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.

Concurrent influenza vaccination reduces anti-FVIII antibody responses in murine hemophilia A.

Inflammatory signals such as pathogen- and danger-associated molecular patterns have been hypothesized as risk factors for the initiation of the anti-factor VIII (FVIII) immune response seen in 25% to 30% of patients with severe hemophilia A (HA). In these young patients, vaccines may be coincidentally administered in close proximity with initial exposure to FVIII, thereby providing a source of such stimuli. Here, we investigated the effects of 3 vaccines commonly used in pediatric patients on FVIII immunogenicity in a humanized HA murine model with variable tolerance to recombinant human FVIII (rhFVIII). Mice vaccinated intramuscularly against the influenza vaccine prior to multiple infusions of rhFVIII exhibited a decreased incidence of rhFVIII-specific neutralizing and nonneutralizing antibodies. Similar findings were observed with the addition of an adjuvant. Upon exposure to media from influenza- or FVIII-stimulated lymph node or splenic lymphocytes, naïve CD4(+) lymphocytes preferentially migrated toward media from influenza-stimulated cells, indicating that antigen competition, by means of lymphocyte recruitment to the immunization site, is a potential mechanism for the observed decrease in FVIII immunogenicity. We also observed no differences in incidence or titer of rhFVIII-specific antibodies and inhibitors in mice exposed to the live-attenuated measles-mumps-rubella vaccine regardless of route of administration. Together, our results suggest that concomitant FVIII exposure and vaccination against influenza does not increase the risk of inhibitor formation and may in fact decrease anti-FVIII immune responses.

CD4+ T-cell epitopes associated with antibody responses after intravenously and subcutaneously applied human FVIII in humanized hemophilic E17 HLA-DRB1*1501 mice.

Today it is generally accepted that B cells require cognate interactions with CD4(+) T cells to develop high-affinity antibodies against proteins. CD4(+) T cells recognize peptides (epitopes) presented by MHC class II molecules that are expressed on antigen-presenting cells. Structural features of both the MHC class II molecule and the peptide determine the specificity of CD4(+) T cells that can bind to the MHC class II-peptide complex. We used a new humanized hemophilic mouse model to identify FVIII peptides presented by HLA-DRB1*1501. This model carries a knockout of all murine MHC class II molecules and expresses a chimeric murine-human MHC class II complex that contains the peptide-binding sites of the human HLA-DRB1*1501. When mice were treated with human FVIII, the proportion of mice that developed antibodies depended on the application route of FVIII and the activation state of the innate immune system. We identified 8 FVIII peptide regions that contained CD4(+) T-cell epitopes presented by HLA-DRB1*1501 to CD4(+) T cells during immune responses against FVIII. CD4(+) T-cell responses after intravenous and subcutaneous application of FVIII involved the same immunodominant FVIII epitopes. Interestingly, most of the 8 peptide regions contained promiscuous epitopes that bound to several different HLA-DR proteins in in vitro binding assays.