The host doesn't sit idly as B. burgdorferi establishes an infection. Invariant natural killer (iNKT) cells are one of the tools deployed by the immune system in its battle against the Lyme spirochetes. Scientists know this because B. burgdorferi-infected mice lacking iNKT cells ended up with more spirochetes in their tissues and greater joint swelling than mice with a complete immune system.2
iNKT cells are an odd type of T cell. Like other T cells, iNKT cells have a T cell receptor (TCR), yet they also express protein markers used to identify natural killer (NK) cells. What makes the iNKT cell invariant is its TCR α chain, which comes only in the version dubbed Vα14 in mice and Vα24 in humans. Even the β chain of the TCR of iNKT cells is restricted to three types in mice and just one in humans. The lack of variation is unusual because the α and β TCR chains of conventional αβ T cells come in many forms in each individual, resulting in millions of varieties of TCRs. This enables conventional αβ T cells to recognize a wide range of microbial peptide antigens when displayed by an MHC molecule on the surface of an antigen-presenting cell (see figure below). In contrast, the TCRs of iNKT cells recognize a limited set of glycolipids displayed by the antigen-presenting cell's CD1d molecule, which structurally resembles MHC. So far these glycolipids have been found only in Sphingomonas and B. burgdorferi.
|Antigen recognition by T cells. The "X" represents variable T cell receptor chains.|
Figure 1 from ref. 3.
|Structures of B. burgdorferi glycolipid antigens recognized by iNKT cells. Figure 3d from ref. 3.|
iNKT cells are activated when their TCR binds to BbGL-II complexed with CD1d.4 The activated iNKT cells secrete cytokines that elicit the appropriate immune response against the spirochetes. How these cytokines promote killing of B. burgdorferi remains unknown.
To view the process of iNKT cell activation, scientists have recently obtained video footage of the early stages of the immune response to Borrelia burgdorferi circulating in the bloodstream of mice.1 The study by Lee et al., which appeared in the April issue of Nature Immunology, complements two earlier studies that revealed how the Lyme disease spirochete escapes from the bloodstream of mice to invade the surrounding tissues.5,6
The investigators employed fluorescence video microscopy to watch the immune cells in action following injection of an engineered B. burgdorferi strain expressing green fluorescent protein (GFP) into the bloodstream. Although the spleen is better known for filtering bloodstream pathogens, the liver was selected for observation because iNKT cells make up 30% of the T cells in the liver. In contrast, iNKT cells represent only 2.5% of T cells in the spleen. Moreover, mice missing their spleen were able to limit B. burgdorferi infection as well as mice having a spleen, suggesting that the spleen is not critical in fighting bloodstream B. burgdorferi.
iNKT cells reside in the liver's sinusoids, which are the specialized capillaries that carry blood through the liver. Similar to what other investigators have observed, the authors saw iNKT cells creeping along the inner surface of the liver sinusoids in healthy mice (see video below).
iNKT cells crawling within the liver sinusoids of a mouse genetically altered to express green fluorescent protein (GFP) in iNKT cells. The iNKT cells glow bright green. The elapsed time is shown at the top right. Video 2 from ref. 1.
The investigators wanted to figure out which of the antigen-presenting cells found in the liver presented borrelial glycolipid to iNKT cells. The answer? After the spirochetes were injected into the bloodstream, they were quickly captured by Kupffer cells, the specialized blood-filtering macrophages that also reside in the liver sinusoids (see figure below). Unlike iNKT cells, Kupffer cell remained stationary.
During the next several hours, the captured spirochetes were engulfed and broken up by the Kupffer cells so that BbGL-II could be loaded onto CD1d and displayed on the cell surface. At 8 hours post injection, iNKT cells started to cluster and form stable contacts with Kupffer cells. The iNKT cells were attracted to Kupffer cells churning out the chemokine CXCL9, a potent iNKT cell attractant. The evidence for this was that injection of antibodies against the CXCL9 receptor, located on the iNKT cell surface, blocked clustering of iNKT cells. Interaction of the Kupffer and iNKT cells was accompanied by increased blood and liver levels of the cytokine IFN-γ (interferon-gamma), a sign that the iNKT cells were being activated.
