Role Of Maternal Immunization Against Low Frequency Human Platelet Antigens (LFHPA) In Neonatal Alloimmune Thrombocytopenia (NAIT)

Neonatal alloimmune thrombocytopenia (NAIT) is largely caused by maternal immunization against Human Platelet Antigens (HPA) inherited by a fetus from its father. In about 70% of apparent NAIT cases, however, maternal HPA antibodies are not detected by standard serologic testing. At least 24 low frequency HPAs (LFHPA) have now been implicated in NAIT. Fathers are not ordinarily typed for these antigens and the corresponding antibodies can be difficult to detect. We studied paternal and maternal blood samples from suspected NAIT cases to investigate the extent to which maternal immunization against LFHPAs causes “antibody-negative” NAIT.

In Phase 1, we studied paternal DNA in 244 NAIT cases not resolved by standard laboratory testing in which there was compelling clinical and/or serologic evidence for NAIT. LFHPAs were identified by full length sequencing of relevant exons encoding platelet glycoproteins (GP). Sixteen LFHPAs (6.6% of cases) were identified. HPA-4b, 6bw, 11bw, 12bw, 13bw, 19bw, 20bw, 21bw, 22bw, and 23bw were found in single cases and HPA-9bw in 6 cases. In 15 of the 16 cases, a maternal antibody reactive with paternal platelets and/or a platelet GP was identified.

Cases studied in Phase 1 were highly selected. To obtain an unbiased estimate of the extent to which LFHPAs trigger NAIT, we determined the prevalence of 22 recognized LFHPAs in fathers of 1067 consecutive “antibody-negative” NAIT cases using high throughput screening methods (Phase 2). Mothers of 1338 cases were similarly typed to assess the normal prevalence of LFHPAs in a population racially/ethnically similar to the fathers. Of the 1067 fathers, 28 (2.6%) were positive for a LFHPA. LFHPAs identified were HPA-4b (1 case), 6bw (2), 8bw (1), 9bw (17), 12bw (4) and 21bw (3). Of the 1338 mothers, 27 (2.0%) were positive for a LFHPA. Maternal LFPAs identified were 4b (2 cases), 6bw (5), 9bw (12), 12bw (4), 13bw (3) and 21bw (1). Of the LFHPAs identified, only HPA-9bw was significantly more common in fathers than in mothers (P=0.02). Serologic evaluations done in the Phase 2 cases provided evidence for maternal antibodies against paternal GPIIb/IIIa in only 2 of 23 instances in which the paternal low frequency antigen was located on this GP complex.

Findings made in Phase 1, show that efforts to identify a paternal LFHPA in unresolved NAIT cases can be rewarding when maternal serum reacts with paternal platelets and the clinical picture strongly suggests NAIT. The Phase 2 studies of consecutive unresolved NAIT cases show that, with the exception of HPA-9b, the prevalence of LFHPAs is about the same in mothers and fathers of apparent NAIT cases not attributable to maternal immunization against “common” HPAs. It is clear that maternal immunization against an LFHPA can account for only a small fraction of the apparent “diagnostic gap” in serologic diagnosis of NAIT. HPA-9bw is by far the most important immunogen among the recognized LFHPAs and consideration should be given to typing fathers of suspected NAIT cases routinely for this antigen. Recent studies suggest that “low avidity” HPA antibodies not detected using conventional serology may account for an unknown fraction of the “antibody-negative” NAIT cases. Whether HLA antibodies cause NAIT more often than has been thought also deserves re-examination.