- Email: [email protected]
A New Vampire Saga: The Molecular Mechanism of T Cell Trogocytosis
Immunity
Previews protect against severe disease. This is consistent with the finding that recombinant IFN-a administration early in PbA infection reduced parasitemia and protected against ECM in a IFN-g-dependent manner (Vigario et al., 2007). A beneficial effect of type I IFNs was also suggested by higher plasma IFN-a levels in African children with mild malaria compared to those with hyperparasitemia or severe malarial anemia (Luty et al., 2000). The heterogeneity and complexity of human severe malaria necessitate examining the role of type I IFNs in diverse patient populations and in multiple murine models representing different aspects of human disease. However, the present findings do caution against administration of type I IFNs as adjunctive therapy for CM, especially because patients typically present with advanced pathology. In summary, Sharma et al. (2011) identify a unique cytosolic DNA sensing pathway and considerably expand our knowledge of the induction and role of
type I IFNs in malaria infection. The presence of AT-rich motifs in the genomes of other protozoa, bacteria, and humans suggests that these motifs could contribute to the pathogenesis of other severe infectious syndromes. In the case of extracellular DNA sources, it is intriguing to consider that other crystals (e.g., urate) may move these motifs into the cytosol for innate sensing. This proposed mechanism, if confirmed, may have relevance for the development of DNA-based vaccine adjuvants.
Herbich, K., Schmid, D., et al. (2000). Infect. Immun. 68, 3909–3915. Parroche, P., Lauw, F.N., Goutagny, N., Latz, E., Monks, B.G., Visintin, A., Halmen, K.A., Lamphier, M., Olivier, M., Bartholomeu, D.C., et al. (2007). Proc. Natl. Acad. Sci. USA 104, 1919–1924. Sharma, S., DeOliveira, R.B., Kalantari, P., Parroche, P., Goutagny, N., Jiang, Z., Chan, J., Bartholomeu, D.C., Lauw, F., Hall, J.P., et al. (2011). Immunity 35, this issue, 194–207. Shimosato, T., Kimura, T., Tohno, M., Iliev, I.D., Katoh, S., Ito, Y., Kawai, Y., Sasaki, T., Saito, T., and Kitazawa, H. (2006). Cell. Microbiol. 8, 485–495. Shio, M.T., Eisenbarth, S.C., Savaria, M., Vinet, A.F., Bellemare, M.J., Harder, K.W., Sutterwala, F.S., Bohle, D.S., Descoteaux, A., Flavell, R.A., and Olivier, M. (2009). PLoS Pathog. 5, e1000559.
REFERENCES Coban, C., Ishii, K.J., Kawai, T., Hemmi, H., Sato, S., Uematsu, S., Yamamoto, M., Takeuchi, O., Itagaki, S., Kumar, N., et al. (2005). J. Exp. Med. 201, 19–25.
van der Heyde, H.C., Nolan, J., Combes, V., Gramaglia, I., and Grau, G.E. (2006). Trends Parasitol. 22, 503–508.
Lovegrove, F.E., Gharib, S.A., Patel, S.N., Hawkes, C.A., Kain, K.C., and Liles, W.C. (2007). Am. J. Pathol. 171, 1894–1903.
Vigario, A.M., Belnoue, E., Gruner, A.C., Mauduit, M., Kayibanda, M., Deschemin, J.C., Marussig, M., Snounou, G., Mazier, D., Gresser, I., and Renia, L. (2007). J. Immunol. 178, 6416–6425.
Luty, A.J., Perkins, D.J., Lell, B., Schmidt-Ott, R., Lehman, L.G., Luckner, D., Greve, B., Matousek, P.,
Wu, X., Gowda, N.M., Kumar, S., and Gowda, D.C. (2010). J. Immunol. 184, 4338–4348.
