Epitope specificity of anti-factor VIII antibodies from inhibitor positive acquired and congenital haemophilia A patients using synthetic peptides spanning A and C domains.

Development of antibodies (Ab) that either block the function of coagulation factor VIII (FVIII) (inhibitors) or clear it from circulation, seriously complicate the treatment of haemophilia A patients with FVIII products. Autoantibodies which develop in subjects without congenital FVIII defects, cause acquired haemophilia, a rare but life-threatening coagulopathy. Identification of the FVIII epitopes to which inhibitor Abs bind will help understanding the mechanisms of inhibitor activity, and perhaps development of new therapies. Here, we examined the FVIII peptide sequence regions recognised by anti-FVIII Ab in the plasma of six congenital and one acquired haemophilia patients with high inhibitor titers (24.4-2000 BU/ml). We used indirect ELISA and overlapping synthetic peptides, 20 residues long, spanning the sequence of the A and C FVIII domains. None of the plasma samples reacted with A1, A3 or C1 domain peptides. Six plasmas reacted with A2 and/or C2 peptides. Peptides spanning residues A2-521-690 and C2-2251-2332 were recognised most frequently and strongly. They include residues that contribute to the binding sites for activated factor IX and phosphatidyl serine/von Willebrand factor. These results suggest that anti-FVIII Abs share a pattern of antigen specificity in our panel of patients, and that exposed regions of the FVIII molecule that form functionally important binding sites elicit an intense Ab response.

Immune deviation by mucosal antigen administration suppresses gene-transfer-induced inhibitor formation to factor IX.

Formation of inhibitory antibodies is a serious complication of protein or gene replacement therapy for hemophilias, congenital X-linked bleeding disorders. In hemophilia B (coagulation factor IX [F.IX] deficiency), lack of endogenous F.IX antigen expression and other genetic factors may increase the risk of antibody formation to functional F.IX. Here, we developed a protocol for reducing inhibitor formation in gene therapy by prior mucosal (intranasal) administration of a peptide representing a human F.IX-specific CD4(+) T-cell epitope in hemophilia B mice. C3H/HeJ mice with a F.IX gene deletion produced inhibitory IgG to human F.IX after hepatic gene transfer with an adeno-associated viral vector. These animals subsequently lost systemic F.IX expression. In contrast, repeated intranasal administration of the specific peptide resulted in reduced inhibitor formation, sustained circulating F.IX levels, and sustained partial correction of coagulation following hepatic gene transfer. This was achieved through immune deviation to a T-helper-cell response with increased IL-10 and TGF-beta production and activation of regulatory CD4(+)CD25(+) T cells.

Immunodominant T-cell epitopes in the factor VIII C2 domain are located within an inhibitory antibody binding site.

Formation of inhibitor antibodies to factor VIII (FVIII) is a major complication of FVIII replacement therapy for hemophilia A patients, and it occurs through a T-cell dependent process. The C2 domain of FVIII contains epitopes that are recognized by antibody inhibitors. We have examined regions of the C2 domain that form epitopes for T cells in mice congenitally deficient in FVIII. We obtained CD4(+)T cells from mice immunized by intravenous infusion of therapeutic doses of recombinant human FVIII (rFVIII), or by subcutaneous injections of rFVIII or recombinant human C2 domain in adjuvant. In all cases, the T cells recognized most strongly and consistently two overlapping peptides that spanned residues 2191 to 2220 of the C2 domain. Analysis of the crystal structure of human factor VIII C2 bound to a human monoclonal antibody, BO2C11, showed these residues also constitute part of a human alloimmune B-cell epitope (Spiegel et al., Blood 2001; 98: 13-19). This region includes one of the "hydrophobic spike" protrusions, consisting of M2199 and F2200, as well as the basic residues R2215 and R2220. These residues contribute to membrane binding and to association with von Willebrand factor (vWF). These findings suggest that a major T-cell epitope in the C2 domain recognized by hemophilic mice is located within the same region that binds to inhibitors, vWF, and activated membranes.

Induction of myasthenia gravis in HLA transgenic mice by immunization with human acetylcholine receptors.

We utilized HLA transgenic mice to identify the dominant epitopes on the human (H)-AChR alpha subunit. The cytoplasmic H-AChR peptide alpha320-337 was the dominant T cell epitope for DQ8, DR3, and DQ8xDQ6 F1 mice. The H-AChR-immunized HLA-DQ8, DR3, DQ8xDR3 F1 and DQ8xDQ6 F1 mice developed clinical EAMG, whereas HLA-DQ6 mice were less susceptible.

Mapping myasthenia gravis-associated T cell epitopes on human acetylcholine receptors in HLA transgenic mice.

Susceptibility to myasthenia gravis (MG) is positively linked to expression of HLA-DQ8 and DR3 molecules and negatively linked to expression of the DQ6 molecule. To elucidate the molecular basis of this association, we have induced experimental autoimmune MG (EAMG) in mice transgenic for HLA-DQ8, DQ6, and DR3, and in DQ8xDQ6 and DQ8xDR3 F(1) transgenic mice, by immunization with human acetylcholine receptor (H-AChR) in CFA. Mice expressing transgenes for one or both of the HLA class II molecules positively associated with MG (DQ8 and DR3) developed EAMG. T cells from DQ8 transgenic mice responded well to three cytoplasmic peptide sequences of H-AChR (alpha320-337, alpha304-322, and alpha419-437), of which the response to alpha320-337 was the most intense. DR3 transgenic mice also responded to this sequence very strongly. H-AChR- and alpha320-337 peptide-specific lymphocyte responses were restricted by HLA class II molecules. Disease resistance in DQ6 transgenic mice was associated with reduced synthesis of anti-AChR IgG, IgG(2b), and IgG(2c) Ab's and reduced IL-2 and IFN-gamma secretion by H-AChR- and peptide alpha320-337-specific lymphocytes. Finally, we show that DQ8 imparts susceptibility to EAMG and responsiveness to an epitope within the sequence alpha320-337 as a dominant trait.

Adoptive protection from experimental myasthenia gravis with T cells from mice treated nasally with acetylcholine receptor epitopes.

Nasal administration of synthetic CD4(+) epitopes of the acetylcholine receptor (AChR) prevents experimental myasthenia gravis (EMG) in C57Bl/6 mice, but not in IL4-deficient C57Bl/6 (IL4(-/-)) mice. Here we verify that nasal tolerance requires IL4, by showing that CD4(+) cells from C57Bl/6 mice treated nasally with a pool of AChR CD4(+) epitopes protected IL4(-/-) mice from EMG and caused a reduced production of anti-AChR antibody. CD4(+) cells from C57Bl/6 mice treated with unrelated peptides or sham-treated did not induce protection. CD4(+) cells from C57Bl/6 mice treated with just one AChR peptide protected IL4(-/-) mice from EMG without affecting antibody synthesis.