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B-cell Signaling: Protein Kinase Cδ Puts the Brakes on
Current Biology, Vol. 12, R554–R556, August 20, 2002, ©2002 Elsevier Science Ltd. All rights reserved.
B-cell Signaling: Protein Kinase Cδδ Puts the Brakes on Diane Mathis and George L. King
The phenotype of mice lacking the delta isoform of protein kinase C reveals that this isoform curtails signaling events after engagement of the antigenspecific receptor on B cells. The result is a state of non-responsiveness, termed anergy, that represents one form of immunological self-tolerance.
Protein kinase C (PKC) is one of the cell’s most important signal-transducing molecules, responding to a diversity of extracellular cues to regulate a number of central processes, including proliferation, metabolism, migration and death [1]. It is also one of the more complex signal transducers, constituting a family of serine/threonine kinases that can phosphorylate a multitude of cellular substrates. The family currently includes 12 isoforms, which are structurally related and have been grouped into 3 classes according to whether or not they require diacylglycerol (DAG) and/or Ca2+ for activation. The various isoforms have different cell-type distributions, and show differential compartmentalizations within cells upon activation. More confounding, a single isoform can have distinct, even opposing, functions in different cell types. It has been a major challenge to penetrate this complexity in order to attribute to specific isoforms specific functions in specific cell types. Fortunately, some key advances have come in the past few years thanks to the development of transgenic mouse lines overexpressing a particular PKC isoform, knockout mouse lines lacking a single isoform, and molecule-specific inhibitors. A good example is provided by two recent papers that reported an important role for PKCδδ in tolerance induction in B lymphocytes [2,3]. PKC activation appears to influence multiple facets of immune system function, from T and B lymphocyte differentiation to various forms of leukocyte activation. α in T lymphocytes exhibMice overexpressing PKCα ited decreased T-cell proliferative responses, a biased pattern of T-cell cytokine secretion and reduced production of antibodies by B cells [4]. Mice lacking β showed reduced B-cell proliferative responses PKCβ and impaired humoral immunity [5]. Since these two molecules belong to the ‘classical’ class of isoforms, which requires both DAG and Ca2+ for activation, investigations into PKC activities focused for some time on immune system functions that depend on both of these intracellular mediators. However, recent reports [2,3,6] of T- and B-cell abnormalities in mice Sections on Immunology and Immunogenetics, and Vascular Cell Biology and Complications, Joslin Diabetes Center and Department of Medicine, Brigham and Women’s Hospital, Harvard Medical School, One Joslin Place, Boston, Massachusetts 02215, USA. E-mail: [email protected]
PII S0960-9822(02)01052-7
Dispatch
θ and PKCδδ, respectively, have changed lacking PKCθ this emphasis because these kinases are both of the ‘novel’ class of isoforms, whose activation requires DAG but not Ca2+. Several observations have piqued interest in examining the role of PKCδδ in the immune system. For example, this isoform has the unusual property of being tyrosine phosphorylated [7], and this phosphorylation event takes place within a minute of engagement of the surface immunoglobulin (Ig) receptors on B cells [8,9]. In addition, this isoform can have a potent negative influence on cell behavior, inhibiting proliferation and enhancing death [7]. When PKCδδ-deficient mice were screened for immune system manifestations, the most overt abnormalities were splenomegaly and lymphadenopathy, both attributed to an increase in the number of B cells of the conventional, B2, type. This augmentation was not observed in the bone marrow, indicating that it is a post-maturation phenomenon. Abnormal B-cell accumulation was also revealed by the unusually high numbers of germinal centers in the spleen and lymph nodes in the