137 Human Leukocyte and Platelet Antigens

Question 1

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 ________________

Answer 1

Chromosome 6p21

Question 2

Their biologic function of human leukocyte antigens (HLAs)

Answer 2

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)

Question 3

Class I HLAs

Answer 3

HLA-A, HLA-B, and HLA-C

Question 4

Class II HLAs

Answer 4

HLA-DR, HLA-DQ, and HLA-DP

Question 5

Type of HLA

Ubiquitous and present on most nucleated somatic cells, including those involved in the innate and adaptive immune response

Answer 5

Class I antigens

Question 6

TRUE OR FALSE

Platelets express HLA-A antigens, but lack some HLA-B and most HLA-C antigens.

Answer 6

TRUE

Platelets express HLA-A antigens, but lack some HLA-B and most HLA-C antigens.

Question 7

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

Answer 7

Class II antigens

Question 8

Nonclassical class Ib antigens

Answer 8

HLA-E, HLA-F, and HLA-G

Question 9

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

Answer 9

Class I HLA-G

Question 10

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)

Answer 10

TRUE

HLA antigens are codominantly inherited so that each individual expresses two antigens, one from each parent, at each locus (A, B, DR, etc)

Question 11

Class I Antigens

Region of the alpha chain comprising the antigen recognition domain (ARD) , the areas of highest polymorphism

Answer 11

α1 and α2 regions

Question 12

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.

Answer 12

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

Question 13

The identification of each HLA antigen of an individual is called

Answer 13

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.

Question 14

The genes inherited from each parent are referred to as a

Answer 14

Haplotype

Question 15

Family studies consist of typing for HLA:

Answer 15

HLA-A, HLA-B, HLA-C, HLA-DR, HLA-DQ, and HLA-DP

Question 16

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

Answer 16

DPA1* and DQA1* antigens

Question 17

What is Linkage Disequilibrium

Answer 17

HLA alleles are inherited together on the same chromosome more often than would be predicted if HLA loci were in equilibrium.

Question 18

Serologic Testing for HLA that has been the benchmark procedure in HLA testing in the field of transplant immunology for more than 50 years.

Answer 18

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.

Question 19

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

Answer 19

Panel reactive antibody (PRA)

Question 20

Low or High-resolution Typing

Used in HSCT

Answer 20

High-resolution Typing

• Typing for solid organ transplantation usually requires a serologic or antigen-level identification commonly referred to as “low-resolution typing.”

Question 21

The most common form of Molecular Human Leukocyte Antigen Typing used in HLA typing is

Answer 21

Dye-terminator sequencing

Question 22

Advantages of NGS compared with other HLA sequencing technologies

Answer 22

• 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.

Question 23

The gold standard for evaluating recipients and donors for allograft transplantation.

Answer 23

Flow cytometric crossmatch

Question 24

TRUE OR FALSE

HLA matching is currently not performed prospectively for renal, hepatic, lung, or cardiac transplantation.

Answer 24

TRUE

HLA matching is currently not performed prospectively for renal, hepatic, lung, or cardiac transplantation.

Question 25

**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.

Answer 25

Kidney, lung, and heart transplants ## Footnote 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).

Question 26

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 __________________-

Answer 26

HLA-A, HLA-B, HLA-C, HLA-DR, HLA-DQ, and HLA-DP

Question 27

Patients requiring platelet transfusions may be broadly sensitized to _________ (ie, have high PRAs) through prior transfusions (particularly nonleukoreduced cellular products) or pregnancies.

Answer 27

HLA-A, and B ## Footnote 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.

Question 28

# HLA disease associations * _______: ankylosing spondylitis * _______: narcolepsy * _______: celiac disease

Answer 28

* HLA-B27: ankylosing spondylitis * HLA-DQ2: narcolepsy * HLA-DQ2 and DQ8: celiac disease

Question 29

HLA typing is also required before the administration of certain drugs to prevent drug hypersensitivity reactions HLA-B*5701 : _______

Answer 29

HLA-B*5701 : abacavir

Question 30

Clinically significant alloantigens expressed only or predominantly by neutrophils

Answer 30

Human neutrophil antigens (HNAs) ## Footnote * The neutrophil-specific HNA-1 antigen system consists of four epitopes—**HNA-1a, -1b, -1c, and -1d**—encoded by five alleles

Question 31

HNA-1 antigens are located on the low-affinity _________________, and are expressed only on neutrophils.

Answer 31

Fcγ receptor IIIb (FcγRIIIb), CD16

Question 32

FcγRIIIb and HNA-1 antigens are expressed on all segmented neutrophils, on approximately one-half of neutrophilic ____________, and on approximately 10% of neutrophilic _________________.

Answer 32

* All segmented neutrophils * One-half of neutrophilic metamyelocytes * 10% of neutrophilic myelocytes

Question 33

# HNA-1 Antigens Among Caucasians, _______ is the most common antigen but in Asian populations, ________ is most common.

Answer 33

Caucasians: HNA-1b Asian: HNA-1a

Question 34

# 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.

Answer 34

FALSE Neutrophils that are **homozygous for HNA-1b have a lower affinity for immunoglobulin (Ig) G3** than those homozygous for HNA-1a. ## Footnote 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

Question 35

Only HNA expressed on subpopulations, but not all neutrophils

Answer 35

HNA-2 ## Footnote HNA-2 is expressed only on neutrophils, neutrophilic metamyelocytes, and neutrophilic myelocytes.

