Summary
The antigen in heparin-induced thrombocytopenia (HIT) is expressed on platelet factor
4 (PF4) when PF4 complexes with polyanions. In recent years, biophysical tools (e.
g. circular dichroism spectroscopy, atomic force microscopy, isothermal titration
calorimetry, x-ray crystallography, electron microscopy) have gained an important
role to complement immunological and functional assays for better understanding the
interaction of heparin with PF4. This allowed identification of those features that
make PF4 immunogenic (e. g. a certain conformational change induced by the polyanion,
a threshold energy of the complexes, the existence of multimeric complexes, a certain
number of bonds formed by PF4 with the polyanion) and to characterize the morphology
and thermal stability of complexes formed by the protein with polyanions. These findings
and methods can now be applied to test new drugs for their potential to induce the
HIT-like adverse drug effect by preclinical in vitro testing. The methods and techniques
applied to characterize the antigen in HIT may also be helpful to better understand
the mechanisms underlying other antibody-mediated disorders in thrombosis and hemostasis
(e. g. acquired hemophilia, thrombotic thrombocytopenic purpura). Furthermore, understanding
the mechanisms making the endogenous protein PF4 immunogenic may help to understand
the mechanisms underlying other autoimmune disorders.
Keywords
Platelet factor 4 - heparin - conformational changes - heparin-induced thrombocytopenia
- biophysics of PF4/polyanion complexes