Microphysiological systems (MPS) are envisioned to improve drug approval success rates. Yet, integration of MPS into drug development processes has been hampered in part by uncertainties in data translation. To speed adoption, development of animal cell-based MPS is advocated by the pharmaceutical industry. In our view, animal MPS availability would fill a key gap in the ability to examine in vitro to in vivo translatability. Since in vivo animal data will be available and guide decision making in regulatory activities for the foreseeable future, there is significant opportunity for translational assessments. In vivo animal study findings that are recapitulated using in vitro models generated from the corresponding animal species provide validation that those models possess the relevant and necessary attributes, e.g., species-specific pharmacodynamics, metabolism, transport, susceptibility to toxicity, etc. Results from the corresponding human models can then be interpreted with greater confidence for the relevant context of use (COU). Some drugs do not get to the clinic due to adverse findings in animals, so there is considerably more data to directly compare to animal in vitro models than human systems. Another benefit of animal MPS is that drug candidates exhibiting animal safety findings might be easier to derisk, for example if the finding was observed in animal but not the corresponding human in vitro model. This paper reviews considerations and recommendations for adopting animal MPS models in drug discovery and development and describes how their deployment is consistent with 3Rs principles.

Drug permeability across epithelial barriers is critical for predicting oral bioavailability and efficacy. Conventional models, including animals and simple cell cultures, lack intestinal complexity and translational value. Organoids and organ-on-a-chip technologies address these limitations, offering physiologically relevant, human-specific platforms for more accurate drug permeability assessment [1]. Intestinal organoids, derived from stem cells, reproduce key structural and functional features of the gut, including crypt–villus organization, transporter and enzyme expression, and absorptive and secretory functions. Organoids are grown inside of microfluidic chips under dynamic media flow that attempt to recapitulate one or more tissue-specific functions, thereby supporting epithelial maturation and enabling more predictive drug transport measurements. Permeability can be studied by introducing compounds at the apical surface and quantifying their translocation to the basolateral compartment using analytical techniques. Compared with animal models, these approaches yield data of greater human relevance while reducing ethical concerns [2]. Furthermore, they enable the evaluation of passive permeability, active transport, intestinal metabolism, and interactions with microbial products [3]. In this study, permeability was investigated for three model compounds with distinct properties: lisinopril (low permeability), metoprolol (moderate permeability), and fluconazole (high permeability). Experiments were conducted using the Emulate human duodenum-on-a-chip platform and benchmarked against the Ralph Russ Canine Kidney (RRCK) cell line. Apparent permeability (Papp) values were correlated with literature-reported effective permeability (Peff) using linear regression. This regression was implemented in Simcyp® Simulator and GastroPlus™ to predict systemic exposure, which was compared against observed plasma concentration–time profiles.

One of the potential risk factors of recombinant adeno-associated virus (rAAV)-based gene therapy is insertional mutagenesis, which has been associated with the development of hepatocellular carcinoma (HCC) in rAAV-treated neonatal mice. The objective of this study was to investigate if well-established in vitro cell transformation assays (CTA) in mouse cell lines can detect AAV2 or AAVdj-mediated cell transformation. Since AAV integration at the Rian locus in neonatal mice has been implicated in AAV-mediated HCC, an rAAV vector specifically targeting the mouse Rian locus and an additional rAAV vector previously shown to cause HCC in neonatal mice were both tested for the induction of cell transformation in NIH3T3 cells. To increase the frequency of AAV DNA integration at the Rian locus in the genome of NIH3T3 cells, double-strand breaks in Rian locus of NIH3T3 cells were created by CRISPR-Cas9 to increase the homologous crossover between viral DNA and the cell genome. When transduced cells were assayed in CTA, the transformation frequency observed in AAV-transduced NIH3T3 cells was not significantly different from that of untreated vehicle cells. The finding that rAAV is unable to transform the NIH3T3 in vitro indicates that either the transformation rate is less than the spontaneous rate of NIH3T3 cellular transformation, or in vitro CTA are not predictive of rAAV-induced HCC in mice.

