Platelet
cold agglutinins (PCA) cause pseudothrombocytopenia, spurious
thrombocytopenia due to ex vivo platelet clumping, complicating clinical diagnosis, but mechanisms and consequences of PCA are not well defined. Here, we characterised an atypical
immunoglobulin (Ig)M PCA in a 37-year-old woman with lifelong
bleeding and chronic moderate
thrombocytopenia, that induces activation and aggregation of autologous or allogeneic platelets via interaction with
platelet glycoprotein (GP)VI. Patient temperature-dependent pseudothrombocytopenia was
EDTA-independent, but was prevented by
integrin αIIbβ3 blockade. Unstimulated patient platelets revealed elevated levels of bound
IgM, increased expression of activation markers (
P-selectin and CD63), low GPVI levels and abnormally high
thromboxane (TX)A2 production. Patient serum induced temperature- and αIIbβ3-dependent decrease of platelet count in allogeneic donor citrated platelet-rich plasma (PRP), but not in PRP from Glanzmann's
thrombasthenia or afibrinogenaemia patients. In allogeneic platelets, patient plasma induced shape change,
P-selectin and CD63 expression, (14)C-serotonin release, and TXA2 production. Activation was not inhibited by
aspirin,
cangrelor or blocking anti-
Fc receptor (FcγRIIA) antibody, but was abrogated by inhibitors of Src and Syk, and by a soluble GPVI-Fc fusion
protein. GPVI-deficient platelets were not activated by patient plasma. These data provide the first evidence for an
IgM PCA causing platelet activation/aggregation via GPVI. The PCA activity persisted over a five-year follow-up period, supporting a causative role in patient chronic
thrombocytopenia and
bleeding.