CRLF2

CRLF2
Available structures PDB Ortholog search: PDBe RCSB List of PDB id codes 4NN5, 4NN6, 4NN7
Identifiers
Aliases	CRLF2, CRL2, CRLF2Y, TSLPR, cytokine receptor-like factor 2, cytokine receptor like factor 2
External IDs	OMIM: 300357, 400023; MGI: 1889506; HomoloGene: 49476; GeneCards: CRLF2; OMA:CRLF2 - orthologs
Gene location (Human) Chr. X chromosome (human) Band Xp22.23;Yp11.32 Start 1,187,549 bp End 1,212,723 bp
Gene location (Mouse) Chr. Chromosome 5 (mouse) Band 5|5 F Start 109,702,575 bp End 109,706,859 bp
RNA expression pattern Bgee Human Mouse (ortholog) Top expressed in testicle appendix gallbladder palpebral conjunctiva smooth muscle tissue rectum bone marrow right lung urinary bladder upper lobe of left lung Top expressed in granulocyte stroma of bone marrow mesenteric lymph nodes ankle subcutaneous adipose tissue fossa substantia nigra decidua barrel cortex tunica media of zone of aorta More reference expression data BioGPS n/a
Gene ontology Molecular function cytokine receptor activity cytokine binding Cellular component membrane integral component of membrane extracellular region plasma membrane external side of plasma membrane receptor complex Biological process interleukin-7-mediated signaling pathway positive regulation of cell population proliferation cytokine-mediated signaling pathway positive regulation of mast cell activation positive regulation of receptor signaling pathway via STAT Sources:Amigo / QuickGO
Orthologs Species Human Mouse Entrez 64109 57914 Ensembl ENSG00000205755 ENSMUSG00000033467 UniProt Q9HC73 Q8CII9 RefSeq (mRNA) NM_001012288 NM_022148 NM_001164735 NM_016715 NM_001310694 RefSeq (protein) NP_001012288 NP_071431 NP_001158207 NP_001297623 NP_057924 Location (UCSC) Chr X: 1.19 – 1.21 Mb Chr 5: 109.7 – 109.71 Mb PubMed search
Wikidata
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Cytokine receptor-like factor 2 (also known as TSLP receptor, TSLP-R) is a protein that in humans is encoded by the CRLF2 gene. It forms a ternary signaling complex with TSLP and interleukin-7 receptor-α, capable of stimulating cell proliferation through activation of STAT3, STAT5 and JAK2 pathways and is implicated in the development of the hematopoietic system. Rearrangement of this gene with immunoglobulin heavy chain gene (IGH) (chromosome 14), or with P2Y purinoceptor 8 gene (P2RY8) (chromosome X or Y) is associated with B-progenitor- and Down syndrome- acute lymphoblastic leukemia (ALL).

Cytokine signals are mediated through specific receptor complexes, the components of which are mostly members of the type I cytokine receptor family. Type I cytokine receptors share conserved structural features in their extracellular domain. Receptor complexes are typically heterodimeric, consisting of alpha chains, which provide ligand specificity, and beta (or gamma) chains, which are required for the formation of high-affinity binding sites and signal transduction.[supplied by OMIM]

Structure

CRLF2 (cytokine receptor-like factor 2) is a single pass type I transmembrane protein of 371 amino acids that belongs to the type I cytokine receptor family. Its N-terminus contains a signal peptide, followed by an extracellular region with fibronectin type III-like domains, a conserved WSXWS motif, and several predicted N-linked glycosylation sites, all typical features of this receptor family.

The transmembrane region is a hydrophobic a-helix that anchors CRLF2 in the plasma membrane. The C-terminal cytoplasmic tail contains a box-1 motif and additional proline-rich sequences that mediate association with Janus kinase 2 (JAK2). Tyrosine residues in this tail become phosphorylated after receptor activation, creating docking sites for STAT transcription factors and other signaling molecules that propagate the signal into the nucleus.

At the genomic level, CRLF2 is located on pseudoautosomal region 1 (PAR1) on the short arms of the X and Y chromosomes.  (Xp22.33 and Yp11.32). The locus spans about 20-22 kb on the reverse strand and produces multiple transcript variants via alternative splicing, including CRLF2-201 to CRLF2-204. This encodes proteins that retain the key extracellular and intracellular signaling domains.

Early work cloned a related receptor in mice, called cytokine receptor-like molecule-2 (CRLM-2), which helped define this receptor subfamily. The cloning and characterization of CRLM-2, as well as the later cloning of human CRLF2/TSLPR, were made possible by the construction of full-length enriched cDNA libraries and improved "oligo-capping" methods that enriched for complete 5' ends of mRNAs. These technical advances increased the chances of capturing the full coding sequence of low-abundance immune receptors such as CRLF2.