BACKGROUND:Presence of hemolysis, the breakdown of red blood cells, in biological samples such as serum and plasma can negatively affect accurate quantification of many biomarkers including commonly measured clinical chemistry and cytokine parameters. Consequently, hemolyzed samples are frequently rejected as unsuitable for biomarker analysis. Currently, the potential interference of hemolysis on cytokine quantification is incompletely understood. Furthermore, the degree of interference with hemolysis may vary by species, cytokine, and quantification platform. Here, we evaluated the effect of various degrees of hemolysis on cytokine quantification accuracy in animal and human samples. METHODS:Frequency of hemolyzed samples in 339 internal animal studies was evaluated and analyzed for various degrees of hemolysis based on visual descriptors converted from H index values. Separately, hemolysis was simulated in vitro by a combination of freeze-thaw, osmotic shock, and shear stress using blood from mice, rats, dogs, cynomolgus macaque monkeys, and humans. Quantification of cytokines interferon gamma, interleukins (IL) IL-10, IL-1β, IL-2, IL-6, interferon gamma-induced protein 10, monocyte chemoattractant protein-1, and tumor necrosis factor-α was evaluated after spiking into hemolyzed and nonhemolyzed serum samples using Meso Scale Discovery and Bio-Rad Bioplex 200 platforms. RESULTS:Comparison of cytokine levels in hemolyzed samples vs nonhemolyzed controls demonstrated that slight hemolysis (H index = 1 equivalent to 11-130 mg/dL of hemoglobin) did not interfere with the accuracy of cytokine quantification in most cases. CONCLUSIONS:Evidence of slight hemolysis was present in most hemolyzed samples across species. We show that, in general, the cytokines can be accurately quantified in animal samples when slight to moderate hemolysis is present.

Most complex in vitro models (CIVM) and microphysiological systems (MPS) are composed of human cells, with the goal of evaluating diseases, efficacy, safety, and pharmacokinetic ques­tions specifically for humans. The hope with CIVM/MPS is that they will eventually improve our predictivity of clinical responses and reduce or replace animal use in research, supporting the 3Rs concept of only using animals in research when necessary. Given the potential of animal-based models to advance this field by comparing existing in vivo animal data with new animal-based MPS responses, there are currently few CIVM and MPS utilizing animal tissues. Animal-based MPS may also have specific utility for cross-species comparisons or species-specific mechanistic questions on zoonotic diseases, and therapies for animals. Animal-based MPS may help expand in-vitro-to-in-vivo correlations, advance the field, and establish confidence in the predictive nature of such platforms. The IQ MPS-FDA workshop provided an interactive venue for pharmaceutical companies and regulatory agencies such as the U.S. Food and Drug Administration (FDA), NC3Rs (UK), Health Canada, NIH/NCATS, NIHS and PMDA (Japan), Danish Medicines Agency, European Com­mission, NIEHS/NICEATM, HHS, NIST, EURL ECVAM, and the IQ MPS Affiliate, a collaboration of pharmaceutical companies, to jointly discuss considerations of animal-based MPS and applica­tions where animal-based MPS are of potential value.

