Immunodeficiencies Flashcards
Define primary immunodeficiency
groups of disorders caused by inherited or genetic defects in the cells and tissues of the immune system. Can be due to gene mutations in a single gene, and can be spontaneous or inherited.
Define secondary immunodeficiency
when the immune system is compromised due to an environmental factor
What are the initial tests done if someone is presenting with the hallmark of ID?
• The ‘hallmark’ of an ID is recurrent infections – this is the first sign of a potential ID
• There are two types of infection, each mainly using one of 2 different arms of the immune system:
− Bacterial → antibody (humoral)
− Viral/fungal → T cell (cell-mediated)
• If someone presents with recurrent infections, they would have their blood count tested (CBC) to see if certain blood cells are missing.
Viral fungal infections:
• As well as CBC, do a DTH response test or HIV test
• If HIV+ → you know they have an acquired immunodeficiency
• If patient has lymphopenia or absent DTH, you can isolate the lymphocytes and do functional assays
Bacterial infections:
• As well as CBC, can do a test for specific antibodies
• You would look to see if antibodies are normal with a normal response
• Can also do functional assays
→ If you have done all of these, can also do genetic testing.
Describe the use of targeted mouse mutants in ID
- Way to identify the genes that lead to ID
- Hard to look in patients directly, as need to compare patient to patient and with their family to identify the gene that is causative
- Easier and faster to create mouse knockouts, and see if they develop the predicted immunodeficiency, then see if you can rescue the gene defect in mice by adding the functional human gene.
Why are IDs much more common in boys?
- Many immunodeficiencies are X-linked, and hence a lot more common in boys
- This is because girls are usually heterozygous, and due to stochastic X chromosome inactivation, they often have the X that is carrying the mutation silenced
- However, in boys, they only have 1 X chromosome, so if they have an X chromosome with the Btk mutation, there is no compensation anymore
Describe the general features of XLA.
X-linked agammaglobulinaemia → Mutations in Bruton’s tyrosine kinase (btk) gene.
• No mature B cells (agammalgobulin = absence of globulin = absence of antibodies)
• Recurrent haemophilus and pneumococcus infections
• Most common form of inherited agammaglobulinaemia – accounts for 85% of all cases
• Mutant Btk:
− BTK plays a crucial role in B cell maturation (proliferation, differentiation and survival of early B cell lineages) and mast cell activation through the IgE receptor
− Cytoplasmic, non-receptor tyrosine kinase expressed at all stages of B cell development, with the exception of long-lived plasma cells.
− Many other haematopoetic cells express BTK, but the functional impairment seems confined to B cells
• Btk mutant mice have the same phenotype, and if you introduce the human Btk gene into Btk mutant mice, you get normal B cell development
• Patients with XLA have normal pre-B cell populations, but there is a developmental block manifested at the transition between the pro-B and pre-B cell stages.
Describe the two treatments for XLA.
Pooled immunoglobulins:
• Sterile, purified IgG products manufactured from pooled plasma and typically contain >95% unmodified IgG, with has infact Fc-dependent effector functions and only trace amounts of IgA or IgM
• Can be given IV or subcut.
• Given prophylactically – needed to maintain the levels
• Also need generous admin. of antibiotics
• If the donor has an infection, it can be transmitted via the pooled Ig, eg) HIV and Hep C can be a problem, although donors are screened. There is also the risk of vCJD due to prion protein transmission
• Expensive
• Considerably improves quality of life, but patients still suffer chronic infections and life expectancy is still reduced.
Splice-correcting oligonucleotides – Bestas et al,:
• Splicing defects identified as an important cause of genetic disease – mutations may disrupt regular splice sites, which can result in the inclusion of intronic sequences or loss of part of the exons
• An A to T transition in intron 4 of the BTK gene generates a novel 5’splice site, resulting in the inclusion of a cryptic exon → this changes the reading frame and completely abolishes BTK expression.
• This prompted investigations into using splice-correcting antisense oligonucleotides
• A key feature of XLA is that BTK is expressed in all B-lineage stages, except for mature plasma cells – so if a transient correction could be restored at the pro-B cell stage, this could result in long-term effect.
• Using both a mouse mouse model and primary cells from XLA patients, they were able to use BTK-specific SCOs to correct aberrant spliced BTK in B lymphocytes. This restored expression of the functional protein.
• It may therefore be feasible to obtain pro-B cells, carry out the treatment ex-vivo and return the splice-corrected cells to the patients.
What are the features of DGS/VCFS?
