Unlocking the Secrets of TLR10
Of the 10 known human TLRs, TLR10, which has no counterpart in mice, is the only one for which no agonist or function has been identified. Guan et al. (p. 5094) sought to resolve this issue by analyzing chimeric receptors consisting of portions of TLR10 fused to portions of other TLRs. TLR10 is most similar to TLR1 and TLR6, which recognize their ligands in heterodimers with TLR2, and was found to also physically associate with TLR2 in phagosomes. Coupled with TLR2, TLR10 shared agonist specificity with TLR1 but not with TLR6 and recruited MyD88 upon ligand binding. However, the TLR2/10 complexes did not activate the expected TLR-induced signaling pathways, and the functional outcome of agonist binding therefore remains unclear. These two TLRs associated in a ligand-dependent manner through their extracellular domains and showed a strikingly similar structure to the TLR2/1 complex that nonetheless differed somewhat at the TLR2 dimer interface, as indicated by computational modeling supported by site-directed mutagenesis. This study identifies the first agonists for TLR10 and suggests that this receptor may compete in vivo with TLR1 for microbial ligands and for interaction with TLR2. In light of these data, much of what we know about TLR2 may need to be re-evaluated, especially before this molecule can be therapeutically targeted.
The Lyn Twins
Mast cell activation is initiated by phosphorylation of FcεR1 by the Src family protein tyrosine kinase Lyn. Lyn exists as two isoforms, Lyn A and Lyn B, which differ by 21 aas in their N-terminal domains and are generated by alternative splicing. To distinguish the functions of these two versions of Lyn, Alvarez-Errico et al. (p. 5000) assessed the results of expressing them individually in mast cells from Lyn-deficient mice. Although Lyn A and Lyn B induced similar levels of FcεR1 phosphorylation and total cellular phosphorylation, Lyn A was more effective at promoting FcεR1-mediated degranulation and Ca2+ mobilization in these cells. However, Lyn B demonstrated an enhanced ability to interact with tyrosine phosphorylated proteins, including LAT, compared with Lyn A. Lyn B also bound to the inhibitory phosphatase SHIP-1 and caused decreases in PLCγ phosphorylation and interaction with LAT compared with Lyn A-expressing or wild-type cells. Expression of both Lyn A and Lyn B in Lyn−/− mast cells allowed their differing activities to complement one another to normalize mast cell responses. Taken together, these data identify distinct functions for the mRNA splice variants Lyn A and Lyn B and provide justification for the presence of both isoforms in mast cells.
Idd18 Gets Taken Apart
Type I diabetes (T1D) development is controlled by multiple genetic and environmental factors. In the mouse, several T1D susceptibility loci have been identified on chromosome 3, and of these, insulin-dependent diabetes (Idd)18 does not contain any published candidate genes. Combining analysis of the mouse genome public database with that of congenic mice, Fraser et al. (p. 5075) dissected the mechanisms of T1D susceptibility conferred by the Idd18 locus. After first refining the Idd18 locus down to 1.27 Mb, the authors compared alleles from T1D-susceptible NOD mice with those from resistant C57BL/6 (B6) mice in a novel congenic mouse strain and were able to identify a genetic interval, Idd18.1, that contained the Vav3 gene. Significant allelic variation between NOD and B6 mice was observed in the intronic regions of Vav3, and the NOD alleles of Vav3 were expressed at higher levels in thymocytes than the B6 alleles. Interestingly, although it is known that the T1D protection conferred by Idd18 does not depend on protective alleles at Idd3, B6-derived Idd18.1 failed to protect against disease in the absence of B6 alleles of Idd3. The authors then identified another interval, designated Idd18.3, that accounted for the Idd3 independence of Idd18. This study suggests that modulation of signaling pathways involving Vav3 may be important in diabetes pathogenesis and highlights the complex interactions among multiple loci that control disease susceptibility.
Sampling Ags and Moving On
Naive CD4+ T cells continuously circulate throughout the body, traveling through secondary lymphoid organs. Tomura et al. (p. 4646) followed T cell migration in steady-state conditions in vivo using transgenic mice expressing the photoconvertible protein Kaede to assess the role of self-Ag recognition in recirculation. Although CD69 is normally regarded as a marker of T cell activation, some naive CD4+ T cells transiently upregulated this marker in the peripheral lymphoid organs. CD69 expression was induced by TCR interactions with MHC:endogenous Ag complexes and promoted retention of T cells in lymphoid organs. Downregulation of CD69 allowed the T cells to exit the lymph nodes and continue on to other lymphoid organs, where they could again sample self-Ags. Following in vitro stimulation through the TCR, CD4+CD69+ naive T cells produced IL-2 and TNF-α much more rapidly and in much greater quantities than CD4+CD69− cells, indicating that these cells were now “primed” for a quick response if met with a foreign Ag. These data suggest that naive CD4+ T cells do not simply circulate passively through the body in the absence of infection but rather constantly seek out self-Ags, and CD69 expression controls retention of these cells in lymphoid organs.