A New Vampire Saga: The Molecular Mechanism of T Cell Trogocytosis Elaine Pashupati Dopfer,1 Susana Minguet,1 and Wolfgang W.A. Schamel1,* 1Department of Molecular Immunology, Faculty of Biology, BIOSS Centre for Biological Signalling Studies, Center for Chronic Immunodeficiency (CCI) and Max-Planck-Institute for Immunbiology and Epigenetics, Stu¨beweg 51, 79108 Freiburg, Germany *Correspondence: [email protected] DOI 10.1016/j.immuni.2011.08.004
In the current issue of Immunity, Martı´nez-Martı´n et al. (2011) describe the central supramolecular activation cluster (cSMAC) as a site of clathrin-independent T cell receptor (TCR) internalization and trogocytosis. Further, they identify small Rho GTPases TC21 and RhoG as key mediators of these processes. Cell-to-cell communication is essential for the orchestration of the immune system and its responses. Although immune cells normally communicate through either soluble or membrane-bound mediators, new types of cellular interchange have been described, including trogocytosis, exosome secretion, and nanotube formation. The term trogocytosis (from Greek trogo-, nibble) was originally coined to describe the intercellular transfer of membrane patches from an antigen-pre-
senting cell (APC) to a lymphocyte (Joly and Hudrisier, 2003). This is in strong contrast to phagocytosis, the process of engulfing whole pathogens and death cell fragments by phagocytes. The first evidence of trogocytosis was the transfer of major histocompatibility complex class II (MHCII) glycoproteins from B to T cells (Cone et al., 1972). Since then, trogocytotic activity has been reported for T, B, natural killer (NK), and dendritic cells and is well documented
in vitro and in vivo. Trogocytosis can be distinguished from other mechanisms of intercellular transfer because it requires close cell-cell contact, is quick (within minutes), and involves transfer of intact proteins. Previous studies focused on the cell types involved and the molecules transferred, but the molecular mechanism that underpins this phenomenon has remained largely unknown. Trogocytosis is an active process in T and NK cells, requiring both receptor signaling and actin cytoskeleton
Immunity 35, August 26, 2011 ª2011 Elsevier Inc. 151
Immunity
Previews APC
MHCp TCR
nanocluster Re-expression
TCR
T cell
microcluster
Clathrin Signal TC21 PI3K RhoG
Trogocytosis
Actin Degradation
pSMAC
cSMAC
Signal
Degradation
Figure 1. TCR Internalization and Trogocytosis in T Cells Constitutive TCR turnover occurs in resting cells. Upon stimulation, TCR nanoclusters aggregate to signaling-active microclusters that can be internalized in the pSMAC in a clathrin-dependent manner. Once the microclusters have reached the cSMAC, patches of the APC containing the pMHC molecules are trogocytosed with a TC21- and RhoG-dependent mechanism. Trogocytosed membrane proteins from the APC can be re-expressed on the T cell.
remodeling. In this issue of Immunity, the group of Balbino Alarco´n provides new insights into the molecular mechanism that drives T cell antigen receptor (TCR)mediated trogocytosis by identifying the small GTPases TC21 and RhoG as key players (Martı´nez-Martı´n et al., 2011). On resting T cells, the TCR coexists in monomeric and nanoclustered forms (Schamel et al., 2005). The amount of TCR expressed on the cell surface is tightly controlled by the rates of TCR synthesis, constitutive clathrin-mediated TCR internalization, recycling and degradation (Figure 1). Once the T cell is stimulated by antigenic peptides presented on MHC (pMHC), the TCRs aggregate to form microclusters. These pMHC-induced microclusters contain phosphorylated TCRs that initiate activation of the cell. Subsequently, the microclusters coalesce to form the central supramolecular activation cluster (cSMAC) surrounded by the peripheral SMAC (pSMAC), together forming an immune synapse. The functional significance of the cSMAC has been the subject of recent controversy. Originally, it had been proposed that accumulation of engaged receptors and signaling molecules in the cSMAC boosts T cell activation. However, observations that the cSMAC forms after the peak of protein tyrosine phosphorylation has been reached and is enriched in ubiquitin ligases and ubiquitinylated proteins suggested a role in TCR internalization and degradation, and thus, in the termination of signaling. Another layer of complexity
was added when analyzing ligands of different qualities, given that the cSMAC can enhance stimulation by weak agonists (Cemerski et al., 2008). TCR stimulation by pMHC leads to TCR downmodulation, mainly by increasing TCR degradation, because clathrin-mediated TCR endocytosis is unchanged (Liu et al., 2000). However, intense TCR stimulation leads to marginal acceleration of TCR internalization by a second, clathrin-independent pathway (Monjas, 2004). Yet, detailed insight into when and where clathrindependent or -independent internalization occurs is still missing. In the present article, Martı´nez-Martı´n et al. identify the cSMAC as the site of clathrin-independent TCR internalization and report that together with the internalized TCR, membrane patches and pMHC complexes from the APC are trogocytosed. Using time-lapse confocal videomicroscopy and primary T cells derived from genetically ablated mice, they have shown that TCR internalization from the cSMAC is dependent on the small GTPases TC21 and RhoG. TC21 is constitutively associated with the TCR and co-translocates with the TCR to the cSMAC upon T cell activation (Delgado et al., 2009). Expression of either inactive or constitutively active TC21 mutants inhibited TCR internalization from the cSMAC, indicating that conversion between the GDP- and GTP-bound forms of TC21 is crucial for this process. Moreover, TC21 and the TCR colocalized in clathrin-deficient intra-
