swelling and itch; only 1-2% progress to anaphylaxis.
Anaphylaxis incidence and definition and presentaiton
• DEFINITION: a severe potentially systemic hypersensitivity reaction. Rapid onset, life-threatening airway, breathing and circulatory problems which is usually but not always associated with skin and mucosal changes
• Incidence: 1.5-8/100,000
Skin is the most frequent organ involved (84%) (angioedema, urticaria)
Cardiovascular (collapse, syncope, drop in BP)
Respiratory compromise (SOB, wheeze, stridor)
Management of anaphylaxis
IM ADRENALINE is the most important treatment for anaphylaxis
• Alpha 1 - causes peripheral vasoconstriction, reverses low BP and mucosal oedema
• Beta 1 - increases heart rate, contractility and BP
• Beta 2 - relaxes bronchial smooth muscle and reduces the release of inflammatory mediators
Supportive Treatments:
• Adjust body position
• 100% Oxygen
• Fluid replacement
• Inhaled bronchodilators
• Hydrocortisone 100 mg IV (prevent late phase response)
• Chlorpheniramine 10 mg IV
Further Management • Referral to allergy/immunology clinic • Investigate cause • Written information on: • Recognition of symptoms • Avoidance of triggers • Indications for self-treatment with an EpiPen • Prescription of emergency kit to manage anaphylaxis
What are APCs?
These are cells that can present antigens to T cells to initiate an acquired immune response APCs include: o Dendritic cells o Macrophages o B lymphocytes
Types of macrophages include: o Langerhans cells o Mesangial cells o Kupffer cells o Osteoclasts o Microglia
How are memory T cells different to other T cells?
- Longevity - Memory T cell persist for a long time in the absence of antigen by the continuous low level proliferation in response to cytokines
- Memory cells have a different pattern of expression of cell surface proteins involved in chemotaxis cell adhesion
- This allows memory cells to rapidly access non-lymphoid tissues (where microbes enter) - Rapid, robust response to subsequent antigen exposure -Lower threshold of activation than naïve cells
Aims of a vaccine
- Generate protective, long-lasting immune response
- No adverse reactions
- Single shot
- Easy storage
What is a haemaglutinin inhibition assay and why is it relevant?
Haemaglutinin (HA) is the receptor-binding and membrane fusion glycoprotein of influenza virus and HA is the target for antibodies.
• If you put normal red cells in a dish, they will clump at the bottom forming a red spot
• If you add the influenza virus to the red cells, the haemaglutinin will make the cells stick together and it will cause a diffuse coloration across the well
• If you add the serum of someone who has a lot of antibodies against HA with the virus and red cells, it will inhibit the HA from causing the above effect - this results in the cells clumping at the bottom as if the virus was not present
• The higher the dilution at which the inhibitory effect can be seen, the greater the level of antibodies the patient has against HA
• The higher the antibody level the lower the likelihood of infection
Antibody protection begins 7 days after vaccine and protection can last for around 6 months
What is the BCG
BCG is an attenuated strain of bovine tuberculosis
Provides some protection against primary infection, mainly provides protection against progression to @ TB
T cell response is important in protection
Mantoux Test:
•Inject a small amount of liquid tuberculin (aka purified protein derivativ (PPD)) intradermally
•The area of injection is examined 48-72 hours after tuberculin injection
•The reaction is an area of swelling around the injection site
Protection usually lasts for 10-15 years
Types of vaccination:
LIVE, attenuated strain e.g. MMR, BCG, oral typhoid, polio
Pros: Lifelong protection after one dose, broad response, more likely to protect against strains
Cons: can’t give to I/C or pregnants, storage, rare reversion to virulence (1: 750,000)
INACTIVATED TOXINS / COMPONENT vaccinations
E.g. component: Hep B (HBsAg); HPV (capsid); Influenza (HA, neuraminidase). Inactivated Toxins: Diptheria and tet
Pros: No mutation or reversion, can be used in immunodeficient patients, Can lead to elimination of wild-type virus from the community, Easier storage, Lower cost
Cons: poor immunogenicity, need multiple injections, may require conjugates or adjuvants
CONJUGATE & ADJUVANT VACCINATIONS:
This is composed of a Polysaccharide + protein carrier
The polysaccharide alone induces a T cell-independent B cell response (transient). Addition of the protein carrier promotes T cell immunity which enhances the B cell/antibody response. E.g. Hib, Meningococcus and Pneumococcus.
Adjuvants:
Increases the immune response without altering its specificity, by mimicing the action of PAMPs on TLR and other PRRs E.g. Aluminium salts, Lipids - monophosphoryl lipid A, Freund’s adjuvant (in animals)
How can we replace missing components of the immune system? When do we use each method?
