Amelioration of Murine Immune Thrombocytopenia by CD44 Antibodies: a Potential Therapy for ITP?

Abstract 2528

Intravenous immunoglobulin (IVIg) is used to treat autoimmune diseases such as ITP. IVIg is a limited resource and its dosage and cost are both high. Although considered safe, it will always carry a theoretical risk of transferring infectious disease. Thus it would thus be highly desirable to improve the efficacy of IVIg or develop monoclonal antibodies, capable of mimicking the clinical effects of IVIg. Work by others has successfully improved the efficaciousness of IVIg in murine models of autoimmunity: It has been shown that enriched sialylated IVIg has a therapeutic effect at a 1 log fold lower dosage than IVIg; and a sialylated recombinant Fc fragment of IgG functions successfully at a 2 log fold lower dosage than IVIg.

CD44 is a widely expressed cell surface polymorphic glycoprotein. It is involved in many processes including tumour metastasis and inflammation. CD44 antibodies have been successfully used to treat several murine autoimmune disease models, including inflammatory arthritis and experimental autoimmune encephalomyelitis, although their mechanism of action remains unclear. To explore the potential for monoclonal antibodies as a treatment for ITP as well as further explore their mechanisms of action, we tested 8 monoclonal CD44 antibodies in murine ITP, and found 4 antibodies which could successfully ameliorate ITP; 2 of these antibodies function at a full 3 log fold lower dosage as compared to IVIg. Further characterization of the 2 most successful antibodies (5035-41.1D and KM114) demonstrated that, similar to IVIg, i) the expression of the inhibitory IgG receptor FcγRIIB was required for their ameliorative function and ii) complement-depleted mice also responded to anti-CD44 treatment. Dissimilar to IVIg, the Fc portion of the CD44 antibody was not required: an F(ab')₂ fragment of antibody KM114 also significantly ameliorated thrombocytopenia at an equivalent molar concentration as intact KM114. Thus while KM114 functions by an FcγRIIB sensitive mechanism, FcγRIIB is unlikely a direct target of the Fc region of this antibody and likely plays a downstream role in the amelioration of immune thrombocytopenia, similar to IVIg. These data demonstrate that CD44 antibodies can function therapeutically in murine ITP and we speculate that they could potentially provide a very low dose recombinant therapy for the treatment of ITP.