Microphysiological systems (MPS) are comprised of one or multiple cell types of human or animal origins that mimic the biochemical/electrical/mechanical responses and blood-tissue barrier properties of the cells observed within a complex organ. The goal of incorporating these in vitro systems is to expedite and advance the drug discovery and development paradigm with improved predictive and translational capabilities. Considering the industry need for improved efficiency and the broad challenges of model qualification and acceptance, the International Consortium for Innovation and Quality (IQ) founded an IQ MPS working group in 2014 and Affiliate in 2018. This group connects thought leaders and end users, provides a forum for crosspharma collaboration, and engages with regulators to qualify translationally relevant MPS models. To understand how pharmaceutical companies are using MPS, the IQ MPS Affiliate conducted two surveys in 2019, survey 1, and 2021, survey 2, which differed slightly in the scope of definition of the complex in vitro models under question. The surveys captured demographics, resourcing, rank order for organs of interest, compound modalities tested, and MPS organ-specific questions, including nonclinical species needs and cell types. The major focus of this manuscript is on results from survey 2, where we specifically highlight the context of use for MPS within safety, pharmacology, or absorption, disposition, metabolism, and excretion and discuss considerations for including MPS data in regulatory submissions. In summary, these data provide valuable insights for developers, regulators, and pharma, offering a view into current industry practices and future considerations while highlighting key challenges impacting MPS adoption. SIGNIFICANCE STATEMENT: The application of microphysiological systems (MPS) represents a growing area of interest in the drug discovery and development framework. This study surveyed 20+ pharma companies to understand resourcing, current areas of application, and the key challenges and barriers to internal MPS adoption. These results will provide regulators, tech providers, and pharma industry leaders a starting point to assess the current state of MPS applications along with key learnings to effectively realize the potential of MPS as an emerging technology.

Pharmaceutical companies continue to advocate for the use of in vitro models towards the reduction of animal use in drug discovery and development while acknowledging that further advancements are needed to heighten the models’ current state of readiness.

Immune responses are heavily involved in the regulation and pathogenesis of human diseases, including infectious diseases, inflammatory and autoimmune conditions, cancer, neurological disorders, and cardiometabolic syndromes. The immune system is considered a double-edged sword serving as a powerful host defense mechanism against infection and cancerous cells and causing detrimental tissue damage when the immune response is exaggerated or uncontrollable. One of the challenges in studying the efficacy and toxicity of drugs that target or modulate the immune system is the lack of suitable preclinical human models that are predictive of human response. Recent advancements in human microphysiological systems (MPS) have provided a promising in vitro platform to evaluate the response of immune organs ex vivo, to investigate the interaction of immune cells with non-lymphoid tissue cells, and to reduce the reliance on animals in preclinical studies. The development, regulation, trafficking, and responses of immune cells have been extensively studied in preclinical animal models and clinically, providing a wealth of knowledge by which to evaluate new in vitro models. Therefore, the application of immunocompetent MPS in drug discovery and development should first verify that the immune response in an MPS model recapitulates the complexity of the human immune physiology. This manuscript reviews biological functions of immune organ systems and tissue-resident immune cells and discusses contexts-of-use for commonly used immunocompetent and immune organ MPS models. Current perspective and recommendations are provided to guide the continued development of immune organ and immunocompetent MPS models and their application in drug discovery and development.

In May 2022, there is an International Regulatory and Pharmaceutical Industry (Innovation and Quality [IQ] Microphysiological Systems [MPS] Affiliate) Workshop on the standardization of complex in vitro models (CIVMs) in drug development. This manuscript summarizes the discussions and conclusions of this joint workshop organized and executed by the IQ MPS Affiliate and the United States Food and Drug Administration (FDA). A key objective of the workshop is to facilitate discussions around opportunities and/or needs for standardization of MPS and chart potential pathways to increase model utilization in the context of regulatory decision making. Participation in the workshop included 200 attendees from the FDA, IQ MPS Affiliate, and 26 global regulatory organizations and affiliated parties representing Europe, Japan, and Canada. It is agreed that understanding global perspectives regarding the readiness of CIVM/MPS models for regulatory decision making and potential pathways to gaining acceptance is useful to align on globally. The obstacles are currently too great to develop standards for every context of use (COU). Instead, it is suggested that a more tractable approach may be to think of broadly applicable standards that can be applied regardless of COU and/or organ system. Considerations and next steps for this effort are described.