Question 36

The encoding gene of the **glycoprotein carrying HNA-2, NB1 glycoprotein**, is located on neutrophil plasma membranes and secondary granules and is GPI-anchored

Answer 36

CD177 ## Footnote Located on chromosome 19q13.31

Question 37

# TRUE OR FALSE The mean fraction of HNA-2–positive neutrophils is 45% to 65% and is gender dependent, higher in women than men.

Answer 37

TRUE The mean fraction of HNA-2–positive neutrophils is 45% to 65% and is gender dependent, higher in **women** than men. ## Footnote Neutrophil expression of HNA-2 increases in **infection** and in Molecular Human Leukocyte Antigen Typing.

Question 38

Function of CD177

Answer 38

Binds to platelet endothelial cell-adhesion molecule-1 (PECAM-1, CD31), functioning as a cell-adhesion molecule and mediating neutrophil transendothelial migration.

Question 39

Antibodies against cochlear _____have been associated with **autoimmune hearing loss**.

Answer 39

HNA-3 ## Footnote The HNA-3 antigen system is expressed by **neutrophils, lymphocytes, platelets, pulmonary endothelial cells, several solid organs, and cells in the inner ear.**

Question 40

Neutrophil antigens are located on **β2 integrins**

Answer 40

HNA-4 and HNA-5 antigens

Question 41

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

Answer 41

HNA-4

Question 42

Was first described as Onda. Expressed on the αL integrin subunit (CD11a) encoded by the ITGAL gene at chromosome 6p11.2.78

Answer 42

HNA-5a

Question 43

Clinical syndromes associated with antibodies against neutrophil antigens

Answer 43

* 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

Question 44

# 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.

Answer 44

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.** ## Footnote * 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**

Question 45

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

Answer 45

HNA-2 antibodies ## Footnote Possibly resulting from G-CSF–induced increase in HNA-2 expression

Question 46

Most commonly observed antibodies in Alloimune Neonatal Neutropenia

Answer 46

Antibodies to HNA-1a, HNA- 1b, FcγRIIIb, and HNA-2 ## Footnote But antibodies to all HNA epitopes have been reported as causative.

Question 47

HNA-related Autoimmune Neutropenia is primarily a disease of children. Typically, the onset of AIN in children begins at ___________ months of age

Answer 47

5 to 15 months of age ## Footnote 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**

Question 48

Treatment for AIN the most reliable and sustained effect on neutrophil counts

Answer 48

G-CSF ## Footnote Others: glucocorticoids, IV immunoglobulin

Question 49

**Febrile transfusion reactions** can be prevented in recipients of platelet and red blood cell component transfusions by

Answer 49

Leukocyte-reduced blood components

Question 50

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

Answer 50

TRALI ## Footnote 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.

Question 51

Mechanism of TRALI HNA and some HLA class I antibodies: HLA class II antibodies:

Answer 51

**HNA and some HLA class I antibodies**: directly activating neutrophils **HLA class II antibodies:** indirectly activate neutrophils via activated monocytes ## Footnote 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**.

Question 52

Among blood donors, ___________ have the highest rate of HLA/HNA antibodies

Answer 52

Previously pregnant women ## Footnote Compared with nonpregnant females and males

Question 53

Clinical syndromes associated with **platelet alloantigens**

Answer 53

* Fetal/neonatal alloimmune thrombocytopenia (FNAIT) * Posttransfusion purpura (PTP) * Passive alloimmune thrombocytopenia * Immune thrombocytopenia after HSCT * Occasionally in unresponsiveness to platelet transfusion ## Footnote 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.

Question 54

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.

Answer 54

Platelet Isoantigens ## Footnote Such patients can make isoantibodies against platelets of virtually all donors that bear the platelet GP. example is **CD36 deficiency**

Question 55

___________: Lack CD36 only on their platelets ____________: Lack CD36 only on their platelets, monocytes, and other cells ## Footnote Platelet GPIV (CD36) Expressed on various human cells including platelets, macrophages, capillary endothelium, myocytes, and adipocytes

Answer 55

**Type II deficiency:** Lack CD36 only on their platelets **Type I deficiency:** Lack CD36 only on their platelets, monocytes, and other cells

Question 56

# TRUE OR FALSE Platelet CD36 deficiency may confer protection from malaria as a receptor for red cells infected with Plasmodium falciparum.

Answer 56

TRUE Platelet CD36 deficiency may confer protection from malaria as a receptor for red cells infected with Plasmodium falciparum. ## Footnote 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.

Question 57

# TESTING FOR PLATELET-SPECIFIC ANTIGENS AND ANTIBODIES The only **major phase 1 assay** remaining in wide use today for platelet antibody detection

Answer 57

Serotonin release assay

Question 58

# TESTING FOR PLATELET-SPECIFIC ANTIGENS AND ANTIBODIES Example of phase 2 tests/assays ## Footnote Detect either surface or total platelet–associated immunoglobulin on patient platelets or on normal platelets after sensitization with patient serum

Answer 58

Solid-phase red cell adherence test ## Footnote Used in platelet crossmatching

Question 59

# TESTING FOR PLATELET-SPECIFIC ANTIGENS AND ANTIBODIES Example of phase 3 tests/assays

Answer 59

Monoclonal antibody immobilization of platelet antigens assay and the modified antigen capture ELISA ## Footnote * 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

Question 60

The principal cause of immunologic platelet transfusion refractoriness

Answer 60

Antibodies to **class I HLA antigens**