Characterization of a mouse model to study mechanisms of hemophilia A pain

Pain is a debilitating feature of hemophilia A (HA), yet it remains understudied. We developed models of chronic and acute pain in factor VIII knockout (FVIIIKO; B6;129S-FVIIItm1Kaz/J) mice, demonstrating characteristic features of hemarthrosis pain and gait. Using these models, we examined the mechanisms involving inflammation and vascular changes triggered by bleeding to produce pain. To recapitulate hemarthrosis, we used an acute knee-injury model to assess acute pain. We observed an increase in nocifensive behaviors, mechanical and deep tissue hyperalgesia, impaired weight bearing, and gait changes 4 days after subpatellar injury, which persisted through 8 weeks. The uninjured FVIIIKO mice showed similar changes in behaviors and gait at 10 weeks of age, suggestive of chronic pain. Dynamic gait changes indicated compensatory behavior. Hemarthropathy induced a significant elevation in circulating serum amyloid P and interleukin-6, and the injured knee joint showed a significant increase in neutrophil elastase, myeloperoxidase, mast cell degranulation, substance P (SP), and calcitonin gene-related peptide (CGRP), suggestive of global inflammation. SP and CGRP stimulate vascular permeability and arteriolar dilatation, respectively, but are also involved in the generation and maintenance of pain. Mast cell tryptase and neutrophil elastase activate protease-activated receptor 2, which causes nociceptor activation, leading to pain. Recombinant FVIII treatment led to partial improvement in pain behaviors. Thus, pain may persist after current HA treatments. Our study shows that a mouse model of HA can be used to study the mechanisms of pain and gait, which will enable us to develop treatable targets for pain in HA.