- Haematopoietic Stem Cell transplant (used in haem malginancies e.g. lymphomas, leukaemias, MMs or used in severe immunodeficiencies e.g. SCID, LAD)
- Antibody replacement (a variety of Immunoglobulin from a pool of human donors. Given for primary antibody deficiency (X-linked agamma…, X-L hyper IgM, Common variable immunodefiency) or for secondary Ab deficiency (CLL, MM or after BMT).
- Specific immunoglobulin replacement - human immunoglobulin used for post exposure prophylaxis
- Cell transfer (4 different types: Virus specific T cells, CAR T cell therapy, TIL T cell therapy and TCR T cell therapy).
How can we boost the immune system by block immune check points?
By blocking immune check points we limit the regulation of the immune system.
Ipilimumab – antibody specific for CTLA4 – blocks downregulatory immune checkpoint and allows T cell activation; indications: metastatic melanoma
Pembrolizumab/Nivolumab – antibody specific for PD-1 - blocks down regulatory immune checkpoint and allows T cell activation; indications: advanced melanoma and metastatic renal cell cancer.
Both of these drugs have a disadvantage of causing autoimmunity e.g. diabetes, thyroidism.
How else can the immune system be boosted?
Cytokine Therapy- The aim is to modify the immune response, unfortunately they haven’t been very effective.
IFN a - Hep B, Hep C, Kaposi sarcoma, CML, MM, Hairy cell leukaemia
IFN B - Behcet’s disease
IFN Gamma - Chronic granulomatous disease
IL 2- Renal cell cancer
How can the immune response be suppressed?
PICCASO Plasmapharesis INhibitors of cell signalling Csf directed agents CK directed agents Antiproliferative Agents Steroids Oh dear..
Plasmapharesis
Remove plasma and filter out pathogenic antibody. Either reinfuse plasma or reinfused albumin instead (PEX)
Issues: Rebound antibody production (because although you’ve got rid of the antibodies, the plasma cells are still there) limits efficacy. Therefore, it is usually given with an anti-proliferative agent.
Indications (severe antibody-mediated disease):
• Goodpasture’s syndrome (to filter out anti-glomerular replacement antibodies and prevent pulmonary haemorrhage and renal failure)
• Severe acute myasthenia gravis
• Severe transplant rejection (Ab against donor HLA)
Inhibitors of cell signalling
- Calcineurin inhibitors (e.g. ciclosporin & tacrolimus) will prevent T cell signalling. Therefore, it blocks IL2 production (IL2 normally acts on T cells and drives proliferation -> calcineurin inhibitors will prevent T cell activation and proliferation
Side Effects: Nephrotoxicity, HTN, Neurotoxicity, Dysmorphic features (ciclosporin) - JAK Inhibitors (JAK hangs out inside the cell attached to a transmembrane receptor. When a CK binds to that receptor, the JAK proteins come together phosphorylate downstream molecules. This inhibits gene transcription and production of inflammatory molecules. Tofacitinib (JAK1 and JAK3 inhibitor) is used in Rheumatoid and Psoriatic Arthritis.
- PDE4 inhibitors - PDE4 metabolises cAMP. Apremilast inhibits PDE4 -> rise in cAMP. This activates PKA which prevents activation of TFs and results in decrease in CK production. Used in psoriatic arthritis and psoriasis.
Cell surface directed agents
- Anti-thymocyte globulin
Thymocyte (lymphocytes from the thymus gland) from humans were injected into rabbits. The rabbits then produced antibodies against the thymocytes of varying specificities (e.g. antibodies to CD2, CD3, CD4, CD8 etc.)
The serum was then taken and injected into patients. This is very effective at targeting T cells, however it is very non-specific. The main aim is to cause T cell depletion (thereby reducing the activation and migration of T cells). This is useful in allograft rejection - Basiliximab
It targets part of the IL2 receptor. Specific for CD25, which causes inhibition of T cell proliferation. Used before and after transplant for prophylaxis of allograft rejection surgery. - Rituximab*
Against CD20 - expressed on mature B lymphocytes but not plasma cells and causes their depletion. Rheum, Lymphoma, SLE, ITP.
S/E: exacerbations cardio vascular disease - Abatacept*
This drug binds to receptors on APCs and prevents them from engaging with T cells -> reduced T cell @. (It blocks CD80 and CD86). Rheum Arthritis. - Natalizumab - inhibits leukocyte migration effective for relapsing-remitting MS
S/E: hepatotoxic - Tocilixumab*
Against IL6 receptor and leads to reduced activation of macrophages, T cells, B cells and neutrophils
Castleman’s disease (IL6 producing tumour) & Rheumatoid arthritis
S/E Hepatotoxic, elevated lipids
List the cytokines have been targeted for cytokine directed agents?
