Over the past month or so, two amazing imaging studies have emerged from the laboratory of Ron Germain at the National Institute of Allergy and Infectious Diseases (NIAID). The techniques involved are fairly recent innovations which allow the activity of mouse immune system cells to be visualized in vivo or in excised tissue sections using special microscopes and fluorescent labels to tag the cells and tissues of interest. In one study, the technique is used to monitor the movement of T cells in lymph nodes. Germain and colleagues show that, in contrast to initial suggestions that T cells wander randomly in these tissues, they in fact move along string-like pathways of fibroblastic reticular cells (FRC). These pathways form a complex traffic system with crossroads, junctions and dead ends. Dendritic cells, whose job it is to present antigens to potentially responsive T cells, gather at the crossroads like streetcorner salesmen plying their wares. The dead ends of the FRC pathways act to prevent T cells from being able to travel into the B cell zone of the lymph nodes. Germain and colleagues also show that B cells have their own similar traffic system, the follicular dendritic cell network in the B cell zone of the lymph node. Although access to the full paper requires a subscription to the journal Immunity, the video images can be viewed on the journal’s website free of charge.
In a second paper in the Journal of Experimental Medicine, the techniques are used to investigate the behavior of dendritic cells in the gut mucosa. While it is known that dendritic cells can reach through the epithelial barrier of the gut to sample antigens in the GI tract, the precise nature of this process has not been well elucidated. Germain’s stunning images show how the long tentacles of dendritic cells can balloon out through epithelia to grab passing antigens before seeming to deflate back behind the mucosal barrier. Additional experiments also demonstrate that the prevalence of this activity varies in different locations in the GI tract, and that it is robustly enhanced by microbial stimuli (such as Salmonella infection). Given that dendritic cells can capture HIV and act as Trojan horses that transport the virus to CD4 target cells, these findings may be very relevant to the mucosal transmission of HIV infection. Again, while access to the full text of the paper is restricted, the movies can be viewed online free of charge (all the movies require a free media player such Windows Media Player or Quicktime).
Abstracts:
Immunity. 2006 Dec;25(6):989-1001. Epub 2006 Nov 16.
Stromal cell networks regulate lymphocyte entry, migration, and territoriality in lymph nodes.
Bajenoff M, Egen JG, Koo LY, Laugier JP, Brau F, Glaichenhaus N, Germain RN.
Lymphocyte Biology Section, Laboratory of Immunology, NIAID, National Institutes of Health, Bethesda, Maryland 20892, USA.
After entry into lymph nodes (LNs), B cells migrate to follicles, whereas T cells remain in the paracortex, with each lymphocyte type showing apparently random migration within these distinct areas. Other than chemokines, the factors contributing to this spatial segregation and to the observed patterns of lymphocyte movement are poorly characterized. By combining confocal, electron, and intravital microscopy, we showed that the fibroblastic reticular cell network regulated naive T cell access to the paracortex and also supported and defined the limits of T cell movement within this domain, whereas a distinct follicular dendritic cell network similarly served as the substratum for movement of follicular B cells. These results highlight the central role of stromal microanatomy in orchestrating cell migration within the LN.
J Exp Med. 2006 Dec 25;203(13):2841-52. Epub 2006 Dec 4.
Chieppa M, Rescigno M, Huang AY, Germain RN.
Lymphocyte Biology Section, Laboratory of Immunology, National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH), Bethesda, MD 20892.
Cells lining the gastrointestinal tract serve as both a barrier to and a pathway for infectious agent entry. Dendritic cells (DCs) present in the lamina propria under the columnar villus epithelium of the small bowel extend processes across this epithelium and capture bacteria, but previous studies provided limited information on the nature of the stimuli, receptors, and signaling events involved in promoting this phenomenon. Here, we use immunohistochemical as well as dynamic explant and intravital two-photon imaging to investigate this issue. Analysis of CD11c-enhanced green fluorescent protein (EGFP) or major histocompatibility complex CII-EGFP mice revealed that the number of trans-epithelial DC extensions, many with an unusual "balloon" shape, varies along the length of the small bowel. High numbers of such extensions were found in the proximal jejunum, but only a few were present in the terminal ileum. The extensions in the terminal ileum markedly increased upon the introduction of invasive or noninvasive Salmonella organisms, and chimeric mouse studies revealed the key role of MyD88-dependent Toll-like receptor (TLR) signaling by nonhematopoietic (epithelial) elements in the DC extension response. Collectively, these findings support a model in which epithelial cell TLR signaling upon exposure to microbial stimuli induces active DC sampling of the gut lumen at sites distant from organized lymphoid tissues.
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