VITT-like Monoclonal Gammopathy of Thrombotic Significance.

Vaccine-induced immune thrombocytopenia and thrombosis (VITT) is associated with antibodies that target platelet factor 4 (PF4) and are heparin-independent. VITT antibodies are implicated in acute, transient prothrombotic disorders that are triggered by adenoviral vector vaccines against coronavirus disease 2019 or by adenovirus infection. We describe chronic prothrombotic disorders featuring anticoagulant-refractory thromboses and intermittent thrombocytopenia that were associated with VITT-like antibodies in five patients (four patients with newly reported cases and the index patient). The patients had low levels of M proteins (median level, 0.14 g per deciliter); in each patient, we found that the M protein was the VITT-like antibody. The antibody clonotype profiles and binding epitopes on PF4 were different from those observed with the acute disorders occurring after vaccination or viral infection, features that reflect distinct immunopathogenesis. Treatment strategies besides anticoagulation alone are needed for the chronic disorders, referred to as VITT-like monoclonal gammopathy of thrombotic significance. (Funded by the Canadian Institutes of Health Research and others.).