In patients with hemophilia A (HA), an analysis of F8 gene haplotypes revealed no clear differences in antibody binding to factor VIII (FVIII) products based on haplotype mismatch. However, the researchers performing the analysis did find differences in inhibitor binding associated with FVIII protein length. The study findings were reported in the journal Blood Advances.
Development of alloimmune inhibitors can complicate treatment of HA using an FVIII treatment product, and inhibitor development is more common in African American patients with the condition. For this reason, and because of differences in prevalence of some F8 gene haplotypes across patient populations, the researchers examined the possibility of a mismatch between FVIII products and certain F8 haplotypes as being a contributor to the development of anti-FVIII antibodies.
In this study, researchers measured anti-FVIII antibody levels in the plasma of patients with HA ranging from mild to severe, and the patient population was based in the United States. A total of 23 family members of patients were included as controls in some analyses.
The researchers analyzed the degree of binding between anti-FVIII antibodies and FVIII proteins associated with several F8 haplotypes, including full-length recombinant H1 and H2 haplotypes and B-domain-deleted (BDD) haplotypes H1/H2, H3/H5, and H4.
The researchers also investigated the possibility that certain polymorphisms associated with F8 haplotype may constitute linear B-cell epitopes, or those based on linear amino acid sequence. Antibody titers, inhibitor titers, and antibody-peptide binding interactions were evaluated using a variety of assays.
The study included 394 patients with HA, of whom 188 patients were Black and 206 were White. The researchers found antibodies that reacted with FVIII in all patients with HA, and levels were higher in patients who had inhibitors.
However, in patients with HA exposed to haplotype-mismatched FVIII, the researchers did not find significant relationships between F8 haplotype mismatches and anti-FVIII antibody titers, in regression analyses adjusted for inhibitor history. Additionally, FVIII peptides with D1241E or M2238V polymorphisms did not appear to form linear B-cell epitopes. A third polymorphism, R484H, occurred in too few of the African American patients of the study to enable its analysis in these patients.
The researchers found, however, that the BDD-FVIII proteins showed greater reactivity with plasma antibodies than the full-length FVIII products did. Binding of antibodies to BDD-FVIII proteins was approximately 2 to 6 times higher, compared with binding to full-length FVIII proteins.
The research team hypothesized that the absence of the B domain may lead to different epitopes being exposed for B-cell activity, which they suggested may relate to conformational differences in these FVIII proteins, compared with full-length FVIII.
“Together with our previous statistical analysis and investigation of potential T-cell responses to these polymorphic sequences,” the researchers wrote in their report. “These data indicate that although immune responses to neo-epitopes resulting from F8 haplotype mismatch may occur rarely, they are not a major contributor to inhibitor development in African American HA patients.”
Disclosures: Some authors have declared affiliations with or received grant support from the pharmaceutical industry. Please refer to the original study for a full list of disclosures.
Pratt KP, Gunasekera D, Vir P, et al. Anti-FVIII antibodies in Black and White hemophilia A subjects: do F8 haplotypes play a role? Blood Adv. Published online December 2, 2022. doi:10.1182/bloodadvances.2021004909