Macrophage Activation Syndrome (MAS) Cell Models for Research

Macrophage activation syndrome (MAS) is a severe, life-threatening hyperinflammatory syndrome that occurs as a complication of rheumatic diseases, most commonly systemic juvenile idiopathic arthritis (sJIA) and adult-onset Still's disease (AOSD). It is classified as a secondary form of hemophagocytic lymphohistiocytosis (HLH). The exact global incidence is not well defined, but MAS complicates approximately 10% of sJIA cases and up to 15% of AOSD cases. Mortality rates range from 10% to 30% in hospitalized patients, with higher rates in developing countries and in patients with delayed diagnosis. According to the World Health Organization (WHO), HLH, including MAS, is recognized as a rare disease with significant underdiagnosis. Key risk factors include genetic mutations in cytolytic pathway genes (e.g., PRF1, UNC13D, STXBP2), infections (especially Epstein-Barr virus), and certain medications. The 5-year survival for MAS is approximately 70-80% with prompt treatment, but drops to 50% in refractory cases. There is no stage-based survival data as MAS is not a cancer. Research is urgently needed to understand the molecular triggers and to develop targeted therapies.

MAS is an ideal disease for mechanistic studies due to its well-defined genetic subtypes, including familial HLH (FHL) and secondary MAS. Public datasets such as those from the NCBI Gene Expression Omnibus (GEO) and the European Genome-phenome Archive (EGA) provide transcriptomic and genomic data from MAS patients. Open questions include: What are the precise triggers of macrophage hyperactivation? How do genetic mutations in cytolytic pathways lead to immune dysregulation? What role do inflammasomes play? The disease's acute and severe nature makes it a compelling model for studying cytokine storms and hyperinflammation, which are also relevant to other conditions like COVID-19 and sepsis.

Although MAS is not a cancer, it shares dysregulated signaling pathways with malignancies. The major pathways involved include:

1. Cytolytic pathway: Defects in perforin (PRF1), MUNC13-4 (UNC13D), syntaxin-11 (STX11), and syntaxin-binding protein 2 (STXBP2) impair CD8+ T cell and NK cell cytotoxicity, leading to uncontrolled immune activation.

2. Inflammasome pathway: Activation of NLRP3 and NLRC4 inflammasomes results in excessive IL-1β and IL-18 production, driving macrophage activation.

3. JAK/STAT pathway: Cytokines such as IFN-γ and IL-6 signal through JAK/STAT, promoting macrophage activation and cytokine release.

4. NF-κB pathway: Toll-like receptor (TLR) signaling and TNF-α activate NF-κB, leading to pro-inflammatory cytokine production.

These pathways are not carcinogenic per se but are central to MAS pathogenesis and are targets for therapeutic intervention.

Genetic alterations in MAS are primarily germline mutations associated with familial HLH, but somatic mutations in acquired MAS are also reported. The table below summarizes key genes and their frequencies based on data from ClinVar and COSMIC (for somatic mutations in related hematologic disorders).

Note: Frequencies are approximate and vary by population. Data from ClinVar and COSMIC.

The signaling networks in MAS are characterized by excessive cytokine production and immune cell activation. Key nodes include:

• • Cytokine receptors: IFN-γR, IL-6R, IL-1R, and IL-18R are overactivated.
• • JAK/STAT: JAK1/2 and STAT1/3 are phosphorylated, driving transcription of pro-inflammatory genes.
• • NF-κB: Activated by TLRs and TNF-α, leading to production of IL-1β, IL-6, and TNF-α.
• • Inflammasome: NLRP3 and NLRC4 assemble with ASC and caspase-1, cleaving pro-IL-1β and pro-IL-18.
• • mTOR: Activated by cytokines, promoting macrophage survival and cytokine production.
• • MAPK: ERK, JNK, and p38 are activated, contributing to cytokine release.

These networks are interconnected, with positive feedback loops that amplify inflammation.

Several cell lines are used to study MAS, including macrophage-like and T cell lines. The table below lists commonly used lines with their key mutations.

Organoids derived from patient iPSCs or primary macrophages are emerging as more physiologically relevant models. They allow study of macrophage activation in a 3D context and can be genetically modified using CRISPR.

Animal models of MAS include:

• • Genetic models: Prf1-/- mice, Unc13d-/- mice, and Nlrc4 gain-of-function mice develop HLH-like symptoms upon immune challenge.
• • Induced models: Repeated TLR9 agonist (CpG) injection in mice induces MAS-like disease.
• • PDX models: Patient-derived xenografts of hematopoietic cells in immunodeficient mice are used to study human MAS.
• • GEMM: Genetically engineered mouse models with mutations in cytolytic genes.

These models are valuable for preclinical testing of anti-cytokine therapies.

CRISPR-Cas9 gene editing enables the creation of isogenic cell lines with specific mutations for MAS research. Examples include:

• • PRF1 knockout THP-1 cells: Loss of perforin function to study impaired cytotoxicity.
• • UNC13D knockout macrophage lines: To model defective degranulation.
• • NLRC4 gain-of-function knock-in: Introduction of activating mutations to study inflammasome hyperactivation.
• • STXBP2 knockout iPSC-derived macrophages: For studying familial HLH type 5.

Commercially available, sequence-verified gene-edited cell models accelerate research by providing reproducible, validated tools. These models are generated using CRISPR and are quality-controlled for off-target effects and genomic integrity. They enable researchers to study MAS pathogenesis and evaluate potential therapeutics in a human cellular context.

Knockout and knock-in cell lines are used to validate gene function in MAS. For example, PRF1 knockout in THP-1 cells confirms its role in cytotoxicity, while NLRC4 knock-in models demonstrate inflammasome hyperactivation. These models allow systematic interrogation of genetic variants identified in patients.

Isogenic pairs (wild-type vs. mutant) are used for high-throughput drug screening to identify compounds that rescue the mutant phenotype. For instance, JAK inhibitors can be tested in STXBP2 knockout cells to assess their ability to reduce cytokine production. Resistance models can be developed by chronic exposure to drugs.

CRISPR synthetic lethality screens in MAS-relevant cell lines can identify genes that are essential only in the context of specific mutations. For example, screening in PRF1-deficient cells may reveal dependencies on specific kinases, which could serve as biomarkers or therapeutic targets.