Autoimmune Lymphoproliferative Syndrome (ALPS) Cell Models for Research

Autoimmune Lymphoproliferative Syndrome (ALPS) is a rare inherited disorder of immune dysregulation, with an estimated prevalence of 1 in 1,000,000 individuals. It is characterized by chronic nonmalignant lymphoproliferation, autoimmune cytopenias, and an increased risk of lymphoma. The disease typically manifests in childhood, with a median age of onset of 2 years. The 5-year survival is high, but morbidity from autoimmune complications and lymphoma is significant. According to the National Cancer Institute (NCI), the risk of Hodgkin and non-Hodgkin lymphoma is increased 14-fold and 10-fold, respectively, compared to the general population.

ALPS is an ideal model for studying apoptosis and immune tolerance. The primary defect is in the Fas apoptotic pathway, leading to defective lymphocyte apoptosis. Research models include cell lines with mutations in FAS, FASLG, and CASP10. Public datasets, such as those from the International ALPS Registry, provide clinical and genetic data. Open questions include the molecular basis of incomplete penetrance and the development of targeted therapies to correct the apoptotic defect.

The core defect in ALPS is impaired Fas-mediated apoptosis. The pathway involves:

1. Fas ligand (FasL) binds to Fas receptor (CD95) on lymphocytes.

2. Fas trimerizes and recruits FADD (Fas-associated death domain) and procaspase-8.

3. Formation of the death-inducing signaling complex (DISC) activates caspase-8.

4. Caspase-8 activates downstream caspases (e.g., caspase-3), leading to apoptosis.

Mutations in FAS, FASLG, or CASP10 disrupt this pathway, preventing lymphocyte apoptosis and causing accumulation of autoreactive lymphocytes.

Data from NCBI Gene and ClinVar.

Beyond the Fas pathway, ALPS involves deregulation of other signaling networks:

• • NF-κB pathway: Chronic activation due to defective apoptosis leads to increased inflammatory cytokine production.
• • PI3K/AKT pathway: Enhanced survival signaling may compensate for defective apoptosis.
• • B-cell receptor signaling: Altered tolerance checkpoints contribute to autoantibody production.

Key nodes include FAS, FADD, CASP8, CASP10, and BID.

Organoids derived from ALPS patient lymphocytes are emerging as more physiologically relevant models, preserving the immune microenvironment.

• • MRL/lpr mice: Spontaneous FAS mutation, develop lymphoproliferation and autoimmune disease.
• • C57BL/6-lpr mice: Similar phenotype.
• • FAS knockout mice: Complete loss of FAS, severe lymphoproliferation.
• • Patient-derived xenograft (PDX) models: Engraftment of ALPS lymphocytes into immunodeficient mice, but limited due to the nonmalignant nature.

CRISPR-based gene editing enables the creation of isogenic cell lines with precise mutations in FAS, FASLG, or CASP10. These models are essential for studying the functional consequences of specific mutations and for drug screening. For example, a FAS knockout Jurkat cell line can be generated to model complete loss of function, while a knock-in of a dominant-negative FAS mutation can mimic patient-specific defects. Commercially available, sequence-verified models accelerate research by providing consistent and validated tools.

Knockout and knock-in lines are used to validate the role of genes in apoptosis. For instance, a FAS knockout cell line can be used to confirm the necessity of FAS in FasL-induced apoptosis. Similarly, introducing a specific CASP10 mutation into a wild-type cell line can assess its impact on apoptotic signaling.

Isogenic pairs (e.g., wild-type vs. FAS knockout) are used to screen for compounds that restore apoptosis in defective cells. This approach can identify drugs that bypass the Fas pathway or enhance alternative death signaling. Resistance mechanisms can be studied by exposing cells to increasing concentrations of apoptosis-inducing agents and selecting for resistant clones.

CRISPR-based synthetic lethality screens can identify genes that, when inhibited, selectively kill ALPS cells with defective apoptosis. This can reveal novel therapeutic targets and biomarkers for patient stratification.