- Email: [email protected]
A 4-midable Connection: CCR7 Tetramers Link GPCR to Src Kinase Signaling
Immunity
Previews Corbett, A.J., Eckle, S.B., Birkinshaw, R.W., Liu, L., Patel, O., Mahony, J., Chen, Z., Reantragoon, R., Meehan, B., Cao, H., et al. (2014). Nature 509, 361–365. Gapin, L. (2014). J. Immunol. 192, 4475–4480. Gherardin, N.A., Keller, A.N., Woolley, R.E., Le Nours, J., Ritchie, D.S., Neeson, P.J., Birkinshaw,
R.W., Eckle, S.B.G., Waddington, J.N., and Liu, L. (2016). Immunity 44, this issue, 32–45. Girardi, E., Maricic, I., Wang, J., Mac, T.T., Iyer, P., Kumar, V., and Zajonc, D.M. (2012). Nat. Immunol. 13, 851–856. Godfrey, D.I., Uldrich, A.P., McCluskey, J., Rossjohn, J., and Moody, D.B. (2015). Nat. Immunol. 16, 1114–1123.
Le Bourhis, L., Mburu, Y.K., and Lantz, O. (2013). Curr. Opin. Immunol. 25, 174–180. Rossjohn, J., Gras, S., Miles, J.J., Turner, S.J., Godfrey, D.I., and McCluskey, J. (2015). Annu. Rev. Immunol. 33, 169–200. Salio, M., Silk, J.D., Jones, E.Y., and Cerundolo, V. (2014). Annu. Rev. Immunol. 32, 323–366.
A 4-midable Connection: CCR7 Tetramers Link GPCR to Src Kinase Signaling Reinhold Fo¨rster,1,* Tim Worbs,1 and Kathrin Werth1 1Institute of Immunology, Hannover Medical School, 30625 Hannover, Germany *Correspondence: [email protected] http://dx.doi.org/10.1016/j.immuni.2015.12.009
Chemokine receptors are known to signal through heterotrimeric G proteins. In this issue, Hauser et al. (2016) report that inflammatory cues can induce tetramers of the chemokine receptor CCR7 that serve as scaffolds integrating G protein with Src kinase signaling. Incredible mobility is the hallmark of the cellular immune system, and chemokines are at the center of orchestrating the directed migration of basically any type of immune cell (Griffith et al., 2014). Data obtained during the last two decades identified the chemokine receptor CCR7 together with its two ligands, CCL19 and CCL21, as arguably the single most important guidance cue of the adaptive immune system (Fo¨rster et al., 2008). CCR7, together with the chemokine receptor CXCR5, guides lymphoid tissueinducer cells into embryonic anlagen, a process essentially required for the development of lymph nodes. During T cell development, CCR7 together with CXCR4 and CCR9 controls the entry of early T cell progenitors into the developing as well as into the adult thymus and helps guiding maturing T cells through the thymic parenchyma to receive appropriate signals during selection processes (Fo¨rster et al., 2008; Griffith et al., 2014). Finally, CCR7 is perhaps best known for its essential function in guiding lymphocytes and dendritic cells (DCs) into lymph nodes. Immune cells can enter lymph nodes via two different routes: either from the blood circulation through specialized high endothelial ve-
nules (HEVs) or from peripheral tissues, such as the skin, via afferent lymphatics, and both pathways heavily rely on signals via CCR7 (Girard et al., 2012). Naive B and T cells recirculate between blood and secondary lymphoid organs and enter lymph nodes via HEVs. During this process, CCL21 (and to a lesser extent CCL19) decorating the luminal surface of HEV endothelial cells triggers a CCR7-mediated signal cascade that leads to the activation of lymphocyteexpressed integrins, thus allowing firm adhesion to the endothelium and the subsequent trans- or paracellular entry into the lymph node parenchyma (Girard et al., 2012). CCR7 is indispensable for the migration of dendritic cells (DCs) as well. Although sessile DCs residing in peripheral organs such as the skin do not express CCR7, this chemokine receptor is strongly upregulated once DCs are exposed to pathogen-associated molecular patterns such as LPS. DCs rely on CCR7-mediated signals during at least two crucial migratory events during their translocation into lymph nodes: (1) migration from the interstitial tissue into terminal lymphatics in the periphery and (2) migration into the deeper lymph node paracortex from the lymph node subcapsular