absence of intentional antigenic stimulation. Interestingly, smooth muscle cells from these knockout mice were previously reported to show aberrant homeostasis, but the mechanism appeared to differ from that for B cells: a normal proliferative capacity [3] and decreased propensity for cell death [2,3] for the former; increased proliferation and normal death for the latter [10]. The B-cell abnormalities in PKCδδ-deficient mice had pathological consequences. Concentrations of serum IgG1 and IgA were increased, and auto-antibodies recognizing a variety of auto-antigens could be detected after 6 months of age. Igs were aberrantly deposited in the kidneys, and B-cell infiltrates were found in perivascular regions of multiple organs. Clearly, then, B-cell tolerance to self-antigens was somehow compromised. To explore the mechanisms underlying the loss of B-cell tolerance in mice lacking PKCδδ, Mecklenbrauker et al. [2] introduced the knockout mutation into a well-studied transgenic mouse system focused on Igs reactive to hen egg lysozyme (HEL). Singletransgenic mice (IgHEL) that carry pre-rearranged Ig genes encoding the heavy and light chains of an antibody that recognizes HEL develop a B-lymphocyte repertoire highly skewed for HEL-reactive cells, and produce large amounts of anti-HEL antibodies upon antigenic stimulation [11]. Double-transgenic mice (IgHEL/mHEL) that also harbor a second transgene encoding a membrane-bound form of HEL as a selfantigen lack mature HEL-responsive B cells because these cells are deleted during maturation in the bone marrow [12]. If, instead, the second transgene (sHEL in IgHEL/sHEL mice) encodes a soluble form of HEL as the self-antigen, significant numbers of mature HEL-reactive B cells are generated, but these do not produce anti-HEL antibodies because they have been rendered non-responsive, or anergic [11]. This has
Current Biology R555
Activation signal (acute)
Tolerance signal (chronic)
Foreign antigen
Self antigen Immunoglobulin
Cell membrane
Ca2+ ERK NFAT NF-κB JNK
Ca2+ ERK NFAT NF-κB JNK
Figure 1. Different signals emanating from the B cell antigen receptor. Left, events downstream of an activation stimulus (e.g. a foreign antigen). Right, events downstream of a tolerogenic stimulus (such as a soluble self-antigen). The upper curve represents the plasma membrane, and the inner represents the nuclear membrane. Ca2+ depicts the calcium signal, with the size of the triangle reflecting the strength and duration of this signal. ERK, extracellular signal regulated kinase; JNK, c-Jun N-terminal kinase; NFAT, nuclear factor of activated T cells; κB, nuclear factor κB. NFκ
Nucleus Positive program Proliferation Migration into follicles Differentiation Antibody production Costimulatory capacity Increased survival
Negative program Block in: Migration Differentiation Antibody production Costimulation Decreased survival
Immunity
Tolerance (anergy) Current Biology
proven a very powerful system for probing the signaling pathways implicated in anergy induction [13–17]. The current picture, schematized in Figure 1, is that the anergic B lymphocytes from IgHEL/sHEL mice are in an activated state compared with antigen-naïve B cells from IgHEL mice, but that the activation is incomplete in comparison with that of antigen-stimulated B cells from IgHEL animals. The aborted activation involves low calcium oscillations, muted protein tyrosine phosphorylation, and mobilization of the transcription factor NFAT and the MAP kinase ERK; but it stops short of a typical biphasic calcium response, a standard pattern of protein tyrosine phosphorylation, κB transcription and additional mobilization of the NF-κ factor and the MAP kinase JNK [13,15,16]. The ultimate result is a negative response: lack of proliferation, reduction in survival, blockade of differentiation, and a failure to express T-cell co-stimulatory molecules [13,14], reflecting induction of genes which encode negative regulators of signaling and transcription but not genes promoting proliferation [17]. Where does PKCδδ come in? Breeding of the PKCδδ knockout mutation into doubletransgenic IgHEL/mHEL mice had no overt effect: as usual, potentially auto-reactive HEL-responsive B cells were deleted in the bone marrow. However, introduction of the mutation into IgHEL/sHEL animals showed a striking influence: HEL-reactive B cells emerged into the periphery as usual, but they were no longer tolerant to HEL, no longer anergic. These animals had significant titers of anti-HEL antibodies in the serum, and their B cells resembled those of non-transgenic or