152 Immunity 35, August 26, 2011 ª2011 Elsevier Inc.
cellular vesicles, and shRNA-mediated silencing of clathrin did not affect TCR and TC21 cointernalization. The authors also have shown by electron microscopy that, in contrast to the pSMAC, the cSMAC is devoid of clathrin-coated pits. Altogether, the authors propose a model in which triggered TCRs are internalized at the cSMAC by a TC21-dependent but clathrin-independent mechanism. In contrast, homeostatic clathrin-mediated internalization might take place in the pSMAC. Additionally, RhoG was identified as a player in TC21-dependent TCR internalization. Like TC21, RhoG must cycle between active and inactive conformations to promote TCR internalization from the cSMAC. Because RhoG was first described in the phagocytosis of apoptotic bodies (Henson, 2005), the authors investigated the capability of T cells to phagocytose anti-TCR-coupled latex beads. It was surprising that primary T cells were able to phagocytose beads that were as big as themselves, which they accommodated through massive reorganization of their cytoplasm and plasma membrane. TCR-driven phagocytosis of beads was impaired in T lymphocytes derived from TC21- or RhoG-deficient mice when compared to wild-type controls. In stimulated T cells from TC21- and RhoG-deficient mice, total downregulation of the TCR was only slightly affected, suggesting that clathrin-mediated internalization was prominent. In contrast, acquisition of membrane patches and MHC molecules by trogocytosis was greatly decreased in CD4+ and CD8+ T cells. Further, acquisition of membrane patches and TCR-triggered phagocytosis were actin dependent and probably equivalent phenomena. Using pharmacological inhibitors, Martı´nez-Martı´n et al. showed that inactivation of phosphatidyl inositol-3 kinase (PI3K) reduced the phagocytosis of beads as well as membrane acquisition by trogocytosis. Furthermore, PI3K inhibitors blocked RhoG-GTP formation without affecting TC21 activation. This led the authors to propose a novel signaling pathway in which TC21, which directly binds to the TCR and activates the p110d-isoform of PI3K (Delgado et al., 2009), is upstream of RhoG. Activated RhoG in turn promotes actin polymerization, which is required for trogocytosis (Figure 1).
