Temporal leukocyte numbers and granulocyte activation in pulsatile and rotary ventricular assist device patients

Increased phagocytosis capacity of circulating neutrophils in patients on continuous flow ventricular assist device support

BACKGROUND

Neutrophils take part in the innate immune response, phagocytosis, and pro-inflammatory cytokine release. The phagocytic capacity of circulating neutrophils in patients on continuous flow (CF) ventricular assist device (VAD) has not been well studied.

METHODS

Blood samples from 14 patients undergoing CF-VAD implantation were collected and analyzed preoperatively (at baseline) and on postoperative days (POD) 3, 7, 14, and 28. Flow cytometry was used to assess the surface expression levels of CD62L, CD162, and macrophage antigen-1 (MAC-1) and neutrophil phagocytic capacity. Interleukin 1 (IL1), IL6, IL8, TNF-α, neutrophil elastase, and myeloperoxidase in plasma were measured using enzyme-linked immunosorbent assays.

RESULTS

Among the 14 patients, seven patients had preoperative bridge device support. Relative to baseline, patients with no bridge device had elevated leukocyte count and neutrophil elastase by POD3 which normalized by POD7. Neutrophil activation level, IL6, IL8, and TNF-α increased by POD3 and sustained elevated levels for 7-14 days postoperatively. Elevated neutrophil phagocytic capacity persisted even until POD28. Similar patterns were observed in patients on a preoperative bridge device.

CONCLUSIONS

Neutrophil activation and phagocytic capacity increased in response to VAD support, while inflammatory cytokines remain elevated for up to 2 weeks postoperatively. These findings may indicate that VAD implantation elicits circulating neutrophils to an abnormal preemptive phagocytotic phenotype.

Hemocompatibility and biophysical interface of left ventricular assist devices and total artificial hearts

ABSTRACT

Over the past 2 decades, there has been a significant increase in the utilization of long-term mechanical circulatory support (MCS) for the treatment of cardiac failure. Left ventricular assist devices (LVADs) and total artificial hearts (TAHs) have been developed in parallel to serve as bridge-to-transplant and destination therapy solutions. Despite the distinct hemodynamic characteristics introduced by LVADs and TAHs, a comparative evaluation of these devices regarding potential complications in supported patients, has not been undertaken. Such a study could provide valuable insights into the complications associated with these devices. Although MCS has shown substantial clinical benefits, significant complications related to hemocompatibility persist, including thrombosis, recurrent bleeding, and cerebrovascular accidents. This review focuses on the current understanding of hemostasis, specifically thrombotic and bleeding complications, and explores the influence of different shear stress regimens in long-term MCS. Furthermore, the role of endothelial cells in protecting against hemocompatibility-related complications of MCS is discussed. We also compared the diverse mechanisms contributing to the occurrence of hemocompatibility-related complications in currently used LVADs and TAHs. By applying the existing knowledge, we present, for the first time, a comprehensive comparison between long-term MCS options.

Influence of Hematocrit Level and Integrin αIIbβIII Function on vWF-Mediated Platelet Adhesion and Shear-Induced Platelet Aggregation in a Sudden Expansion

Purpose

Shear-mediated thrombosis is a clinically relevant phenomenon that underlies excessive arterial thrombosis and device-induced thrombosis. Red blood cells are known to mechanically contribute to physiological hemostasis through margination of platelets and vWF, facilitating the unfurling of vWF multimers, and increasing the fraction of thrombus-contacting platelets. Shear also plays a role in this phenomenon, increasing both the degree of margination and the near-wall forces experienced by vWF and platelets leading to unfurling and activation. Despite this, the contribution of red blood cells in shear-induced platelet aggregation has not been fully investigated-specifically the effect of elevated hematocrit has not yet been demonstrated.