IgHEL mice in many features of their behavior, culminating in a normal response to HEL. Thus, chronic exposure of PKCδδ-deficient B cells to self-antigen stimulation failed to induce tolerance, via anergy, as it does for wild-type B cells; in marked contrast, acute exposure to the same antigen was perfectly capable of eliciting immunity. The search is now on for the molecular basis of the B-lymphocyte abnormalities exhibited by PKCδδ-deficient mice. One group [3] observed an augmentation of interleukin-6 production upon engagement of surface Ig (and the costimulatory molecule CD40), attributed to an increase in the transcription factor NF-IL6. This seems an attractive explanation for at least some of the B-cell aberrancies because mice lacking PKCδδ and transgenic mice overexpressing IL-6 exhibit a number of similarities. The other group [2] focused on the tranκB, no doubt prompted by a previscription factor NF-κ ous report [15] that this factor is not activated in anergic B cells, neither ex vivo nor after cognate antigen stimulation in vitro. This lack of activity is due κB, a reflection to reduced nuclear translocation of NF-κ of inefficient degradation of an inhibitor that sequesκB. Indeed, the non-anergic B ters it in the cytoplasm, Iκ cells from IgHEL/sHEL mice lacking PKCδδ showed κB activation, greater than that of anergic enhanced NF-κ B cells from double-transgenic animals expressing PKCδδ, both with and without exogenous HEL stimulation, and even greater than that of non-anergic B cells from IgHEL animals, whether or not they expressed κB was PKCδδ. Rather surprisingly, degradation of Iκ not diminished in parallel with enhanced activation of
Dispatch R556
κB, suggesting that there might be an alternative NF-κ κB route to boost activation. The differences in NF-κ behavior in the presence and absence of PKCδδ were a property of the anergic state, as they were not observed with B cells from either non-transgenic or IgHEL mice. These new findings pointing to a ‘braking’ role for PKCδδ during anergy induction in B cells are of significant interest from a number of perspectives. Firstly, they pose again the question, in a perfectly matched setting, of how the same signal transducer can have divergent effects in two cell types: increased proliferation [3], normal death [2,3] in B cells; and normal proliferation, increased death in smooth muscle cells [10]. The answer must await painstaking dissection of the sequence of events downstream of PKCδδ activation in the two cell types. Secondly, these studies raise the issue of what transducer might have an analogous function in T cells, which also undergo anergy induction when confronted with a self-antigen under the appropriate circumstances. In fact, there are similarities in the activation state of anergic T and B cells, κB notably normal NFAT [15,18] but defective NF-κ [15,19] activation. No T-cell abnormalities were θ has a observed in the PKCδδ-deficient mice. PKCθ number of features in common with PKCδδ, such as not requiring Ca2+ for activation and perhaps being more dependent on phosphatidylinositol 3-kinase than DAG θ as an upstream regulator [8,20]. The results on PKCθ knockout mice argue for this isoform having a positive role in the T-cell response, however, being essential for activation of mature T cells through the antigen receptor [6]. It is imperative now to look elsewhere. Finally, one cannot help but evoke the potential of PKCδδ-specific modulators to prevent or reverse Bcell-mediated autoimmune diseases, in particular systemic lupus erythematosus. References 1. Liu, W.S. and Heckman, C.A. (1998). The sevenfold way of PKC regulation. Cell. Signal. 10, 529–542. 