sinus (SCS) after passive transport by lymph flow (Girard et al., 2012). Of interest, the atypical chemokine receptor ACKR4, which scavenges CCL19 and CCL21 (in addition to CCL25 and CXCL13), is expressed only by endothelial cells lining the SCS ceiling—not by those of the SCS floor. Based on its scavenging activity and its characteristic expression pattern, this receptor was recently shown to actively pattern CCL21 gradients that facilitate the CCR7-mediated translocation of DCs from the SCS into the lymph node parenchyma (Ulvmar et al., 2014). All chemokine receptors, including CCR7, belong to the large superfamily of seven transmembrane-spanning G protein-coupled receptors. A unique feature of chemokine receptors (with the exception of the four atypical chemokine receptors) is the highly conserved DRY motif in transmembrane 3 next to the second intracellular loop (Randolph et al., 2008). In sharp contrast to its broad relevance for many immunological processes, relatively little is known about how signaling through CCR7 translates into cell motility. Interestingly, CCL21 and CCL19 elicit different signaling events through CCR7. CCL21 gets immobilized to glycosaminoglycans, mediating integrin-dependent
Immunity 44, January 19, 2016 ª2016 Elsevier Inc. 9
Immunity
Previews
Figure 1. Signaling through CCR7 Left: Upon ligand binding, guanosine diphosphate (GDP) gets exchanged by guanosine triphosphate (GTP) leading to the dissociation of the a-b-g heterotrimeric G protein complex into a a-GTP and a b-g subunit. These in turn activate several parallel cascades of downstream molecules including phospholipase C (PLC), phosphatidylinositol 3-kinase (PI3K), mitogen-activated protein kinase (MAPK), and protein kinase C (PKC) that, among others, activate second messengers such as inositol trisphosphate (IP3), diacylglycerol (DAG), and Ca2+ ions (Randolph et al., 2008). Independent of G protein signaling, G protein receptor kinases (GRKs) phosphorylate C-terminal residues, allowing b-arrestin (b-arr)-mediated receptor internalization (Randolph et al., 2008). Right: Inflammatory signals such as PGE2 provoke oligomerization of CCR7. These oligomers serve as scaffolds that integrate signaling through G proteins and Src kinase. Src tyrosine phosphorylates CCR7 which then serves as a docking site for SH2-domain bearing molecules such as phosphatase SHP2.
adhesion and lymph node homing. Of interest, CCL21, but not CCL19, has been shown to take on an autoinhibited conformation that can be released by interacting polysialic acid conjugated to CCR7 that subsequently allows binding to its receptor (Kiermaier et al., 2015). In contrast, CCL19 induces phosphorylation of CCR7 by G protein-coupled receptor kinase 3 leading to receptor internalization via b-arrestin (Figure 1, left; Zidar et al., 2009). Classical, but not the atypical, chemokine receptors signal through activating heterotrimeric G proteins containing a pertussis toxin (PTX)-sensitive Gai subunit (Figure 1). Consequently, treating cells with PTX completely blocks homing of naive lymphocytes to lymph nodes. However, it was shown earlier that CCR7-expressing monocyte-derived DCs had to be activated with Prostaglandin E2 (PGE2) to gain full sensitivity toward CCR7 ligands, and a recent study suggests that effector T cells, in contrast to naive T cells, can actually bypass Gai signaling by using Src kinase to adhere
to and crawl on inflamed endothelial cells (Shulman et al., 2012), indicating that CCR7 might signal not only through G proteins and b-arrestin but also via alternative pathways. In the current issue of Immunity, Hauser et al. (2016) now report that oligomerization of CCR7 enables a previously unknown, G protein-parallel intracellular signaling pathway of CCR7 via Src kinase and SHP2 (Figure 1, right). Interestingly, some CCR7 oligomers seem to form spontaneously within the cell membrane of (for example) naive T lymphocytes even under steady-state conditions. However, they are much more abundantly present on effector and central memory T cells as well as on mature DCs. Inflammatory signals, in particular PGE2, increase the CCR7 oligomerization status of DCs by modulating genes of cholesterol metabolism and transport, thus lowering cellular cholesterol levels. Additionally, binding of the CCR7 ligands CCL19 and CCL21 promotes CCR7 oligomerization as well. Using cysteine-cross-