Complex in vitro models (CIVM) offer the potential to improve pharmaceutical clinical drug attrition due to safety and/ or efficacy concerns. For this technology to have an impact, the establishment of robust characterization and qualifi­cation plans constructed around specific contexts of use (COU) is required. This article covers the output from a workshop between the Food and Drug Administration (FDA) and Innovation and Quality Microphysiological Systems (IQ MPS) Affiliate. The intent of the workshop was to understand how CIVM technologies are currently being applied by pharma­ceutical companies during drug development and are being tested at the FDA through various case studies in order to identify hurdles (real or perceived) to the adoption of microphysiological systems (MPS) technologies, and to address evaluation/qualification pathways for these technologies. Output from the workshop includes the alignment on a working definition of MPS, a detailed description of the eleven CIVM case studies presented at the workshop, in-depth analysis, and key take aways from breakout sessions on ADME (absorption, distribution, metabolism, and excretion), pharmacology, and safety that covered topics such as qualification and performance criteria, species differences and concordance, and how industry can overcome barriers to regulatory submission of CIVM data. In conclusion, IQ MPS Affiliate and FDA scientists were able to build a general consensus on the need for animal CIVMs for preclinical species to better determine species concordance. Furthermore, there was acceptance that CIVM technologies for use in ADME, pharmacology and safety assessment will require qualification, which will vary depending on the specific COU.

BACKGROUND:The blood coagulation factor fibrin(ogen) can modulate inflammation by altering leukocyte activity. Analyses of fibrin(ogen)-mediated proinflammatory activity have largely focused on leukocyte integrin binding activity revealed by conversion of fibrinogen to a stabilized fibrin polymer by blood coagulation enzymes. In addition to coagulation enzymes, fibrinogen is a substrate for tissue transglutaminase-2 (TG2), a widely expressed enzyme that produces unique fibrinogen Aα-γ chain cross-linked products.OBJECTIVES:We tested the hypothesis that TG2 dependent cross-linking alters the proinflammatory activity of surface-adhered fibrinogen.METHODS:Mouse bone marrow-derived macrophages (BMDMs) were cultured on tissue culture plates coated with fibrinogen or TG2-cross-linked fibrinogen (10 µg/ml) and then stimulated with lipopolysaccharide (LPS, 1 ng/ml) or vehicle for various times.RESULTS:In the absence of LPS stimulation, TG2-cross-linked fibrin(ogen) enhanced inflammatory gene induction (e.g., Tnfα) compared with unmodified fibrinogen. LPS stimulation induced mitogen-activated protein kinase phosphorylation, IκBα degradation, and expression of proinflammatory cytokines (e.g., tumor necrosis factor α) within 60 min. This initial cellular activation was unaffected by unmodified or TG2-cross-linked fibrinogen. In contrast, LPS induction of interleukin-10 mRNA and protein and STAT3 phosphorylation was selectively attenuated by TG2-cross-linked fibrinogen, which was associated with enhanced proinflammatory cytokine secretion by LPS-stimulated BMDMs at later time points (6 and 24 h).CONCLUSIONS:The results indicate that atypical cross-linking by TG2 imparts unique proinflammatory activity to surface-adhered fibrinogen. The results suggest a novel coagulation-independent mechanism controlling fibrinogen-directed macrophage activation.

Microphysiological systems (MPS) are making advances to provide more standardized and predictive physiologically relevant responses to test articles in living tissues and organ systems. The excitement surrounding the potential of MPS to better predict human responses to medicines and improving clinical translation is overshadowed by their relatively slow adoption by the pharmaceutical industry and regulators. Collaboration between multiorganizational consortia and regulators is necessary to build an understanding of the strengths and limitations of MPS models and closing the current gaps. Here, we review some of the advances in MPS research, focusing on liver, intestine, vascular system, kidney and lung and present examples highlighting the context of use for these systems. For MPS to gain a foothold in drug development, they must have added value over existing approaches. Ideally, the application of MPS will augment in vivo studies and reduce the use of animals via tiered screening with less reliance on exploratory toxicology studies to screen compounds. Because MPS support multiple cell types (e.g. primary or stem-cell derived cells) and organ systems, identifying when MPS are more appropriate than simple 2D in vitro models for understanding physiological responses to test articles is necessary. Once identified, MPS models require qualification for that specific context of use and must be reproducible to allow future validation. Ultimately, the challenges of balancing complexity with reproducibility will inform the promise of advancing the MPS field and are critical for realization of the goal to reduce, refine and replace (3Rs) the use of animals in nonclinical research.

