Activation or anergy: NKT cells are stunned by α-galactosylceramide

Abstract

Invariant natural killer T (iNKT) cells are T lymphocytes that behave similarly to cells of the innate immune system. The glycolipid α-galactosylceramide (α-GalCer) is a potent and specific activator of mouse and human iNKT cells and has been used in cancer clinical trials to drive NKT cell–mediated immune responses. However, little is known about the dynamics of the iNKT cell response to α-GalCer in vivo. In this issue of the JCI, Parekh and colleagues demonstrate that administration of α-GalCer causes iNKT cells to become unresponsive, for at least 1 month, in mice. This leads us to ask, should sequential administration of α-GalCer still be used to activate iNKT cells given the anergic state it has been shown here to induce? This intriguing article raises the issue of the avoidance of anergy induction in the design of treatment regimens that use α-GalCer as a specific activator of iNKT cells.

Natural killer T (NKT) cells are specialized T cells of the immune system that express markers of the NK cell lineage, such as NK1.1. They recognize glycolipid antigens presented by the MHC class I–related protein CD1d (1). In mice, these cells are sometimes referred to as invariant NKT (iNKT) cells, because they express an invariant TCR consisting of a single Vα-to-Jα rearrangement (Vα14-Jα18), paired with a restricted set of Vβ chains (Vβ8.2, Vβ7, and Vβ2 are most common). Human iNKT cell TCRs express orthologous V segments, an invariant Vα24-Jα18 paired with Vβ11. Human and mouse iNKT cells react to the same glycolipid antigens, including α-galactosylceramide (α-GalCer), which is presented by CD1d. Among T lymphocytes, iNKT cells are unique in terms of the conservation of their specificity and activated phenotype; this suggests that studies elucidating key components of mouse iNKT cell biology will likely also prove important for human iNKT cells. However, humans have fewer CD1d-restricted iNKT cells than mice.

α-GalCer activates and anergizes iNKT cells

The glycolipid α-GalCer (also called KRN7000) was originally identified in a screen for antitumor activity of compounds derived from marine sponges (2). In mice, α-GalCer activates iNKT cells within hours of injection, causing these cells to produce copious amounts of both regulatory and proinflammatory cytokines. Proinflammatory cytokines lead to the downstream transactivation of NK, T, and B cells as well as DCs. The NK cell activation that is dependent on activated iNKT cells is essential for α-GalCer–mediated tumor clearance in mice. Coadministration of a model protein antigen with α-GalCer leads to the enhanced induction of specific T cell memory (3, 4). α-GalCer is highly potent because of its strong binding to the iNKT cell TCR and its long half-life (5). Additionally, α-GalCer targets only iNKT cells, as there is little bystander activation and little transactivation in mice lacking CD1d. Numerous investigators have used synthetic α-GalCer, or its variants, in mouse models to prevent tumor metastases, to reduce autoimmunity in experimental autoimmune encephalomyelitis (EAE) and diabetes models, and to enhance the responses to viral and parasitic infections (6). Furthermore, α-GalCer has been used in 4 phase I cancer clinical trials to date, with the ultimate purpose of determining whether the antitumor effects obtained in mice also will be observed in humans (7–10). Despite the interest in using α-GalCer in the clinic to activate iNKT cells, little is known about the effects of repeated α-GalCer administration in vivo. In this issue of the JCI, Parekh et al. provide what we believe to be the first description of the detailed long-term effects of α-GalCer administration on iNKT cells in vivo, in mice (11).

Parekh et al. (11) have demonstrated that a single injection of α-GalCer induces long-term anergy of iNKT cells in mice. What exactly is anergy? Ronald Schwartz, a pioneer in this field of study, calls it “a tolerance mechanism in which the lymphocyte is intrinsically functionally inactivated following an antigen encounter, but remains alive for an extended period of time in a hyporesponsive state” (12). Simply put, conventional T cells can be “stunned” by strong TCR signals delivered in the absence of costimulation. The iNKT cell anergy described by Parekh et al. has several features in common with the anergy of conventional T cells, as it is long lasting, independent of persistent antigen, cell autonomous, and reversible with administration of IL-2 (11). The authors also show that, as for conventional T cells, the block in activation of anergized iNKT cells is at the level of the TCR. Culture of anergized iNKT cells with PMA plus ionomycin, which bypasses proximal steps in TCR signaling, allowed the iNKT cells to respond by producing cytokines.