10 Immunity 44, January 19, 2016 ª2016 Elsevier Inc.
linking, molecular modeling, and directed evolutionary screens, the authors identified a hydrophobic interaction surface proximate to the NPXXY motif of the transmembrane spanning region TM7 of CCR7 as essential for the formation of CCR7 oligomers. Furthermore, they found a naturally occurring CCR7 SNP with ‘‘super-oligomerization’’ properties due to an enlargement of this hydrophobic region. Most importantly, immune cell lines transfected with oligomerization-prone CCR7 variants displayed higher migratory activity in transwell migration assays while chemokine binding and G protein activation were unaltered, indicating a G-protein-independent pro-migratory signaling function of CCR7 oligomers. Based on their characterization of CCR7 oligomerization interfaces and previous structural analysis, the authors propose a scenario in which a CCR7 tetramer, containing two bound sets of heterotrimeric G proteins, recruits the tyrosine kinase Src to form a signaling scaffold integrating G protein-dependent as well as Src-dependent signaling pathways. After CCR7-induced auto-phosphorylation, Src is able to tyrosine-phosphorylate CCR7 within the highly conserved DRY-motif, thereby creating an active SH2-domain binding site. This in turn allows for the recruitment of the tyrosine phosphatase SHP2 into the complex and its activation by Src-mediated phosphorylation (Figure 1, right). Importantly, these events were triggered by binding of both CCL19 and CCL21 to their receptor CCR7. However, only CCR7 triggering by CCL21 resulted in the accumulation of a large cytosolic pool of catalytically active SHP2, representing another example of so-called ‘‘biased signaling’’ of the CCR7 ligand CCL21. Consequently, inhibition of SHP2 interfered with T cell migration mediated by CCL21, but not CCL19, as the latter probably primarily relies on the well-known classical signaling pathway via Gai. Representing some limitations of this important study, Hauser et al. (2016) analyzed the influence of CCR7 oligomerization on immune cell migration almost exclusively in vitro, primarily using 2D and 3D transwell assays. It would be extremely interesting to see the effect of varying cellular CCR7 oligomerization levels on the migration characteristics of DCs and lymphocytes in situ, studying their
Immunity
Previews behavior during crucial migration events within intact, living tissues. In this context, it is tempting to speculate that the immobilization of CCL21, but not CCL19, on glycosaminoglycan-bearing biological surfaces (fibroblastic reticular cells, interstitial stroma) might actually favor the formation of CCR7 tetramer complexes by mechanically approximating CCR7 molecules within the cell membrane of migrating immune cells, thus unlocking additional Src-mediated signals. Another important question is how the formation of tetrameric CCR7 signaling hubs might influence CCR7-dependent haptokinetic ‘‘gradient sensing’’ (Schumann et al., 2010). This mechanism enables immune cells to be not only randomly motile, but to exhibit highly persistent directional migration, crucial for (for example) the entry of skin DCs into primary lymphatics or their migration into the deep paracortical T cell zone of draining lymph nodes. The elegant study by Hauser et al. (2016) brings a fascinating new twist into the well-studied field of CCR7 biology: identifying and characterizing CCR7 tetramers as signaling hubs for not only G
protein- but also Src-SHP2-dependent intracellular signals. Their findings open up new ways to potentially interfere with CCR7-mediated cell migration in a number of scientifically as well as clinically relevant situations, e.g., anti-inflammatory and anti-metastatic drug development. Due to the strong influence of inflammatory signals on cellular CCR7 oligomerization levels, the Src-SHP2 side of CCR7-driven migration might indeed be of particular importance for ‘‘battle-proven’’ immune cells such as mature DCs and effector and/or memory T cells, further emphasizing the therapeutic potential of such interventions.