Methods

Here, a microfluidic model of a sudden expansion is presented as a platform for investigating platelet adhesion at hematocrits ranging from 0 to 60% and shear rates ranging from 1000 to 10,000 s-1. The sudden expansion geometry models nonphysiological flow separation characteristic to mechanical circulatory support devices, and the validatory framework of the FDA benchmark nozzle. PDMS microchannels were fabricated and coated with human collagen. Platelets were fluorescently tagged, and blood was reconstituted at variable hematocrit prior to perfusion experiments. Integrin function of selected blood samples was inhibited by a blocking antibody, and platelet adhesion and aggregation over the course of perfusion was monitored.

Results

Increasing shear rates at physiological and elevated hematocrit levels facilitate robust platelet adhesion and formation of large aggregates. Shear-induced platelet aggregation is demonstrated to be dependent on both αIIbβIII function and the presence of red blood cells. Inhibition of αIIbβIII results in an 86.4% reduction in overall platelet adhesion and an 85.7% reduction in thrombus size at 20-60% hematocrit. Hematocrit levels of 20% are inadequate for effective platelet margination and subsequent vWF tethering, resulting in notable decreases in platelet adhesion at 5000 and 10,000 s-1 compared to 40% and 60%. Inhibition of αIIbβIII triggered dramatic reductions in overall thrombus coverage and large aggregate formation. Stability of platelets tethered by vWF are demonstrated to be αIIbβIII-dependent, as adhesion of single platelets treated with A2A9, an anti-αIIbβIII blocking antibody, is transient and did not lead to sustained thrombus formation.

Conclusions

This study highlights driving factors in vWF-mediated platelet adhesion that are relevant to clinical suppression of shear-induced thrombosis and in vitro assays of platelet adhesion. Primarily, increasing hematocrit promotes platelet margination, permitting shear-induced platelet aggregation through αIIbβIII-mediated adhesion at supraphysiological shear rates.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-024-00796-0.

Bioprosthetic Total Artificial Heart Implantation Does Not Induce Chronic Inflammation

The Aeson total artificial heart (A-TAH) has been developed for patients at risk of death from biventricular failure. We aimed to assess the inflammatory status in nine subjects implanted with the A-TAH in kinetics over one year. Laboratory assessment of leukocyte counts, inflammatory cytokines assay, and peripheral blood mononuclear cell collection before and after A-TAH implantation. Leukocyte counts were not significantly modulated according to time after A-TAH implantation (coefficient of the linear mixed effect model with 95% CI, -0.05 (-0.71 to -0.61); p = 0.44). We explored inflammatory cytokine after A-TAH and did not observe, at any time, a modified profile compared to pre-implantation values (all p -values > 0.05). Finally, we compared the distribution of circulating immune cell subpopulations identified based on sequential expression patterns for multiple clusters of differentiation. None of the population explored had significant modulation during the 12-month follow-up (all p -values > 0.05). In conclusion, using a cytokine multiplex assay combined with a flow cytometry approach, we demonstrated the absence of inflammatory signals in peripheral blood over a period of 12 months following A-TAH implantation.

Neutrophil dysfunction due to continuous mechanical shear exposure in mechanically assisted circulation in vitro

OBJECTIVE

Leukocytes play an important role in the body's immune system. The aim of this study was to assess alterations in neutrophil phenotype and function in pump-assisted circulation in vitro.

METHODS

Human blood was circulated for four hours in three circulatory flow loops with a CentriMag blood pump operated at a flow of 4.5 L/min at three rotational speeds (2100, 2800, and 4000 rpm), against three pressure heads (75, 150, and 350 mm Hg), respectively. Blood samples were collected hourly for analyses of neutrophil activation state (Mac-1, CD62L, CD162), neutrophil reactive oxygen species (ROS) production, apoptosis, and neutrophil phagocytosis.

RESULTS

Activated neutrophils indicated by both Mac-1 expression and decreased surface expression of CD62L and CD162 receptors increased with time in three loops. The highest level of neutrophil activation was observed in the loop with the highest rotational speed. Platelet-neutrophil aggregates (PNAs) progressively increased in two loops with lower rotational speeds. PNAs peaked at one hour after circulation and decreased subsequently in the loop with the highest rotational speed. Neutrophil ROS production dramatically increased at one hour after circulation and decreased subsequently in all three loops with similar levels and trends. Apoptotic neutrophils increased with time in all three loops. Neutrophil phagocytosis capacity in three loops initially elevated at one hour after circulation and decreased subsequently. Apoptosis and altered phagocytosis were dependent on rotational speed.

