Maintenance and break of immune tolerance against human factor VIII in a new transgenic hemophilic mouse model.

Replacement of the missing factor VIII (FVIII) is the current standard of care for patients with hemophilia A. However, the short half-life of FVIII makes frequent treatment necessary. Current efforts focus on the development of longer-acting FVIII concentrates by introducing chemical and genetic modifications to the protein. Any modification of the FVIII protein, however, risks increasing its immunogenic potential to induce neutralizing antibodies (FVIII inhibitors), and this is one of the major complications in current therapy. It would be highly desirable to identify candidates with a high risk for increased immunogenicity before entering clinical development to minimize the risk of exposing patients to such altered FVIII proteins. In the present study, we describe a transgenic mouse line that expresses a human F8 cDNA. This mouse is immunologically tolerant to therapeutic doses of native human FVIII but is able to mount an antibody response when challenged with a modified FVIII protein that possesses altered immunogenic properties. In this situation, immunologic tolerance breaks down and antibodies develop that recognize both the modified and the native human FVIII. The applicability of this new model for preclinical immunogenicity assessment of new FVIII molecules and its potential use for basic research are discussed.

Induction of immune tolerance by oral IVIG.

In the last years evidence has been provided for the importance of B cells in the pathogenesis of rheumatoid arthritis (RA). Several studies have supported the concept that humoral immunity, manifested by the production of autoantibodies, such as rheumatoid factors (RFs), plays a significant role in the course of the disease. Specific targeting of autoantibody-producing B cells, such as RF-producing B cells, should therefore be a promising new approach in the treatment of RA. We used a mouse model to induce human RF responses and asked the question whether oral treatment with the antigen (human IgG) recognized by RFs could induce immune tolerance to RF responses. Balb/c mice were orally treated with polyvalent human IgG before and after immunization with insoluble immune complexes (ICs) that triggered the induction of RFs. Serum titers of RFs were significantly reduced after both primary and booster immunization when human IgG was given as a single oral dose or continuously in drinking water. Continuous treatment with human IgG even prevented booster effects on RFs when treatment started after primary immunization. Treatment with IgG fragments provided evidence that the observed effect of human IgG was mediated by the Fc part and not the Fab part of IgG. Furthermore, transfer of spleen cells obtained from mice after oral treatment with human IgG suppressed RF responses in recipient mice. These data give promising indications that oral human IgG might represent an alternative approach for immunosuppressive B-cell targeted therapies in RA.

Preventing restimulation of memory B cells in hemophilia A: a potential new strategy for the treatment of antibody-dependent immune disorders.

Memory B cells are responsible for the rapidly emerging antibody response after antigen reexposure. The signals required for the restimulation of memory B cells have not been fully explained. We used a murine model of anti-factor VIII (FVIII) antibody responses in hemophilia A to study the requirements for the restimulation of FVIII-specific memory B cells and their differentiation into anti-FVIII antibody-producing cells. We were particularly interested in the significance of activated T cells and costimulatory interactions. Our results indicate that the restimulation of FVIII-specific memory B cells is strictly dependent on interactions with activated T cells. These activated T cells can be specific for either FVIII or third-party antigens. Restimulation by T cells specific for third-party antigens requires the presence of FVIII, indicating that signals induced by B-cell receptor (BCR) triggering and by interactions with activated T cells are important. The blockade of B7-1 or B7-2 as well as the blockade of CD40L inhibits the restimulation and differentiation of FVIII-specific memory B cells in vitro and in vivo. The interference with inducible costimulator-inducible costimulator ligand (ICOS-ICOSL) interactions, however, does not cause any modulation. As expected, the production of anti-FVIII antibodies by plasma cells is not dependent on any of the costimulatory interactions tested.