Polyreactive IgM initiates complement activation by PF4/heparin complexes through the classical pathway

Abstract

<jats:title>Abstract</jats:title> <jats:p>The mechanisms by which exposure to heparin initiates antibody responses in many, if not most, recipients are poorly understood. We recently demonstrated that antigenic platelet factor 4 (PF4)/heparin complexes activate complement in plasma and bind to B cells. Here, we describe how this process is initiated. We observed wide stable variation in complement activation when PF4/heparin was added to plasma of healthy donors, indicating a responder “phenotype” (high, intermediate, or low). Proteomic analysis of plasma from these healthy donors showed a strong correlation between complement activation and plasma immunoglobulin M (IgM) levels (r = 0.898; P &amp;lt; .005), but not other Ig isotypes. Complement activation response to PF4/heparin in plasma displaying the low donor phenotype was enhanced by adding pooled IgM from healthy donors, but not monoclonal IgM. Depletion of IgM from plasma abrogated C3c generation by PF4/heparin. The complement-activating features of IgM are likely mediated by nonimmune, or natural, IgM, as cord blood and a monoclonal polyreactive IgM generate C3c in the presence of PF4/heparin. IgM facilitates complement and antigen deposition on B cells in vitro and in patients receiving heparin. Anti-C1q antibody prevents IgM-mediated complement activation by PF4/heparin complexes, indicating classical pathway involvement. These studies demonstrate that variability in plasma IgM levels correlates with functional complement responses to PF4/heparin. Polyreactive IgM binds PF4/heparin, triggers activation of the classical complement pathway, and promotes antigen and complement deposition on B cells. These studies provide new insights into the evolution of the heparin-induced thrombocytopenia immune response and may provide a biomarker of risk.</jats:p>

Ex vivoobservation of granulocyte activity during thrombus formation

“…S1H). This allowed us to claim, in agreement with previously published data (23), that heparin weakly pre-activates granulocytes in our experimental setting. Thus, hirudin-treated blood samples were used in all further experiments.…”

Targeting the Initiator Protease of the Classical Pathway of Complement Using Fragment-Based Drug Discovery

“…Even in the absence of underlying genetic causes, dysregulation of complement can cause inappropriate activation leading to adverse effects, as is the case in ischemia/reperfusion injury or transplant rejection [ 8 ]. In other diseases, including neurological disorders like Alzheimer’s disease [ 12 , 13 , 14 ] or thrombotic conditions like heparin-induced thrombocytopenia (HIT) [ 15 , 16 ], the underlying mechanisms of complement involvement are only now becoming more clear. Nevertheless, mounting evidence for the involvement of complement in many human diseases has spurred a resurgence of activity in the field of complement-directed therapeutics [ 8 ].…”

Update on Mechanisms, Pathogenicity, Heterogeneity of Presentation, and Laboratory Diagnosis of Heparin-Induced Thrombocytopenia

“…More rarely, IgM can be present at high concentration in patients with HIT, without IgGs. The mechanisms involved are not totally understood, but recently it was demonstrated that anti-HPF4 IgM antibodies can activate complement and induce platelet destruction [39]. Altogether, the different activities described here above help to understand why HPF4 antibodies, including IgG isotypes, can remain asymptomatic in many patients and produce (especially IgG isotypes with high HPF4 affinity) HIT or HITT only in a few of them.…”

“…The three isotypes (IgG, IgA, or IgM) can be present [13,19], but IgGs are formed very rapidly, which is unusual in the early stage of the immune response, and IgGs can become rapidly pathogenic [19]. In rare cases, only IgA (especially in patients with cancer) or IgM isotypes are identified [39]. Following heparin treatment cessation, antibodies disappear from blood circulation within about 3 months.…”