Identification of a Tolerogenic Factor V Peptide and Its Potential Role in FVIII Tolerance Induction

Inhibitory Anti-Drug Antibody (ADA) responses interfere with FVIII replacement efficacy in 25-30% of Hemophilia A (HA), greatly increasing patient morbidity and treatment costs. As an extension of previous work on tolerance-inducing peptides in IgG (Tregitopes) we investigated whether there were peptides in other ubiquitous serum proteins that could have high homology to peptides found in Factor VIII. We hypothesized that tolerance to cross-conserved peptides in other prevalent proteins might explain why anti-FVIII antibodies fail to develop in some severely FVIII-deficient HA patients. Using advanced computational modeling tools, we discovered in Factor V a potent regulatory peptide (FV1) with an immunologic profile (HLA and TCR binding) that is homologous to a non-identical peptide in FVIII. We postulated that treatment with a FV1-peptide containing biologic may be able to invoke tolerance to Factor VIII in patients who have anti-FVIII antibodies (Figure 1).

Our preliminary studies support this hypothesis. We developed an ex vivo assay using human PBMC (peripheral blood mononuclear cells) in which the recall response to tetanus toxoid, a well-known immunogenic protein for which there is a T memory response, was found to be robust and strongly inhibited by FV1 but not by other FV peptides. Other FV peptides with similar HLA-binding properties did not suppress tetanus toxoid response.

FV1 peptide also strongly inhibits proliferation of central and effector memory CD4 T and effector CD8 T cells without affecting cell viability. FV1 peptide at 4 mM reduced proliferation of effector and central memory T cells by more than 80% over the levels seen with 0.5 mg/ml TT and no peptide on a wide set of donors with HLA-DRB1 haplotypes covering the nine major supertypes in the human population. A peptide homologue to FV1 found in Factor VIII shows similar inhibitory activity on activated CD4 T cells.

Using this assay, we are determining the evolution of markers on regulatory T cells, Antigen Presenting Cells, and effector T cells as well as the cytokine secretion profile over time in order to identify specific parameters associated with cell populations and soluble factors mediating immune suppression in response to FV1. This is the first time that we have reported that FV1 peptide may play a role in the induction of tolerance to FVIII in HA patients; the important implications of this work for treatment of HA, ADA and reduction of immunogenicity discussed.

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