Prevalence and Clinical Significance of Low Avidity HPA-1a Antibodies in Women Exposed to HPA-1a During Pregnancy

Abstract 266

Neonatal alloimmune thrombocytopenia (NAIT) is caused by maternal antibodies specific for fetal platelet antigens and is the most common cause of intracranial hemorrhage in full term infants. The antigen HPA-1a, carried on beta 3 integrin (GPIIIa), is the most common trigger for NAIT and the 2% of women who are HPA-1a-negative are at risk to produce such antibodies. Most HPA-1a antibodies causing NAIT can be easily detected but it is not rare for an HPA-1a negative mother who lacks detectable antibodies to give birth to an infant with thrombocytopenia. Several recent reports suggest that low avidity HPA-1a antibodies not detected by conventional serologic methods are responsible for some of these cases (Socher 2009, Bakchoul 2011).

To examine this question, we retroactively analyzed a cohort of 3478 suspected NAIT cases referred for laboratory diagnosis. Among 677 HPA-1a-negative mothers, we identified 61 in whom HPA-1a-specific antibodies were not detected by conventional methods. Surface plasmon resonance (SPR) analysis enables ligand-receptor interaction to be studied in real time without washing the target. Using this approach, we studied reactions of IgG from the 61 archived serum samples against purified GPIIb/IIIa isolated from group O platelets and identified 18 samples that reacted preferentially with the HPA-1a-positive version of the integrin in comparison with HPA-1a-negative integrin. Information defining clinical status was obtained on 13 of these cases by follow-up communication. Seven cases had been referred because of neonatal thrombocytopenia. Platelet nadirs ranged from 8,000/ul to 141,000/ul (median 38,000/ul). Five of the 7 infants had bleeding and were given maternal platelet transfusions and/or IVIgG. Normal platelet counts were achieved after 4 to 70 days (median 7 days). One infant had a normal platelet count, however IVIgG had been given to its mother throughout pregnancy. The remaining 5 cases were referred in mid-pregnancy because an HPA-1a-negative sister previously gave birth to an infant with NAIT. Four of these infants had normal platelets at birth; one had mild TP (platelets 125,000/ul). Only 3 of 12 mothers typed for HLA were positive for HLA-DRB3*0101, a marker found in >95% of women who make “conventional” HPA-1a antibodies during pregnancy (p < 0.001). The ability of human antibodies to cause destruction of human platelets in vivo can be studied in the NOD/SCID mouse, which lacks xenoantibodies normally found in other species (Newman 2007). Serum from 4 mothers was available in quantities sufficient for mouse studies; three of the four maternal sera caused accelerated destruction of HPA-1a-positive, but not HPA-1a-negative platelets.

These findings indicate that low-avidity HPA-1a antibodies not detectable by conventional serologic methods are made by a subset of women exposed to the HPA-1a antigen during pregnancy and that some, but not all of these antibodies are capable of causing NAIT, which is usually mild, but can be severe. Women negative for HLA-DRB3*0101 may be especially prone to produce antibodies of this type. Maternal-fetal incompatibility for platelet antigens other than HPA-1a is very common and many apparent NAIT cases not involving HPA-1a go unresolved. The possibility that low avidity antibodies specific for antigens other than HPA-1a are responsible for some of these cases deserves study.