Griffith, J.W., Sokol, C.L., and Luster, A.D. (2014). Annu. Rev. Immunol. 32, 659–702.
ACKNOWLEDGMENTS
Shulman, Z., Cohen, S.J., Roediger, B., Kalchenko, V., Jain, R., Grabovsky, V., Klein, E., Shinder, V., Stoler-Barak, L., Feigelson, S.W., et al. (2012). Nat. Immunol. 13, 67–76.
This work has been supported by ERC Advanced Grant 322645 to R.F.
REFERENCES Fo¨rster, R., Davalos-Misslitz, A.C., and Rot, A. (2008). Nat. Rev. Immunol. 8, 362–371. Girard, J.P., Moussion, C., and Fo¨rster, R. (2012). Nat. Rev. Immunol. 12, 762–773.
Hauser, M.A., Schaeuble, K., Kindinger, I., Impellizzieri, D., Krueger, W.A., Hauck, C.R., Boyman, O., and Legler, D.F. (2016). Immunity 44, this issue, 59–72. Kiermaier, E., Moussion, C., Veldkamp, C.T., Gerardy-Schahn, R., de Vries, I., Williams, L.G., Chaffee, G.R., Phillips, A.J., Freiberger, F., Imre, R., et al. (2015). Science, aad0512, http://dx.doi.org/ 10.1126/science.aad0512. Randolph, G.J., Ochando, J., and Partida-Sa´nchez, S. (2008). Annu. Rev. Immunol. 26, 293–316. Schumann, K., La¨mmermann, T., Bruckner, M., Legler, D.F., Polleux, J., Spatz, J.P., Schuler, G., Fo¨rster, R., Lutz, M.B., Sorokin, L., and Sixt, M. (2010). Immunity 32, 703–713.
Ulvmar, M.H., Werth, K., Braun, A., Kelay, P., Hub, E., Eller, K., Chan, L., Lucas, B., Novitzky-Basso, I., Nakamura, K., et al. (2014). Nat. Immunol. 15, 623–630. Zidar, D.A., Violin, J.D., Whalen, E.J., and Lefkowitz, R.J. (2009). Proc. Natl. Acad. Sci. USA 106, 9649–9654.
AIF Is ‘‘Always In Fashion’’ for T Cells Marc O. Johnson1 and Jeffrey C. Rathmell1,* 1Vanderbilt Center for Immunobiology, Department of Pathology, Microbiology, and Immunology, Vanderbilt University, Nashville, TN 37232, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.immuni.2015.12.015
AIF has been known to have both apoptotic and metabolic roles. Green and colleagues show that T cells, but not B cells, rely on AIF to maintain mitochondrial electron transport and that metabolic, rather than apoptotic, pathways mediate this dependence. The newly emerging field of immunometabolism has several origins, not the least of which is through the recognition that mitochondrial proteins play critical roles in lymphocyte apoptosis. Cytochrome C, SMAC/DIABLO, and apoptosis inducing factor (AIF) were all identified as mitochondrial proteins that were released and drove apoptosis upon stress. Of these, Cytochrome C was long known as a component of the electron transport
chain. Metabolic roles for AIF have also been shown over the past years, and AIF is thought to promote electron transport complex I assembly (Hangen et al., 2010). Interestingly, the harlequin mouse strain, which has a proviral insertion that results in reduced AIF expression, has immune defects in T cells, but not B cells. In this issue, Milasta et al. (2016) examine the metabolic and apoptotic effects of AIF and show that this discrepancy is
due to key differences in the metabolic demands of B and T cells, rather than apoptosis. Though their functions are very different, B cells and T cells form the basis for adaptive immunity and share a close lymphocyte progenitor cell. Each undergoes selection and proliferation during development and persists in a resting state while surveilling for antigen. After activation, each cell type is induced to
Immunity 44, January 19, 2016 ª2016 Elsevier Inc. 11
Previews Corbett, A.J., Eckle, S.B., Birkinshaw, R.W., Liu, L., Patel, O., Mahony, J., Chen, Z., Reantragoon, R., Meehan, B., Cao, H., et al. (2014). Nature 509, 361–365. Gapin, L. (2014). J. Immunol. 192, 4475–4480. Gherardin, N.A., Keller, A.N., Woolley, R.E., Le Nours, J., Ritchie, D.S., Neeson, P.J., Birkinshaw,
R.W., Eckle, S.B.G., Waddington, J.N., and Liu, L. (2016). Immunity 44, this issue, 32–45. Girardi, E., Maricic, I., Wang, J., Mac, T.T., Iyer, P., Kumar, V., and Zajonc, D.M. (2012). Nat. Immunol. 13, 851–856. Godfrey, D.I., Uldrich, A.P., McCluskey, J., Rossjohn, J., and Moody, D.B. (2015). Nat. Immunol. 16, 1114–1123.
