High-Throughput Screening to Develop a Biologically Relevant Netosis Induction Model

Sepsis is a life-threatening organ dysfunction caused by a dysregulated host response to infection (1). It is responsible for ~370,000 US deaths annually (2). Neutrophils are an integral part of the innate immune response and rapidly clear pathogens from circulation using neutrophil extracellular traps (NETs), which are released through a process called NETosis (3). NETs prevent dissemination of pathogens by entrapment in externalized chromatin containing deactivating enzymes. While we have learned much about the mechanisms underlying NETosis, we are yet to translate it to improved therapies or patient outcomes. This gap may be attributable to the models used to study NETosis. Current models used to investigate NETosis are limited and routinely employ unnatural triggers such as phorbol 12-myristate 13-acetate (PMA). PMA is not a physiological trigger present in the immune system and may bypass the natural pathways that regulate NETs production. Mouse models that use isolated neutrophils and neutrophil-like cells induced from immortalized cell lines do not completely reflect the complex cellular and molecular biology underlying neutrophil activation and NETosis, especially in a whole-blood environment. Therefore, it is crucial to study how specific factors, known to be upregulated in disease, interact and potentially induce NETosis. Here we use high-throughput screening and natural NETosis triggers to develop a more biologically relevant ex vivo NETosis (Synthetic-Sepsis™) model.

Whole blood was collected from healthy donors and aliquoted into a 384 well plate using a Formulatrix Mantis liquid handler. This plate contained small molecules associated with neutrophils or NETosis activation, such as interleukins: Il-1b, IL-5, IL-6, IL-8, IL-15, IL-17, IL-18 and other molecules TNF- α, LT-α, IFN-γ, G-CSF, GM-CSF, E-selectin, PAF-16, CXCL1, CXCL2, LTB4, CXCL5, CCL2, CCL3, fMLP, Ferritin, HMGB1, C5a and LPS. We used a combinatorial pooling strategy designed using JMP software to identify which combinations of small molecules could stimulate NET formation. NETosis was assessed using Sytox green intercalation at 5 minute intervals for up to 24 hours using a Molecular Devices plate reader. PMA was utilized as a positive control for NETosis induction at varying concentrations.

Using our combinatorial pooling approach of the various factors, we successfully induced NETosis in an ex vivo whole blood system using naturally occurring cytokines and chemokines at physiologically relevant concentrations. We found that different combinations of factors evoke distinct neutrophil responses both in the time of NET generation and/or magnitude of NET-associated intercalation signal. We observed inter-donor variability in response time and amplitude however, similar small molecule pools induced consistent responses across donors. Furthermore, our findings suggest that at least four naturally occurring factors are necessary to induce NETosis in our system. Although some factors activate similar pathways, they are unable to induce a signal alone and as the number of factors increased beyond four, there was an enhanced NET response. Interestingly, we found either TNF-α or LT-α was required to cause a NETosis response, underlining the potentially significant roles these factors play in inflammatory disease. These results suggest an underlying master regulatory mechanism, such that certain factors are essential but not individually sufficient to trigger NETosis.

To our knowledge, we report the first ex-vivo model using naturally occurring cytokines and chemokines to induce NETosis in whole blood. These findings emphasize the importance of expanding our understanding of neutrophil physiology in a biologically relevant context with physiological triggers to induce NETosis. This approach could reveal new dimensions in our understanding of disease pathology and risk factors and might unearth potential therapeutic targets providing novel strategies for disease intervention and treatment. Further investigation of these factors is underway to further understand the release of NETs in natural and pathological states.

1. Singer M, et al. The Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3). JAMA. 2016 Feb 23;315(8):801.
2. https://www.cdc.gov/sepsis/index.html.
3. Brinkman, et al. Neutrophil Extracellular Traps Kill Bacteria. 2004 Mar 5;303(5663):1532–5.