Immunity
Previews Although the current view is that TCR internalization dampens signaling by removing the antigen and promoting receptor degradation, internalization of a pMHC-TCR complex by trogocytosis opens new questions. Is internalization by trogocytosis a way to prolong the TCR-pMHC interaction even after APC separation? Are those TCRs still signaling competent? In this regard, RhoG- or TC21-deficient T cells stimulated by APCs showed increased upregulation of early TCR-mediated activation markers, such as CD69. This could suggest that signaling of TCRs from the cell surface is required to stimulate TC21- and RhoG-independent signaling cascades that upregulate CD69. In contrast, T cell proliferation was reduced in RhoG- or TC21-deficient T cells, either suggesting that trogocytosed TCRs are required to stimulate proliferation or that RhoG and TC21 are involved in the signaling pathways that promote proliferation. Taken together, these data open novel possibilities to study the functional consequences of trogocytosis. Lineage-specific deletion of RhoG could be useful to study the contribution of trogocytosis in different cell types in the course of immune
responses, helping to characterize the stimulatory or suppressive effects that have been attributed to trogocytosis. T helper cells that have captured pMHC via trogocytosis can present these pMHC complexes to other T cells, amplifying an immune response. Cytotoxic T cells (CTLs) that have captured agonistic pMHC by trogocytosis become susceptible to cytolysis by neighboring CTLs, which could result in a dampening of an immune response. While trying to understand the purpose and consequences of trogocytosis, energetic concerns arise: how do T cells generate the force needed to tear off the APC-membrane patch containing pMHC? The force required to pull a protein and surrounding lipids from a membrane is on the same order of magnitude as the force needed to break a high-affinity protein-protein interaction (Bell, 1978). Therefore, trogocytosis could be energetically beneficial for the T cell, given that the acquired lipids could be recycled or metabolized. This might increase the capacity of the T cell to proliferate. In that sense, T cells that are blood cells themselves and take up protein complexes in membrane ‘‘bites’’ from other cells could be fancied as little vampires
that feed from their victims without killing them. REFERENCES Bell, G.I. (1978). Science 200, 618–627. Cemerski, S., Das, J., Giurisato, E., Markiewicz, M.A., Allen, P.M., Chakraborty, A.K., and Shaw, A.S. (2008). Immunity 29, 414–422. Cone, R.E., Sprent, J., and Marchalonis, J.J. (1972). Proc. Natl. Acad. Sci. USA 69, 2556–2560. Delgado, P., Cubelos, B., Calleja, E., Martı´nezMartı´n, N., Cipre´s, A., Me´rida, I., Bellas, C., Bustelo, X.R., and Alarco´n, B. (2009). Nat. Immunol. 10, 880–888. Henson, P.M. (2005). Curr. Biol. 15, R29–R30. Joly, E., and Hudrisier, D. (2003). Nat. Immunol. 4, 815. Liu, H., Rhodes, M., Wiest, D.L., and Vignali, D.A. (2000). Immunity 13, 665–675. Martı´nez-Martı´n, N., Fernandez-Arenas, E., Cemerski, S., Delgado, P., Turner, M., Heuser, J., Irvine, D., Huang, B., Bustelo, X., Shaw, A.S., and Alarcon, B. (2011). Immunity 35, this issue, 208–222. Monjas, A. (2004). J. Biol. Chem. 279, 55376– 55384. Schamel, W.W., Arechaga, I., Risueno, R.M., van Santen, H.M., Cabezas, P., Risco, C., Valpuesta, J.M., and Alarcon, B. (2005). J. Exp. Med. 202, 493–503.
Intracellular Pathogens and CD8+ Dendritic Cells: Dangerous Liaisons Boris Reizis1,* 1Department of Microbiology and Immunology, Columbia University Medical Center, New York, NY 10032, USA *Correspondence: [email protected] DOI 10.1016/j.immuni.2011.08.003
CD8+ dendritic cells comprise a distinct cell type whose function is unclear. In this issue of Immunity, Mashayekhi et al. (2011) show these cells are essential for protection against the parasite Toxoplasma, but Edelson et al. (2011) show they are hijacked by Listeria during initial spreading. Dendritic cells (DCs) are a distinct lineage of mononuclear phagocytes that excel at pathogen sensing, cytokine secretion, and antigen presentation. The classical DCs comprise two distinct subsets, distinguished in the mouse by the expression of CD8a (Shortman and Heath, 2010). This subset dichotomy exists not only in