Le Bourhis, L., Mburu, Y.K., and Lantz, O. (2013). Curr. Opin. Immunol. 25, 174–180. Rossjohn, J., Gras, S., Miles, J.J., Turner, S.J., Godfrey, D.I., and McCluskey, J. (2015). Annu. Rev. Immunol. 33, 169–200. Salio, M., Silk, J.D., Jones, E.Y., and Cerundolo, V. (2014). Annu. Rev. Immunol. 32, 323–366.
A 4-midable Connection: CCR7 Tetramers Link GPCR to Src Kinase Signaling Reinhold Fo¨rster,1,* Tim Worbs,1 and Kathrin Werth1 1Institute of Immunology, Hannover Medical School, 30625 Hannover, Germany *Correspondence: [email protected] http://dx.doi.org/10.1016/j.immuni.2015.12.009
Chemokine receptors are known to signal through heterotrimeric G proteins. In this issue, Hauser et al. (2016) report that inflammatory cues can induce tetramers of the chemokine receptor CCR7 that serve as scaffolds integrating G protein with Src kinase signaling. Incredible mobility is the hallmark of the cellular immune system, and chemokines are at the center of orchestrating the directed migration of basically any type of immune cell (Griffith et al., 2014). Data obtained during the last two decades identified the chemokine receptor CCR7 together with its two ligands, CCL19 and CCL21, as arguably the single most important guidance cue of the adaptive immune system (Fo¨rster et al., 2008). CCR7, together with the chemokine receptor CXCR5, guides lymphoid tissueinducer cells into embryonic anlagen, a process essentially required for the development of lymph nodes. During T cell development, CCR7 together with CXCR4 and CCR9 controls the entry of early T cell progenitors into the developing as well as into the adult thymus and helps guiding maturing T cells through the thymic parenchyma to receive appropriate signals during selection processes (Fo¨rster et al., 2008; Griffith et al., 2014). Finally, CCR7 is perhaps best known for its essential function in guiding lymphocytes and dendritic cells (DCs) into lymph nodes. Immune cells can enter lymph nodes via two different routes: either from the blood circulation through specialized high endothelial ve-
nules (HEVs) or from peripheral tissues, such as the skin, via afferent lymphatics, and both pathways heavily rely on signals via CCR7 (Girard et al., 2012). Naive B and T cells recirculate between blood and secondary lymphoid organs and enter lymph nodes via HEVs. During this process, CCL21 (and to a lesser extent CCL19) decorating the luminal surface of HEV endothelial cells triggers a CCR7-mediated signal cascade that leads to the activation of lymphocyteexpressed integrins, thus allowing firm adhesion to the endothelium and the subsequent trans- or paracellular entry into the lymph node parenchyma (Girard et al., 2012). CCR7 is indispensable for the migration of dendritic cells (DCs) as well. Although sessile DCs residing in peripheral organs such as the skin do not express CCR7, this chemokine receptor is strongly upregulated once DCs are exposed to pathogen-associated molecular patterns such as LPS. DCs rely on CCR7-mediated signals during at least two crucial migratory events during their translocation into lymph nodes: (1) migration from the interstitial tissue into terminal lymphatics in the periphery and (2) migration into the deeper lymph node paracortex from the lymph node subcapsular
