Researchers believe they have identified a therapeutic target for heparin-induced thrombocytopenia (HIT).
The team noted that HIT is caused by antibodies to complexes that form between platelet factor 4 (PF4), which is released from activated platelets, and heparin or cellular glycosaminoglycans.
The researchers elucidated the crystal structure of 3 PF4 complexes and found evidence suggesting that tetramerization of PF4 is targetable.
Zheng Cai, PhD, of the University of Pennsylvania in Philadelphia, and his colleagues described this work in Nature Communications.
Previously, the researchers identified KKO, a murine monoclonal antibody to PF4/heparin complexes that causes HIT in a murine model. The team said human HIT antibodies compete with KKO for binding to PF4/heparin, and KKO augments the formation of pathogenic immune complexes.
The researchers also identified RTO, an isotype-matched, anti-PF4 antibody that binds to PF4 but does not generate pathogenic complexes.
For the current study, the team described and compared the crystal structures of PF4 in complex with Fabs derived from KKO and RTO to the structure of PF4 in complex with fondaparinux.
The researchers noted that PF4 molecules can exist singly as monomers, doubly as dimers, and as a 4-part complex called a tetramer, which have an “open” end and a “closed” end.
The crystal structure of PF4 in complex with fondaparinux showed that fondaparinux binds to the “closed” end of the PF4 tetramer, which stabilizes the tetramer.
The crystal structure of PF4 in complex with KKO showed that KKO binds to the “open” end of the stabilized tetramer, making contact with 3 of 4 monomers in the tetramer.
The researchers said this helps explain the requirement for heparin as a backbone for the complex. They also said this finding provides new insight into how a normal host protein such as PF4 can be converted into a target of the host immune system, which leads to an autoimmune disorder.
The crystal structure of PF4 in complex with RTO showed that RTO binds to PF4 monomers rather than tetramers. And RTO binds to the monomers in a way that prevents them from combining into tetramers.
Via cell experiments, the researchers confirmed that RTO prevents the formation of antigenic complexes, as well as the activation of platelets by KKO and human HIT antibodies. RTO also prevented clot formation caused by KKO in a mouse model of HIT.
These results suggest that binding of RTO to PF4 monomers prevents the formation of pathogenic complexes that are central to the pathology of HIT. So the researchers believe RTO can provide the basis for new diagnostics and may pave the way for a therapy to stop HIT early in its progression.