- anti TNF alpha
- TNF alpha/beta receptor (Etanercept- Rheum, Ankspon, Psor)
- IL12 (Ustekinumab - Psor & Crohn’s)
- anti IL 17A (Secukinumab- Psor & Ank Spon )
- anti-RANK ligand antibody (Denosumab - Osteoporosis, can cause avascular necrosis of jaw)
What are the anti TNF alpha agents?
infliximab, adalimumab, certolizumab, golimumab
TNF-alpha is a critical molecule within the cytokine cascade that is responsible for the inflammatory response in inflammatory arthritis .
Indications: • Rheumatoid arthritis, Ankylosing spondylitis, Psoriasis and psoriatic arthritis, IBD
S/E:
• Infusion or injection site reactions
• Infection (TB, HBV, HCV)
• Lupus-like conditions
• Demyelination
• Malignancy
MoA of antiproliferative agents
Stop proliferation, immune cells proliferate more rapidly than other cells; therefore with correct dosing, they can be selective for immune cells.
Examples: Cyclophosphamide (most toxic), Mycophenolate, Azathioprine, Methotrexate
MoA: Inhibit DNA synthesis, targeting cells with rapid turnover
S/E: BM suppression, Teratogenic, malignancy and infection
Cyclophosphamide
MoA: Alkylating agent, alkylates the guanine base of DNA and this damages the DNA ensuring it wont replicate. Initially affects B cells more than T cells, but at high doses it targets all cells with high turnover.
Indication: Used to induce remission in short term, then move on to a less toxic drug. Anti-cancer, multisystem connective tissue disease, Lupus, GPA/Wegners
S/E:
Toxic to proliferating cells (BM Depression, hair loss, irreversibly sterility M>F
Toxic to bladder (Haemorrhagic cystitis caused by the toxic metabolite of cyclophosphamide called acrolein which is excreted in the urine) ( acrolein destroys the urothelium, the underlying detrusor smooth muscle and blood vessels become exposed to urine,causing further cell death)
Malignant (Bladder cancer, haem cancers, non-melanoma skin cancer)
Infection (Pneumocystitis jiroveci- yeast like fungus which causes pneumonia in the immunosuppressed particularly important in HIV discovery)
Azathioprine
MoA: Anti-metabolite. In liver, azathioprine is metabolised to 6-mercaptopurine, which is a purine analogue i.e. interferes with DNA production and inhibits proliferating cells. T>B cells, and affects innate cells too.
Indication: Widely used in transplantation, auto-immune disease and auto-inflamm.
S/E:
BM suppression (rapid turnover are effect)
1:300 individuals are v suscpetible to BM suppression due to a thiopurine polymorphism called TPMT polymorphism (Thiopurine methyltransferase). These pts are unable to metabolise azathioprine and result in huge huge BM suprression and neutropenia. Therefore always check TPMT activity or gene variants before starting and always check FBC after starting thearpy. Homozygous for abnormal allele = don’t use. Heterozygous = try half dose or consdier other dryg
Hepatotoxic (uncommon)
Infection (less common than cyclophosphamide)
Mycophenoalte
used in place of azathiopurine these days
MoA: Anti-metabolite purine, blodcks de novo guanosine nucleotide synthesis and prevents replcation fo DNA again T> B population
Indication: Widely used in transplantation instead of azathioprine and also used in AI disease and vasculitis instead of cyclophos
SE: BM suprresion, infection (particular herpes reactivation - due to mycolphenolates effect on t cells,also PML (Progressive multifocal leukocencelopathy cause dby JC virus - a normal common virus)
Steroids MoA
1) Effects on Prostaglandins
o Corticosteroids inhibit phospholipase A2, which converts phospholipids into AA which is then converted to eicosanoids (e.g. PG and leukotrienes) by COX
o By inhibiting phospholipase A2, corticosteroids will block AA and PG formation thereby reducing inflammation
2) Effects on Phagocytes
o Decrease traffic of phagocytes to inflamed tissue by:
• Reduce adhesion molecules on the endothelium
• They also block the signals that tell immune cells to move from the bloodstream into tissues
• This leads to a transient increase in neutrophil count
o Decreased phagocytosis
o Decreased release of proteolytic enzymes
3) Effects on Lymphocyte Function o Lymphopaenia (Sequestration of lymphocytes in lymphoid tissue. Affects: CD4 > CD8 > B cells) o Blocks cytokine gene expression o Decreased antibody production o Promotes apoptosis
SIDE EFFECTS:
Metabolic (Central obesity, Moon face, Diabetes, Lipid disorders, Osteoporosis, Hirstuitism, Adrenal suppression)
Cataracts, glaucoma, peptic ulceration, pancreatitis
Immunosuppression
NB - This all refers to prednisolone (it has NO mineralocorticoid activity just glucocorticoid)
Most commonly transplanted organ?
