Activated Immune Response in Ventricular Assist Device Implanted Patients Identified through a Platelet Activation Biomarker

Introduction: Mechanical circulatory support using an implanted ventricular assist device (VAD) is an important means of enhancing or maintaining the quality of life for heart failure patients awaiting heart transplant (bridge to transplant), during the recovery of their own heart (bridge to recovery) or for long-term destination therapy (no transplant). The non-physiologic continuous blood flow of a VAD (CF-VAD) exposes blood and vasculature to abnormal shear stress and leads to a variety of hemostatic derangements that likely contribute to the development of bleeding complications, neurologic dysfunction and venous thromboembolism in these patients. Additionally, the implanted CF-VAD likely stimulates the body's natural innate immune response to a foreign object. We hypothesize that the continuous activation of platelets due to the non-physiologic blood flow and the foreign nature of the CF-VAD combine to stimulate an immune response. Previous studies have suggested thatanti-PF4/heparin antibodies [as targeted in a diagnosis of heparin-induced thrombocytopenia (HIT)] can develop in the absence of heparin. It is believed that these antibodies are not related to heparin per se but rather are generated as a physiological response by platelets, in their capacity to generate an innate immune response, reacting to non-self substances. This study evaluated the time course of anti-PF4/heparin titers in patients supported by CF-VADs.

Methods: Blood samples were collected from 13 randomly selected patients prior to implantation of a HeartMate II CF-VAD and at routine clinic visits following implantation. Median follow-up was 183 days (range: 32-352 days). Anti-PF4/heparin antibody titers were determined by the PF4 Enhanced X-45 Assay (Immucor GTI Diagnostics, Waukesha, WI). Patients were treated with heparin at the time of implant; long-term anticoagulation was maintained with warfarin (INR 1.5-2.0) and low-dose aspirin.

Results: Following CF-VAD implantation, there was an acute increase in median platelet count from 169,000 to 420,000/µl. With increasing time post-implant, the median platelet count returned to pre-implant levels; no patients became thrombocytopenic. Pre-implant, 6 of 13 patients had 'positive' anti-PF4/heparin titers (OD >0.4 using the HIT criteria); the median OD for all patients was 0.433. There was a progressive increase in median titer with increasing time post-implant. At 1 month the median OD was 0.54. By 2 months all patients had an OD >0.4 with a median value of 0.82 (>0.750 is high clinical thrombosis risk using the HIT criteria). The median OD continued to rise through 8 months post-implant. At 5 months and later OD values >1.5 were common. OD values >1.5 were present in some patients as early as 1 month post-implant.

Discussion: The mechanism(s) associated with CF-VAD pump thrombosis and other adverse events remains unclear. The above data shows that long-term implant of a CF-VAD leads to the development of anti-PF4/heparin antibodies. Such a response may result from shear-induced platelet activation and subsequent release of PF4. This response is similar to a previous report in which patients who underwent total knee or hip arthroplasty and subsequently received dynamic mechanical thromboprophylaxis demonstrated an anti-PF4/heparin immune response even in the absence of heparin (Bito et al, Blood 2016). CF-VAD-related infection may also contribute to the generation of anti-PF4/heparin antibodies as PF4 bound to bacteria has previously been shown to induce antibody formation (Krauel et al, Blood 2011). Data from our lab has shown that patients with implanted CF-VADs are frequently in heightened states of inflammation (ASH 2016 abstract) with excessive levels of C-reactive protein in patients who develop pump thrombosis. CRP has been shown to activate platelets and accelerate thrombogenesis (Xu et al, BMC Immunology 2015). Thus, it is plausible that the development of anti-PF4/heparin antibodies may be an indicator of an early activation mechanism of adverse events in CF-VAD implanted patients through the cross-talk between the hemostatic and immune systems. Larger studies are required to determine whether the presence of such antibodies confers any risk to LVAD patients. The presence of anti-PF4/heparin antibodies may be of particular concern for LVAD patients undergoing a subsequent surgery for pump replacement or heart transplant.