Anti-ADAMTS13 Inhibitor Boosting During Plasma Exchange Therapy in Acquired TTP Is the Expression of a General Dysregulation of the Immune Response,

Thrombotic thrombocytopenic purpura (TTP) is a rare disorder characterized by thrombocytopenia, microangiopathic hemolytic anemia with schistocytes on the peripheral blood smear and a variable degree of ischemic organ dysfunction due to microvascular thrombosis. In the case of acquired TTP, circulating autoantibodies inhibit ADAMTS13 activity and/or increase ADAMTS13 clearance. The first-line treatment of an acute TTP bout is plasma exchange (PEX) therapy with the replacement of fresh frozen plasma. Up to 30% of TTP patients show an exacerbation under PEX. After initial improvement, platelet counts drop again which is typically associated with a strong increase of ADAMTS13 inhibitor titers, and therefore the phenomenon is also referred to as inhibitor boosting. Interestingly it is apparently not seen in other autoimmune diseases treated with PEX, like acquired hemophilia A or myasthenia gravis. The aim of this study was therefore to investigate if inhibitor boosting in acquired TTP is a general or ADAMTS13 specific phenomenon.

Plasma samples, obtained day-to-day during PEX therapy supplemented with steroids until remission, of 9 patients suffering from acute TTP due to severe acquired ADAMTS13 deficiency, were analyzed for the following laboratory parameters: (a) C-reactive protein (CRP) and total IgG, (b) ADAMTS13 activity, (c) anit-ADAMTS13 antibody titers by ELISA, (d) functional inhibitor titers by FRETS assay, and (e) anti-TG and anti-TPO antibody titers.

At initial presentation inhibitor titers ranged from 0.7 - >10 BU/ml (a positive functional inhibitor titer is defined as >0.4 BU/ml by FRETS). All patients initially improved when PEX was started, however, in 7/9 cases a drop in platelet count was noted on day 7 (n=5), 8 or 9 (each n=1), which was associated with a substantial increase in anti-ADAMTS13 antibody and functional inhibitor titer in all seven patients. CRP was increased (>30mg/L) in one patient only, during the three days before the raise in inhibitor titer and a drop in platelet count was noted, while total IgG remained stable in all 9 patients throughout PEX. At presentation none of the patients had a positive result for the thyroid antibodies, however, all 9 patients developed positive anti-TPO antibody titers during treatment and three patients tested positive for anti-TG antibodies at the same time. Anti-TPO antibody peaks occurred after the anti-ADAMTS13 antibody peak in all patients (range days 9–17), except the two patients where no raise in anti-ADAMTS13 antibody titers was observed (anti-TPO peak on day 3 in both). The two patients without inhibitor boosting required only five and six PEX sessions, respectively to get into remission, of the other seven patients two didn't overcome the inhibitor boosting and became refractory and were subsequently successfully treated by splenectomy or Rituximab, and one patient died of sepsis on day 11.

Inhibitor boosting in acquired TTP due to antibody-mediated severe ADAMTS1313 deficiency treated with PEX is common (78%) and cannot be overcome by standard first line treatment in half of the patients. The fact that significant titers of other autoantibodies than anti-ADAMTS13 antibodies occur during the acute disease episode is striking and points to a more general dysregulation of the immune response in acquired TTP. Investigation of further autoantibodies, cytokines, etc. to explore the role of B-cells and T-cells in this phenomenon in more detail are underway.

No relevant conflicts of interest to declare.