P48 Molecular Characterisation of the Variable Domains of an ΑIIbβ3 Specific IgM κ Platelet Cold Agglutinin in a Follicular Lymphoma Patient with Treatment Refractory Thrombocytopenia

Cold agglutinins (CA) were originally defined as IgM κ antibodies capable of agglutinating erythrocytes at temperatures below 37°C and cold haemagglutinin disease may ensue in patients with high titre cold agglutinins at low environmental temperature. Platelet CA are rare and have been poorly characterised. We present the first molecular characterisation of a pathological, platelet reactive, CA. This antibody was identified in a patient with follicular lymphoma (FL) who had profound, refractory, thrombocytopenia that resolved in response to Rituximab. Serological and molecular studies of this IgM κ that recognises αIIbβ3 shows that binding is temperature dependent but independent of the integrin activation state. The VH1-02 and Vλ3-20 encoded variable domains of the heavy and light chains have characteristics typical of a low affinity autoantibody. Maldi-Tof analysis of the platelet reactive IgM showed unequivocally that the IgM was derived from the FL clone. Interestingly we observed idiotypic cross-reactivity between this IgM paraprotein, several other FL idiotypes and our β3-leucine 33 (HPA-1a) specific human recombinant antibody. Our results suggest that binding of the latter and other αIIbβ3 specific monoclonal antibodies induces a subtle conformational change that enhances platelet binding of the patient IgM.