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Donald M. Arnold MDCM, FRCPCAssist professor, McMaster UniversityMcMaster Platelet immunology LaboratoryCBS Hamilton

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“Difficulties with Platelet Transfusion, HLA Serology & Management of NAIT & PTT”

NomenclatureLimited evidenceVariability in practiceSerious illnessesCostly, potentially dangerous

treatments 2

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Learning Objectives

1. Understand the mechanism of platelet antigen sensitization

2. Explore pathophysiology of: platelet refractoriness fetal or neonatal alloimmune thrombocytopenia post-transfusion purpura

3. Discuss management of PLT refractoriness, FNAT and PTP

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Blood group antigens (ABO) Common tissue antigens

(HLA)Platelet specific antigens

(HPA)

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Chosen name, or name related to individual with antigen (PlA1, Zav, Gov).

Currently all antigens are designated as human platelet antigen (HPA).

Antigens numbered in order of discovery. Higher frequency allele is “a”.e.g. PLA1 = HPA-1a, PLA2 = HPA-1b

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Nomenclature

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To date, more than 22 platelet specific alloantigens identified, with 6 diallelic system (HPA-1, 2, 3, 4, 5, 15). Almost all are associated with a single amino acid substitution (SNP).

Antibodies against HPA-1a are most commonly implicated in NAT, PTP.

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Platelet Transfusion Refractoriness

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Definition 1h corrected count incr < 5 – 10 x109/L Percent PLT recovery <20% 1h CCI < 5 x109/L x2 using ABO-identical

fresh platelets A post-transfusion platelet count that is

less than expected

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Immune Anti-HLA, anti-ABO, anti-HPA

Non-immune Fever Sepsis Splenomegaly DIC Bleeding VOD GVHD

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Alloimmunization occurs after transfusion, pregnancy, transplantation.

Alloimmunization does not mean refractory

Strategies to reduce refractoriness:1. Leukoreduction2. Platelet dose?3. Apheresis vs. whole blood derived platelets

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Decline in PLT responsiveness

A: In all patients (n=6334 transfusions; 533 patients)

B: In HLA-antibody-negative patients (n= 5484 transfusions; 477 patients)

Slichter Blood 2005

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Heddle, Transfusion 2008

Apheresis vs. Whole blood PLTs

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2020Hod BMJ 2008

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Low platelets and bleeding in a fetus or neonate caused by maternal antibodies directed against fetal platelet antigens inherited from the father.

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Definition:

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Incidence: 1 in 1,000 to 1 in 2,000 births

N IncidenceBurrows and Kelton, 1993 15,932 1 in 1,700

Uhrynowska, 2000 24,101 1 in 2,400

Turner, 2005 26,000 1 in 5,000

Kjeldsen-Kragh, 2007 100,448 1 in 1,700

FNAT

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Most common cause of severe TCP in infant Most common cause of ICH in term newborns First pregnancies, without warning Otherwise healthy babies

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Fetal platelet antigen: Inherited from father

Sensitization: Transplacental Immunization: Mother forms IgG allo-

antibodies Maternal antibodies: React with fetal

platelets Fetal platelet destruction: Spleen and RES

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2828Arnold et al, Trans Med Rev 2008

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<150,000 - 1 in 100< 50,000 - 1 in 400

Infection Immune mediated Chromosomal abnormalities

<20,000 - NAT

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Burrows, Kelton 1993

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NAT HDN

Affected cells Platelets Red blood cells

Most common antigen HPA-1a Rh-DAffected pregnancy First Second +

Timing of sensitization 16 weeks onwards At birth, or during a procedure

Affected Infant TCP, bleeding Hemolysis, jaundice, kernicterus

Affected fetus ICH Hydrops fetalis

Main risk factor Previously affected infant Rh-negative mothers

Treatment Supportive Supportive

Prevention IVIG RhIg (anti-D)

Efficacy of prevention ? 99%

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Present without warning Require prompt recognition and treatment IVIg (2g/kg)

Effective in 75% of affected neonatesMueller-Eckhardt C, Blut 1989

Platelet transfusions: Antigen-negative (maternal) platelets

Allen, Blood 2007 Random-donor platelets

Kiefel et al Blood 2006

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IVIg 1g/kg/wk (2g/kg/wk for refractory) IVIg + corticosteroids Intrauterine platelet transfusions Fetal blood sampling (FBS)

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High Risk (n= 40) Standard Risk (n= 39) (previous ICH or PLT<20) (neither)

IVIg vs. IVIg + pred IVIg vs. pred

(1g/kg/wk) (1mg/kg) (1g/kg/wk) (0.5mg/kg)

PUBS (20 wks, repeat 3-8 wks)

Outcome: increase in fetal platelet count

Berkowitz, Bussel, 2006

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HIGH RISK Mothers(platelet count)

Pre 2-8 wks BirthIVIg alone* 7,000 17,000

67,000IVIg + pred 8,000 67,000

99,000

* One ICH 34

Berkowitz, Bussel, 2006

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STANDARD RISK Mothers*(platelet count)

Pre 3-8 wksIVIg alone >20,000 31,000

Pred alone >20,000 26,000

* 2 fetal deaths, 2 ICH

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Berkowitz, Bussel, 2006

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11 SERIOUS COMPLICATIONS OF 175 PUBS (6%) - 1 Fetal Death - 9 Emergency C-Sections (14%)

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IVIG Cochrane collaboration 2006Corticosteroids Berkowitz, Bussel, 2006 Invasive vs. non-invasive approach?

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• Fetal blood sampling• FBS + intrauterine PLT

transfusions• Cesarean section

• No FBS• Empiric treatment• Vaginal delivery

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N= 100,448 pregnancies screened (for HPA-1a)

Offered early c/s with compatible platelets.

3 of 161 (6%) screen-positive infants died or had ICH; versus 10 of 51 (20%) unscreened infants.

Kjeldsen-Kragh J, Blood, 2007

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Frequency: uncertain 1 per 2 million RBCs (2008, SHOT)

Adults (~50 years); females (5x) PLT <15 x109/L, Bleeding ++ Onset: 9 days post blood transfusion Recovery: 7 – 48 days after onset Mortality ~10% (intracerebral hemorrhage)

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Sensitization Sensitization by previous transfusions,

pregnancy Usually HPA-1bb recipients Other at risk HPA genotypes

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Transfusion of incompatible PLTs Typically, anti-HPA-1a Abs Destroy transfused and autologous

PLTs Theories:

1. Immune complexes2. Adsorbed PLT antigens3. Concomitant auto-Abs

Further research needed

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Supportive: Ag-negative platelet transfusions

IVIGPlasma exchangeCorticosteroids

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Summary

Platelet sensitization

HLA antibodies are ubiquitous, transient Example: PLT refractoriness

HPA antibodies are rare, persistent Examples: FNAT, PTP

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Summary

PLT refractoriness: Consider HLA matched, XM compatible PLTs

FNAT: Newborn PLT <20; FNAT until proven

otherwise

PTP: Severe thrombocytopenia (PLT <15) and

bleeding