Anti-ADAMTS13 Autoantibody-specific Anti-idiotypic Antibodies Restore ADAMTS13 Activity in Immune-mediated Thrombotic Thrombocytopenic Purpura (iTTP) Patients

Objectives: The severe deficiency of ADAMTS13 in patients suffering from iTTP is due to anti-ADAMTS13 antibodies. At least 50% of iTTP patients surviving a first acute episode experience subsequently further episodes. The mechanisms, which uphold the balance during ongoing autoimmune processes in remission, are not yet understood. We hypothesize that anti-ADAMTS13 antibodies are counteracted by neutralizing anti-idiotypic antibodies.

Methods: From splenic mononuclear cells of two iTTP patients (A and C), splenectomized after a relapsing disease course, the anti-idiotypic IgG₁ Fab κ/λ repertoire was amplified by phage display technology. Selecting antibodies consisted of three sets of monoclonal anti-ADAMTS13 Fabs, previously generated (Schaller et al., Blood 2014), each set corresponding to specific paratopes, CDR3 motifs 1, 2 or 3. The usage of the immunoglobulin variable heavy (IGHV) and light (IGKV/IGLV) chain genes of the anti-ADAMTS13-specific anti-idiotypic Fabs was determined by sequencing. Eight purified anti-idiotypic Fabs, pooled according to their patient's origin (pool A and pool C, equimolar, 200nM), were tested for their ability to neutralize anti-ADAMTS13 abs, the selecting monoclonal abs and in plasma of nine randomly picked iTTP patients (functional inhibitor were 0.5-2BU/ml in five, and >2BU/ml in four patients).

Results: Seven of eight heavy chain anti-idiotypic Fabs were paired with a productive light chain (one λ, six κ, with IGKV1-39*01 shared by both patients). Pool A and pool C fully neutralized functional ADAMTS13 inhibitor titers (from >>2BU/ml to ≤0.4BU/ml) of the three selecting monoclonal anti-ADAMTS13 pools, thereby completely restoring ADAMTS13 activity. In mixtures with iTTP patient's plasma, the anti-idiotypic Fabs pools decreased functional ADAMTS13 inhibitor titers in 7/9 patients, full neutralization (≤0.4BU/ml) with either pool was seen in three, partial neutralization in four patients.

Conclusions: In the splenic immunoglobulin repertoire of two iTTP patients, we have identified functionally effective anti-ADAMTS13 anti-idiotypic IgG1 antibodies, supporting our hypothesis of an anti-idiotypic immune response as antagonizing mechanisms in iTTP. The anti-ADAMTS13 anti-idiotypic antibodies fully or partially restored ADAMTS13 activity in plasma of 7/9 randomly picked iTTP patients hinting at a ubiquitous action of the selected anti-idiotypes. Analysis of further iTTP patients, with tittered functional ADAMTS13 inhibitors beyond 2BU/ml, as well as full characterization of the functional properties of the anti-idiotypic Fabs (single and as pools) are underway.