CONCLUSIONS

Our study revealed that the pump-assisted circulation induced neutrophil activation, apoptosis, and functional impairment. The alterations were strongly associated with pump operating condition and duration.

Preclinical Device Thrombogenicity Assessments: Key Messages From the 2018 FDA, Industry, and Academia Forum

Device-related thrombosis and thromboembolic complications remain a major clinical concern and often impact patient morbidity and mortality. Thus, improved preclinical thrombogenicity assessment methods that better predict clinical outcomes and enhance patient safety are needed. However, there are several challenges and limitations associated with developing and performing preclinical thrombogenicity assessments on the bench and in animals (e.g., the clinical relevance of most in vitro tests has not been established, animal studies may not accurately predict clinical thrombotic events). To facilitate a discussion on how to overcome some of these challenges and to promote collaboration between the Food and Drug Administration (FDA), industry, and academia for the development of more reliable test methods, a scientific forum was organized by FDA and held in Washington, DC, on June 15, 2018 at the ASAIO 64th Annual Conference. Three subject matter experts from the medical device industry and FDA presented their perspectives at this forum, and several audience experts provided input during the open dialogue session. This article summarizes the key messages from the forum regarding the current status and challenges of preclinical thrombogenicity testing, important areas of needed research, and mechanisms for working with FDA to further improve thrombogenicity evaluations of medical devices.

Temporal expression of cytokines and B-cell phenotypes during mechanical circulatory support

BACKGROUND

Allosensitization during mechanical circulatory support (MCS) is a well-described phenomenon, although its mechanism remains unknown. Although immune-mediated interactions from devices or blood transfusions have been proposed, the role of inflammation in this development is less clear. This study was undertaken to further investigate the temporal association of cytokines and B-cell phenotypes in the MCS population.

METHODS

Adult patients who received the Heartmate II (Thoratec, Pleasanton, Calif) at our center between September 2012 and March 2015 were prospectively followed after device implantation. Blood draws for anti-human leukocyte antigen (HLA) antibody, cytokine expression, and B-cell immunophenotyping were performed before implantation and for 3 weeks postoperatively. Time courses for cytokines and B-cell subsets were expressed using visual representations of median levels as heat maps, and mixed modeling analysis was used to model changes with time and patient factors.

RESULTS

Twenty patients who received the Heartmate II (Thoratec) were analyzed during the study period. Four patients showed measureable levels of anti-HLA antibody during the follow-up period, although 3 of these had evidence of antibodies preoperatively. Analysis of cytokine trends revealed early (interleukin [IL]-6, IL-8, and IL-10) and late peaking (IL-3, IL-4, fibroblast growth factor 2, and CD40L) patterns. Upregulation of switched memory, transitional, and plasma blast B cells occurred over time. Right ventricular assist device use and low Interagency Registry for Mechanically Assisted Circulatory Support score were associated with decreased mature naive and increased antibody-secreting cells.

CONCLUSIONS

MCS device implantation was associated with increased inflammatory cytokines and maturation of B-cell phenotypes. No patients developed de novo HLA antibodies, whereas several showed increases in anti-HLA antibody levels detected before implantation. This suggests that inflammation and maturation of existing sensitized B cells might play an important role in the pathogenesis of allosensitization in MCS.

