Anti-Glycoprotein V Autoantibodies in Patients with Immune Thrombocytopenia

Background: Platelet autoantibody (aab)-induced platelet clearance represents a major pathomechanism in immune thrombocytopenia (ITP). Evidence for clinical differences between anti-GP IIb/IIIa and anti-GP Ib/IX mediated ITP is currently accumulating. Glycoprotein (GP) V is a well characterized target antigen in Varicella-associated and drug-induced thrombocytopenia, but its relavance in ITP has never been investigated systematically.

Methodology: We conducted a systematic study assessing the prevalence and functional capacity of aabs against GP V in ITP. A total of 1,140 patients were included. Clinical data were evaluated. All patients were investigated by monoclonal antibody immobilization of platelet-specific antigens (MAIPA) assay both for platelet-bound and free antibodies. A subgroup was studied by surface plasmon resonance (SPR) technology. Functional capacity of GPV antibodies was investigated in a phagocytosis model using human splenic macrophages and in a murine NOD/SCID model of human platelet elimination.

Findings: In one third of patients, platelet-bound aabs against GP Ib/IX, GP IIb/IIIa, or GP V were detected in the MAIPA assay; platelet bound autoanti-GP V was present in the majority of samples (222/343 = 64.7%). Investigating patient sera revealed the presence of free aabs against GP V in 13.5% of these patients by an indirect MAIPA, but in 39.6% by SPR technology. SPR-only antibodies showed significantly lower avidity (association/dissociation ratio 0.32±0.13 versus 0.73±0.14, p < 0.001). High- and low-avidity antibodies induced comparable amounts of platelet uptake in a phagocytosis assay using CD14+ positively-selected human macrophages isolated from spleens (p=0.555). In a NOD/SCID mouse model, IgG prepared from both types of anti-GP V aabs eliminated human platelets with no detectable difference between the groups from the murine circulation (mean platelet survival at 300 min: 40% [range 27-55] versus 35% [16-46], p=0.025).

Conclusions: Our data establish GP V as a relevant immune target in ITP. We have demonstrated that anti-GP V autoantibodies are of clinical relevance since they can remove platelets from the circulation. We could also, for the first time, demonstrate that low platelet autoantibody avidity is a central reason why current serology does not detect platelet autoantibodies more frequently. Apparently, before ITP treatment can be tailored according to platelet autoantibody specificities, studies including GP V as immunotarget are required.

No conflict of interest has been declared by the author(s).