Recombinant Mini-β3 Integrin Fragments for the Detection of HPA-1 Antibodies.

The αIIbβ3 integrin (GPIIbIIIa, CD61) is the platelet receptor for fibrinogen, fibronectin and vitronectin. A single nucleotide polymorphism (dbSNP Id= rs5918) in the ITGB3 gene defines the Human Platelet Antigen (HPA)-1 system encoding either a Leucine (1a) or Proline (1b) at position 33. HPA-1a immunisation is of clinical relevance in neonatal alloimmune thrombocytopenia (NAIT), post transfusion purpura (PTP) and platelet refractoriness. Currently, the detection of HPA-1 antibodies is based on a cumbersome sandwich ELISA requiring HPA genotyped platelets as source of antigen. We set out to express soluble calmodulin (CaM) tagged β3 fragments suitable for HPA-1 antibody detection. The design of fragments was informed by domain structural information, analysis of homology between β3 and integrin-like monomeric proteins and the evolutionary β3 domain phylogeny. Combined with computer modelling, this was the basis of the successful expression in Drosophila S2 cells of eight β3 fragments with either Leu33 or Pro33 (ΔSDL, ΔβA, PSI-Hybrid and PSI alone, both alleloforms for each of the four). Murine monoclonal antibodies Y2/51, SZ21 and CRC54 reacted with the β3 fragments as expected with only CRC54 reacting with the PSI fragment. Two of 3 human HPA-1a specific phage antibodies (19–7, 23–15, both derived from a PTP patient) reacted with all 4 Leu33 fragments but CamTran007 derived from a NAIT patient only reacted with the ΔSDL-Leu33 and ΔβA-Leu33 fragments, defining an epitope split. By ELISA, reactivity of both potency (NIBSC-03/152) and sensitivity (NIBSC-93/710) standards were comparable to those seen by MAIPA. Twelve polyclonal samples from NAIT patients (5 with cerebral haemorrhage [CH], 5 with platelet counts <40x10⁹/L but no CH and 2 with counts >40x10⁹/L) of potency ranging from 5–193 au/ml were detected by ELISA with Leu33 forms of ΔSDL and ΔβA. In conclusion soluble mini-β3-Leu33 fragments have been successfully expressed and shown to react with all polyclonal anti-HPA-1a samples tested from NAIT patients. The CaM tagged β3 fragments are now being further validated with a repository of 150 HPA-1a antibody samples from a NAIT case series with well defined treatment and clinical outcome histories to establish the relation between HPA-1a antibody potency, epitope reactivity and clinical phenotype.