Immunology and Cell Biology (2004) 82, 276–284 doi:10.1111/j.1440-1711.2004.01264.x © 2004 Australasian Society for Immunology Inc. Special Feature Genetic control of NKT cell numbers MARGARET A JORDAN, JULIE FLETCHER and ALAN G BAXTER Comparative Genomics Centre, James Cook University, Townsville, Queensland, Australia Summary NKT cells play a critical role in shaping the character and strength of a wide range of immune responses, including those against pathogens, tumours, allografts and autologous tissues. Because numbers of NKT cells affect clinical outcomes in a wide range of disease models, and this characteristic demonstrates allelic variation, the mapping of the locations and identification of the coding sequences of these genes has become a matter of significant importance. Here, we review the results to date that examine the effects of targeted deletion of a number of candidate genes, as well as the congenic and genetic linkage analyses that have attempted to localize allelic loci that affect NKT cell numbers. Although a number of candidate genes have been examined, there is no evidence that any of these contribute to variation in NKT cell numbers in natural populations. Two of the most important genetic regions controlling NKT cell numbers are Nkt1 on chromosome 1, which may contribute to lupus susceptibility, and Nkt2 on chromosome 2, which appears to contribute to diabetes susceptibility. Of great interest is a third locus on chromosome 18, identified in a novel congenic line, which can confer an absolute deficiency in this important immunoregulatory lymphocyte population. Key words: autoimmunity, diabetes, disease susceptibility, genes, linkage analysis, lupus, NK cells. Background NKT cells Lymphocytes that express surface markers characteristic of both conventional T cells and NK cells have been variably termed NK-like T cells, NK T cells and NKT cells (reviewed in Godfrey et al. 2000). 1 Two major subsets have been identified. The subset with the most potent immuno- modulatory activity is termed the invariant NKT (iNKT) population, as it is characterized by the presence of an invariant TCR α-chain composed of Vα14 and Jα281 seg- ments in mice, and Vα24 and JαQ segments in humans. 2 This population is also highly biased toward Vβ8.2, Vβ7 and Vβ2 usage in mice and Vβ11 in humans. Invariant NKT cells constitute most of the NK1.1 + T-cell population in the murine spleen and represent more than 80% of NK1.1 + T-cells in the thymus and liver. 3 They either express CD4 at intermediate levels, or are CD4 – CD8 – (double negative; DN), and are uniformly reactive to the marine-sponge-derived glycolipid α-galactosylceramide (αGalCer) when presented in the context of non-classical MHC class I-like β2 microglobulin (β2M)-dependent leucocyte surface antigen CD1d. This latter characteristic has been used to good advantage with the development of highly specific fluorescent CD1d/ αGalCer tetramers, which can be used in flow cytometric analyses. Invariant NKT cells are key immunoregulators that can control the strength and nature of responses against viruses, 4–10 bacteria, 11–13 fungi, 14,15 parasites, 16–19 tumours, 20–25 allografts 26,27 and autologous tissues 28–34 (reviewed in articles by Kronenberg and Gapin 35 and Godfrey et al. 1 ). They are positively selected by CD1d, 36 which has a highly hydrophobic antigen-binding cleft and binds and presents glycolipids, such as glyco- sylceramides and glycosylphosphatidyl inositol (GPI). 37–39 In ontogeny, iNKT cells probably branch from the develop- mental pathway of conventional T cells at the CD4 + CD8 + (double positive; DP) stage following generation of the canonical Vα14Jα281 TCR. These cells are positively selected by CD1d-expressing DP thymocytes, rather than thymic epithelial cells, as for conventional T cells. 40,41 The earliest NKT cells identified by binding of CD1d/ αGalCer tetramers are CD4 + CD8 – NK1.1 – and are precursors of the NK1.1 + CD4 + and NK1.1 – DN subsets. Expression of NK1.1 is not required for migration to the periphery since the majority of recent thymic immigrants are NK1.1 – . Both NK1.1 – and NK1.1 + iNKT cells can rapidly produce IL-4 and IFN-γ on stimulation; NK1.1 + NKT cells produce high levels of both cytokines, whereas the NK1.1 – subset produces high levels of IL-4 and relatively lower levels of IFN- γ. 40 Natural variation in NKT cell numbers Strain-dependent differences in NKT cell numbers have been identified, with non-obese diabetic (NOD)/Lt mice having relatively lower numbers of NKT cells in the thymus, liver and peripheral lymphoid organs than eight other commonly used mouse strains. 42–44 These studies were complicated by the use of surrogate markers for NKT cells in the absence of a definitive surface marker such as the binding of CD1d/ αGalCer tetramers. More recent studies have corrected this deficiency and have compared NKT cell numbers and func- tion between NOD.Nkrp1 b , C57BL/6 and BALB.Cmv1 r mice using tetramers and confirmed that NOD mice have numeri- cal deficiencies in NKT cells. 45,46 In addition to numerical deficiencies, NKT cells derived from NOD mice produce relatively less IL-4, so that this strain has 20-fold fewer IL-4- secreting thymic NKT cells than C57BL/6 mice. 46 Correspondence: Alan G. Baxter, Comparative Genomics Centre, Molecular Sciences Bldg 21, James Cook University, Townsville, Queensland 4811, Australia. Email: Alan.Baxter@jcu.edu.au Received 11 March 2004; accepted 11 March 2004.