Artificial shear stress effects on leukocytes at a biomaterial interface

Medical devices, such as ventricular assist devices (VADs), introduce both foreign materials and artificial shear stress to the circulatory system. The effects these have on leukocytes and the immune response are not well understood. Understanding how these two elements combine to affect leukocytes may reveal why some patients are susceptible to recurrent device-related infections and provide insight into the development of pump thrombosis. Biomaterials-DLC: diamond-like carbon-coated stainless steel; Sap: single-crystal sapphire; and Ti: titanium alloy (Ti₆ Al₄ V) were attached to the parallel plates of a rheometer. Whole human blood was left between the two discs for 5 minutes at +37°C with or without the application of shear stress (0 s^-1 or 1000 s^-1 ). Blood was removed and used for complete blood cell counts, flow cytometry (leukocyte activation, cell death, microparticle generation, phagocytic ability, and reactive oxygen species [ROS] production), and the production of pro-inflammatory cytokines. L-selectin expression on monocytes was decreased when blood was exposed to the biomaterials both with and without shear. Applying shear stress to blood on a Sap and Ti surface led to activation of neutrophils shown as decreased L-selectin expression. Sap and Ti blunted the LPS-stimulated macrophage migration inhibitory factor (MIF) production, most notably when sheared on Ti. The biomaterials used here have been shown to activate leukocytes in a static environment. The introduction of shear appears to exacerbate this activation. Interestingly, a widely accepted biocompatible material (Ti) utilized in many different types of devices has the capacity for immune cell activation and inhibition of MIF secretion when combined with shear stress. These findings contribute to our understanding of the contribution of biomaterials and shear stress to recurrent infections and vulnerability to sepsis in some VAD patients as well as pump thrombosis.

Mechanical shear stress and leukocyte phenotype and function: Implications for ventricular assist device development and use

Heart failure remains a disease of ever increasing prevalence in the modern world. Patients with end-stage heart failure are being referred increasingly for mechanical circulatory support. Mechanical circulatory support can assist patients who are ineligible for transplant and stabilise eligible patients prior to transplantation. It is also used during cardiopulmonary bypass surgery to maintain circulation while operating on the heart. While mechanical circulatory support can stabilise heart failure and improve quality of life, complications such as infection and thrombosis remain a common risk. Leukocytes can contribute to both of these complications. Contact with foreign surfaces and the introduction of artificial mechanical shear stress can lead to the activation of leukocytes, reduced functionality and the release of pro-inflammatory and pro-thrombogenic microparticles. Assessing the impact of mechanical trauma to leukocytes is largely overlooked in comparison to red blood cells and platelets. This review provides an overview of the available literature on the effects of mechanical circulatory support systems on leukocyte phenotype and function. One purpose of this review is to emphasise the importance of studying mechanical trauma to leukocytes to better understand the occurrence of adverse events during mechanical circulatory support.

The Inflammatory Response to Ventricular Assist Devices

The therapeutic use of ventricular assist devices (VADs) for end-stage heart failure (HF) patients who are ineligible for transplant has increased steadily in the last decade. In parallel, improvements in VAD design have reduced device size, cost, and device-related complications. These complications include infection and thrombosis which share underpinning contribution from the inflammatory response and remain common risks from VAD implantation. An added and underappreciated difficulty in designing a VAD that supports heart function and aids the repair of damaged myocardium is that different types of HF are accompanied by different inflammatory profiles that can affect the response to the implanted device. Circulating inflammatory markers and changes in leukocyte phenotypes receive much attention as biomarkers for mortality and disease progression. However, they are seldom used to monitor progress during and outcomes from VAD therapy or during the design phase for new devices. Even the partial reversal of heart damage associated with heart failure is a desirable outcome from VAD use. Therefore, improved understanding of the interplay between VADs and the recipient's inflammatory response would potentially increase their uptake, improve patient lives, and fuel research related to other blood-contacting medical devices. Here we provide a review of what is currently known about inflammation in heart failure and how this inflammatory profile is altered in heart failure patients receiving VAD therapy.