2. Mecklenbrauker, I., Saijo, K., Zheng, N.Y., Leitges, M. and Tarakhovsky, A. (2002). Protein kinase Cdelta controls self-antigeninduced B-cell tolerance. Nature 416, 860–865. 3. Miyamoto, A., Nakayama, K., Imaki, H., Hirose, S., Jiang, Y., Abe, M., Tsukiyama, T., Nagahama, H., Ohno, S., Hatakeyama, S. et al. (2002). Increased proliferation of B cells and auto-immunity in mice lacking protein kinase Cdelta. Nature 416, 865–869. 4. Ohkusu, K., Du, J., Isobe, K.I., Yi, H., Akhand, A.A., Kato, M., Suzuki, H., Hidaka, H. and Nakashima, I. (1997). Protein kinase C alphamediated chronic signal transduction for immunosenescence. J. Immunol. 159, 2082–2084. 5. Leitges, M., Schmedt, C., Guinamard, R., Davoust, J., Schaal, S., Stabel, S. and Tarakhovsky, A. (1996). Immunodeficiency in protein kinase c beta-deficient mice. Science 273, 788–791. 6. Sun, Z., Arendt, C.W., Ellmeier, W., Schaeffer, E.M., Sunshine, M.J., Gandhi, L., Annes, J., Petrzilka, D., Kupfer, A., Schwartzberg, P.L. et al. (2000). PKC-theta is required for TCR-induced NF-kappaB activation in mature but not immature T lymphocytes. Nature 404, 402–407. 7. Gschwendt, M. (1999). Protein kinase C delta. Eur. J. Biochem. 259, 555–564. 8. Popoff, I.J. and Deans, J.P. (1999). Activation and tyrosine phosphorylation of protein kinase C delta in response to B cell antigen receptor stimulation. Mol. Immunol. 36, 1005–1016. 9. Barbazuk, S.M. and Gold, M.R. (1999). Protein kinase C-delta is a target of B-cell antigen receptor signaling. Immunol. Lett. 69, 259–267. 10. Leitges, M., Mayr, M., Braun, U., Mayr, U., Li, C., Pfister, G., Ghaffari-Tabrizi, N., Baier, G., Hu, Y. and Xu, Q. (2001). Exacerbated vein graft arteriosclerosis in protein kinase Cdelta-null mice. J. Clin. Invest. 108, 1505–1512.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
Goodnow, C.C., Crosbie, J., Adelstein, S., Lavoie, T.B., Smith-Gill, S.J., Brink, R.A., Pritchard-Briscoe, H., Wotherspoon, J.S., Loblay, R.H., Raphael, K. et al. (1988). Altered immunoglobulin expression and functional silencing of self-reactive B lymphocytes in transgenic mice. Nature 334, 676–682. Hartley, S.B., Crosbie, J., Brink, R., Kantor, A.B., Basten, A. and Goodnow, C.C. (1991). Elimination from peripheral lymphoid tissues of self-reactive B lymphocytes recognizing membrane-bound antigens. Nature 353, 765–769. Cooke, M.P., Heath, A.W., Shokat, K.M., Zeng, Y., Finkelman, F.D., Linsley, P.S., Howard, M. and Goodnow, C.C. (1994). Immunoglobulin signal transduction guides the specificity of B cell-T cell interactions and is blocked in tolerant self-reactive B cells. J. Exp. Med. 179, 425–438. Cyster, J.G. and Goodnow, C.C. (1995). Antigen-induced exclusion from follicles and anergy are separate and complementary processes that influence peripheral B cell fate. Immunity 3, 691–701. Healy, J.I., Dolmetsch, R.E., Timmerman, L.A., Cyster, J.G., Thomas, M.L., Crabtree, G.R., Lewis, R.S. and Goodnow, C.C. (1997). Different nuclear signals are activated by the B cell receptor during positive versus negative signaling. Immunity 6, 419–428. Dolmetsch, R.E., Lewis, R.S., Goodnow, C.C. and Healy, J.I. (1997). Differential activation of transcription factors induced by Ca2+ response amplitude and duration. Nature 386, 855–858. Glynne, R., Akkaraju, S., Healy, J.I., Rayner, J., Goodnow, C.C. and Mack, D.H. (2000). How self-tolerance and the immunosuppressive drug FK506 prevent B-cell mitogenesis. Nature 403, 672–676. Li, W., Whaley, C.D., Mondino, A. and Mueller, D.L. (1996). Blocked signal transduction to the ERK and JNK protein kinases in anergic CD4+ T cells. Science 271, 1272–1276. Sundstedt, A., Sigvardsson, M., Leanderson, T., Hedlund, G., Kalland, T. and Dohlsten, M. (1996). In vivo anergized CD4+ T cells express perturbed AP-1 and NF-kappa B transcription factors. Proc Natl. Acad. Sci U.S.A. 93, 979–984. Villalba, M., Bi, K., Hu, J., Altman, Y., Bushway, P., Reits, E., Neefjes, J., Baier, G., Abraham, R.T. and Altman, A. (2002). Translocation of PKC[theta] in T cells is mediated by a nonconventional, PI3-K- and Vav-dependent pathway, but does not absolutely require phospholipase C. J. Cell Biol. 157, 253–263.
