XCL1

XCL1, a C class chemokine also known as Lymphotactin, is mainly produced by activated CD8⁺ T cells and natural killer cells. XCL1 signals by binding to the XCR1 receptor. XCL1 is involved in infectious, inflammatory and immunological diseases. XCL1 is expressed by various immune cells, including activated CD8⁺ T cells, CD4⁺ T cells, NK cells, NKT cells, γδ T cells, and thymic medullary epithelial cells. XCL1 elicits its chemotactic function by binding to the receptor called XCR1. XCR1 is expressed by a dendritic cell (DC) subpopulation^[1].
There are two highly homologous C class chemokine genes in human, XCL1 and XCL2 (also known as SCYC1 and SCYC2, respectively). The XCL1 and XCL2 genes in human are localized closely on chromosome 1 and encode highly homologous proteins (also known as SCM-1K and SCM-1L, respectively) with only two amino acid differences from each other. Unlike CXC, CC, and CX3C class chemokines that carry two disulfide bonds with four cysteine residues at the amino termini, XCL1 has only one disulfide bond with two cysteine residues at the amino terminus. Human and mouse XCL1s share 60% amino acid identity. XCL1 transcripts are detected in spleen, thymus, intestine, and peripheral blood leukocytes. XCL1 expression is also detectable in lung, colon, prostate gland, testis, and ovary. In leukocytes, XCL1 is highly expressed in CD8⁺ and CD4^-CD8^- TCRαβ⁺ T cells in blood and thymus, particularly when the cells are activated. TCRαβ⁺ T cells in epidermis and intestinal epithelium also express XCL1^[1].