sinus (SCS) after passive transport by lymph flow (Girard et al., 2012). Of interest, the atypical chemokine receptor ACKR4, which scavenges CCL19 and CCL21 (in addition to CCL25 and CXCL13), is expressed only by endothelial cells lining the SCS ceiling—not by those of the SCS floor. Based on its scavenging activity and its characteristic expression pattern, this receptor was recently shown to actively pattern CCL21 gradients that facilitate the CCR7-mediated translocation of DCs from the SCS into the lymph node parenchyma (Ulvmar et al., 2014). All chemokine receptors, including CCR7, belong to the large superfamily of seven transmembrane-spanning G protein-coupled receptors. A unique feature of chemokine receptors (with the exception of the four atypical chemokine receptors) is the highly conserved DRY motif in transmembrane 3 next to the second intracellular loop (Randolph et al., 2008). In sharp contrast to its broad relevance for many immunological processes, relatively little is known about how signaling through CCR7 translates into cell motility. Interestingly, CCL21 and CCL19 elicit different signaling events through CCR7. CCL21 gets immobilized to glycosaminoglycans, mediating integrin-dependent
Immunity 44, January 19, 2016 ª2016 Elsevier Inc. 9
Immunity
Previews
Figure 1. Signaling through CCR7 Left: Upon ligand binding, guanosine diphosphate (GDP) gets exchanged by guanosine triphosphate (GTP) leading to the dissociation of the a-b-g heterotrimeric G protein complex into a a-GTP and a b-g subunit. These in turn activate several parallel cascades of downstream molecules including phospholipase C (PLC), phosphatidylinositol 3-kinase (PI3K), mitogen-activated protein kinase (MAPK), and protein kinase C (PKC) that, among others, activate second messengers such as inositol trisphosphate (IP3), diacylglycerol (DAG), and Ca2+ ions (Randolph et al., 2008). Independent of G protein signaling, G protein receptor kinases (GRKs) phosphorylate C-terminal residues, allowing b-arrestin (b-arr)-mediated receptor internalization (Randolph et al., 2008). Right: Inflammatory signals such as PGE2 provoke oligomerization of CCR7. These oligomers serve as scaffolds that integrate signaling through G proteins and Src kinase. Src tyrosine phosphorylates CCR7 which then serves as a docking site for SH2-domain bearing molecules such as phosphatase SHP2.
adhesion and lymph node homing. Of interest, CCL21, but not CCL19, has been shown to take on an autoinhibited conformation that can be released by interacting polysialic acid conjugated to CCR7 that subsequently allows binding to its receptor (Kiermaier et al., 2015). In contrast, CCL19 induces phosphorylation of CCR7 by G protein-coupled receptor kinase 3 leading to receptor internalization via b-arrestin (Figure 1, left; Zidar et al., 2009). Classical, but not the atypical, chemokine receptors signal through activating heterotrimeric G proteins containing a pertussis toxin (PTX)-sensitive Gai subunit (Figure 1). Consequently, treating cells with PTX completely blocks homing of naive lymphocytes to lymph nodes. However, it was shown earlier that CCR7-expressing monocyte-derived DCs had to be activated with Prostaglandin E2 (PGE2) to gain full sensitivity toward CCR7 ligands, and a recent study suggests that effector T cells, in contrast to naive T cells, can actually bypass Gai signaling by using Src kinase to adhere
to and crawl on inflamed endothelial cells (Shulman et al., 2012), indicating that CCR7 might signal not only through G proteins and b-arrestin but also via alternative pathways. In the current issue of Immunity, Hauser et al. (2016) now report that oligomerization of CCR7 enables a previously unknown, G protein-parallel intracellular signaling pathway of CCR7 via Src kinase and SHP2 (Figure 1, right). Interestingly, some CCR7 oligomers seem to form spontaneously within the cell membrane of (for example) naive T lymphocytes even under steady-state conditions. However, they are much more abundantly present on effector and central memory T cells as well as on mature DCs. Inflammatory signals, in particular PGE2, increase the CCR7 oligomerization status of DCs by modulating genes of cholesterol metabolism and transport, thus lowering cellular cholesterol levels. Additionally, binding of the CCR7 ligands CCL19 and CCL21 promotes CCR7 oligomerization as well. Using cysteine-cross-
