137 Human Leukocyte and Platelet Antigens Flashcards
The human leukocyte antigens (HLAs) are highly polymorphic glycoproteins encoded by a region of genes known as the major histocompatibility complex (MHC) located on the short arm of chromosome ________________
Chromosome 6p21
Their biologic function of human leukocyte antigens (HLAs)
Present donor-specific antigenic peptides to T lymphocytes and/or stimulate B cells to further differentiate to plasma cells and produce donor antigen–specific antibodies (DSA)
Class I HLAs
HLA-A, HLA-B, and HLA-C
Class II HLAs
HLA-DR, HLA-DQ, and HLA-DP
Type of HLA
Ubiquitous and present on most nucleated somatic cells, including those involved in the innate and adaptive immune response
Class I antigens
TRUE OR FALSE
Platelets express HLA-A antigens, but lack some HLA-B and most HLA-C antigens.
TRUE
Platelets express HLA-A antigens, but lack some HLA-B and most HLA-C antigens.
Type of HLA
More restricted distribution, with varying levels of expression on B cells, dendritic cells, monocytes, and macrophages
Present peptides from exogenous sources, such as bacterial pathogens, to CD4+ T cells.
Initial biomarker of inflammation and rejection
Class II antigens
Nonclassical class Ib antigens
HLA-E, HLA-F, and HLA-G
Type of HLA
Group of genes and their products may have a pivotal role in maternal acceptance of a fetus as well as in other pathways involved in immune tolerance
Class I HLA-G
TRUE OR FALSE
HLA antigens are codominantly inherited so that each individual expresses two antigens, one from each parent, at each locus (A, B, DR, etc)
TRUE
HLA antigens are codominantly inherited so that each individual expresses two antigens, one from each parent, at each locus (A, B, DR, etc)
Class I Antigens
Region of the alpha chain comprising the antigen recognition domain (ARD) , the areas of highest polymorphism
α1 and α2 regions
TRUE OR FALSE
The β chain of HLA-DR is constant for all HLA-DR molecules, whereas the α chain is polymorphic and determines specificity of the molecule.
FALSE
The α chain of HLA-DR is constant for all HLA-DR molecules, whereas the β chain is polymorphic and determines specificity of the molecule.
In contrast, HLA-DQ and DP molecules exhibit polymorphism in both α and β chains, with the β chain displaying the greatest variability
The identification of each HLA antigen of an individual is called
Phenotype
- Each individual has one allele for each locus per chromosome and therefore encodes two HLA antigens per locus.
- Because HLA genes are closely linked, recombination within the MHC is rare (≤1%), and a complete set of HLA genes usually is inherited from each parent as a unit.
The genes inherited from each parent are referred to as a
Haplotype
Family studies consist of typing for HLA:
HLA-A, HLA-B, HLA-C, HLA-DR, HLA-DQ, and HLA-DP
Added to the list of determinants for transplant compatibility
Used in the in the solid organ arena, where antibodies to these antigens have been implicated in rejection
DPA1* and DQA1* antigens
What is Linkage Disequilibrium
HLA alleles are inherited together on the same chromosome more often than would be predicted if HLA loci were in equilibrium.
Serologic Testing for HLA that has been the benchmark procedure in HLA testing in the field of transplant immunology for more than 50 years.
Complement-dependent microlymphocytotoxicity (CDC) assay
- It can be used for both HLA typing and the detection of HLA-specific antibodies.
- The third use of the CDC assay is in performance of a crossmatch between a potential donor’s cells and patient serum.
CDC Assay
The number of wells that show cell death in the panel divided by the total number of microtiter wells gives one a relative indication of the serum’s antibody “strength” is called
Panel reactive antibody (PRA)
Low or High-resolution Typing
Used in HSCT
High-resolution Typing
- Typing for solid organ transplantation usually requires a serologic or antigen-level identification commonly referred to as “low-resolution typing.”
The most common form of Molecular Human Leukocyte Antigen Typing used in HLA typing is
Dye-terminator sequencing
Advantages of NGS compared with other HLA sequencing technologies
- The ability to batch samples from several patients into a single sequencing reaction
- Ability to obtain sequencing results from a single DNA molecule, with near complete coverage of all exons and introns comprising the 11 HLA genes.
- NGS eliminates phasing ambiguities that occur in SBT.
The gold standard for evaluating recipients and donors for allograft transplantation.
Flow cytometric crossmatch
TRUE OR FALSE
HLA matching is currently not performed prospectively for renal, hepatic, lung, or cardiac transplantation.
TRUE
HLA matching is currently not performed prospectively for renal, hepatic, lung, or cardiac transplantation.
Detection of alloantibody by screening techniques and the donor-specific crossmatch is of prime importance for _________________transplants, where, if undetected, their existence could cause a hyperacute rejection and graft failure.
Kidney, lung, and heart transplants
HLA typing for matching of kidneys and pancreases is performed at the HLA-A, HLA-B, and HLA-DR loci at low resolution (serologic or antigen level by DNA).