- Kidneys are the most commonly transplanted organs (lasts 12 years)
- 2nd most common is liver transplant
What are the most relevant protein variations in clinical transplantation?
- ABO blood group
- HLA (Coded on chromosome 6 by MHC )
• NOTE: sometimes HLA is used to refer to the proteins and MHC refers to the genes but they are often used interchangeably - There are some other minor histocompatibility genes
What are the two mechanisms of rejection?
- T cell mediated
- Antibody mediated
- Both
What are the HLA classes?
HLA Class I (A, B and C) - expressed on ALL cells
HLA Class II (DR, DQ, DP) - expressed on antigen-presenting cells but can also be upregulated on other cells under stress
HLA are cell surface proteins. The presentation of foreign antigens on HLA molecules to T cells is a vital part of T cell activation.
Why are HLA classes such an issue?
They are highly polymorphic with hundreds of alleles for each locus (The areas of protein lining the peptide-binding groove are responsible for the high degree of variability between HLA)
The high variability has evolved so that we are able to present a wide variety of antigens in that peptide-binding groove to the cells of the immune system
The variability in antigens is an issue in transplantation because they provide a key difference that the immune system can react with i.e. different people’s HLA will recognise different range of things as foreign. If this discrepancy is large - (high mismatch) it will increase the risk of rejection.
• A, B and DR are thought to be the MOST IMMUNOGENIC
• The number of mismatches is a major determinant of the risk of rejection
Describe T cell mediated rejection
T cells require presentation of the foreign HLA antigens by an APC (+ Costimulation) to initiate activation of alloreactive T cells- this tends to occur in the LNs. Activated T cellls (who are specific for that foreign HLA antigen) then produce CKs (IL2), help CD8+ cells, recruit phagocyte cells, provide help for AB production etc etc.
The inflammation caused by this process will lead to graft dysfunction (i.e. characterised by a raise in creatinine).
This new clonal population of T cells will attack the graft
They will tether, roll and arrest on the endothelial cell surface. They then crawl into the interstitium and start attacking the tubular epithelium. This results in:
• Lymphocytic interstitial infiltration
• Ruptured basement membrane of tubular epithelium
• Tubulitis (inflammatory cells within the tubular epithelium that have been recruited)
• The inflammatory cells can also attack the endothelium of blood vessels leading to arteritis
[NB the above is for kidney transplant - but it can occur in any transplanted organ.]
Describe antibody mediated rejection
Antibodies will bind to antigens (HLA) on the endothelium of the blood vessels in the transplanted organ. These antibodies can then fix complement which assembles to form membrane attack complexes (MAC) resulting in endothelial cell lysis & recruit inflammatory cells to the microcirculation- one of the cardinal features of antibody-mediated rejection is the presence of inflammatory cells within the capillaries of the kidney which then cause endothelial injury (capillaritis)
These processes will result in procoagulant tendencies and closure of the microcirculation leading to graft fibrosis
Antibodies against graft endothelial epitopes can also cause damage by cross-linking the MHC molecules and activating them
Histology of antibody mediated rejection
The little capillaries in between the tubules contain inflammatory cells
This is inflammation of the microcirculation (whereas in T cell mediated rejection you get inflammatory cells in the tubular epithelium and the interstitial space)
Immunohistochemistry looking for fixation of complement fragments on the endothelial cell surface may also be useful
How can we reduce the difference between the transplant and donor?
Determine donor and recipient blood group and HLA type (esp. BM, kidney) . We sequence the HLA using PCR.
Check recipient’s pre-formed Ab against ABO and HLA – via CDC, FACS and Luminex. Before during and after the transplanation.
Cross match – via CDC and FACS. Tests if serum from recipient is able to bind/kill donor lymphocytes- positive crossmatch is contraindication for transplantation.
After transplant check again for new antibodies vs the graft
CDC - complement dependent cytotoxicity (Looks at whether the host’s serum will kill the lymphocytes of the donor in the presence of complement. Positive crossmatch suggests that there is cell lysis)
FACS - Flow cytometry (Looks at whether the host’s serum binds to the donor’s lymphocytes irrespective of complement. Bound antibody is detected by fluorescently-labelled anti-human immunoglobulin)
How do we prevent rejection?