In addition to its core domain organization, CRLF2 can be placed within the broader interleukin receptor family by sequence homology and motif conservation. Alignment with related receptors, including murine cytokine receptor-like molecule-2 (CRLM-2) and rat TSLP receptor, shows conservation of key cysteine residues and the WSXWS motif that stabilizes the extracellular region. These similarities support a common evolutionary origin and suggest that CRLF2 shares signaling mechanisms with other type I cytokine receptors.

Expression and regulation

CRLF2 is expressed most strongly in hematopoietic tissues such as bone marrow, thymus, spleen, and lymph nodes. Expression profiling shows higher transcript and protein levels in myeloid dendritic cells, basophils, and activated monocytes, with lower but detectable expression in subsets of T and B lymphocytes, consistent with a role in shaping immune responses.

Outside of the main immune organs, CRLF2 is found at epithelial barrier sites, including the skin, respiratory mucosa, and gastrointestinal tract. At these locations, CRLF2 is often co-expressed with interleukin-7 receptor alpha (IL-7Ra) on stromal or hematopoietic cells, while neighboring epithelial cells are major producers of thymic stromal lymphopoietic (TSLP). This arrangement allows epithelial-derived TSLP to act on local CRLF2-expressing cells during infection, tissue damage, or allergen exposure.

Regulation of CRLF2 expression is context-dependent. Pattern-recognition receptor ligands and pro-inflammatory cytokines can upregulate CRLF2 on monocytes and dendritic cells, enhancing their sensitivity to TSLP during inflammation. In leukemia, chromosomal rearrangements such as IGH-CRLF2 or P2RY8-CRLF2 place the gene under strong heterologous promoters, driving very high expression compared with normal progenitor cells and setting the stage for oncogenic signaling.

Early expression analysis of cytokine receptor-like 2 (CRL2/CRLF2) using full-length cDNA libraries showed that transcripts are enriched in immune tissues and myeloid leukemia cell lines, but largely absent from most non-hematopoietic organs. Studies constructing full-length enriched and 5'-end enriched cDNA libraries using oligo-capping approaches were crucial for detecting these transcripts, which are low-abundance but biologically important.

Function

CRLF2 is the high-affinity receptor chain for TSLP and is also known as the TSLP receptor (TSLPR). It does not signal on its own, so instead it forms a heterodimeric receptor complex with IL-7Ra. TSLP first binds to CRLF2 and then recruits IL-7Ra to form a ternary complex at the cell surface, as demonstrated by structural studies of the human TSLP-CRLF2-IL-7Ra complex.

When this receptor complex is engaged, intracellular signaling cascades are activated. The cytoplasmic tail of CRLF2 associates mainly with JAK2, while IL-7Ra binds JAK1. TSLP binding brings these kinases together, leading to their activation and phosphorylation of tyrosine residues within the receptor tails. These phosphotyrosines recruit STAT transcription factors, more significantly in STAT5 and STAT3, which are then phosphorylated, dimerize, and move into the nucleus to control genes involved in cell survival, proliferation, and cytokine production.

Beyond the JAK-STAT pathway, CRLF2 signaling can also activate the PI3K-AKT-mTOR and MAPK/ERK pathways. In CRLF2-positive B-cell acute lymphoblastic leukemia (B-ALL), TSLP stimulation increases phosphorylation of AKT,  ribosomal protein S6, and other mTOR targets, which promotes cell growth and resistance to apoptosis. JAK inhibitors not only block STAT activation but also dampen PI3K/mTOR signaling, showing cross-talk between these pathways downstream of the receptor and providing a rationale for kinase-targeted therapies.

Protein-protein interactions

CRLF2 is known to interact with the following proteins and protein complexes:

• IL-7Ra (Interleukin-7 receptor alpha) - forms the heterodimeric TSLP receptor complex
• TSLP (Thymic stromal lymphopoietin) - Cytokine ligand that binds the CRLF2-IL-7Ra receptor
• JAK2 (Janus kinase 2) - associates with the intracellular box-1 motif of CRLF2
• JAK1 - associates with IL-7Ra within the same receptor complex
• STAT5 and STAT3 - bind to phosphorylated tyrosine residues on the cytoplasmic tail and are activated downstream
• Adaptor proteins - link CRLF2 to PI3K-AKT-mTOR and MAPK/ERK pathways

The main binding partner of CRLF2 is IL-7Ra, with which it forms the functional TSLP receptor complex. This heterodimeric receptor is required for high-affinity binding of TSLP and for efficient recruitment of JAK1 and JAK2, and it is therefore essential for known CRLF2-mediated signaling. On the cytoplasmic side, CRLF2 interacts with JAK2 through its box-1 motif and with signaling proteins that recognize phosphorylated tyrosine residues, including STAT5, STAT3, and adaptor molecules that link to the PI3K and MAPK pathways. Gain-of-function mutations in JAK2 or IL-7Ra can enhance these interactions and produce ligand-independent signaling in leukemic cells, especially when combined with high CRLF2 expression.