lymphoid organs but also in tissues, in which CD103+ DCs represent a genetic and functional equivalent of CD8+ DCs. Notably, CD8+ and CD103+ DCs are found in relatively low numbers in vivo (0.1%–0.2% of murine splenocytes), to the dismay of researchers who study them. A DC subset similar to CD8+ DCs
has been identified in humans, with this conservation probably reflecting an essential role in immunity. An overwhelming body of evidence suggests that CD8+ DCs are particularly efficient at cross-presentation, i.e., the presentation of exogenously acquired antigens on MHC class I molecules to CD8+ T cells. Although this
Immunity 35, August 26, 2011 ª2011 Elsevier Inc. 153
Previews protect against severe disease. This is consistent with the finding that recombinant IFN-a administration early in PbA infection reduced parasitemia and protected against ECM in a IFN-g-dependent manner (Vigario et al., 2007). A beneficial effect of type I IFNs was also suggested by higher plasma IFN-a levels in African children with mild malaria compared to those with hyperparasitemia or severe malarial anemia (Luty et al., 2000). The heterogeneity and complexity of human severe malaria necessitate examining the role of type I IFNs in diverse patient populations and in multiple murine models representing different aspects of human disease. However, the present findings do caution against administration of type I IFNs as adjunctive therapy for CM, especially because patients typically present with advanced pathology. In summary, Sharma et al. (2011) identify a unique cytosolic DNA sensing pathway and considerably expand our knowledge of the induction and role of
type I IFNs in malaria infection. The presence of AT-rich motifs in the genomes of other protozoa, bacteria, and humans suggests that these motifs could contribute to the pathogenesis of other severe infectious syndromes. In the case of extracellular DNA sources, it is intriguing to consider that other crystals (e.g., urate) may move these motifs into the cytosol for innate sensing. This proposed mechanism, if confirmed, may have relevance for the development of DNA-based vaccine adjuvants.
Herbich, K., Schmid, D., et al. (2000). Infect. Immun. 68, 3909–3915. Parroche, P., Lauw, F.N., Goutagny, N., Latz, E., Monks, B.G., Visintin, A., Halmen, K.A., Lamphier, M., Olivier, M., Bartholomeu, D.C., et al. (2007). Proc. Natl. Acad. Sci. USA 104, 1919–1924. Sharma, S., DeOliveira, R.B., Kalantari, P., Parroche, P., Goutagny, N., Jiang, Z., Chan, J., Bartholomeu, D.C., Lauw, F., Hall, J.P., et al. (2011). Immunity 35, this issue, 194–207. Shimosato, T., Kimura, T., Tohno, M., Iliev, I.D., Katoh, S., Ito, Y., Kawai, Y., Sasaki, T., Saito, T., and Kitazawa, H. (2006). Cell. Microbiol. 8, 485–495. Shio, M.T., Eisenbarth, S.C., Savaria, M., Vinet, A.F., Bellemare, M.J., Harder, K.W., Sutterwala, F.S., Bohle, D.S., Descoteaux, A., Flavell, R.A., and Olivier, M. (2009). PLoS Pathog. 5, e1000559.
REFERENCES Coban, C., Ishii, K.J., Kawai, T., Hemmi, H., Sato, S., Uematsu, S., Yamamoto, M., Takeuchi, O., Itagaki, S., Kumar, N., et al. (2005). J. Exp. Med. 201, 19–25.
van der Heyde, H.C., Nolan, J., Combes, V., Gramaglia, I., and Grau, G.E. (2006). Trends Parasitol. 22, 503–508.
Lovegrove, F.E., Gharib, S.A., Patel, S.N., Hawkes, C.A., Kain, K.C., and Liles, W.C. (2007). Am. J. Pathol. 171, 1894–1903.
Vigario, A.M., Belnoue, E., Gruner, A.C., Mauduit, M., Kayibanda, M., Deschemin, J.C., Marussig, M., Snounou, G., Mazier, D., Gresser, I., and Renia, L. (2007). J. Immunol. 178, 6416–6425.
Luty, A.J., Perkins, D.J., Lell, B., Schmidt-Ott, R., Lehman, L.G., Luckner, D., Greve, B., Matousek, P.,
Wu, X., Gowda, N.M., Kumar, S., and Gowda, D.C. (2010). J. Immunol. 184, 4338–4348.
