Exploring Anti-FVIII Antibodies in Haemophilia A - Role in In Vitro Haemostasis and Clinical Disease

Abstract: Haemophilia A (HA) is caused by defective synthesis of coagulation factor VIII(FVIII), which has serious effects on haemostasis; joints being the most common site of bleeding. The development of FVIII replacements has improved the situation for patients with haemophilia such that chronic arthropathy can be prevented, and life expectancy and the quality of life have increased. However, approximately 20-30% of patients suffering from severe HA develop neutralizing
antibodies (inhibitors) against FVIII. Alternative treatment, using by-passing agents, is available for patients exhibiting inhibitors, although these can only be used for the short-term treatment of acute haemorrhage and as prophylaxis during surgery. Furthermore, the clinical response to by-passing products is unpredictable. Two of the studies included in this thesis evaluated the response to by-passing therapy in plasma from patients with HA. The variation in thrombin
production within families was found to be significantly lower than the variation between families, indicating that a familial predisposition may influence thrombin formation in response to by-passing agents (Paper I). Moreover, FVIII clotting factors were found to potentiate the in vitro effect of by-passing agents on thrombin formation in plasma from patients with HA exhibiting inhibitors,
indicating that further assessment of this treatment strategy in a clinical context is warranted (Paper II). Not all anti-FVIII antibodies have neutralizing capacity. In the studies presented in Papers III & V, non-neutralizing anti-FVIII antibodies(NNAs) were investigated in two different cohorts, using an enzyme-linked
immunosorbent assay (ELISA). NNAs were detected in 18.9% of siblings with HA, and in 12.8% of unrelated HA subjects followed for four years. The antibody response was assayed using three different rFVIII products. The antibody response was found to be heterogeneous, to vary considerably between individuals (Papers III and V), and also over time (Paper V). None of the patients in the cohort with NNAs observed longitudinally developed inhibitors (Paper V). However, in one patient with moderate HA, the detection of Bethesda-negative anti-FVIII antibodies coincided with a change in bleeding phenotype four years prior to FVIII inhibitor development. This finding suggests that immunoassays may be a useful complement in evaluating the immune response to FVIII (Paper IV). The potential clinical impact of NNAs was evaluated in the long term study (Paper V), showing no association between age, F8 mutation, or the influence of immune system challenges on NNA development. Interestingly, patients with NNAs had significantly fewer bleeding episodes than NNA-negative patients (p=0.048), raising questions about the possibility of yet undefined types of anti-FVIII antibodies with protective or potentiating effects on FVIII.