B-cell Signaling: Protein Kinase Cδδ Puts the Brakes on Diane Mathis and George L. King
The phenotype of mice lacking the delta isoform of protein kinase C reveals that this isoform curtails signaling events after engagement of the antigenspecific receptor on B cells. The result is a state of non-responsiveness, termed anergy, that represents one form of immunological self-tolerance.
Protein kinase C (PKC) is one of the cell’s most important signal-transducing molecules, responding to a diversity of extracellular cues to regulate a number of central processes, including proliferation, metabolism, migration and death [1]. It is also one of the more complex signal transducers, constituting a family of serine/threonine kinases that can phosphorylate a multitude of cellular substrates. The family currently includes 12 isoforms, which are structurally related and have been grouped into 3 classes according to whether or not they require diacylglycerol (DAG) and/or Ca2+ for activation. The various isoforms have different cell-type distributions, and show differential compartmentalizations within cells upon activation. More confounding, a single isoform can have distinct, even opposing, functions in different cell types. It has been a major challenge to penetrate this complexity in order to attribute to specific isoforms specific functions in specific cell types. Fortunately, some key advances have come in the past few years thanks to the development of transgenic mouse lines overexpressing a particular PKC isoform, knockout mouse lines lacking a single isoform, and molecule-specific inhibitors. A good example is provided by two recent papers that reported an important role for PKCδδ in tolerance induction in B lymphocytes [2,3]. PKC activation appears to influence multiple facets of immune system function, from T and B lymphocyte differentiation to various forms of leukocyte activation. α in T lymphocytes exhibMice overexpressing PKCα ited decreased T-cell proliferative responses, a biased pattern of T-cell cytokine secretion and reduced production of antibodies by B cells [4]. Mice lacking β showed reduced B-cell proliferative responses PKCβ and impaired humoral immunity [5]. Since these two molecules belong to the ‘classical’ class of isoforms, which requires both DAG and Ca2+ for activation, investigations into PKC activities focused for some time on immune system functions that depend on both of these intracellular mediators. However, recent reports [2,3,6] of T- and B-cell abnormalities in mice Sections on Immunology and Immunogenetics, and Vascular Cell Biology and Complications, Joslin Diabetes Center and Department of Medicine, Brigham and Women’s Hospital, Harvard Medical School, One Joslin Place, Boston, Massachusetts 02215, USA. E-mail: [email protected]
PII S0960-9822(02)01052-7
Dispatch
θ and PKCδδ, respectively, have changed lacking PKCθ this emphasis because these kinases are both of the ‘novel’ class of isoforms, whose activation requires DAG but not Ca2+. Several observations have piqued interest in examining the role of PKCδδ in the immune system. For example, this isoform has the unusual property of being tyrosine phosphorylated [7], and this phosphorylation event takes place within a minute of engagement of the surface immunoglobulin (Ig) receptors on B cells [8,9]. In addition, this isoform can have a potent negative influence on cell behavior, inhibiting proliferation and enhancing death [7]. When PKCδδ-deficient mice were screened for immune system manifestations, the most overt abnormalities were splenomegaly and lymphadenopathy, both attributed to an increase in the number of B cells of the conventional, B2, type. This augmentation was not observed in the bone marrow, indicating that it is a post-maturation phenomenon. Abnormal B-cell accumulation was also revealed by the unusually high numbers of germinal centers in the spleen and lymph nodes in the absence of intentional antigenic