Sepsis Cell Models for Research

Sepsis is a life-threatening organ dysfunction caused by a dysregulated host response to infection. According to the World Health Organization (WHO), sepsis affects approximately 49 million people globally each year, with an estimated 11 million deaths, representing about 20% of all global deaths. The incidence is particularly high in low- and middle-income countries, but it remains a major cause of mortality in high-income settings as well. Key risk factors include age (extremes of age), immunosuppression, chronic diseases (e.g., diabetes, cancer), and invasive procedures. The 5-year survival rate for sepsis survivors is significantly lower than the general population, with many experiencing long-term physical, cognitive, and psychological sequelae (NCI).

Sepsis is a complex syndrome with heterogeneous clinical presentations, making it an ideal model for studying the interplay between infection, immunity, and organ dysfunction. Research models are essential to dissect the molecular mechanisms underlying the dysregulated inflammatory response, endothelial dysfunction, and immune suppression. Public datasets, such as those from the Gene Expression Omnibus (GEO), provide transcriptomic profiles of sepsis patients, enabling the identification of key pathways and potential therapeutic targets. Open questions include the identification of biomarkers for early diagnosis, the mechanisms of immune paralysis, and the development of targeted therapies that can modulate the host response without compromising antimicrobial defense.

Sepsis pathogenesis involves a complex cascade of events:

1. Pathogen recognition: Pattern recognition receptors (PRRs) such as Toll-like receptors (TLRs) and NOD-like receptors (NLRs) recognize pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs).

2. Inflammatory signaling: Activation of NF-κB and MAPK pathways leads to the production of pro-inflammatory cytokines (e.g., TNF-α, IL-6, IL-1β).

3. Endothelial activation: Cytokines and other mediators activate endothelial cells, leading to increased vascular permeability, coagulation activation, and microvascular thrombosis.

4. Immune dysregulation: An initial hyper-inflammatory phase is often followed by a hypo-inflammatory phase, characterized by immune suppression and increased susceptibility to secondary infections.

5. Organ dysfunction: The combination of microvascular dysfunction, tissue hypoxia, and mitochondrial dysfunction leads to multiple organ failure.

While sepsis is not a cancer, genetic variations in immune-related genes influence susceptibility and outcome. The table below lists key genes with common polymorphisms or mutations that have been associated with sepsis risk or severity.

Data from NCBI and literature.

Sepsis involves the dysregulation of several signaling networks:

• • NF-κB pathway: Central to the inflammatory response. Key nodes include TLRs, MyD88, IRAK, TRAF6, and IKK complex.
• • MAPK pathway: Includes ERK, JNK, and p38, which regulate cytokine production and apoptosis.
• • PI3K/AKT pathway: Modulates cell survival and inflammation.
• • JAK/STAT pathway: Mediates signaling of many cytokines and interferons.
• • Coagulation cascade: Tissue factor and thrombin activation contribute to microvascular thrombosis.
• • Complement system: Activation leads to opsonization and inflammation.

Common cell lines used in sepsis research include:

Organoids, such as intestinal and lung organoids, offer a more physiologically relevant 3D model that recapitulates tissue architecture and cell-cell interactions.

Animal models are crucial for studying sepsis pathophysiology and testing therapies. Common models include:

• • Cecal ligation and puncture (CLP): The gold standard for polymicrobial sepsis.
• • Lipopolysaccharide (LPS) injection: Induces endotoxemia.
• • Colon ascendens stent peritonitis (CASP): Reproduces polymicrobial peritonitis.
• • Patient-derived xenografts (PDX): Used in cancer research but less relevant for sepsis.
• • Genetically engineered mouse models (GEMM): Knockout or transgenic mice for specific genes (e.g., TLR4, TNF-α) to study their roles.

CRISPR-based gene editing enables the creation of isogenic cell lines with precise genetic modifications, providing powerful tools for sepsis research. For example:

• • TLR4 knockout cell lines: THP-1 or HEK293 cells with TLR4 knockout are used to study LPS signaling and validate TLR4 as a therapeutic target.
• • NF-κB reporter cell lines: Knock-in of a reporter gene (e.g., luciferase or GFP) under the control of NF-κB response elements allows real-time monitoring of pathway activation.
• • Cytokine gene knockout lines: Knockout of TNF-α or IL-6 in macrophages helps dissect their roles in the inflammatory cascade.
• • Endothelial cell lines with mutations in adhesion molecules (e.g., ICAM-1) can be used to study leukocyte-endothelial interactions.

Commercially available, sequence-verified gene-edited cell models accelerate research by providing consistent and reproducible results, reducing the time and effort required for generating custom models.

Gene-edited cells are essential for functional genomics studies. For example:

• • Knockout of a specific gene (e.g., TLR4) in macrophages can reveal its role in cytokine production and bacterial clearance.
• • Knock-in of disease-associated variants (e.g., TNF-α -308A) allows the study of their impact on gene expression and inflammatory response.
• • CRISPR screens using pooled libraries can identify genes that modulate sepsis-related phenotypes, such as cell death or cytokine release.

Isogenic cell line pairs (wild-type vs. knockout) are used to screen for drugs that target specific pathways. For example:

• • A TLR4 knockout cell line can be used to confirm the on-target specificity of a TLR4 inhibitor.
• • Drug resistance studies: Cells with mutations in genes involved in the inflammatory response can be used to test the efficacy of drugs in resistant backgrounds.
• • High-throughput screening: Gene-edited reporter cell lines enable rapid and sensitive readouts for drug candidates.

CRISPR-based synthetic lethality screens can identify novel biomarkers and therapeutic targets. For example:

• • In sepsis, screening for genes that when knocked out sensitize cells to LPS-induced cell death can reveal potential drug targets.
• • Gene-edited cells can be used to validate candidate biomarkers by measuring their expression or secretion in response to stimuli.