An update on the 'danger theory' in inhibitor development in hemophilia A.

INTRODUCTION: Nowadays, one of the most serious treatment complications in hemophilia A is the formation of neutralizing antibodies against coagulation factor VIII (FVIII). These so-called inhibitors develop in about 30% of all patients with severe hemophilia A. Once formed, inhibitors reduce FVIII efficacy in blood coagulation, which has a negative impact on patients' health and quality of life and significantly increases hemophilia A treatment costs. The pathophysiology of inhibitor development is a complex and multi-causal process, in which both genetic factors as well as environmental factors participate. So-called 'danger signals' are considered contributors to inhibitor formation, and can be triggered by surgery, joint bleeds or infections. A pro-inflammatory tissue micro-environment is thereby established, which is characterized by the upregulation of costimulatory molecules on antigen-presenting cells (APCs), that can facilitate the alloimmunization to FVIII and thereby inhibitor formation. Here, the authors will discuss evidence from (pre)clinical studies about this theory in hemophilia A. Areas covered: In this review, the current knowledge regarding the 'danger theory' with regard to inhibitor development in hemophilia A is summarized. Expert opinion: Danger signals might contribute to inhibitor development; however, the evidence is scarce and not conclusive. Future studies, like multinational registries, are warranted but challenging.