Frequency and Epitope Specificity of Anti-Factor VIII C1 Domain Antibodies in Acquired and Hereditary Hemophilia Inhibitor Patient Plasma

Acquired hemophilia A (AHA) is a rare autoimmune disease caused by development of inhibitory anti-factor VIII (fVIII) antibodies (also called inhibitors) resulting in severe hemorrhages. In addition, inhibitor development is the most serious complication of today's replacement therapy in patients with hereditary X-linked hemophilia A (HA) disorder. Earlier studies showed that antibodies in AHA and HA inhibitor plasmas are both primarily directed to the A2 and C2 domains suggesting that these two domains are the predominant immunogenic fVIII regions (Fulcher et al, 1987; Prescott et al, 1997; Lollar, 2004). However, the C1 domain also makes a major contribution to the humoral anti-fVIII immune response in hemophilic mice (Healey et al, 2007), which motivated us to analyze the frequency and epitope specificity of anti-C1 antibodies in AHA and HA inhibitor patient plasma.

The frequency of domain-specific antibodies were studied by antibody binding to human A2, C1 and C2 domains presented as (i) single human domain (SHD) human/porcine hybrid fVIII and (ii) HSA-fusion proteins. While similar frequencies of A2- and C2-specific antibodies were observed for both applied mapping strategies the use of isolated C1 domain resulted in much higher detection level of anti-C1 antibodies compared to the use of the human C1 domain human/porcine hybrid fVIII protein. As homologue-scanning mutagenesis relies on differences among human and porcine sequences these results suggest the presence of a large number of cross-reactive anti-C1 antibodies binding to species-conserved epitopes. Overall, anti-C1 antibodies were detected in 90 of 115 (78%) AHA and 36 of 63 (57%) HA inhibitor patients.

Two well-characterized monoclonal C1 inhibitors, human LE2E9 (Jacquemin et al, 2000) and murine MAb 2A9 (ASH 2014 poster, Batsuli et al) were used for indirect epitope mapping of anti-C1 antibodies in AHA patients by competition binding studies. Our results for AHA patients with non-crossreactive anti-C1 antibodies only (n=11) show that antibody binding to human C1 domain human/porcine hybrid fVIII (HP53) protein was completely blocked in the presence of MAb 2A9. In contrast, antibody binding to the isolated C1 domain was only partially reduced in the presence of MAb 2A9 for a selected number of (high responding) AHA patients (n=10) suggesting the presence of a second population of crossreactive anti-C1 antibodies that exclusively bind to conserved amino acid residues.

Competition binding to native and denatured fVIII and HP53 proteins revealed that MAb 2A9 and LE2E9 bind mutually exclusive to a conformational C1 epitope involving amino acid residues that are not conserved between humans and pigs. Consequently, essential binding residues were identified for both C1 inhibitors via the use of HP53 variants, in which surface exposed non-conserved amino acid residues on the human C1 domain were substituted for porcine residues. The results of this mutational analysis showed that despite their competitive binding different amino acid residues are essential for binding of MAb 2A9 and LE2E9. These findings are in agreement with the different specific inhibitory activities of the two C1 inhibitors (97 BU/mg vs 10000 BU/mg).

Finally, HSA-C1 point mutants were used to directly map essential epitope residues of anti-C1 antibodies in AHA and HA inhibitor patient plasma.

Our study demonstrates that a large number of AHA and HA inhibitor patients (126 of 178; 71%) have anti-C1 antibodies that comprise at least two different populations, crossreactive and non-crossreactive to porcine fVIII. Therefore, in addition to the A2 and C2 domains, the C1 domain seems to significantly contribute to the immune response to fVIII in these patients. As recent data point toward a functional role of the fVIII C1 domain for membrane-, fX-, and von Willebrand factor-binding (Lü et al, 2011) the clinical relevance of anti-C1 antibodies should be analyzed in further studies.