Ovine Leukocyte Microparticles Generated by the CentriMag Ventricular Assist Device In Vitro

Ventricular assist devices (VADs) are a life-saving form of mechanical circulatory support in heart failure patients. However, VADs have not yet reached their full potential due to the associated side effects (thrombosis, bleeding, infection) related to the activation and damage of blood cells and proteins caused by mechanical stress and foreign materials. Studies of the effects of VADs on leukocytes are limited, yet leukocyte activation and damage including microparticle generation can influence both thrombosis and infection rates. Therefore, the aim was to develop a multicolor flow cytometry assessment of leukocyte microparticles (LMPs) using ovine blood and the CentriMag VAD as a model for shear stress. Ovine blood was pumped for 6 h in the CentriMag and regular samples analyzed for hemolysis, complete blood counts and LMP by flow cytometry during three different pump operating conditions (low flow, standard, high speed). The high speed condition caused significant increases in plasma-free hemoglobin; decreases in total leukocytes, granulocytes, monocytes, and platelets; increases in CD45⁺ LMPs as well as two novel LMP populations: CD11b^bright /HLA-DR^- and CD11b^dull /HLA-DR⁺ , both of which were CD14^- /CD21^- . CD11b^bright /HLA-DR^- LMPs appeared to respond to an increase in shear magnitude whereas the CD11b^dull /HLA-DR⁺ LMPs significantly increased in all pumping conditions. We propose that these two populations are released from granulocytes and T cells, respectively, but further research is needed to better characterize these two populations.

The effect of ventricular assist device-associated biomaterials on human blood leukocytes

Ventricular assist devices (VADs) are an effective bridging or destination therapy for patients with advanced stage heart failure. These devices remain susceptible to adverse events including infection, bleeding, and thrombus; events linked to the foreign body response. Therefore, the biocompatibility of all biomaterials used is crucial to the success of medical devices. Biomaterials common in VADs-DLC: diamond-like carbon coated stainless steel; Sap: single-crystal sapphire; SiN: silicon nitride; Ti: titanium alloy; and ZTA: zirconia-toughened alumina-were tested for their biocompatibility through incubation with whole human blood for 2 h with mild agitation. Blood was then removed and used for: complete cell counts; leukocyte activation and death, and the production of key inflammatory cytokines. All were compared to time 0 and an un-exposed 2 h sample. Monocyte numbers were lower after exposure to DLC, SiN, and ZTA and monocytes showed evidence of activation with DLC, Sap, and SiN. Neutrophils and lymphocytes were unaffected. This approach allows comprehensive analysis of the potential blood damaging effects of biomaterials. Monocyte activation by DLC, Sap, ZTA, and SiN warrants further investigation linking effects on this cell type to unfavorable inflammatory/thrombogenic responses to VADs and other blood handling devices. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1730-1738, 2018.

Visualization and analysis of biomaterial-centered thrombus formation within a defined crevice under flow

The blood flow pathway within a device, together with the biomaterial surfaces and status of the patient's blood, are well-recognized factors in the development of thrombotic deposition and subsequent embolization. Blood flow patterns are of particular concern for devices such as blood pumps (i.e. ventricular assist devices, VADs) where shearing forces can be high, volumes are relatively large, and the flow fields can be complex. However, few studies have examined the effect of geometric irregularities on thrombus formation on clinically relevant opaque materials under flow. The objective of this study was to quantify human platelet deposition onto Ti6Al4V alloys, as well as positive and negative control surfaces, in the region of defined crevices (∼50-150 μm in width) that might be encountered in many VADs or other cardiovascular devices. To achieve this, reconstituted fresh human blood with hemoglobin-depleted red blood cells (to achieve optical clarity while maintaining relevant rheology), long working optics, and a custom designed parallel plate flow chamber were employed. The results showed that the least amount of platelet deposition occurred in the largest crevice size examined, which was counterintuitive. The greatest levels of deposition occurred in the 90 μm and 53 μm crevices at the lower wall shear rate. The results suggest that while crevices may be unavoidable in device manufacturing, the crevice size might be tailored, depending on the flow conditions, to reduce the risk of thromboembolic events. Further, these data might be used to improve the accuracy of predictive models of thrombotic deposition in cardiovascular devices to help optimize the blood flow path and reduce device thrombogenicity.