Do iNKT cells become anergic when activated without costimulation? Parekh et al. do not address this issue directly; however, they do show that B cells presenting α-GalCer can induce anergy in iNKT cells whereas DCs do not (11). They speculate that this is due to the numbers of costimulatory molecules present on the surface of each cell type, with B cells having low numbers and DCs having higher numbers. Unlike conventional T cells, iNKT cells do not require costimulation for rapid cytokine release (13). Mouse iNKT cells may be activated initially in response to antigen and expand in number without costimulation, but may then adopt an anergic state. iNKT cell anergy therefore may not be mechanistically identical to conventional T cell anergy.

In some cases, anergic cells also regulate the activation of other T cells. This does not seem to be the case for iNKT cells, and they do not express Foxp3 (11), the transcription factor that acts as a master regulator for the generation and function of CD25⁺ Tregs. iNKT cells normally produce both IFN-γ and IL-4 when activated by α-GalCer, but Parekh et al. also show that while iNKT cell anergy is characterized by a reduction in IL-4 production, there is a more severe drop in IFN-γ production (11). iNKT cell anergy may therefore potentially skew Th2 cytokine production, a fact that might explain the beneficial effects of repeated α-GalCer treatment in the prevention of spontaneous diabetes in NOD mice (14, 15).

Are glycolipid antigens ready for the clinic?

Could the induction of iNKT cell anergy be useful in the clinic? In some animal models, iNKT cells have deleterious effects, as shown in airway hypersensitivity models of human asthma (16, 17). In these cases, B cells loaded with α-GalCer could be used to anergize mouse iNKT cells, and, potentially, this concept could be extended to humans as well. However, even when presented by a B cell, α-GalCer induces the expression of numerous cytokines before the iNKT cells become anergic, and this could lead to morbidity. It is therefore perhaps more likely that avoiding anergy will be the key issue in future therapies aimed at activating iNKT cells. Along those lines, phase I clinical trials have examined the utility of immunization with either α-GalCer alone (7) or α-GalCer–loaded immature (8, 9) or mature DCs (10) for the treatment of cancer. Consistent with mouse studies, delivery of α-GalCer on mature DCs has proven to currently be the best way to expand iNKT cell numbers: the iNKT cell population was shown to expand dramatically (more than 100-fold) after 3 immunizations with α-GalCer–loaded mature DCs (10).

What are the prospects for using glycolipid agonists to activate iNKT cells in vivo? iNKT cells are an exceptional target for immunotherapy, because of their swift and directed activation of the adaptive immune system. However, it might be worthwhile to investigate iNKT cell anergy further in the context of targeting α-GalCer to different APCs. Furthermore, new glycolipid ligands for iNKT cells have been recently identified, including synthetic and naturally occurring antigens. The naturally occurring antigens isoglobotrihexosylceramide (iGb3; an autologous mouse antigen [ref. 18]) and GSL-1′, a glycosphingolipid antigen from Sphingomonas bacteria that is structurally related to α-GalCer (19, 20), stimulate iNKT cells less strongly than α-GalCer and might be good candidates for immunotherapy. Repeated injections of weaker compounds may be more effective in the avoidance of anergy, and therefore it will be critical to determine whether iGb3 and/or GSL-1′ induce anergy in iNKT cells. This is needed for the development of glycolipid-containing adjuvants and vaccines and for any immunotherapy designed to stimulate iNKT cells.

Footnotes

See the related article beginning on page 2572.

Nonstandard abbreviations used: α-GalCer, α-galactosylceramide; iNKT, invariant natural killer T (cell); NKT, natural killer T (cell).

Conflict of interest: The authors have declared that no conflict of interest exists.

