Human Platelet Antigen 3 (HPA-3): Localization of the Determinant of the Alloantibody Leka (HPA-3a) to the C-Terminus of Platelet Glycoprotein IIb Heavy Chain and Contribution of O-Linked Carbohydrates

Abstract

SummaryThe human platelet antigen (HPA) 3 system is expressed on GPIIb, one subunit of GPIIb-IIIa, the platelet fibrinogen receptor. It was recently shown that HPA-3 was associated with an Ile843/Ser polymorphism. To investigate further HPA-3 determinant structure, we localized an HPA-3a determinant, recognized by the alloantiserum Leka, within the last 29 amino acids of GPIIbα. This region encompasses the polymorphic Ile843, which, as expected, is substituted into Ser in Leka-negative individuals, as shown by DNA sequence after polymerase chain reaction on platelet RNA. In addition, contribution of glycosylation to the determinant structure was demonstrated since the Leka antigenicity was strongly decreased after specifically removing nonterminal O-linked sugars, but not terminal sialic acids. We have thus refined the localization of an HPA-3a determinant within the last 29 amino acids, including Ile843, of GPIIb heavy chain, and shown that the Leka HPA-3a determinant is dependent, in part, upon the serine-linked carbohydrates adjacent to Ile/Ser843.

Severe neonatal alloimmune thrombocytopenia caused by antibodies to human platelet antigen 3a (Baka) detectable only in whole platelet assays

“…Studies with neuraminidase‐treated intact PLTs provided evidence that the epitope recognized by two of the antibodies was dependent on a glycosylation event for its expression as has been demonstrated for the MN antigen system of RBCs 22 . A possible role of glycosylation in determining HPA‐3 alloantigen expression was also suggested in two other biochemical studies of HPA‐3 alloantigens 11,23 …”

Platelet biogenesis and functions require correct protein O-glycosylation

“…Although the detailed N-and O-glycan structures of human αIIbβ3 are unknown, prior studies on biosynthesis of αIIbβ3 in human HEL cells explored modifications such as N-glycans, but did not report the presence of O-glycans (28). However, human αIIb appears to contain at least one O-glycan important for expression of the Baka (HPA-3a) alloantigen (29,30). The functions of O-glycans on αIIbβ3 are unknown, but our results suggest that complex O-glycan structures beyond the Tn antigen are required for conformation/ proteolytic stability of αIIb and its movement to the cell surface and activation.…”

Three novel β3 domain-deletion peptides for the sensitive and specific detection of HPA-4 and six low frequency β3-HPA antibodies

“…The weak cross‐reactivity seen with three of the anti‐HPA‐4b samples (B1, B2, and B4) may reflect similar cross‐reactivity already described for other alloantigen systems such as the HPA‐2 system [38]. Alternatively, the increased binding may reflect the aspecific binding of IgG in ELISA observed with some inert samples [22,39].…”