Biology:XCL1
 Generic protein structure example |
Chemokine (C motif) ligand 1 also known as lymphotactin is a protein that in humans is encoded by the XCL1 gene. XCL1 is a small cytokine belonging to the C chemokine family that signals exclusively through its receptor XCR1.[1] Produced primarily by activated CD8+ T cells and natural killer (NK) cells, XCL1 functions as a chemoattractant for specific immune cell populations, particularly XCR1-positive conventional dendritic cells (cDC1s), thereby orchestrating immune responses to infection and inflammation.[2]
Chemokines are known for their function in inflammatory and immunological responses. This family C chemokines differs in structure and function from most chemokines.[3][4] There are only two chemokines in this family and what separates them from other chemokines is that they only have two cysteines; one N-terminal cysteine and one cysteine downstream. These both are called lymphotactin, alpha and beta form, and claim special characteristics only found between the two. Lymphotactins can go through a reversible conformational change which influences its binding.[5]
Gene
In humans, XCL1 is closely related to another chemokine, XCL2, which is located at the same genomic locus on the long arm of chromosome 1 (band q24.2).[6] Both genes share strong genetic and functional similarities; however, XCL2 has only been identified in humans and not in mice.[7]
The XCL1 gene spans approximately 6,017 base pairs and contains three exons and two introns, along with multiple transcription start sites.[8] It encodes a 114-amino acid protein that differs from most chemokines by lacking the first and third conserved cysteine residues. As a result, XCL1 contains only one disulfide bond rather than the typical two or three found in other chemokines.[3] Despite their similarity, the genes for XCL1 and XCL2 exhibit subtle but notable differences. Both belong to the C chemokine subfamily, characterized by a single disulfide bond and nearly identical tertiary structures.[8] Their genomic sequences include conserved flanking regions, such as promoter regions, and other non-coding elements important for gene regulation.[8]
Gene mapping has revealed that the structure of XCL1 and XCL2 is largely conserved, with a key distinction in the first intron. XCL1 contains a complete sequence encoding the 60S ribosomal protein L7a, whereas in XCL2, part of this region is truncated.[8] The only difference in the mature proteins is the amino acid composition at positions 7 and 8, which may contribute to functional differences between the two chemokines.[8][7] One limitation in comparative studies of XCL1 and XCL2 is that XCL2 has not been observed in mice, making functional comparisons across species more difficult.[7]
Tissue distribution
In normal tissues, XCL1 is found in high levels in the spleen, thymus, small intestine, and peripheral blood leukocytes, and at lower levels in the lung, prostate gland, and ovary. Secretion of XCL1 is responsible for the increase of intracellular calcium in peripheral blood lymphocytes.[8] Cellular sources for XCL1 include activated thymic and peripheral blood CD8+ T cells.[9][10][6] NK cells also secrete XCL1 along with other chemokines early in infections.[7] XCR1-expressing dendritic cells (DC) are a major target of XCL1.[7]
Structure
A defining feature of XCL1 is its unique structural configuration.[5] Unlike most chemokines, which possess two disulfide bonds linking the N-terminus to the protein core, XCL1 contains only a single disulfide bond.[3] This structural simplification alters its protein tertiary structure, distinguishing it from other members of the chemokine family.
XCL1 is classified as a metamorphic protein, capable of reversibly switching between two distinct conformations—Ltn10 and Ltn40—both of which are biologically active.[11][5] At lower temperatures (10 °C), XCL1 exists predominantly as a monomeric form known as Ltn10, while at higher temperatures (40 °C), it adopts a dimeric conformation called Ltn40.[12] These reversible structural states are essential to its function, influencing receptor binding and chemokine activity.[5]
Function
XCL1 exerts its chemotactic activity by binding to its cognate chemokine receptor, XCR1.[13] XCL1 is expressed by various cell types, including macrophages, fibroblasts, and specific lymphocytes.[4] The XCL1–XCR1 axis plays a critical role in antigen cross-presentation, antigen uptake, and the induction of both innate and adaptive cytotoxic immune responses.[7] XCR1 is selectively expressed on a subset of conventional dendritic cells, which are specialized for presenting extracellular antigens via MHC class I to CD8+ T cells. XCL1 is secreted by activated NK cells and antigen-specific CD8+ T cells, often alongside other cytokines such as IFN-γ.[7] This interaction facilitates effective antigen cross-presentation by dendritic cells.