To determine the best match and diminish the possibility of GVHD, HLA typing for HSCT are generally performed by molecular methods at high resolution (allele-level) typing for __________________-
HLA-A, HLA-B, HLA-C, HLA-DR, HLA-DQ, and HLA-DP
Patients requiring platelet transfusions may be broadly sensitized to _________ (ie, have high PRAs) through prior transfusions (particularly nonleukoreduced cellular products) or pregnancies.
HLA-A, and B
Determining the HLA type and/or HLA antibody profiles of these patients allows for the selection of more appropriate platelet donors (either HLA-matched, or donors selected to avoid a patient’s HLA antibody specificity), enabling these refractory patients to achieve improved platelet increments.
HLA disease associations
- _______: ankylosing spondylitis
- _______: narcolepsy
- _______: celiac disease
- HLA-B27: ankylosing spondylitis
- HLA-DQ2: narcolepsy
- HLA-DQ2 and DQ8: celiac disease
HLA typing is also required before the administration of certain drugs to prevent drug hypersensitivity reactions
HLA-B*5701 : _______
HLA-B*5701 : abacavir
Clinically significant alloantigens expressed only or predominantly by neutrophils
Human neutrophil antigens (HNAs)
- The neutrophil-specific HNA-1 antigen system consists of four epitopes—HNA-1a, -1b, -1c, and -1d—encoded by five alleles
HNA-1 antigens are located on the low-affinity _________________, and are expressed only on neutrophils.
Fcγ receptor IIIb (FcγRIIIb), CD16
FcγRIIIb and HNA-1 antigens are expressed on all segmented neutrophils, on approximately one-half of neutrophilic ____________, and on approximately 10% of neutrophilic _________________.
- All segmented neutrophils
- One-half of neutrophilic metamyelocytes
- 10% of neutrophilic myelocytes
HNA-1 Antigens
Among Caucasians, _______ is the most common antigen but in Asian populations, ________ is most common.
Caucasians: HNA-1b
Asian: HNA-1a
TRUE OR FALSE
Function of HNA-1 Antigens
Neutrophils that are homozygous for HNA-1a have a lower affinity for immunoglobulin (Ig) G3 than those homozygous for HNA-1b.
FALSE
Neutrophils that are homozygous for HNA-1b have a lower affinity for immunoglobulin (Ig) G3 than those homozygous for HNA-1a.
Neutrophils from those who are homozygous for HNA-1b phagocytize erythrocytes sensitized with IgG1 and IgG3 anti-Rh monoclonal antibodies and bacteria opsonized with IgG1 at a lower level than neutrophils homozygous for HNA-1a
Only HNA expressed on subpopulations, but not all neutrophils
HNA-2
HNA-2 is expressed only on neutrophils, neutrophilic metamyelocytes, and neutrophilic myelocytes.
The encoding gene of the glycoprotein carrying HNA-2, NB1 glycoprotein, is located on neutrophil plasma membranes and secondary granules and is GPI-anchored
CD177
Located on chromosome 19q13.31
TRUE OR FALSE
The mean fraction of HNA-2–positive neutrophils is 45% to 65% and is gender dependent, higher in women than men.
TRUE
The mean fraction of HNA-2–positive neutrophils is 45% to 65% and is gender dependent, higher in women than men.
Neutrophil expression of HNA-2 increases in infection and in Molecular Human Leukocyte Antigen Typing.
Function of CD177
Binds to platelet endothelial cell-adhesion molecule-1 (PECAM-1, CD31), functioning as a cell-adhesion molecule and mediating neutrophil transendothelial migration.
Antibodies against cochlear _____have been associated with autoimmune hearing loss.
HNA-3
The HNA-3 antigen system is expressed by neutrophils, lymphocytes, platelets, pulmonary endothelial cells, several solid organs, and cells in the inner ear.
Neutrophil antigens are located on β2 integrins
HNA-4 and HNA-5 antigens
First described as Marta, is expressed on the αM chain (CD11b) of αM/β2 and is present on neutrophils, monocytes, and natural killer cells
With leukocyte activation, the complex mediates vascular adhesion and migration, oxidative burst activity, and phagocytosis
HNA-4
Was first described as Onda.
Expressed on the αL integrin subunit (CD11a) encoded by the ITGAL gene at chromosome 6p11.2.78
HNA-5a
Clinical syndromes associated with antibodies against neutrophil antigens
- ANN
- Autoimmune neutropenia (AIN)
- Immune neutropenia after HSCT
- Transfusion-related acute lung injury (TRALI)
- Transfusion-related alloimmune neutropenia
- Refractoriness to granulocyte transfusion
- Febrile transfusion reactions
TRUE OR FALSE
In Alloimmune Neonatal Neutropenia, isolated neutropenia can be complicated by infections, but the cytopenia resolves as the antibody is cleared over 1 to 6 months.
TRUE
In Alloimmune Neonatal Neutropenia, isolated neutropenia can be complicated by infections, but the cytopenia resolves as the antibody is cleared over 1 to 6 months.
- Most often the neutropenia is detected in the first week of life when the neonate becomes febrile or develops an infection and a neutrophil count is performed.
