XCL1

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). Both genes share strong genetic and functional similarities; however, XCL2 has only been identified in humans and not in mice.

The XCL1 gene spans approximately 6,017 base pairs and contains three exons and two introns, along with multiple transcription start sites. 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. 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. Their genomic sequences include conserved flanking regions, such as promoter regions, and other non-coding elements important for gene regulation.

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. 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. 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.

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. Cellular sources for XCL1 include activated thymic and peripheral blood CD8+ T cells. NK cells also secrete XCL1 along with other chemokines early in infections. XCR1-expressing dendritic cells (DC) are a major target of XCL1.

Structure

A defining feature of XCL1 is its unique structural configuration. Unlike most chemokines, which possess two disulfide bonds linking the N-terminus to the protein core, XCL1 contains only a single disulfide bond. 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. 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. These reversible structural states are essential to its function, influencing receptor binding and chemokine activity.

Function

XCL1 exerts its chemotactic activity by binding to its cognate chemokine receptor, XCR1. XCL1 is expressed by various cell types, including macrophages, fibroblasts, and specific lymphocytes. 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. 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. 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.

References

References

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