10 Immunity 44, January 19, 2016 ª2016 Elsevier Inc.
linking, molecular modeling, and directed evolutionary screens, the authors identified a hydrophobic interaction surface proximate to the NPXXY motif of the transmembrane spanning region TM7 of CCR7 as essential for the formation of CCR7 oligomers. Furthermore, they found a naturally occurring CCR7 SNP with ‘‘super-oligomerization’’ properties due to an enlargement of this hydrophobic region. Most importantly, immune cell lines transfected with oligomerization-prone CCR7 variants displayed higher migratory activity in transwell migration assays while chemokine binding and G protein activation were unaltered, indicating a G-protein-independent pro-migratory signaling function of CCR7 oligomers. Based on their characterization of CCR7 oligomerization interfaces and previous structural analysis, the authors propose a scenario in which a CCR7 tetramer, containing two bound sets of heterotrimeric G proteins, recruits the tyrosine kinase Src to form a signaling scaffold integrating G protein-dependent as well as Src-dependent signaling pathways. After CCR7-induced auto-phosphorylation, Src is able to tyrosine-phosphorylate CCR7 within the highly conserved DRY-motif, thereby creating an active SH2-domain binding site. This in turn allows for the recruitment of the tyrosine phosphatase SHP2 into the complex and its activation by Src-mediated phosphorylation (Figure 1, right). Importantly, these events were triggered by binding of both CCL19 and CCL21 to their receptor CCR7. However, only CCR7 triggering by CCL21 resulted in the accumulation of a large cytosolic pool of catalytically active SHP2, representing another example of so-called ‘‘biased signaling’’ of the CCR7 ligand CCL21. Consequently, inhibition of SHP2 interfered with T cell migration mediated by CCL21, but not CCL19, as the latter probably primarily relies on the well-known classical signaling pathway via Gai. Representing some limitations of this important study, Hauser et al. (2016) analyzed the influence of CCR7 oligomerization on immune cell migration almost exclusively in vitro, primarily using 2D and 3D transwell assays. It would be extremely interesting to see the effect of varying cellular CCR7 oligomerization levels on the migration characteristics of DCs and lymphocytes in situ, studying their
Immunity
Previews behavior during crucial migration events within intact, living tissues. In this context, it is tempting to speculate that the immobilization of CCL21, but not CCL19, on glycosaminoglycan-bearing biological surfaces (fibroblastic reticular cells, interstitial stroma) might actually favor the formation of CCR7 tetramer complexes by mechanically approximating CCR7 molecules within the cell membrane of migrating immune cells, thus unlocking additional Src-mediated signals. Another important question is how the formation of tetrameric CCR7 signaling hubs might influence CCR7-dependent haptokinetic ‘‘gradient sensing’’ (Schumann et al., 2010). This mechanism enables immune cells to be not only randomly motile, but to exhibit highly persistent directional migration, crucial for (for example) the entry of skin DCs into primary lymphatics or their migration into the deep paracortical T cell zone of draining lymph nodes. The elegant study by Hauser et al. (2016) brings a fascinating new twist into the well-studied field of CCR7 biology: identifying and characterizing CCR7 tetramers as signaling hubs for not only G
protein- but also Src-SHP2-dependent intracellular signals. Their findings open up new ways to potentially interfere with CCR7-mediated cell migration in a number of scientifically as well as clinically relevant situations, e.g., anti-inflammatory and anti-metastatic drug development. Due to the strong influence of inflammatory signals on cellular CCR7 oligomerization levels, the Src-SHP2 side of CCR7-driven migration might indeed be of particular importance for ‘‘battle-proven’’ immune cells such as mature DCs and effector and/or memory T cells, further emphasizing the therapeutic potential of such interventions.
