TRENDSin Immunology Vol.23 No.2 February 2002 http://immunology.trends.com 1471-4906/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. 66 News& Comment News& Comment Journal Club The immunological synapse, the molecular structure underlying antigen-specific communication between a T cell and an antigen-presenting cell (APC), has attracted tremendous interest in the past few years. To date, synapse formation has been investigated on artificial lipid bilayers containing adhesion molecules and MHC–peptide complexes, as well as by computerized reconstruction of the three-dimensional composition of T-cell–B-cell contacts. However, the synapses between T cells and dendritic cells (DCs), the most effective type of APC, have not been investigated in detail yet. A first attempt has been undertaken by Revy and colleagues, who have made the striking observation that functional synapses form between DCs and T cells even in the absence of specific antigen [1]. Revy et al. have followed up on an earlier observation made by the same group; namely, that contact between DCs and T cells induces a low, but measurable, calcium signal in the T cells, even when no recognizable antigen is present on the DCs. In their new study, the authors use polyclonal, naive CD4 + or CD8 + T cells to show that in >50% of all T cells, physical contacts with DCs were established, and the interaction zone was enriched for the same molecules that have been described previously to be located in the center of B-cell–T-cell synapses. Strikingly, these contacts were observed in the absence of specific antigen on the surface of the DCs, and even with MHC-deficient DCs, suggesting that these synapses are indeed MHC–TCR-independent. Moreover, the contacts with DCs were functional by several criteria: (1) induction of a calcium signal; (2)relocalization of surface molecules; (3)localized phosphorylation events in the T cells; (4) rescue of the T cells from cell death; and (5) induction of very slow proliferation of the T cells. These experiments demonstrate that DCs interact with T cells even in the absence of a cognate antigen and suggest that DC–T-cell synapses differ fundamentally from B-cell–T-cell synapses. In the light of these data, it will be very interesting to reveal the dynamics and structural composition of cognate DC–T-cell synapses, which might well differ also from the previously investigated B-cell–T-cell synapses. 1 Revy, P. et al . (2001) Functional antigen- independent synapses formed between T cells and dendritic cells. Nat. Immunol . 2, 925–931 Stephan Grabbe Grabbe@uni-muenster.de M atthias Gunzer mgunzer@uni-muenster.de DC–T-cell synapses Gp96: Sw iss-army knife or tollgate? The past decade has seen a reassertion of the importance of the innate immune system in the response to infectious-disease pathogens. This has led to an explosion in our knowledge of receptors and responses of the human innate immune system, resulting particularly from comparisons with the immune systems of more-primitive organisms. For example, the identification of mammalian homologs of the Drosophila Toll receptors has led to the characterization of a rapidly expanding family of human Toll-like receptors (TLRs), which play several roles in the initiation of the innate immune response. These receptors appear to recognize molecular structures that are associated inherently with microorganisms and they activate transcriptional factors (e.g. NF-κB) that initiate the innate immune response to pathogens. How this signaling is mediated is an area of active interest, both to academic and pharmaceutical researchers. An elegant approach to answering this question involves the use of NF-κB-dependent green fluorescent protein reporter constructs in cells expressing various Toll receptors to identify mutants in the signaling pathway by the absence of fluorescence. Using this approach, Randow and Seed used a murine pre-B-cell line expressing the cell-surface protein CD14 to identify the components involved in signaling from the Toll receptor for bacterial lipopolysaccharide [1]. Complementation of a mutant cell line, which had greatly reduced responsiveness to CD14, identified the endoplasmic reticulum (ER) chaperone gp96 as harboring the frameshift mutation responsible for this defect. Consistent with the proposed role of ER chaperones, the effect appeared to be mediated by the intracellular retention of TLR4, the product of the lps gene in mice. Most interestingly, a similar retention of other TLRs was seen also in these cells, although the trafficking of other cell-surface proteins (e.g. the IL-1 receptor and CD16) was unaffected. This apparent specificity of the chaperone–client-protein interaction was studied further and the authors found that all gp96 clients were recruited from TLRs or integrins, both key players in the induction of the innate immune response. However, this new role for gp96 is in marked contrast to previous reports of the multiple functions of this chaperone in the immune system, which compared gp96 to a Swiss-army knife [2]. Previous studies had argued for a role for gp96 not only in the stimulation of expression of proinflammatory cytokines and maturation of antigen- presenting cells (APCs) but also, as a carrier of antigens. Although disparate, all these previously reported roles are consistent with an extracellular function for gp96 and are supported by the identification of a putative receptor for gp96 on APCs. By contrast, the new role of gp96 in ‘gating’ the expression of TLRs and integrins is dependent on its intracellular location. This is consistent with the normal cellular function of chaperones in the trafficking of proteins. Interestingly, the present study [1] showed also that gp96 is not required for cell viability, which is the major function of most chaperones. Most intriguingly, gp96 was identified originally as the glucose-regulated protein grp94, although how this role fits in with the hypothesized functions of ‘Swiss-army knife’ or ‘tollgate’ is difficult to see. Watch this space… 1 Randow, F. and Seed, B. (2001) Endoplasmic reticulum chaperone gp96 is required for innate immunity and not cell viability. Nat. Cell Biol . 3, 891–895 2 Schild, H. and Rammensee, H.G. (2000) Gp96 – the immune system’s Swiss-army knife. Nat. Immunol . 1, 100–101 Camilo Colaco camilo.colaco@immunobiology.co.uk