Dexamethasone promotes durable factor VIII-specific tolerance in hemophilia A mice via thymic mechanisms.

The development of inhibitory antibodies to factor VIII is the most serious complication of replacement therapy in hemophilia A. Activation of the innate immune system during exposure to this protein contributes to inhibitor development. However, avoidance of factor VIII exposure during innate immune system activation by external stimuli (e.g., vaccines) has not been consistently shown to prevent inhibitors. We hypothesized that dexamethasone, a drug with potent anti-inflammatory effects, could prevent inhibitors by promoting immunologic tolerance to factor VIII in hemophilia A mice. Transient dexamethasone treatment during ainitial factor VIII exposure reduced the incidence of anti-factor VIII immunoglobulin G in both a conventional hemophilia A mouse model (E16KO, 77% vs 100%, P=0.048) and a hemophilia A mouse model with a humanized major histocompatibility complex type II transgene (E17KO/hMHC, 6% vs 33%, P=0.0048). More importantly, among E17KO/hMHC mice that did not develop anti-factor VIII immunoglobulin G after initial exposure, dexamethasone-treated mice were less likely to develop a response after re-exposure six (7% vs 52%, P=0.005) and 16 weeks later (7% vs 50%, P=0.097). Similar results were obtained even when factor VIII re-exposure occurred in the context of lipopolysaccharide (30% vs 100%, P=0.069). The ability of these mice to develop immunoglobulin G to human von Willebrand factor, a structurally unrelated antigen, remained unaffected by treatment. Transient dexamethasone administration therefore promotes antigen-specific immunologic tolerance to factor VIII. This effect is associated with an increase in the percentage of thymic regulatory T cells (12.06% vs 4.73%, P<0.001) and changes in the thymic messenger ribonucleic acid transcription profile.

War and peace: Factor VIII and the adaptive immune response.

The development of neutralizing anti-factor VIII (FVIII) antibodies (inhibitors) remains a major challenge for FVIII replacement therapy in hemophilia A patients. The adaptive immune response plays a crucial role in the development and maintenance of inhibitors. In this review, we focus on our current understanding of FVIII interactions with cells of the adaptive immune system and the phenotype of the resultant response. Additionally, we examine both current and novel FVIII tolerance induction methods that function at the level of the adaptive immune response.

Recombinant Factor VIII Fc Inhibits B Cell Activation via Engagement of the FcγRIIB Receptor.

The development of neutralizing antibodies (inhibitors) against factor VIII (FVIII) is a major complication of hemophilia A treatment. The sole clinical therapy to restore FVIII tolerance in patients with inhibitors remains immune tolerance induction (ITI) which is expensive, difficult to administer and not always successful. Although not fully understood, the mechanism of ITI is thought to rely on inhibition of FVIII-specific B cells (1). Its efficacy might therefore be improved through more aggressive B cell suppression. FcγRIIB is an inhibitory Fc receptor that down-regulates B cell signaling when cross-linked with the B cell receptor (BCR). We sought to investigate if recombinant FVIII Fc (rFVIIIFc), an Fc fusion molecule composed of FVIII and the Fc region of immunoglobulin G1 (IgG1) (2), is able to inhibit B cell activation more readily than FVIII. rFVIIIFc was able to bind FVIII-exposed and naïve B cells from hemophilia A mice as well as a FVIII-specific murine B cell hybridoma line (413 cells). An anti-FcγRIIB antibody and FVIII inhibited binding, suggesting that rFVIIIFc is able to interact with both FcγRIIB and the BCR. Furthermore, incubation of B cells from FVIII-exposed mice and 413 cells with rFVIIIFc resulted in increased phosphorylation of SH-2 containing inositol 5-phosphatase (SHIP) when compared to FVIII. B cells from FVIII-exposed hemophilia A mice also exhibited decreased extracellular signal-regulated kinase (ERK) phosphorylation when exposed to rFVIIIFc. These differences were absent in B cells from naïve, non-FVIII exposed hemophilic mice suggesting an antigen-dependent effect. Finally, rFVIIIFc was able to inhibit B cell calcium flux induced by anti-Ig F(ab)2. Our results therefore indicate that rFVIIIFc is able to crosslink FcγRIIB and the BCR of FVIII-specific B cells, causing inhibitory signaling in these cells.