Physiological roles

In normal immunity, the TSLP-CRLF2-IL-7Ra axis is a major driver of type 2 immune responses at barrier surfaces. When epithelial cells in the skin, lungs, or gut encounter allergens, helminths, or mechanical injury, they release TSLP, which acts on nearby dendritic cells, basophils, and innate lymphoid cells that express CRLF2. These cells respond by up-regulating co-stimulatory molecules and producing cytokines such as IL-4, IL-5, and IL-13, promoting T helper 2 (Th2) differentiation, eosinophil recruitment, and class switching to IgE.

The TSLP-CRLF2 pathway is particularly important in human atopic disease. In asthma and atopic dermatitis, epithelial cells are exposed to allergens and pollutants produce high amounts of TSLP, which then activate CRLF2-expressing dendritic cells and T cells to favor Th2 polarization. Elevated TSLP and TSLP receptor expression have been detected in the inflamed airway and skin tissues from patients, supporting the idea that excessive CRLF2 signaling contributes directly to chronic allergic inflammation.

For several years, it was thought that TSLP acts on T cells only indirectly by conditioning dendritic cells. However, a "Cutting Edge" study in The Journal of Immunology showed that activated human CD4+ T cells themselves express functional TSLP receptors, including CRLF2 and IL-7Ra. In that work, TSLP directly promoted CD4+ T-cells survival, proliferation, and production of Th2-associated cytokines, indicating that CRLF2 signaling can act both on antigen-presenting cells and directly on T cells to amplify type 2 immune responses.

Beyond allergy, TSLP-CRLF2 signaling influences early B-cell development and may help maintain regulatory T cells and mucosal immune homeostasis. Knockout and transgenic mouse studies suggest that the effect of CRLF2 on adaptive immunity is context-dependent, so it can enhance protective responses against helminths and extracellular pathogens in some settings but can also worsen chronic inflammation or tissue damage in others, depending on the balance of effector and regulatory cells and the local cytokine environment.

Clinical significance

B-cell acute lymphoblastic leukemia

CRLF2 is most clinically important in B-cell precursor acute lymphoblastic leukemia (B-ALL), where its overexpression defines a high-risk subgroup. Two main genomic lesions account for most of these cases, where IGH-CRLF2 translocations and P2RY8-CRLF2 fusions. Both put CRLF2 under strong regulatory elements, leading to much higher receptor expression on leukemic blasts than in normal progenitor cells.

CRLF2 rearranged B-ALL often carries cooperating mutations in JAK2, IL-7Ra, or other regulators of cytokine signaling. Together with high receptor density, these lesions cause constitutive activation of JAK-STAT and PI3K/AKT/mTOR pathways, promoting survival and proliferation of leukemic cells even with little or no exogenous TSLP. Gene-expression profiling shows that most CRLF2 rearranged leukemias fall within the "Philadelphia chromosome-like" ALL subtype, which has a BCR-ABL1-like signaling signature in the absence of the BCR-ABL1 fusion.

Epidemiologic studies report CRLF2 alternations in a minority of B-ALL overall but enriched in specific groups. Pediatric and adolescent/young-adult cohorts show CRLF2 rearrangement or overexpression in roughly 5-15% of cases, with the highest frequencies in Down syndrome-associated ALL and in patients of Hispanic or Latino ancestry. Across several trials, CRLF2 rearranged ALL is linked to higher presenting white blood cell counts, increased minimal residual disease after induction, and worse event-free and overall survival compared with CRLF2-negative disease, although exact outcomes vary by age and treatment regimen.

Flow cytometry can detect CRLF2 overexpression in leukemic blasts, and multiple groups have demonstrated high concordance between surface staining and underlying CRLF2 rearrangements confirmed by molecular tests. Because immunophenotyping is rapid and relatively inexpensive, it is increasingly used as a screening tool to identify patients who should undergo confirmatory cytogenetic or sequencing studies and possibly enroll in targeted-therapy trials.

Treating CRLF2 rearranged B-ALL remains challenging, especially in adults and relapsed/refractory cases. Preclinical work shows that JAK inhibitors such as ruxolitinib can suppress JAK-STAT and PI3K/mTOR signaling and reduce the viability of CRLF2 positive leukemia cells, providing a rationale for JAK-directed therapy combined with standard chemotherapies. Early clinical experience suggests responses are variable and resistance via parallel pathways such as RAS-MAPK is common, which is driving interest in combination strategies and new targeted agents, including approaches that degrade JAK kinases or target multiple signaling nodes at once.