Deletion of chromosome 22q11
• Wide range of developmental anomalies in the heart, glands and facial structures (underdeveloped chin, eyes with heavy eyelids)
• Thymic aplasia → very few T cells
• Patients with DGS develop autoimmune disease at a rate that is higher than in the general population – the most common being idiopathic thrombocytopenia, autoimmune haemolytic anaemia and autoimmune arthritis.
• Clinical features highly variable in penetrance, even in twins with identical deletions
• Autosomal dominant
• Approx 90 % occurs spontaneously
• The TBX1 transcription factor has been found to be important (Jerome & Papaloannou):
− Recently, aortic arch defects have been reported in mice heterozygous for a deletion that includes the TBX1 gene
− TBX2 identified as a candidate for this syndrome both due to its chromosomal location and by its expression in the head mesenchyme
− TBX1 knockout mice display a spectrum of phenotypic effects encompassing most of the common DGS/VCFS malformations.
• No cure, but there are some therapies aimed at correcting the defects in affected organs eg) corrective heart surgery.
• For the ID, just involved monitoring overall immune system
− Complete lack of T cells is called ‘complete DGS’, and is potentially fatal and similar to SCID → need a thymus transplant (only available on a research basis) or stem cells transplant to reconstitute the T cells.
− In some patients, the T-lymphocyte defect is enough to cause the B-lymphocytes to fail to make sufficient antibody.
Describe the features of ADA SCID.
• T- B- NK-
• Autosomal recessive
• ADA is a key enzyme in the purine pathway, and is ubiquitously expressed
• Mutation leads to accumulation of lymphotoxic deoxyadenosine → purine metabolite, accumulates in plasma and cells.
• Results in depletion of ATP, inhibits proliferation of T and B lymphocytes
− Accordingly, broad lymphopenia, recurrent infections and failure to thrive are the prominent features
− Systemic accumulation of purine metabolites can cause alterations in several organs inc. skeleton, lung, liver, GI tract and CNS.
Cassani et al, :
• CD4+ T cells from ADA-SCID patients have severely compromised TCR/CD28 driven proliferation and cytokine production
• Impairment is associated with intrinsically reduced ZAP-70 phosphorylation, Ca2+ influx and ERK1/2 signalling
• Exposure to deoxyadenosine results in stronger inhibition of T cell activation
How is ADA SCID treated?
- HSC transplant is curative, but dependent on a good donor match
- Enzyme replacement therapy with PEG-adenosine deaminase bovine (PEG-ADA)
− Used in over 150 patients worldwide
− Allowed stabilization of patients awaiting more definitive treatment
− Affords metabolic deotoxification but immune reconstitution is suboptimal, and may not be long lived
− Gives partial restoration of T cell immunity - Gene therapy
− First disease to be treated with gene therapy (4 year old girl in 1990)
− Remove blood and grow T cells, add inactivated retrovirus carrying functional ADA gene, infuse transduced cells back into patient
− T cell function found ot be restored for 2 years
Describe the features and treatment of Omenn Syndrome SCID
- Oligoclonal T cells
- Autosomal recessive
Genetics:
• Associated with hypomorphic missense mutations in immunologically relevant genes of T and B cells, eg, RAG genes, IL-7R, ZAP-70, DNA-ligase
• Partial loss of RAG gene function is common, loss of this ability means cant perform recombination of TCR and BCR.
• For Omenn syndrome, it is the loss of T cell function that is important → results in only a few T cell clones
• Leads to engraftment of maternal lymphocytes in the feoteus and a co-existence of autologous and maternal lymphocytes.
Symptoms:
• Similar to GvHD → the T cells present have limited levels of recombination, and have specific affinity for self-antigen found in the thymus and periphery
• Characteristic symptom is chronic inflammation of the skin, appearing as a red rash
• Eosinophilia
• Failure to thrive
• Swollen lymph nodes and spleen
• Low Ig levels
Treatment:
• Only treatment is HSC transplant , which is curative - without this, it is rapidly fatal.
Describe the features and diagnosis of X-linked SCID.
- T- B+ NK-
- 50-60% of SCIDs
- X-linked recessive
- Although B cells are present, in the absence of T cell help they will become defective
- No C/M demarcation in thymus
- Hypoplastic lymphoid tissue
Symptoms:
• Infections before 3 months of age due to decreased IgG in the infant
• Followed by viral infections such as pneumonitis
• Tell-tale sign of SCID is candidiasis
• Recurrent eczema-like rashes common
• Other common infections include diarrhea and sepsis
• Failure to thrive, gut problems, skin problems and muscle hypotonia
• Syptoms may not appear for the first 6 months due to passive immunity from the mother
Genetics:
• Mutation in the xq13.1 locus of the X chromosome
• Only tends to affect males, as recessive – males only have 1 X whereas females have 2.