Evaluation of Four Veterinary Hematology Analyzers for Bovine and Ovine Blood Counts for In Vitro Testing of Medical Devices

Small affordable automated hematology analyzers that produce rapid and accurate complete blood cell counts are a valuable tool to researchers developing blood-handling medical devices, such as ventricular assist devices, for in vitro safety assessments. In such studies, it is common to use the blood of large animals such as cattle and sheep. However, the commercially available instruments have not been evaluated for their ability to measure the blood counts of these animals. In this study, we compare, for the first time, four veterinary analyzers for blood counts on bovine and ovine blood samples. We look at ease of use, repeatability and agreement with a view to inform researchers of the benefits of these instruments in routine measurement of ovine and bovine bloods during in vitro testing. Complete blood cell counts and a three-part differential (granulocytes, monocytes, and lymphocytes) were measured by each of the instruments, and the results compared to those obtained from two additional analyzers used in a reference laboratory. Repeatability and agreement were evaluated using the Bland-Altman method; bias and 95% limits of agreement between the instruments, and between the instruments and two reference instruments, were used to evaluate instrument performance. In summary, there are advantages and disadvantages with all instruments. Of the four instruments tested, the repeatability and agreement was fairly similar for all instruments except one instrument which cannot be recommended for bovine or ovine blood counts.

The CentriMag centrifugal blood pump as a benchmark for in vitro testing of hemocompatibility in implantable ventricular assist devices

Implantable ventricular assist devices (VADs) have proven efficient in advanced heart failure patients as a bridge-to-transplant or destination therapy. However, VAD usage often leads to infection, bleeding, and thrombosis, side effects attributable to the damage to blood cells and plasma proteins. Measuring hemolysis alone does not provide sufficient information to understand total blood damage, and research exploring the impact of currently available pumps on a wider range of blood cell types and plasma proteins such as von Willebrand factor (vWF) is required to further our understanding of safer pump design. The extracorporeal CentriMag (Thoratec Corporation, Pleasanton, CA, USA) has a hemolysis profile within published standards of normalized index of hemolysis levels of less than 0.01 g/100 L at 100 mm Hg but the effect on leukocytes, vWF multimers, and platelets is unknown. Here, the CentriMag was tested using bovine blood (n = 15) under constant hemodynamic conditions in comparison with a static control for total blood cell counts, hemolysis, leukocyte death, vWF multimers, microparticles, platelet activation, and apoptosis. The CentriMag decreased the levels of healthy leukocytes (P < 0.006), induced leukocyte microparticles (P < 10(-5) ), and the level of high molecular weight of vWF multimers was significantly reduced in the CentriMag (P < 10(-5) ) all compared with the static treatment after 6 h in vitro testing. Despite the leukocyte damage, microparticle formation, and cleavage of vWF multimers, these results show that the CentriMag is a hemocompatible pump which could be used as a standard in blood damage assays to inform the design of new implantable blood pumps.

Real time visualization and characterization of platelet deposition under flow onto clinically relevant opaque surfaces

Although the thrombogenic nature of the surfaces of cardiovascular devices is an important aspect of blood biocompatibility, few studies have examined platelet deposition onto opaque materials used for these devices in real time. This is particularly true for the metallic surfaces used in current ventricular assist devices (VADs). Using hemoglobin depleted red blood cells (RBC ghosts) and long working distance optics to visualize platelet deposition, we sought to perform such an evaluation. Fluorescently labeled platelets mixed with human RBC ghosts were perfused across six opaque materials (a titanium alloy (Ti6Al4V), silicon carbide (SiC), alumina (Al2O3, 2-methacryloyloxyethyl phosphorylcholine polymer coated Ti6Al4V (MPC-Ti6Al4V), yttria partially stabilized zirconia (YZTP), and zirconia toughened alumina (ZTA)) for 5 min at wall shear rates of 400 and 1000 s(-1). Ti6Al4V had significantly increased platelet deposition relative to MPC-Ti6Al4V, Al2 O3 , YZTP, and ZTA at both wall shear rates (p < 0.01). For all test surfaces, increasing the wall shear rate produced a trend of decreased platelet adhesion. The described system can be a utilized as a tool for comparative analysis of candidate blood-contacting materials with acute blood contact.