Other malignancies and disorders

CRLF2 overexpression has also been reported in some myeloid neoplasms and mixed-phenotype acute leukemias, where it may similarly drive JAK-STAT signaling together with other lesions. Small series describes CRLF2-positive myeloid malignancies with IGH-CRLF2 or P2R8-CRLF2 rearrangements, suggesting that deregulation of this receptor is not completely restricted to B-lineage cells.

Germline variants affecting CRLF2 or nearby regulatory elements in PAR1 have been linked to altered blood cell traits and possibly to variation in leukemia risk, though these associations are still being clarified. Case reports have also described CRLF2 rearranged ALL that later switches lineage to acute myeloid leukemia after targeted immunotherapies, highlighting the dynamic nature of CRLF2-driven disease and the need for careful molecular monitoring during treatment.

Therapeutic targeting of the TSLP-CRLF2 axis

Most strategies for CRLF2-driven leukemia focus on blocking downstream kinases, but the TSLP-CRLF2 axis has also been targeted at the ligand level in allergic disease. Tezepelumab is a human monoclonal antibody that neutralizes TSLP, preventing it from binding the CRLF2-IL-7Ra receptor complex. Phase 3 trials in severe uncontrolled asthma showed that tezepelumab significantly reduces exacerbations and improves lung function, leading to regulatory approval as an add-on maintenance therapy for patients 12 years and older.

The indication for tezepelumab has since been expanded to include chronic rhinosinusitis with nasal polyps (CRSwNP), another epithelial-driven inflammatory disease where TSLP-CRLF2 signaling plays a role. In the phase 3 WAYPOINT trial, tezepelumab reduced polyp size, nasal congestion, and the need for surgery or systemic steroids, making it the first biologic targeting TSLP approved for CRSwNP and showing how blocking TSLP can modulate CRLF2-dependent pathways in non-malignant disease.

In oncology, experimental strategies are being developed to target CRLF2 itself, including monoclonal antibodies, antibody-drug conjugates, and cell-based or bispecific therapies directed against CRLF2 on leukemic blasts. Because normal hematopoietic cells usually express lower levels of CRLF2 than leukemic blasts, the receptor may offer a therapeutic window; however, clinical data are still limited, and potential effects on normal immune function remain under investigation.

Looking forward, several groups are exploring combined strategies that target both upstream and downstream components of the TSLP-CRLF2 pathway. One approach is to pair JAK inhibitors or JAK-directed degraders with CRLF2 or TSLP-targeted antibodies, intending to block ligand binding at the surface and signal propagation inside the cell at the same time. As more is learned about resistance mechanisms in CRLF2 rearranged leukemia, these multitarget regimens may offer more durable disease control than single-agent therapies.

Further reading

• Maruyama K, Sugano S (January 1994). "Oligo-capping: a simple method to replace the cap structure of eukaryotic mRNAs with oligoribonucleotides". Gene. 138 (1–2): 171–174. doi:10.1016/0378-1119(94)90802-8. PMID 8125298.
• Suzuki Y, Yoshitomo-Nakagawa K, Maruyama K, Suyama A, Sugano S (October 1997). "Construction and characterization of a full length-enriched and a 5'-end-enriched cDNA library". Gene. 200 (1–2): 149–156. doi:10.1016/S0378-1119(97)00411-3. PMID 9373149.
• Hiroyama T, Iwama A, Morita Y, Nakamura Y, Shibuya A, Nakauchi H (May 2000). "Molecular cloning and characterization of CRLM-2, a novel type I cytokine receptor preferentially expressed in hematopoietic cells". Biochemical and Biophysical Research Communications. 272 (1): 224–229. Bibcode:2000BBRC..272..224H. doi:10.1006/bbrc.2000.2764. hdl:2241/1571. PMID 10872831. S2CID 38299268.
• Reche PA, Soumelis V, Gorman DM, Clifford T, Travis M, Zurawski SM, et al. (July 2001). "Human thymic stromal lymphopoietin preferentially stimulates myeloid cells". Journal of Immunology. 167 (1): 336–343. doi:10.4049/jimmunol.167.1.336. PMID 11418668.
• Blagoev B, Nielsen MM, Angrist M, Chakravarti A, Pandey A (February 2002). "Cloning of rat thymic stromal lymphopoietin receptor (TSLPR) and characterization of genomic structure of murine Tslpr gene". Gene. 284 (1–2): 161–168. doi:10.1016/S0378-1119(02)00380-3. PMID 11891057.
• Rochman I, Watanabe N, Arima K, Liu YJ, Leonard WJ (June 2007). "Cutting edge: direct action of thymic stromal lymphopoietin on activated human CD4+ T cells". Journal of Immunology. 178 (11): 6720–6724. doi:10.4049/jimmunol.178.11.6720. PMID 17513717.

External links

• Human CRLF2 genome location and CRLF2 gene details page in the UCSC Genome Browser.