- Typically, the counts are 0.1 to 0.2 × 109/L.
- The mean duration of neutropenia has been reported to be approximately 11 weeks
In Alloimmune Neonatal Neutropenia, granulocyte colony-stimulating factor (G-CSF) has been successfully used in more severe cases, but resistance has been reported with HNA-_______ antibodies
HNA-2 antibodies
Possibly resulting from G-CSF–induced increase in HNA-2 expression
Most commonly observed antibodies in Alloimune Neonatal Neutropenia
Antibodies to HNA-1a, HNA- 1b, FcγRIIIb, and HNA-2
But antibodies to all HNA epitopes have been reported as causative.
HNA-related Autoimmune Neutropenia is primarily a disease of children.
Typically, the onset of AIN in children begins at ___________ months of age
5 to 15 months of age
Most studies have found that neutrophil counts recover spontaneously in most patients by the age of 5 years, with a median duration of neutropenia of 13 to 20 months.
The antibodies are predominantly specific to epitopes located on FcγRIIIb
Treatment for AIN the most reliable and sustained effect on neutrophil counts
G-CSF
Others: glucocorticoids, IV immunoglobulin
Febrile transfusion reactions can be prevented in recipients of platelet and red blood cell component transfusions by
Leukocyte-reduced blood components
Can result from the transfusion of neutrophil-activating antibodies or nonantibody biological response modifiers from cellular products in the plasma portion of the blood component
Diagnosed within 6 hours of a transfusion when hypoxia (oxygen saturation <90% or a partial pressure of arterial oxygen-to-fraction of inspired oxygen ratio <300) and noncardiogenic pulmonary edema occur
TRALI
Symptoms and signs should be present without risk factors for acute respiratory distress syndrome or mild acute respiratory distress syndrome, or in their presence when pulmonary status has been stable in the 12 hours before transfusion.
Mechanism of TRALI
HNA and some HLA class I antibodies:
HLA class II antibodies:
HNA and some HLA class I antibodies: directly activating neutrophils
HLA class II antibodies: indirectly activate neutrophils via activated monocytes
Case series have associated TRALI with the transfusion of (in decreasing frequency) monocyte-activating HLA class II antibodies, anti-HNA, and HLA class I antibodies with granulocyte agglutination activity.
Among blood donors, ___________ have the highest rate of HLA/HNA antibodies
Previously pregnant women
Compared with nonpregnant females and males
Clinical syndromes associated with platelet alloantigens
- Fetal/neonatal alloimmune thrombocytopenia (FNAIT)
- Posttransfusion purpura (PTP)
- Passive alloimmune thrombocytopenia
- Immune thrombocytopenia after HSCT
- Occasionally in unresponsiveness to platelet transfusion
Can be an unusual complication of solid organ transplantation in which donor lymphocytes make alloantibodies specific for the platelets produced by the recipient of an organ allograft.
A condition similar to alloimmune platelet destruction occurs in patients who lack part or all of a particular platelet GP because of defective alleles of the GP-encoding genes.
Platelet Isoantigens
Such patients can make isoantibodies against platelets of virtually all donors that bear the platelet GP.
example is CD36 deficiency
___________: Lack CD36 only on their platelets
____________: Lack CD36 only on their platelets, monocytes, and other cells
Platelet GPIV (CD36)
Expressed on various human cells including platelets, macrophages, capillary endothelium, myocytes, and adipocytes
Type II deficiency: Lack CD36 only on their platelets
Type I deficiency: Lack CD36 only on their platelets, monocytes, and other cells
TRUE OR FALSE
Platelet CD36 deficiency may confer protection from malaria as a receptor for red cells infected with Plasmodium falciparum.
TRUE
Platelet CD36 deficiency may confer protection from malaria as a receptor for red cells infected with Plasmodium falciparum.
However, monocyte CD36 deficiency may actually increase the risk for more severe forms of malarial infection because of the lack of clearance of infected cells.
TESTING FOR PLATELET-SPECIFIC ANTIGENS AND ANTIBODIES
The only major phase 1 assay remaining in wide use today for platelet antibody detection
Serotonin release assay
TESTING FOR PLATELET-SPECIFIC ANTIGENS AND ANTIBODIES
Example of phase 2 tests/assays
Detect either surface or total platelet–associated immunoglobulin on patient platelets or on normal platelets after sensitization with patient serum
Solid-phase red cell adherence test
Used in platelet crossmatching
TESTING FOR PLATELET-SPECIFIC ANTIGENS AND ANTIBODIES
Example of phase 3 tests/assays
Monoclonal antibody immobilization of platelet antigens assay and the modified antigen capture ELISA
- Used to detect alloantibodies in the evaluation of suspected FNAIT and PTP cases as well as autoantibodies in some cases of idiopathic thrombocytopenic purpura
- Have an advantage over both phase 1 and phase 2 tests in that they detect antibodies that bind to platelet GPs, and not to non–platelet-specific epitopes, such as class I HLA
The principal cause of immunologic platelet transfusion refractoriness
Antibodies to class I HLA antigens