• Mutates the gene for the common y chain of the IL-2RG → shared between the receptors for IL-2, 4,7,9,15 and 21
− IL-7 important for the development of lymphocytes
− Il-15 required for development of NK cells
− IL-21 needed to promote Th17 response
− Il-4 needed for Th2 and Ig production
• Usually passed on through mother being a carrier, but can also arise through de novo mutations in a germ cell.
− Since only 1/3rd SCID patients have a positive family history of SCID, it is thought that de novo mutations account for a significant percentage of cases.
- Absence of thymic shadow on chest X rays
- Genetic testing for IL2RG mutations
Cell count:
Normal Approx 5000,
Describe bone marrow transplant as a treatment for X-linked SCID.
− First successfully reported for SCID in 1968
− Results in full immune reconstitution if the treatment is successful because bone marrow contains multipotent HSCs
− Major problem is GvHD
− BMT requires HLA match between donor & recipient
− Autologous BMT involves full HLA match, allogeneic can be full or half HLA match
− In allogeneic BMT, the chances of GvHD is high if the match is not close enough
− By this, we mean that there are various HLA haplotypes that can result in mismatch, and some are worse than others
− In the case of mismatch, donor bone marrow attack the patients cells.
− BM donor and recipient must share at least one haplotype to restore immune function, eg)
➢ HLA a/b donor & HLA c/d recipient → HLAc or d restricted T cells cannot be activated by HLA a/b APCs.
➢ HLA a/b donor and HLA b/d recipient → HLAb-restricted T cells can be activated by HLAa/b APCs and therefore recognize HLAb/d infected cells.
Symptoms GvHD include rash, high serum bilirubin and diarrhea.
− Depletion of donor T cells and close HLA match will reduce the chance of GvHD, however still not 100% chance of successful cure. In haploidentical twins, still 20% chance of death.
− Other sources of HSCs include:
− Peripheral blood – but can be difficult to get enough cells
− Umbilical cord → the best source
− Would otherwise be discarded
− Rich in stem cells
− Quicker engraftment
− Lower risk of GvHD
− No donor discomfort
− In families where a diagnosis has been previously made (ie, have other ID children), in utero bone marrow transplantation may be done. This has been successful in a number of cases.
Discuss gene therapy as a treatment for X-SCID
− Available only for clinical trials
− Monogenic disorder, so gene therapy can theoretically easily replace mutated IL2RG
− HSCs can be isolated for SCID gene therapy – needs to be in a stem cell because this cell will repopulate the niche. Wouldn’t get long term reconstitution if did in a differentiated cell
− Viral vectors can be employed
− Insert normal gene into the virus
− Infect bone marrow cell with the virus
− Viral DNA inserts into the chromosome
− Inject cells into the patient
− Potential treatment option for many of the SCID classifications (already discussed ADA SCID)
− Cavazzana-Calvo et al:
− 10 children treated with gene therapy at infancy for X SCID
− 9/10 cured
− However, 3 years after treatment, 2 children developed T cell leukemia due to insertion of the IL2RG gene near the LMO2 gene (a known oncogene), resulting in its activation → insertional mutagenesis.
− Insertional mutategenesis is a problem in X-SCID and CGD, but not in ADA SCID.
− Research into the use if insulator and suicide genes is warranted as this may prevent cancer from development
− Insulator genes → inhibit the activation of adjacent genes
− Suicide gene → stimulated when a tumour begins to form, and will result in the deactivation of the therapeutic gene.
− Restriction enzymes such as the zinc-finger nuclease allows the research to choose the site of gene integration
− Vectors that self-inactivate the promoter and enhancer are being research
− CRISPR/Cas9 may also prove highly effective, as will bypass the need for retroviral vectors.
Gene therapy may not always be long-term:
− In X-linked and ADA SCID, the cells that have the repaired gene have a cellular advantange. Repairing both genes allows the cells to proliferate, so they grow over the endogenous cells – so it is long term.
− In CGD
Describe the features of JAK3-SCID.
• T- B+ NK- SCID
• JACK3 is a tyrosine kinase functionally coupled to cytokine receptors sharing the common gamma chain
• Mediates the signal transduction from the cytokines that share the common gamma chain, so mutation affects multiple immune cells:
− IL-7 important for the development of lymphocytes
− Il-15 required for development of NK cells
− IL-21 needed to promote Th17 response
− Il-4 needed for Th2 and Ig production
• Autosomal recessive
• Patients present with the classical features of SCID – respiratory infections, diarrhea, thrush and failure to thrive
• Rare, only 6% of all cases of SCID