Not all spirochetes were captured. The investigators saw B. burgdorferi escaping from the sinusoids into the surrounding liver tissue even as other spirochetes were trapped by nearby Kupffer cells (see figures above). Spirochetes circulating throughout the host probably escaped into other organs in the same manner. Indeed, large amounts of B. burgdorferi DNA were detected by PCR in several organs, including the liver, three days after the spirochetes were injected. Although one doesn't usually think about the effects of Lyme disease on the liver, the authors pointed out that a mild hepatitis is common in Lyme disease patients. In one prospective study, 40% of Lyme disease patients had at least one liver test abnormality.7
By now you may be wondering why the liver would devote such a high percentage of its T cells towards recognizing glycolipids that aren't found on most bacteria. One answer is that microbes lacking the proper glycolipids may activate iNKT cells indirectly.3 For example, Salmonella typhimurium uses its LPS to coax antigen-presenting cells into making an endogenous glycolipid that gets presented to the iNKT cell by CD1d.8 It is also possible that glycolipids that are recognized by iNKT cells are present in other bacteria but are yet to be discovered.
1. Lee, W.Y., Moriarty, T.J., Wong, C.H.Y., Zhou, H., Strieter, R.M., van Rooijen, N., Chaconas, G., & Kubes, P. (2010). An intravascular immune response to Borrelia burgdorferi involves Kupffer cells and iNKT cells Nature Immunology, 11 (4), 295-302 DOI: 10.1038/ni.1855
2. Tupin, E., Benhnia, M.R., Kinjo, Y., Patsey, R., Lena, C.J., Haller, M.C., Caimano, M.J., Imamura, M., Wong, C., Crotty, S., Radolf, J.D., Sellati, T.J., and Kronenberg, M. (2008). NKT cells prevent chronic joint inflammation after infection with Borrelia burgdorferi. Proc. Natl. Acad. Sci. USA 105(50):19863-19868. DOI: 10.1073/pnas.0810519105
3. Tupin, E., Kinjo, Y., and Kronenberg, M. (2007). The unique role of natural killer T cells in the response to microorganisms. Nature Reviews Microbiology 5(6):405-417. DOI: 10.1038/nrmicro1657
4. Kinjo, J., Tupin, E., Wu, D., Fujio, M., Garcia-Navarro, R., Benhnia, M. R., Zajonc, D.M., Ben-Menachem, G., Ainge, G.D., Painter, G.F., Khurana, A., Hoebe, K., Behar, S.M., Beutler, B., Wilson, I.A., Tsuji, M., Sellati, T.J., Wong, C., and Kronenberg, M. (2006). Nature Immunology 7(9):978-986. DOI: 10.1038/ni1380
5. Moriarty, T.J., Norman, M.U., Colarusso, P., Bankhead, T., Kubes, P., and Chaconas, G. (June 20, 2008). Real-time high resolution 3D imaging of the Lyme disease spirochete adhering to and escaping from the vasculature of a living host. PLoS Pathogens 4(6):e1000090. DOI: 10.1371/journal.ppat.1000090
6. Norman, M.U., Moriarty, T.J., Dresser, A.R., Millen, B., Kubes, P., and Chaconas, G. (October 3, 2008). Molecular mechanisms involved in vascular interactions of the Lyme disease pathogen in a living host. PLoS Pathogens 4(10):e1000169. DOI: 10.1371/journal.ppat.1000169
7. Horowitz, H.W., Dworkin, B., Forseter, G., Nadelman, R.B., Connolly, C., Luciano, B.B., Nowakowski, J., O'Brien, T.A., Calmann, M., Wormser, G.P. (June 1996). Liver function in early Lyme disease. Hepatology 23(6):1412-1417. DOI: 10.1002/hep.510230617
8. Mattner J., DeBord, K.L., Ismail, N., Goff, R.D., Cantu III, C., Zhou, D., Saint Mezard, P., Wang, V., Gao, Y., Yin, N, Hoebe, K., Schneewind, O., Walker, D., Beutler, B., Teyton, L, Savage, P.B., and Bendelac, A. (March 24, 2005). Exogenous and endogenous glycolipid antigens activate NKT cells during microbial infections. Nature 434(7032):525-529. DOI: 10.1038/nature03408