Sepsis induces platelet‑activating anti‑PF4 antibodies with potential protective role against secondary bacterial infection

Abstract Background: Increasing evidence indicates the presence of anti–platelet factor 4 (PF4)/heparin antibodies in some patients with sepsis. These antibodies typically arise following heparin exposure; a subset can activate platelets, contribute to a prothrombotic state, and increase mortality. Notably, in our retrospective cohort of 440 sepsis patients, elevated anti–PF4/heparin IgG levels were significantly associated with increased thrombotic complications but, unexpectedly, showed a trend toward improved rather than worsened survival. These findings suggest that such antibodies may have alternative, potentially beneficial roles in the host response to sepsis beyond their well-established prothrombotic effects. Previous studies have shown that PF4/heparin-reactive antibodies are induced in a mouse model of polymicrobial sepsis without heparin exposure. In the present study, we investigated the induction of platelet-activating antibodies and their potential protective role using the cecal ligation and puncture (CLP) mouse septic model. Methods and Results: Following CLP, nearly 80% of mice developed antibodies recognizing PF4/heparin, as detected by ELISA. The temporal pattern reflected a primary immune response, characterized by an early IgM surge peaking at day 3, followed by a delayed IgG rise peaking at day 14. Approximately 25% of septic mice produced IgG capable of activating platelets, as measured by P-selectin expression on mouse platelets expressing transgenic human FcγRIIa. This platelet activation was PF4- and FcγRIIa-dependent and was inhibited by high-dose heparin. Importantly, the development of platelet-activating antibodies in FcγRIIa transgenic, but not wild-type mice positively correlated with P-selectin expression on their platelets, demonstrating an FcγRIIa-dependent platelet-activating function of these antibodies in vivo. To investigate the biological function of platelet-activating antibodies in sepsis, we established a model that mimics the clinical susceptibility of septic patients to secondary infection. In this model, mice in the late stage of CLP were subsequently challenged intratracheally with a clinically relevant strain of Achromobacter xylosoxidans (GN050), an opportunistic pathogen capable of bloodstream invasion. FcγRIIa transgenic mice that developed platelet-activating IgG exhibited significantly improved survival following this “second hit” of acute disseminating pneumonia, accompanied by enhanced bacterial clearance within the lungs and peripheral blood. Consistently, in vitro, platelet-activating IgG markedly enhanced the direct killing of Achromobacter xylosoxidans by platelets. Conclusions: Sepsis can induce the production of platelet-activating antibodies, whose platelet-mediated effects may enhanced antibacterial defense and offer a potential avenue for therapeutic intervention.