Intravascular fibrin clot formation follows a well-ordered series of reactions catalyzed by thrombin cleavage of fibrinogen leading to fibrin polymerization and cross-linking by factor XIIIa (FXIIIa). Extravascular fibrin(ogen) deposits are observed in injured tissues; however, the mechanisms regulating fibrin(ogen) polymerization and cross-linking in this setting are unclear. The objective of this study was to determine the mechanisms of fibrin polymerization and cross-linking in acute liver injury induced by acetaminophen (APAP) overdose. Hepatic fibrin(ogen) deposition and cross-linking were measured following APAP overdose in wild-type mice, mice lacking the catalytic subunit of FXIII (FXIII-/-), and in FibAEK mice, which express mutant fibrinogen insensitive to thrombin-mediated fibrin polymer formation. Hepatic fibrin(ogen) deposition was similar in APAP-challenged wild-type and FXIII-/- mice, yet cross-linking of hepatic fibrin(ogen) was dramatically reduced (>90%) by FXIII deficiency. Surprisingly, hepatic fibrin(ogen) deposition and cross-linking were only modestly reduced in APAP-challenged FibAEK mice, suggesting that in the APAP-injured liver fibrin polymerization is not strictly required for the extravascular deposition of cross-linked fibrin(ogen). We hypothesized that the oxidative environment in the injured liver, containing high levels of reactive mediators (eg, peroxynitrite), modifies fibrin(ogen) such that fibrin polymerization is impaired without impacting FXIII-mediated cross-linking. Notably, fibrin(ogen) modified with 3-nitrotyrosine adducts was identified in the APAP-injured liver. In biochemical assays, peroxynitrite inhibited thrombin-mediated fibrin polymerization in a concentration-dependent manner without affecting fibrin(ogen) cross-linking over time. These studies depict a unique pathology wherein thrombin-catalyzed fibrin polymerization is circumvented to allow tissue deposition and FXIII-dependent fibrin(ogen) cross-linking.

Acute and chronic liver disease are associated with substantial alterations in the hemostatic system, including elevated levels of the platelet-adhesive protein von Willebrand factor (VWF). Carbon tetrachloride-induced liver fibrosis is reduced in VWF-deficient mice, but it is unclear if VWF plays a pathologic role in all settings of liver fibrosis. Indeed, several studies suggest an anti-fibrotic role for components of the hemostatic system, including platelets, in experimental settings of bile duct fibrosis. However, the role of VWF in this specific pathology has not been examined. We tested the hypothesis that VWF exerts hepatoprotective effects in experimental bile duct injury. Wild-type and VWF-deficient (VWF-/-) mice were challenged with the bile duct toxicant alpha-naphthylisothiocyanate (ANIT) and the impact of VWF deficiency on acute cholestatic liver injury and chronic liver fibrosis was determined. Acute ANIT (60 mg/kg, po)-induced cholestatic liver injury was associated with increased VWF plasma antigen and activity levels. VWF deficiency enhanced ANIT-induced hepatocellular injury, evidenced by increased plasma ALT activity and area of hepatocellular necrosis. Surprisingly, platelet accumulation within necrotic areas was increased in ANIT-challenged VWF-/- mice compared to wild-type mice. Compared to acute ANIT challenge, hepatic platelet accumulation was modest and appeared to be VWF-dependent in mice exposed to ANIT diet (0.05 %) for 6 weeks. However, contrasting the role of VWF after acute ANIT challenge, VWF deficiency did not impact biliary fibrosis induced by chronic ANIT exposure. The results suggest that VWF plays dichotomous roles in experimental acute and chronic ANIT-induced cholestatic liver injury.

