Lecture 9 - Human Leucocyte Antigen (HLA) Flashcards
What does MHC and HLA stand for?
- MHC = Major Histocompatibility Complex
* HLA = Human Leucocyte Antigen
What are the “minor histocompatibility antigens” and what are their significance?
- HA-1, HA-2 and male associated H-Y antigens
- Other surface proteins can act as antigens in transplantation
- Genes for these are scattered throughout genome
- Non HLA antigens associated with graft rejection
- Complete matching for HLA often lead to graft rejection (non HLA mismatch), requirement for immunosuppressive therapy
Describe the structures of MHC class I and its functions.
- Single gene coding for transmembrane glycoprotein (α or heavy chain) forming three globular domains (α1, α2, and α3)
- Linked to β2-microglobulin (not coded in MHC)
- Sequence differences between alleles are limited to α1 and α2 (regions most distant from cell surface)
- Variation in α1 and α2 domains determine peptide binding and antigenicity (types)
- Class 1 molecules present antigen to cytotoxic T cells (CTL)
- HLA-A, HLA-B and HLA-C
What is “Linkage disequilibrium”?
• Mendelian genetics suggests that the frequency of one locus is not influenced by another
– So what is inherited at HLA-A should be independent of what is inherited at HLA-DR
– But this is not true for HLA, some HLA antigens are found in higher frequencies with other HLA antigens (more than could be expected randomly)
– e.g. in Caucasians where the HLA -
A1, B8, DR3, DQ2 haplotype (8.1 haplotype) is so conserved that even the alleles at the complement genes (Class III) can be predicted with great accuracy
What are the four types of transplantations?
- Autograft
- Isograft
- Allograft
- Xenograft
What is the cause of “hyper acute” allograft rejection?
– Few minutes / hours of transplantation
– Pre-existing humoral antibodies
– Blood group incompatibility (ABO)
– Pre-sensitization to class I MHC (blood transfusion, previous transplant, pregnancy)
– Complement activation, thrombosis, swelling, interstitial haemorrhage, fibrinoid necrosis
– Cell mediated immunity not involved
Describe the complement dependent cytotoxicity (CDC) assay
- Serum containing known specific anti-HLA antibodies in tray wells
- Lymphocytes added to serum (react with specific antibody if express specific HLA type)
- Rabbit complement added (Lymphocytes with Ag-Ab complex killed, they enlarge)
- Detect killing by addition of eosin dye, ethidium bromide (Killed cells larger and stained, live cells are smaller and retractile)
What are the three methods for HLA typing?
– Sequence Specific Primers (SSP)
– Sequence Specific Oligonucleotides (SSO)
– DNA sequencing
Why is testing for HLA antibodies important in terms of transplantation
• For allo-transplantation, antibodies reactive with donor organ may result in hyperacute rejection
What is a PRA?
- Percent/Panel Reactive Antibody
- A measure of a patient’s level of sensitization to donor antigens.
• It is the percentage of cells from a panel of
blood donors against which a potential recipient’s serum reacts.
• The PRA reflects the percentage of the general population
that a potential recipient makes antibodies against.
- high PRA = more sensitized = more difficult to find suitable donor.
- A patient may become sensitized as a result of pregnancy, a blood transfusion, or a previous transplant.
Describe how a cross-match is done by flow-cytometry
- Fast and sensitive test
- T & B cells assayed in single tube
- Recipient serum
- Add donor cells
- If recipient has antibodies to HLA type on cells – binding
- Add anti- human IgG labelled antibodies
- Analyse by Flow-cytometry
How is a “virtual” cross-match done?
- Identify anti-HLA antibodies in recipient (ELISA or Luminex bead array)
- Determine HLA type of donor cells (DNA)
- Compare: if recipient has antibodies to donor HLA type = positive
Give three examples of immunosuppressive agents
- Inhibitors of Lymphocyte gene expression (e.g. corticosteroids)
- Inhibitors of cytokine signal transduction (e.g. Anti-CD25)
- Inhibitors of nucleotide synthesis (e.g. Azathioprine)
What is GVHD and how does it happen?
- Graft vs Host Disease
- Patient is Immune-compromised due to disease and pre-transplant treatment
- Immune-competent T cells transplanted from donor
- Host cannot reject the graft due to decreased immunity
- Graft T cells perceive recipients tissue as foreign (different HLA type)
- Activation of CD4 and CD8 T cells
- Inflammation and destruction of host cells death
HLA-B27 is associated with what disease?
- Ankylosing spondylitis (95% of AS patients have HLA-B27)
- Only 8-10% of people with HLA-B27 will develop the disease
- HLA – B27 has slower loss of CD4 and
slower progression of disease
