SCMM organises a seminar by Dr. Sanjay Khandelwal

Special Centre for Molecular Medicine

Jawaharlal Nehru University

Seminar Notice

Title: Mechanism of immune activation against PF4/heparin complexes in heparin induced thrombocytopenia

Speaker

Dr. Sanjay Khandelwal, DVM & PhD

Research Scientist

Division of Hematology, Department of Medicine, Duke Medical Center

Duke University, Durham, NC, USA (an alumni of SLS, JNU)

Abstract: Heparin is a commonly used anticoagulant agent for patients presenting with thrombotic disease. Some  patients  who  receive  heparin  develop  an  immune-mediated  thrombotic  disorder  called Heparin Induced Thrombocyopenia (HIT). HIT is caused by antibodies to a multimeric antigenic complex  consisting  of  a  positively  charged  protein,  platelet  factor  4  (PF4)  and  heparin,    a negatively charged polysaccharide.    The mechanism that incites antibody production against PF4/heparin  complexes  is  poorly  understood.  Since  2013,  my  studies  have  focused  on understanding  the  mechanism  of  immune  activation  by  PF4/heparin  complexes,  with  recent studies showing a major role for complement activation by PF4/heparin complexes (Khandelwal et al., Blood 2016).  In these studies, we showed that PF4/heparin complexes robustly activate complement in plasma and whole blood and that complement activation results in uniform antigen deposition  on  circulating  B  cells  via  the  complement  receptor  CD21.   In  the  course  of  these studies, we developed a novel assay for measuring complement activation (Khandelwal et al., Thromb  Haemost  2018)  and  observed  wide,  but  stable  variation,  among  donor  plasmas  with respect to complement activation by PF4/heparin complexes with donor responses segregating into  a    "phenotype"  (high,  intermediate,  or  low).  Subsequent  studies  have  shown  that  this phenotype was mediated by polyreactive naturally occurring IgM initiating complement activation through the classical pathway and C1q (Khandelwal et al., Blood 2018).   Finally, we showed the clinical  relevance  of  these  findings  by  demonstrating  that  patients  receiving  heparin  have circulating B cells with PF4/heparin antigen, the complement activation fragment C3 and IgM. Based on these findings, we have developed the following working model for immune activation following  heparin  exposure:    Polyreactive  natural  IgM  1)  binds  to  circulating  PF4/heparin complexes  in  plasma  and  2)  activates  classical  pathway  of  complement  which  leads  to  3) depositon  of  complement  fragment  C3  to  PF4/heparin  antigenic  complexes  and  4)  promotes binding of these complement coated complexes to B cells via CD21 (Figure 1). As complement activation by natural IgM against particulate antigen and binding of complement coated antigen to CD21 on B cells markedly potentiates its immunogenicity, natural IgM mediated complement activation  by  PF4/heparin  complexes  and  subsequent  binding  of  these  complement  coated complexes to B cells may represent early, sensitizing events and antibody generation against PF4/heparin complexes. Our results also suggest that patients who have high IgM might be at higher  risk  of  antibody  generation  against  PF4/heparin  complexes.    These  findings  not  only provide the mechanism of immune activation in HIT but also provide mechanistic insights into other pathological conditions involving auto or exogenous antigens such as antibody generation in antiphospholipid syndrome and FVIII inhibitor development and opens up future avenues to develop biomarkers and therapeutic interventions in these diseases.