Page 175 - Haematologica August 2018

Dexamethasone promotes tolerance to factor VIII
not result in altered expression of immune-relevant genes. A number of prior studies have examined the ability of immunomodulatory agents to prevent FVIII inhibitors in murine models of HA: anti-CD3 monoclonal antibody,18 anti-CD4 monoclonal antibody with adjuvant,36 rapamycin,22 CD40/CD40-ligand interaction blockade,37 and IL2/ anti-IL2 monoclonal antibody complexes.38 However, some of these strategies only induced transient tolerance to FVIII37 or tolerance that was not tested for long-term durability.18 Furthermore, the effect on immune responses to other antigens was not assessed in some cases.18,37 Importantly, none of these other immunomodu-
latory agents is commonly used in clinical practice.
Dex and other glucocorticoids are widely available, rou- tinely used by hematologists and are known to have good oral bioavailability.39 Glucocorticoids have been used in clinical practice to reduce humoral immune responses to protein therapeutics (e.g., infliximab),40 and have even been successful in hemophilia patients with inhibitors.41 Moreover, because the greatest risk of FVIII inhibitors occurs early (~25 first exposure days), clinical application of this approach might only require Dex coverage of a few early FVIII exposures, until the inhibitor risk is reduced. For these reasons, translation of this approach to the bed- side seems feasible. A clinical trial examining the use of Dex during early FVIII exposures expected to span several
days would be the most immediate application.
This study does have some limitations. As previously mentioned, E17KO/hMHC mice have a mixed genetic background. While this variability between animals may be responsible for some of the observed variability in anti- FVIII immune responses, it may also indicate some biolog- ical robustness of the tolerance effect. Due to the genetic variability of E17KO/hMHC, experiments identifying the mechanistic basis of the effect observed were carried out using the inbred E16KO mouse model. The ability of our
treatment protocol to diminish the anti-FVIII immune response in this model with high propensity for inhibitor development further confirms the robustness of the effect.
Although the E17KO/hMHC mouse model of HA reca- pitulates the epidemiology of anti-FVIII immune respons- es among humans with severe HA, this does not imply that the immunological mechanisms of these responses are identical. There are significant differences between these species, such as the absence of the IgG4 isotype in mice, which is the dominant IgG subclass associated with inhibitors in HA patients.42 In addition, this mouse model has only one MHC allele and will therefore have non- physiologic antigen presentation. The age of the model might also be a limitation, as our mice would be consid- ered “young adults”. In contrast, HA patients who are at the greatest risk for inhibitor development are toddlers. The effects of these age differences on the ability of Dex to prevent inhibitors in humans cannot be predicted.
Our dose of Dex (~3mg/kg/day, demonstrated to have anti-inflammatory effects in rodents)43 is higher than that typically used in humans. Small animals require higher per-weight doses of medications than humans to achieve equivalent dosing relative to body surface area.44 Dex is given to children in doses of 0.3 – 0.6mg/kg/day for the treatment of croup45 and asthma.46 In these applications, Dex exerts the anti-inflammatory and anti-lymphocytic effects that may be important for promoting immunologic tolerance to FVIII. Therefore, similar dosing would be rea- sonable to use in a clinical study addressing the mitigation of FVIII inhibitor development. Although Dex can be immunosuppressive, the risk of invasive infections is low after intermittent exposure in young children.47
We also used FVIII doses (~240units/kg/dose) higher than those used in most clinical applications. However, this is the FVIII dose required to provoke FVIII immune responses in E17KO/hMHC mice. Although lower doses
AB
Figure 7. Administration of Dex during initial FVIII exposure alters the thymic but not splenic mRNA transcript profile of E16KO mice. A. Thymic and B. splenic mRNA transcript counts three days after treatment with FVIII+Dex versus mRNA transcript counts three days after treatment with FVIII alone. Each point corresponds with the average mRNA transcript count of three different tissue samples. Labeled points indicate genes that exhibited a ≥2-fold change as a result of treatment with FVIII+Dex. These genes had a gene count ratio ≥2:1 in the same direction when comparing both FVIII+Dex against FVIII and Dex against HBSS. n=3 for each condition. FVIII: factor VIII; Dex: dexamethasone; mRNA: messenger ribonucleic acid.
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