A New Vampire Saga: The Molecular Mechanism of T Cell Trogocytosis Elaine Pashupati Dopfer,1 Susana Minguet,1 and Wolfgang W.A. Schamel1,* 1Department of Molecular Immunology, Faculty of Biology, BIOSS Centre for Biological Signalling Studies, Center for Chronic Immunodeficiency (CCI) and Max-Planck-Institute for Immunbiology and Epigenetics, Stu¨beweg 51, 79108 Freiburg, Germany *Correspondence: [email protected] DOI 10.1016/j.immuni.2011.08.004
In the current issue of Immunity, Martı´nez-Martı´n et al. (2011) describe the central supramolecular activation cluster (cSMAC) as a site of clathrin-independent T cell receptor (TCR) internalization and trogocytosis. Further, they identify small Rho GTPases TC21 and RhoG as key mediators of these processes. Cell-to-cell communication is essential for the orchestration of the immune system and its responses. Although immune cells normally communicate through either soluble or membrane-bound mediators, new types of cellular interchange have been described, including trogocytosis, exosome secretion, and nanotube formation. The term trogocytosis (from Greek trogo-, nibble) was originally coined to describe the intercellular transfer of membrane patches from an antigen-pre-
senting cell (APC) to a lymphocyte (Joly and Hudrisier, 2003). This is in strong contrast to phagocytosis, the process of engulfing whole pathogens and death cell fragments by phagocytes. The first evidence of trogocytosis was the transfer of major histocompatibility complex class II (MHCII) glycoproteins from B to T cells (Cone et al., 1972). Since then, trogocytotic activity has been reported for T, B, natural killer (NK), and dendritic cells and is well documented
in vitro and in vivo. Trogocytosis can be distinguished from other mechanisms of intercellular transfer because it requires close cell-cell contact, is quick (within minutes), and involves transfer of intact proteins. Previous studies focused on the cell types involved and the molecules transferred, but the molecular mechanism that underpins this phenomenon has remained largely unknown. Trogocytosis is an active process in T and NK cells, requiring both receptor signaling and actin cytoskeleton
Immunity 35, August 26, 2011 ª2011 Elsevier Inc. 151
Immunity
Previews APC
MHCp TCR
nanocluster Re-expression
TCR
T cell
microcluster
Clathrin Signal TC21 PI3K RhoG
Trogocytosis
Actin Degradation
pSMAC
cSMAC
Signal
Degradation
Figure 1. TCR Internalization and Trogocytosis in T Cells Constitutive TCR turnover occurs in resting cells. Upon stimulation, TCR nanoclusters aggregate to signaling-active microclusters that can be internalized in the pSMAC in a clathrin-dependent manner. Once the microclusters have reached the cSMAC, patches of the APC containing the pMHC molecules are trogocytosed with a TC21- and RhoG-dependent mechanism. Trogocytosed membrane proteins from the APC can be re-expressed on the T cell.
remodeling. In this issue of Immunity, the group of Balbino Alarco´n provides new insights into the molecular mechanism that drives T cell antigen receptor (TCR)mediated trogocytosis by identifying the small GTPases TC21 and RhoG as key players (Martı´nez-Martı´n et al., 2011). On resting T cells, the TCR coexists in monomeric and nanoclustered forms (Schamel et al., 2005). The amount of TCR expressed on the cell surface is tightly controlled by the rates of TCR synthesis, constitutive clathrin-mediated TCR internalization, recycling and degradation (Figure 1). Once the T cell is stimulated by antigenic peptides presented on MHC (pMHC), the TCRs aggregate to form microclusters. These pMHC-induced microclusters contain phosphorylated TCRs that initiate activation of the cell. Subsequently, the microclusters coalesce to form the central supramolecular activation cluster (cSMAC) surrounded by the peripheral SMAC (pSMAC), together forming an immune synapse. The functional significance of the cSMAC has been the subject of recent controversy. Originally, it had been proposed that accumulation of engaged receptors and signaling molecules in the cSMAC boosts T cell activation. However, observations that the cSMAC forms after the peak of protein tyrosine phosphorylation has been reached and is enriched in ubiquitin ligases and ubiquitinylated proteins suggested a role in TCR internalization and degradation, and thus, in the termination of signaling. Another layer of complexity
was added when analyzing ligands of different qualities, given that the cSMAC can enhance stimulation by weak agonists (Cemerski et al., 2008). TCR stimulation by pMHC leads to TCR downmodulation, mainly by increasing TCR degradation, because clathrin-mediated TCR endocytosis is unchanged (Liu et al., 2000). However, intense TCR stimulation leads to marginal acceleration of TCR internalization by a second, clathrin-independent pathway (Monjas, 2004). Yet, detailed insight into when and where clathrindependent or -independent internalization occurs is still missing. In the present article, Martı´nez-Martı´n et al. identify the cSMAC as the site of clathrin-independent TCR internalization and report that together with the internalized TCR, membrane patches and pMHC complexes from the APC are trogocytosed. Using time-lapse confocal videomicroscopy and primary T cells derived from genetically ablated mice, they have shown that TCR internalization from the cSMAC is dependent on the small GTPases TC21 and RhoG. TC21 is constitutively associated with the TCR and co-translocates with the TCR to the cSMAC upon T cell activation (Delgado et al., 2009). Expression of either inactive or constitutively active TC21 mutants inhibited TCR internalization from the cSMAC, indicating that conversion between the GDP- and GTP-bound forms of TC21 is crucial for this process. Moreover, TC21 and the TCR colocalized in clathrin-deficient intra-
