Two groups of investigators, reporting in Nature Immunology, have visualized invariant natural killer T (iNKT) cells being activated in vivo and provide new insights into how and where this occurs. Both studies reveal a crucial role for specialized tissue-resident macrophage populations — subcapsular sinus CD169+ macrophages in the lymph nodes and Kupffer cells in the liver sinusoids — in early iNKT cell activation.
Using multiphoton microscopy, Barral et al. tracked the behaviour of fluorescently labelled iNKT cells in lymph nodes following adoptive transfer into wild-type mice. In resting lymph nodes, iNKT cells were mainly found in the paracortex alongside CD4+ T cells and sometimes close to the subcapsular sinus (SCS), but were mainly excluded from the B cell follicles. The average speed and random pattern of their movements were similar to co-transferred fluorescently labelled CD4+ T cells. The transferred iNKT cells were activated in vivo by injection of silica particles coated with the iNKT cell glycolipid antigen α-galactosylceramide (α-GalCer). After injection of these stimulatory particles, but not particles coated with non-stimulatory lipids, iNKT cell movements in the draining lymph nodes slowed, were more confined and involved more long-lasting stops. This did not occur when the cells were transferred into CD1d-deficient mice, indicating that this change in behaviour depended on the recognition of α-GalCer presented by CD1d molecules.
Further imaging revealed that, within minutes of injection, the lipid-coated particles could be detected in the SCS and medullary regions of the draining lymph nodes, in particular, co-localizing with CD169+ macrophages. Within 6 hours, transferred iNKT cells were retained in the antigen-rich areas and established direct contacts with CD169+ macrophages. Experiments involving depletion of macrophages in the lymph nodes or in vitro analysis of purified populations confirmed that SCS CD169+ macrophages can present antigenic lipids — even those that require internalization and processing — to iNKT cells, inducing their activation, cytokine production and proliferation.
Lee et al. visualized the behaviour of iNKT cells in the liver following infection of mice with Borrelia burgdorferi, a spirochete responsible for human Lyme disease. In uninfected livers, iNKT cells were found to reside mainly in the sinusoids and to move in a random pattern, whereas Kupffer cells were largely immobile with extended processes reaching into several sinusoids. Within a few hours of infection, most B. burgdorferi spirochetes became trapped by Kupffer cells and iNKT cells became more stationary, forming clusters with Kupffer cells containing B. burgdorferi. The reduced motility and clustering of iNKT cells was shown to depend on CXC-chemokine receptor 3 and CD1d and resulted in the production of interferon-γ by the iNKT cells. Studies involving the depletion of Kupffer cells confirmed that these cells are responsible for rapid CD1d-mediated activation of iNKT cells in the liver in response to B. burgdorferi. Moreover, this pathway was shown to be crucial to limit the spread of B. burgdorferi from the blood to the liver parenchyma and joints.
Together, these studies establish a key role for tissue macrophages in the early activation of iNKT cells, which is likely to facilitate the induction of later immune responses.
ORIGINAL RESEARCH PAPERS
Barral, P. et al. CD169+ macrophages present lipid antigens to mediate early activation of iNKT cells in lymph nodes. Nature Immunol. 14 Mar 2010 (doi:10.1038/ni.1853)
Lee, W.-Y. et al. An intravascular immune response to Borrelia burgdorferi involves Kupffer cells and iNKT cells. Nature Immunol. 14 Mar 2010 (doi:10.1038/ni.1855)
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Bird, L. Picturing activation in action. Nat Rev Immunol 10, 218 (2010). https://doi.org/10.1038/nri2756