Clinical significance
XCL1 appears to be involved in the pathogenesis of rheumatoid arthritis (RA). It is expressed on synovial lymphocytes and contributes to the accumulation of T cells in inflamed joints.[4] A recent study shows that neutralizing XCL1 mitigates brain damage and reduces lymphocyte and dendritic cell recruitment after intracerebral hemorrhage in mice.[14]
References
- ↑ "The multifaceted role of XCL1 in health and disease". Protein Science 34 (2). February 2025. doi:10.1002/pro.70032. PMID 39840812.
- ↑ "XCL1 and XCR1 in the immune system". Microbes and Infection 14 (3): 262–267. March 2012. doi:10.1016/j.micinf.2011.10.003. PMID 22100876.
- ↑ 3.0 3.1 3.2 "Chemokine oligomerization in cell signaling and migration". Progress in Molecular Biology and Translational Science. 117. 2013. pp. 531–578. doi:10.1016/B978-0-12-386931-9.00020-9. ISBN 978-0-12-386931-9.
- ↑ 4.0 4.1 4.2 "Cell Recruitment and Angiogenesis". Kelly and Firestein's Textbook of Rheumatology. Elsevier. 2017. pp. 384–395. doi:10.1016/B978-0-323-31696-5.00025-5. ISBN 978-0-323-31696-5.
- ↑ 5.0 5.1 5.2 5.3 "Chapter 3 Lymphotactin Structural Dynamics". Chemokines, Part B. Methods in Enzymology. 461. Elsevier. 2009. pp. 51–70. doi:10.1016/s0076-6879(09)05403-2. ISBN 978-0-12-374907-9.
- ↑ 6.0 6.1 "Structure and expression of two highly related genes encoding SCM-1/human lymphotactin". FEBS Letters 395 (1): 82–88. October 1996. doi:10.1016/0014-5793(96)01004-6. PMID 8849694. Bibcode: 1996FEBSL.395...82Y.
- ↑ 7.0 7.1 7.2 7.3 7.4 7.5 7.6 "The role of XCR1 and its Ligand XCL1 in antigen cross-presentation by murine and human dendritic cells". Frontiers in Immunology 3 (14): 14. February 10, 2012. doi:10.3389/fimmu.2012.00014. PMID 22566900.
- ↑ 8.0 8.1 8.2 8.3 8.4 8.5 "Structure and expression of two highly related genes encoding SCM-1/human lymphotactin". FEBS Letters 395 (1): 82–88. October 14, 1996. doi:10.1016/0014-5793(96)01004-6. PMID 8849694. Bibcode: 1996FEBSL.395...82Y.
- ↑ "Lymphotactin: a cytokine that represents a new class of chemokine". Science (New York, N.Y.) 266 (5189): 1395–1399. November 1994. doi:10.1126/science.7973732. PMID 7973732. Bibcode: 1994Sci...266.1395K.
- ↑ "Molecular cloning and functional characterization of human lymphotactin". Journal of Immunology (Baltimore, Md.) 155 (1): 203–209. July 1995. doi:10.4049/jimmunol.155.1.203. PMID 7602097.
- ↑ "Metamorphic proteins: the Janus proteins of structural biology". Open Biology 11 (4). 2021-04-21. doi:10.1098/rsob.210012. PMID 33878950.
- ↑ "Native-state interconversion of a metamorphic protein requires global unfolding". Biochemistry 50 (33): 7077–7079. August 2011. doi:10.1021/bi200750k. PMID 21776971.
- ↑ "Identification of single C motif-1/lymphotactin receptor XCR1". The Journal of Biological Chemistry 273 (26): 16551–16554. June 1998. doi:10.1074/jbc.273.26.16551. PMID 9632725.
- ↑ "Neutralizing XCL1 Attenuates Brain Injury and Reduces Lymphocyte and Dendritic Cell Recruitment Following Intracerebral Hemorrhage in Mice". Mol Neurobiol 63 (1): 70. November 2025. doi:10.1007/s12035-025-05391-6. PMID 41254348.
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