Alveolar type II (ATII) epithelial cells contain lamellar bodies (LBs) which synthesize and store lung surfactants. In animals, the inhibition or knockout of leucine-rich repeat kinase 2 (LRRK2) causes abnormal enlargement of LBs in ATII cells. This effect of LRRK2 inhibition in lung is largely accepted as being mediated directly through blocking of the kinase function; however, downstream consequences in the lung remain unknown. In this work we established an in vitro alveolar epithelial cell (AEC) model that recapitulates the in vivo phenotype of ATII cells and developed an assay to quantify changes in LB size in response to LRRK2 inhibitors. Culture of primary human AECs at the air-liquid interface on matrigel and collagen-coated transwell inserts in the presence of growth factors promoted the LB formation and apical microvilli and induced expression of LRRK2 and ATII cell markers. Treatment with a selective LRRK2 inhibitor resulted in pharmacological reduction of phospho-LRRK2 and a significant increase in LB size; effects previously reported in lungs of non-human primates treated with LRRK2 inhibitor. In summary, our human in vitro AEC model recapitulates the abnormal lung findings observed in LRRK2-perturbed animals and holds the potential for expanding current understanding of LRRK2 function in the lung.

Fatty liver disease is a potential risk factor for drug-induced liver injury (DILI). Despite advances in nonclinical in vitro and in vivo models to assess liver injury during drug development, the pharmaceutical industry is still plagued by idiosyncratic DILI. Here, we tested the hypothesis that certain features of asymptomatic metabolic syndrome (namely hepatic steatosis) increase the risk for DILI in certain phenotypes of the human population. Comparison of the Zucker Lean (ZL) and Zucker Fatty rats fed a high fat diet (HFD) revealed that HFD-fed ZL rats developed mild hepatic steatosis with compensatory hyperinsulinemia without increases in liver enzymes. We then challenged steatotic HFD-fed ZL rats and Sprague-Dawley (SD) rats fed normal chow, a nonclinical model widely used in the pharmaceutical industry, with acetaminophen overdose to induce liver injury. Observations in HFD-fed ZL rats included increased liver injury enzymes and greater incidence and severity of hepatic necrosis compared with similarly treated SD rats. The HFD-fed ZL rats also had disproportionately higher hepatic drug accumulation, which was linked with abnormal hepatocellular efflux transporter distribution. Here, we identify ZL rats with HFD-induced hepatic steatosis as a more sensitive nonclinical in vivo test system for modeling DILI compared with SD rats fed normal chow.

Obesity is a prevalent prothrombotic risk factor marked by enhanced fibrin formation and suppressed fibrinolysis. Fibrin both promotes thrombotic events and drives obesity pathophysiology, but a lack of essential analytical tools has left fibrinolytic mechanisms affected by obesity poorly defined. Using a plasmin-specific fluorogenic substrate, we developed a plasmin generation (PG) assay for mouse plasma that is sensitive to tissue plasminogen activator, α2-antiplasmin, active plasminogen activator inhibitor (PAI-1), and fibrin formation, but not fibrin crosslinking. Compared with plasmas from mice fed a control diet, plasmas from mice fed a high-fat diet (HFD) showed delayed PG and reduced PG velocity. Concurrent to impaired PG, HFD also enhanced thrombin generation (TG). The collective impact of abnormal TG and PG in HFD-fed mice produced normal fibrin formation kinetics but delayed fibrinolysis. Functional and proteomic analyses determined that delayed PG in HFD-fed mice was not due to altered levels of plasminogen, α2-antiplasmin, or fibrinogen. Changes in PG were also not explained by elevated PAI-1 because active PAI-1 concentrations required to inhibit the PG assay were 100-fold higher than circulating concentrations in mice. HFD-fed mice had increased circulating thrombomodulin, and inhibiting thrombomodulin or thrombin-activatable fibrinolysis inhibitor (TAFI) normalized PG, revealing a thrombomodulin- and TAFI-dependent antifibrinolytic mechanism. Integrating kinetic parameters to calculate the metric of TG/PG ratio revealed a quantifiable net shift toward a prothrombotic phenotype in HFD-fed mice. Integrating TG and PG measurements may define a prothrombotic risk factor in diet-induced obesity.

