Novel Methods of Factor VIII Tolerance Induction in Hemophilia A Mouse Models
The most serious complication of factor VIII (FVIII) replacement therapy in hemophilia A (HA) patients remains the development of anti-FVIII antibodies (inhibitors). At this time, there are no methods to prevent inhibitor development and immune tolerance induction (ITI) therapy is difficult and expensive to administer. As such, novel approaches to establish FVIII tolerance are of great interest. The presence of pro-inflammatory signals during early FVIII exposure has been identified as a risk factor for inhibitor development. Here we demonstrate that in HA mice, administration of a corticosteroid to pharmacologically suppress such signals during initial exposure to FVIII leads to antigen-specific tolerance durable for a minimum of 18 weeks. This was associated with a short-term increase in the proportion of thymic regulatory T cells and changes in thymic genes responsible for immune regulation. Failure to establish central FVIII tolerance is also thought to contribute towards inhibitor development in HA patients. This process begins in the fetal period and therefore requires in utero exposure to the protein via transplacental transfer. Our data suggest that FVIIIFc, a novel fusion protein composed of FVIII and the Fc region of immunoglobulin G1, can undergo transplacental transfer in HA mice. However, the resultant in utero exposure offered minimal protection from inhibitor development later in life, and did so only in the context of HA mice that also lacked VWF. FVIIIFc has also been associated with decreased immunogenicity and improved ITI performance when compared to conventional FVIII. We provide a potential molecular basis for this observation by showing the ability of FVIIIFc to engage the inhibitory FcγRIIB receptor of FVIII-exposed murine B cells. The protein was also able to induce suppressive downstream signaling such as increased SHIP phosphorylation, decreased ERK phosphorylation and decreased calcium flux in FVIII-exposed but not naïve murine B cells. The work presented here provides preclinical evidence of three novel approaches to FVIII tolerance induction and increases our understanding of FVIII immunology.