152 Immunity 35, August 26, 2011 ª2011 Elsevier Inc.
cellular vesicles, and shRNA-mediated silencing of clathrin did not affect TCR and TC21 cointernalization. The authors also have shown by electron microscopy that, in contrast to the pSMAC, the cSMAC is devoid of clathrin-coated pits. Altogether, the authors propose a model in which triggered TCRs are internalized at the cSMAC by a TC21-dependent but clathrin-independent mechanism. In contrast, homeostatic clathrin-mediated internalization might take place in the pSMAC. Additionally, RhoG was identified as a player in TC21-dependent TCR internalization. Like TC21, RhoG must cycle between active and inactive conformations to promote TCR internalization from the cSMAC. Because RhoG was first described in the phagocytosis of apoptotic bodies (Henson, 2005), the authors investigated the capability of T cells to phagocytose anti-TCR-coupled latex beads. It was surprising that primary T cells were able to phagocytose beads that were as big as themselves, which they accommodated through massive reorganization of their cytoplasm and plasma membrane. TCR-driven phagocytosis of beads was impaired in T lymphocytes derived from TC21- or RhoG-deficient mice when compared to wild-type controls. In stimulated T cells from TC21- and RhoG-deficient mice, total downregulation of the TCR was only slightly affected, suggesting that clathrin-mediated internalization was prominent. In contrast, acquisition of membrane patches and MHC molecules by trogocytosis was greatly decreased in CD4+ and CD8+ T cells. Further, acquisition of membrane patches and TCR-triggered phagocytosis were actin dependent and probably equivalent phenomena. Using pharmacological inhibitors, Martı´nez-Martı´n et al. showed that inactivation of phosphatidyl inositol-3 kinase (PI3K) reduced the phagocytosis of beads as well as membrane acquisition by trogocytosis. Furthermore, PI3K inhibitors blocked RhoG-GTP formation without affecting TC21 activation. This led the authors to propose a novel signaling pathway in which TC21, which directly binds to the TCR and activates the p110d-isoform of PI3K (Delgado et al., 2009), is upstream of RhoG. Activated RhoG in turn promotes actin polymerization, which is required for trogocytosis (Figure 1).