Acetaminophen overdose is the leading cause of acute liver failure in the United States. In mice, interleukin‐10 (IL‐10) deficiency dramatically increased early inflammation and hepatotoxicity after acetaminophen challenge. Paradoxically, IL‐10 levels are highest in acute liver failure patients with the poorest prognosis. The mechanistic basis for this apparent discrepancy is not understood. We examined the function of IL‐10 in a mouse model that recapitulates failed liver repair analogous to acute liver failure in patients, wherein mice are treated with a large dose of acetaminophen (i.e., 600 mg/kg). Hepatic IL‐10 mRNA expression was dramatically increased 24 hours after challenge of mice with 600 mg/kg APAP, whereas IL‐10 induction was not observed at this time point in mice given 300 mg/kg APAP. At 24 hours, few IL‐10 expressing cells were detected in the livers of mice challenged with 300 mg/kg acetaminophen, whereas numerous macrophages and other hepatic cells expressed IL‐10 in mice treated with 600 mg/kg acetaminophen. Necrotic lesions were effectively cleared in mice given 300 mg/kg APAP, whereas inefficient macrophage trafficking into the necrotic lesions was associated with failed repair in mice given 600 mg/kg APAP. Remarkably, administration of an IL‐10 neutralizing antibody, 24 hours after treatment of mice with 600 mg/kg APAP, fully restored macrophage accumulation in areas of hepatic necrosis, suggesting exaggerated IL‐10 expression causes macrophage dysfunction essential for repair. Consistent with this hypothesis, administration of recombinant IL‐10 to mice with established APAP (300 mg/kg)‐induced liver necrosis delayed liver repair alongside a marked reduction in macrophage infiltration. Collectively, these studies indicate that dysregulation of IL‐10 in the late repair phase after acute liver injury impairs intrahepatic macrophage trafficking, which may explain failed liver repair and a poor outcome in acute liver failure patients.Support or Funding InformationSupported by NIH Grant DK103895 to BLC and NIH Grant DK105099 to JPL.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

BACKGROUND:Equine herpesvirus type 1 (EHV-1) is the cause of respiratory disease, abortion storms, and outbreaks of herpesvirus myeloencephalopathy (EHM). Infection of the spinal cord is characterised by multifocal regions of virally infected vascular endothelium, associated with vasculitis, thrombosis and haemorrhage that result in ischaemia and organ dysfunction. However, the mechanism of thrombosis in affected horses is unknown.OBJECTIVES:To evaluate tissue factor (TF) procoagulant activity and thrombin-antithrombin complex (TAT) levels in horses following infection with EHV-1.STUDY DESIGN:In vitro and in vivo studies following experimental EHV-1 infection.METHODS:Horses were infected with EHV-1 and levels of peripheral blood mononuclear cell (PBMC)-associated TF activity; plasma and cerebrospinal fluid (CSF)-derived microvesicle (MV)-associated TF activity and TAT complexes in plasma were examined.RESULTS:EHV-1 infection increased PBMC TF procoagulant activity in vitro and in vivo. In infected horses, this increase was observed during the acute infection and was most marked at the onset and end of viraemia. However, no significant differences were observed between the horses that showed signs of EHM and the horses that did not develop EHM. Significant changes in MV-associated TF procoagulant activity and TAT complexes were not observed in infected horses.MAIN LIMITATIONS:A small number of horses typically exhibit clinical EHM following experimental infection.CONCLUSIONS:The results indicate that EHV-1 infection increases PBMC-associated TF procoagulant activity in vivo and in vitro. Additional in vivo studies are needed to better understand the role of TF-dependent coagulation during EHM pathogenesis in horses.