Finicky IgE Binding
Allergens are recognized by specific IgE Abs in allergic individuals. However, multiple isoforms of allergens, or iso-allergens, can exist and the biological importance of these variants in an allergic response is somewhat unclear. To address the potential interplay of polyclonal Abs and variations on the surface of an allergen, Christensen et al. (p. 4966) analyzed the human IgE response to iso-allergens of the common house dust mite allergen Der p 2. Der p 2-specific monoclonal IgE Abs were cloned from an allergic patient, and their binding profiles were characterized. Individual Abs demonstrated binding affinities differing by as much as 100-fold between different Der p 2 isoforms, and these variations in binding strength translated to differences in the abilities of these Abs to induce basophil degranulation. However, different iso-allergens did not elicit different levels of basophil degranulation when the basophils were sensitized with polyclonal IgE. These data demonstrate that an exceptionally high level of polyclonality exists within the IgE repertoire, even the repertoire directed against a single allergen. Although the extent of this variability may not be apparent when analyzing a polyclonal allergic response, the underlying variations in reactivity to iso-allergens are important for a complete understanding of allergy.
Will the Real Mucosal APC Stand Up?
Vaccination via mucosal routes often leads to more effective immunity in the vaginal tract than does parenteral immunization. A better understanding of the development of vaginal CTL responses is necessary for the therapeutic generation of protective immunity against sexually transmitted infections. Little is known about mucosal Langerhans cells (LCs) and their potential role in immunity initiated at mucosal sites, leading Hervouet et al. (p. 4842) to analyze the contribution of different mucosal dendritic cell (DC) subsets to the generation of CTL responses. A small population of LCs and a larger one of Langerin− DCs were identified in vaginal epithelium, and both cell types took up Ag and transported it to draining lymph nodes upon intravaginal immunization. Whereas mucosal Langerin− DCs could effectively present Ag and activate both CD4+ and CD8+ T cells, mucosal LCs were less effective at stimulating CD8+ T cells and did not induce CD4+ T cell proliferation. Interestingly, T cells primed by vaginal LCs produced IL-17 and IL-10, whereas those primed by Langerin− DCs produced IFN-γ. Transient ablation of LCs prior to intravaginal immunization led to enhanced vaginal CTL responses, suggesting that mucosal LCs could have a regulatory function. The possibility that mucosal LCs may induce IL-17 responses and inhibit CTL activity has important implications for the design of mucosal vaccines and DC-targeting therapies.
Viral infection often induces cellular apoptosis by activating the caspase family of cysteine proteases, which are involved in inflammation as well as cell death. López et al. (p. 5193) found that in the absence of proteasomal Ag processing, caspase activity could also generate antigenic viral peptides that were presented on MHC class I and recognized by CTLs. The 19-aa m19 miniprotein, which includes an immunodominant CMV epitope, could be processed in vitro either by the proteasome or by caspases 5 and 10 to generate products that stimulated CTLs. The composition and quantity of the peptide pool produced by the caspases differed from that produced by the proteasome and indicated that the caspases had cleaved m19 at noncanonical sites. However, analysis of endogenously processed peptides in virally infected cells revealed that similar peptides with similar antigenic activities were produced via the proteasomal and caspase-mediated pathways. Infection of cells with vaccinia viruses expressing the B13 protein, which is known to block apoptosis, inhibited caspase-mediated Ag processing. Thus, caspase activation during infection-induced apoptosis may modulate the pool of Ags available to CTLs and can, at least in this setting, render the proteasome redundant for Ag processing.
The All-Powerful CCR5
Eradication of the fungal pathogen Histoplasma capsulatum requires a Th1 response and is inhibited by regulatory T cells (Tregs). The chemokine receptor CCR5 mediates trafficking of both inflammatory cells and Tregs and could therefore be involved in either the promotion or the inhibition of pathogen persistence. Kroetz and Deepe, Jr. (p. 5224) investigated the role of CCR5 in H. capsulatum infection and identified unexpected activities for this receptor. Early in infection, CCR5 and its ligand CCL4 were important for inflammatory cell recruitment into H. capsulatum-infected lungs, and mice lacking CCR5 activity had a higher fungal burden than did wild-type mice. Later in infection, however, CCR5-deficient mice cleared infection more rapidly than their wild-type counterparts. This enhanced fungal clearance was found to involve CCR5-mediated modulation of the Treg/Th17 balance. That is, CCR5 deficiency was associated with decreases in Treg infiltration and proliferation of both CD4+ effector T cells and Tregs in the lungs. The authors also observed a corresponding increase in Th17 cell activity but no significant change in Th1 cytokines in the absence of CCR5. Neutralization of IL-17 demonstrated that Th17 cells were responsible for the reductions in fungal burden and Treg expansion in CCR5−/− mice. Thus, in addition to its roles in cellular migration, CCR5 can control the equilibrium between Treg and Th17 responses.
Summaries written by Jennifer Hartt Meyers, Ph.D.