References

1. Kronenberg, M, Rudensky, A. Regulation of immunity by self-reactive T cells. Nature. 2005. 435:598-604.
   View this article via: PubMed Google Scholar
2. Kobayashi, E, Motoki, K, Uchida, T, Fukushima, H, Koezuka, Y. KRN7000, a novel immunomodulator, and its antitumor activities. Oncol. Res. 1995. 7:529-534.
   View this article via: PubMed Google Scholar
3. Fujii, S, Shimizu, K, Smith, C, Bonifaz, L, Steinman, RM. Activation of natural killer T cells by alpha-galactosylceramide rapidly induces the full maturation of dendritic cells in vivo and thereby acts as an adjuvant for combined CD4 and CD8 T cell immunity to a coadministered protein. J. Exp. Med. 2003. 198:267-279.
   View this article via: PubMed Google Scholar
4. Gonzalez-Aseguinolaza, G, et al. Natural killer T cell ligand alpha-galactosylceramide enhances protective immunity induced by malaria vaccines. J. Exp. Med. 2002. 195:617-624.
   View this article via: PubMed Google Scholar
5. Sidobre, S, et al. The T cell antigen receptor expressed by Valpha14i NKT cells has a unique mode of glycosphingolipid antigen recognition. Proc. Natl. Acad. Sci. U. S. A. 2004. 101:12254-12259.
   View this article via: PubMed Google Scholar
6. Godfrey, DI, Kronenberg, M. Going both ways: immune regulation via CD1d-dependent NKT cells [review]. J. Clin. Invest. 2004. 114:1379-1388. doi:10.1172/JCI200423594.
   View this article via: JCI PubMed Google Scholar
7. Giaccone, G, et al. A phase I study of the natural killer T-cell ligand alpha-galactosylceramide (KRN7000) in patients with solid tumors. Clin. Cancer Res. 2002. 8:3702-3709.
   View this article via: PubMed Google Scholar
8. Nieda, M, et al. Therapeutic activation of Valpha24+Vbeta11+ NKT cells in human subjects results in highly coordinated secondary activation of acquired and innate immunity. Blood. 2004. 103:383-389.
   View this article via: PubMed Google Scholar
9. Ishikawa, A, et al. A phase I study of alpha-galactosylceramide (KRN7000)-pulsed dendritic cells in patients with advanced and recurrent non-small cell lung cancer. Clin. Cancer Res. 2005. 11:1910-1917.
   View this article via: PubMed Google Scholar
10. Chang, DH, et al. Sustained expansion of NKT cells and antigen-specific T cells after injection of alpha-galactosyl-ceramide loaded mature dendritic cells in cancer patients. J. Exp. Med. 2005. 201:1503-1517.
    View this article via: PubMed Google Scholar
11. Parekh, VV, et al. Glycolipid antigen induces long-term natural killer T cell anergy in mice. J. Clin. Invest. 2005. 115:2572-2583. doi:10.1172/JCI24762.
    View this article via: JCI PubMed Google Scholar
12. Schwartz, RH. T cell anergy. Annu. Rev. Immunol. 2003. 21:305-334.
    View this article via: PubMed Google Scholar
13. Matsuda, JL, et al. Mouse V alpha 14i natural killer T cells are resistant to cytokine polarization in vivo. Proc. Natl. Acad. Sci. U. S. A. 2003. 100:8395-8400.
    View this article via: PubMed Google Scholar
14. Sharif, S, et al. Activation of natural killer T cells by alpha-galactosylceramide treatment prevents the onset and recurrence of autoimmune type 1 diabetes. Nat. Med. 2001. 7:1057-1062.
    View this article via: PubMed Google Scholar
15. Hong, S, et al. The natural killer T-cell ligand alpha-galactosylceramide prevents autoimmune diabetes in non-obese diabetic mice. Nat. Med. 2001. 7:1052-1056.
    View this article via: PubMed Google Scholar
16. Lisbonne, M, et al. Cutting edge: invariant V alpha 14 NKT cells are required for allergen-induced airway inflammation and hyperreactivity in an experimental asthma model. J. Immunol. 2003. 171:1637-1641.
    View this article via: PubMed Google Scholar
17. Akbari, O, et al. Essential role of NKT cells producing IL-4 and IL-13 in the development of allergen-induced airway hyperreactivity. Nat. Med. 2003. 9:582-588.
    View this article via: PubMed Google Scholar
18. Zhou, D, et al. Lysosomal glycosphingolipid recognition by NKT cells. Science. 2004. 306:1786-1789.
    View this article via: PubMed Google Scholar
19. Kinjo, Y, et al. Recognition of bacterial glycosphingolipids by natural killer T cells. Nature. 2005. 434:520-525.
    View this article via: PubMed Google Scholar
20. Mattner, J, et al. Exogenous and endogenous glycolipid antigens activate NKT cells during microbial infections. Nature. 2005. 434:525-529.
    View this article via: PubMed Google Scholar