Autoimmune diseases 2 Flashcards
Molecular mimicry definition
Epitopes relevant to the pathogen are shared with host antigens
Molecular mimicry
Viral infection: presentation of viral peptides to a CD4 T cell via MHC 2, causing T cell activation
Viral peptides happen to be similar to a host derived peptide; the T cell would normally recognise these peptides, but not react to them
The activated T cell now reacts strongly yo the self peptide and initiates inflammation
The process depends on having the correct MHC molecules to present this critical epitope that is common to both virus and host
Also depends on having the correct T cell to recognise it
Examples of molecular mimicry
Autoimmune haemolysis after mycoplasma pneumoniae
Rheumatic fever
Autoimmune haemolysis after mycoplasma pneumoniae
Mycoplasma antigen has homology to I antigen on red blood cells
IgM antibody to mycoplasma may cause transient haemolysis
Rhueumatic fever
Inflammatory disease occurring after streptococcal infection affecting heart, joints, skin and brain
Anti-streptococcal antibodies believed to cross react with connective tissue
Type 1 diabetes
Lack of insulin impairs cellular update of glucose, leading to polyuria, polydipsia, polyphagia and weight loss
Onset at any age, but typically childhood
Disease prevalence around 0.8%; rising by around 5% per anum
Treatment by injection of insulin and diet
Immunology of T1DM: evidence for autoimmune disease
Islet cell antibodies detectable for months to years before the onset of clinical disease
HLA associations
Mouse model
Early pancreatic biopsy shows infiltration with CD4/8 T cells
Antibodies present do not appear to be directly relevant to destruction of the pancreas
By the time patient has established diabetes, generally no active inflammation in pancreatic biopsy
Genetics and type 1 diabetes
Genetic background definitely important- con-concordance in monozygotic twins is close to 100% if they are observed for long enough
HLA class II alleles are the major defined genetic risk factor
Believed these molecules are required to present relevant islet cell antigens to CD4 T cells
Autoimmune response may occur if appropriate T cell receptors are present, together with other genetic and environmental co factors
Precipitating events for type 1 diabetes
Autoantibodies to islet cell antigens present for months- years before onset of clinical disease
Gap between initiation of disease and its presentation makes identification of triggers difficult
Some evidence for coxsackie virus
Autoimmune serology for diagnosis
Some autoantibodies have diagnostic value
- in some cases the antibodies are pathogenic
- in others they are simply a bystander effect
Broadly three methods for detection
- indirect immunofluroscence
- solid phase immunoassay
- direct immunofluorescence
Detecting antibodies in blood: indirect immunofluorescence
INCUBATE
- patient serum containing (or not) relevant antibodies
DETECT
- add detection antibody labelled with fluorescent marker
READ
- look for fluorescence under microscope
Important to identify type 1 DM because
Risk of ketoacidosis
Requires insulin
Monogenic diabetes and type 2 diabetes require a different apprach
Introduction to ELISA practical
Uses a plastic 96 well plate
First step is to coat each well with tTG antibody
tTG solution is added to the wells; because it’s a protein, it sticks to the plastic
Excess tTG is then washed off
Block the well using milk powder solution- this gets rid of the spare spaces on the plastic that biological molecules can bind to
After blocking, excess milk solution is washed away
Principles of the experiment
Patient samples are added to the wells
Samples are incubated, to allow time for tTG antibody in the samples to react with the tTG antibody
Excess antibody is washed off
If there was tTG antibody in the sample, it will now be captured by the immobilised tTG antigen
Secondary antibody is added
Secondary antibody is anti-IgA, which binds to IgA Fc regions
Covalently linked to an enzyme such as horse radish peroxidase
Excess secondary antibody is washed away
Substrate added, reacts with horse radish peroxidase
Produces colour change
Detecting antibodies bound to tissue: direct immunofluorescence
Take a biopsy of affected tissue e.g. skin; if damage mediated by antibody, antibody will already be stuck to ints antigen in the tissue
Add detection antibody labelled with fluorescent marker
Look for fluorescence under microscope
