Chemokine (C motif) ligand 1 also known as lymphotactin is a protein that in humans is encoded by the XCL1gene. XCL1 is a small cytokine belonging to the C chemokine family that signals exclusively through its receptor XCR1.[5] 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.[6]
Chemokines are known for their function in inflammatory and immunological responses. This family C chemokines differs in structure and function from most chemokines.[7][8] 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.[9]
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).[10] Both genes share strong genetic and functional similarities; however, XCL2 has only been identified in humans and not in mice.[11]
The XCL1 gene spans approximately 6,017 base pairs and contains three exons and two introns, along with multiple transcription start sites.[12] 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.[7]
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.[12] Their genomic sequences include conserved flanking regions, such as promoter regions, and other non-coding elements important for gene regulation.[12]
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.[12] 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.[12][11] 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.[11]
A defining feature of XCL1 is its unique structural configuration.[9] Unlike most chemokines, which possess two disulfide bonds linking the N-terminus to the protein core, XCL1 contains only a single disulfide bond.[7] 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.[15][9]
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.[16] These reversible structural states are essential to its function, influencing receptor binding and chemokine activity.[9]
Function
XCL1 exerts its chemotactic activity by binding to its cognate chemokine receptor, XCR1.[17] XCL1 is expressed by various cell types, including macrophages, fibroblasts, and specific lymphocytes.[8]
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.[11] 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.[11] 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.[8] A recent study shows that neutralizing XCL1 mitigates brain damage and reduces lymphocyte and dendritic cell recruitment after intracerebral hemorrhage in mice.[18]
123Szekanecz Z, Koch AE (2017). "Cell Recruitment and Angiogenesis". In Firestein GS, Budd RC, Gabriel SE, McInnes IB, O'Dell JR (eds.). Kelly and Firestein's Textbook of Rheumatology. Elsevier. pp.384–395. doi:10.1016/B978-0-323-31696-5.00025-5. ISBN978-0-323-31696-5.
↑Kelner GS, Kennedy J, Bacon KB, Kleyensteuber S, Largaespada DA, Jenkins NA, etal. (November 1994). "Lymphotactin: a cytokine that represents a new class of chemokine". Science. 266 (5189). New York, N.Y.: 1395–1399. Bibcode:1994Sci...266.1395K. doi:10.1126/science.7973732. PMID7973732.