1 . Suppress T cells
Calcineurin Inhibitors (Tacrolimus and Cyclosporin)
Cell Cycle Inhibitors (MMF)
Target TCR (Anti-thymocyte globulin)
• Alemtuzumab is an anti-CD52 monoclonal antibody that causes lysis of T cells
• Daclizumab is an anti-CD25 monoclonal antibody which targets the cytokine signal
- Suppress Ab mediated Rejection
•B cells can be depleted using rituximab (anti-CD20)
•BAFF inhibitors target cytokines that promote B cell activation and growth
•Proteasome inhibitors such as bortezemib can block the production of antibodies by plasma cells
•Complement binding to the surface of endothelial cells can be blocked using complement inhibitors such as eculizimab
Modern Transplant Immunosuppression
•Induction agent: e.g. OXT3/ATG, anti-CD52, anti-CD25
oThis is given at the time of transplantation or just before in order to prepare the patient to receive the foreign organ
•Baseline immunosuppression: calcineurin inhibitor + mycofenolat mofetil or azathioprine with or without steroids
•Treatment of episodes of acute rejection
oCellular: steroids, OKT3/ATG
oAntibody-mediated: IVIG, plasma exchange, anti-C5, anti-CD20
NOTE: IVIG can reduce antibody production through feedback and it can displace troublesome antibodies so that they cannot exert their harmful effects
How do we prevent GvHD in HSCT?
o During the process of SCT, the host immune system is eliminated (using total body irradiation and drugs)
o It is then replaced by own (autologous) or HLA-matched donor (allogeneic) bone marrow
o Allogeneic stem cell transplantation leads to reaction of donor lymphocytes against host tissues
o Related to a degree of HLA-incompatibility
o If there is a malignancy (e.g. leukaemia), the graft can help kill these cells (graft-versus-tumour)
o GVHD Prophylaxis- Methotrexate/cyclosporine
o It is thought that the damage to the GI tract by the irradiation and cytotoxic drugs before the transplant has an important role in liberating antigens which will subsequently be presented to the donor’s immune cells
oSymptoms - Rash, GI (N&V&D, Jaundice(
oTreatment: STEROIDS
What are the issues post transplant?
Infections
• Increased risk of conventional infections
• Also increased risk of opportunistic infection:
o CMV
o BK virus
o PCP
Post-Transplantation Malignancy
• Viral-associated malignancies are much more common, such as:
o Kaposi sarcoma (HHV8)
o Lymphoproliferative disease (EBV)
• Skin cancer is 20 x more common
• Risk of other cancers is also increased
Complement deficiency one-liners /stereotypes
- C1q deficiency = severe childhood-onset SLE with normal levels of C3 and C4
- C3 deficiency with presence of a nephritic factor = membranoproliferative nephritis and abnormal fat distribution
- C7 deficiency = meningococcus meningitis with family history of sibling dying of the same condition aged 6
- MBL deficiency = recurrent infections when neutropaenic following chemotherapy but previously well
C1q deficiency
o C1q deficiency is an inherited form of complement deficiency that tends to present with SLE in childhood
o In patients with C1q deficiency, they will not be able to activate their classical pathway so CH50 will be low
functional complement deficiency
- Active lupus causes the persistent production of immune complexes
- This leads to the consumption of complement components resulting in a functional complement deficiency
- C3 and C4 can be measured and they tend to be low in severe active disease. In active disease C4 is low. Inactive both normal.
How do nephritic factors affect complement
- Nephritic factors are autoantibodies that are directed against components of the complement pathway
- Nephritic factors will stabilise C3 convertases (which break down C3) resulting in C3 activation and consumption
- Nephritic factors are often associated with GLOMERULONEPHRITIS (classically membranoproliferative)
- It may be associated with partial lipodystrophy
Properdin
This joins Bb to C3(H2O) in the alternative pathway.
Properdin is the only known positive regulator of complement activation that stabilizes the alternative pathway convertases.
Immunodeficiency signs
o TWO major OR ONE major + recurrent minor infections in one year o Unusual organisms o Unusual sites o Unresponsive to treatment o Chronic infections o Early structural damage
How do phagocytes interact with other cells?
- Cells have pattern recognition receptors (e.g. Toll-like receptors) which recognise generic motifs known as pathogen-associated molecular patterns (PAMPs) such as bacterial sugars, DNA and RNA
- Cells have Fc receptors to allow the detection of immune complexes
- They also have phagocytic capacity meaning that they can engulf the pathogens
- Cells can secrete cytokines and chemokines to regulate the immune response
