Summary
Heparin-induced thrombocytopenia (HIT) is the most frequent drug-induced immune reaction
affecting blood cells. Its antigen is formed when the chemokine platelet factor 4
(PF4) complexes with polyanions. By assessing polyanions of varying length and degree
of sulfation using immunoassay and circular dichroism (CD)-spectroscopy, we show that
PF4 structural changes resulting in antiparallel β-sheet content >30% make PF4/polyanion
complexes antigenic. Further, we found that polyphosphates (polyP-55) induce antigenic
changes on PF4, whereas fondaparinux does not. We provide a model suggesting that
conformational changes exposing antigens on PF4/polyanion complexes occur in the hairpin
involving AA 32–38, which form together with C-terminal AA (66–70) of the adjacent
PF4 monomer a continuous patch on the PF4 tetramer surface, explaining why only tetrameric
PF4 molecules express “HIT antigens”. The correlation of antibody binding in immunoassays
with PF4 structural changes provides the intriguing possibility that CD-spectroscopy
could become the first antibody-independent, in vitro method to predict potential immunogenicity of drugs. CD-spectroscopy could identify
compounds during preclinical drug development that induce PF4 structural changes correlated
with antigenicity. The clinical relevance can then be specifically addressed during
clinical trials. Whether these findings can be transferred to other endogenous proteins
requires further studies.
Keywords
Heparin-induced thrombocytopenia - platelet factor 4 - GAGs - CD spectroscopy - antigenicity