Immunity
Previews Although the current view is that TCR internalization dampens signaling by removing the antigen and promoting receptor degradation, internalization of a pMHC-TCR complex by trogocytosis opens new questions. Is internalization by trogocytosis a way to prolong the TCR-pMHC interaction even after APC separation? Are those TCRs still signaling competent? In this regard, RhoG- or TC21-deficient T cells stimulated by APCs showed increased upregulation of early TCR-mediated activation markers, such as CD69. This could suggest that signaling of TCRs from the cell surface is required to stimulate TC21- and RhoG-independent signaling cascades that upregulate CD69. In contrast, T cell proliferation was reduced in RhoG- or TC21-deficient T cells, either suggesting that trogocytosed TCRs are required to stimulate proliferation or that RhoG and TC21 are involved in the signaling pathways that promote proliferation. Taken together, these data open novel possibilities to study the functional consequences of trogocytosis. Lineage-specific deletion of RhoG could be useful to study the contribution of trogocytosis in different cell types in the course of immune
responses, helping to characterize the stimulatory or suppressive effects that have been attributed to trogocytosis. T helper cells that have captured pMHC via trogocytosis can present these pMHC complexes to other T cells, amplifying an immune response. Cytotoxic T cells (CTLs) that have captured agonistic pMHC by trogocytosis become susceptible to cytolysis by neighboring CTLs, which could result in a dampening of an immune response. While trying to understand the purpose and consequences of trogocytosis, energetic concerns arise: how do T cells generate the force needed to tear off the APC-membrane patch containing pMHC? The force required to pull a protein and surrounding lipids from a membrane is on the same order of magnitude as the force needed to break a high-affinity protein-protein interaction (Bell, 1978). Therefore, trogocytosis could be energetically beneficial for the T cell, given that the acquired lipids could be recycled or metabolized. This might increase the capacity of the T cell to proliferate. In that sense, T cells that are blood cells themselves and take up protein complexes in membrane ‘‘bites’’ from other cells could be fancied as little vampires
that feed from their victims without killing them. REFERENCES Bell, G.I. (1978). Science 200, 618–627. Cemerski, S., Das, J., Giurisato, E., Markiewicz, M.A., Allen, P.M., Chakraborty, A.K., and Shaw, A.S. (2008). Immunity 29, 414–422. Cone, R.E., Sprent, J., and Marchalonis, J.J. (1972). Proc. Natl. Acad. Sci. USA 69, 2556–2560. Delgado, P., Cubelos, B., Calleja, E., Martı´nezMartı´n, N., Cipre´s, A., Me´rida, I., Bellas, C., Bustelo, X.R., and Alarco´n, B. (2009). Nat. Immunol. 10, 880–888. Henson, P.M. (2005). Curr. Biol. 15, R29–R30. Joly, E., and Hudrisier, D. (2003). Nat. Immunol. 4, 815. Liu, H., Rhodes, M., Wiest, D.L., and Vignali, D.A. (2000). Immunity 13, 665–675. Martı´nez-Martı´n, N., Fernandez-Arenas, E., Cemerski, S., Delgado, P., Turner, M., Heuser, J., Irvine, D., Huang, B., Bustelo, X., Shaw, A.S., and Alarcon, B. (2011). Immunity 35, this issue, 208–222. Monjas, A. (2004). J. Biol. Chem. 279, 55376– 55384. Schamel, W.W., Arechaga, I., Risueno, R.M., van Santen, H.M., Cabezas, P., Risco, C., Valpuesta, J.M., and Alarcon, B. (2005). J. Exp. Med. 202, 493–503.
Intracellular Pathogens and CD8+ Dendritic Cells: Dangerous Liaisons Boris Reizis1,* 1Department of Microbiology and Immunology, Columbia University Medical Center, New York, NY 10032, USA *Correspondence: [email protected] DOI 10.1016/j.immuni.2011.08.003
CD8+ dendritic cells comprise a distinct cell type whose function is unclear. In this issue of Immunity, Mashayekhi et al. (2011) show these cells are essential for protection against the parasite Toxoplasma, but Edelson et al. (2011) show they are hijacked by Listeria during initial spreading. Dendritic cells (DCs) are a distinct lineage of mononuclear phagocytes that excel at pathogen sensing, cytokine secretion, and antigen presentation. The classical DCs comprise two distinct subsets, distinguished in the mouse by the expression of CD8a (Shortman and Heath, 2010). This subset dichotomy exists not only in
lymphoid organs but also in tissues, in which CD103+ DCs represent a genetic and functional equivalent of CD8+ DCs. Notably, CD8+ and CD103+ DCs are found in relatively low numbers in vivo (0.1%–0.2% of murine splenocytes), to the dismay of researchers who study them. A DC subset similar to CD8+ DCs
has been identified in humans, with this conservation probably reflecting an essential role in immunity. An overwhelming body of evidence suggests that CD8+ DCs are particularly efficient at cross-presentation, i.e., the presentation of exogenously acquired antigens on MHC class I molecules to CD8+ T cells. Although this
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