Griffith, J.W., Sokol, C.L., and Luster, A.D. (2014). Annu. Rev. Immunol. 32, 659–702.
ACKNOWLEDGMENTS
Shulman, Z., Cohen, S.J., Roediger, B., Kalchenko, V., Jain, R., Grabovsky, V., Klein, E., Shinder, V., Stoler-Barak, L., Feigelson, S.W., et al. (2012). Nat. Immunol. 13, 67–76.
This work has been supported by ERC Advanced Grant 322645 to R.F.
REFERENCES Fo¨rster, R., Davalos-Misslitz, A.C., and Rot, A. (2008). Nat. Rev. Immunol. 8, 362–371. Girard, J.P., Moussion, C., and Fo¨rster, R. (2012). Nat. Rev. Immunol. 12, 762–773.
Hauser, M.A., Schaeuble, K., Kindinger, I., Impellizzieri, D., Krueger, W.A., Hauck, C.R., Boyman, O., and Legler, D.F. (2016). Immunity 44, this issue, 59–72. Kiermaier, E., Moussion, C., Veldkamp, C.T., Gerardy-Schahn, R., de Vries, I., Williams, L.G., Chaffee, G.R., Phillips, A.J., Freiberger, F., Imre, R., et al. (2015). Science, aad0512, http://dx.doi.org/ 10.1126/science.aad0512. Randolph, G.J., Ochando, J., and Partida-Sa´nchez, S. (2008). Annu. Rev. Immunol. 26, 293–316. Schumann, K., La¨mmermann, T., Bruckner, M., Legler, D.F., Polleux, J., Spatz, J.P., Schuler, G., Fo¨rster, R., Lutz, M.B., Sorokin, L., and Sixt, M. (2010). Immunity 32, 703–713.
Ulvmar, M.H., Werth, K., Braun, A., Kelay, P., Hub, E., Eller, K., Chan, L., Lucas, B., Novitzky-Basso, I., Nakamura, K., et al. (2014). Nat. Immunol. 15, 623–630. Zidar, D.A., Violin, J.D., Whalen, E.J., and Lefkowitz, R.J. (2009). Proc. Natl. Acad. Sci. USA 106, 9649–9654.
AIF Is ‘‘Always In Fashion’’ for T Cells Marc O. Johnson1 and Jeffrey C. Rathmell1,* 1Vanderbilt Center for Immunobiology, Department of Pathology, Microbiology, and Immunology, Vanderbilt University, Nashville, TN 37232, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.immuni.2015.12.015
AIF has been known to have both apoptotic and metabolic roles. Green and colleagues show that T cells, but not B cells, rely on AIF to maintain mitochondrial electron transport and that metabolic, rather than apoptotic, pathways mediate this dependence. The newly emerging field of immunometabolism has several origins, not the least of which is through the recognition that mitochondrial proteins play critical roles in lymphocyte apoptosis. Cytochrome C, SMAC/DIABLO, and apoptosis inducing factor (AIF) were all identified as mitochondrial proteins that were released and drove apoptosis upon stress. Of these, Cytochrome C was long known as a component of the electron transport
chain. Metabolic roles for AIF have also been shown over the past years, and AIF is thought to promote electron transport complex I assembly (Hangen et al., 2010). Interestingly, the harlequin mouse strain, which has a proviral insertion that results in reduced AIF expression, has immune defects in T cells, but not B cells. In this issue, Milasta et al. (2016) examine the metabolic and apoptotic effects of AIF and show that this discrepancy is
due to key differences in the metabolic demands of B and T cells, rather than apoptosis. Though their functions are very different, B cells and T cells form the basis for adaptive immunity and share a close lymphocyte progenitor cell. Each undergoes selection and proliferation during development and persists in a resting state while surveilling for antigen. After activation, each cell type is induced to
Immunity 44, January 19, 2016 ª2016 Elsevier Inc. 11