stimulation. Interestingly, smooth muscle cells from these knockout mice were previously reported to show aberrant homeostasis, but the mechanism appeared to differ from that for B cells: a normal proliferative capacity [3] and decreased propensity for cell death [2,3] for the former; increased proliferation and normal death for the latter [10]. The B-cell abnormalities in PKCδδ-deficient mice had pathological consequences. Concentrations of serum IgG1 and IgA were increased, and auto-antibodies recognizing a variety of auto-antigens could be detected after 6 months of age. Igs were aberrantly deposited in the kidneys, and B-cell infiltrates were found in perivascular regions of multiple organs. Clearly, then, B-cell tolerance to self-antigens was somehow compromised. To explore the mechanisms underlying the loss of B-cell tolerance in mice lacking PKCδδ, Mecklenbrauker et al. [2] introduced the knockout mutation into a well-studied transgenic mouse system focused on Igs reactive to hen egg lysozyme (HEL). Singletransgenic mice (IgHEL) that carry pre-rearranged Ig genes encoding the heavy and light chains of an antibody that recognizes HEL develop a B-lymphocyte repertoire highly skewed for HEL-reactive cells, and produce large amounts of anti-HEL antibodies upon antigenic stimulation [11]. Double-transgenic mice (IgHEL/mHEL) that also harbor a second transgene encoding a membrane-bound form of HEL as a selfantigen lack mature HEL-responsive B cells because these cells are deleted during maturation in the bone marrow [12]. If, instead, the second transgene (sHEL in IgHEL/sHEL mice) encodes a soluble form of HEL as the self-antigen, significant numbers of mature HEL-reactive B cells are generated, but these do not produce anti-HEL antibodies because they have been rendered non-responsive, or anergic [11]. This has
Current Biology R555
Activation signal (acute)
Tolerance signal (chronic)
Foreign antigen
Self antigen Immunoglobulin
Cell membrane
Ca2+ ERK NFAT NF-κB JNK
Ca2+ ERK NFAT NF-κB JNK
Figure 1. Different signals emanating from the B cell antigen receptor. Left, events downstream of an activation stimulus (e.g. a foreign antigen). Right, events downstream of a tolerogenic stimulus (such as a soluble self-antigen). The upper curve represents the plasma membrane, and the inner represents the nuclear membrane. Ca2+ depicts the calcium signal, with the size of the triangle reflecting the strength and duration of this signal. ERK, extracellular signal regulated kinase; JNK, c-Jun N-terminal kinase; NFAT, nuclear factor of activated T cells; κB, nuclear factor κB. NFκ
Nucleus Positive program Proliferation Migration into follicles Differentiation Antibody production Costimulatory capacity Increased survival
Negative program Block in: Migration Differentiation Antibody production Costimulation Decreased survival
Immunity
Tolerance (anergy) Current Biology
proven a very powerful system for probing the signaling pathways implicated in anergy induction [13–17]. The current picture, schematized in Figure 1, is that the anergic B lymphocytes from IgHEL/sHEL mice are in an activated state compared with antigen-naïve B cells from IgHEL mice, but that the activation is incomplete in comparison with that of antigen-stimulated B cells from IgHEL animals. The aborted activation involves low calcium oscillations, muted protein tyrosine phosphorylation, and mobilization of the transcription factor NFAT and the MAP kinase ERK; but it stops short of a typical biphasic calcium response, a standard pattern of protein tyrosine phosphorylation, κB transcription and additional mobilization of the NF-κ factor and the MAP kinase JNK [13,15,16]. The ultimate result is a negative response: lack of proliferation, reduction in survival, blockade of differentiation, and a failure to express T-cell co-stimulatory molecules [13,14], reflecting induction of genes which encode negative regulators of signaling and transcription but not genes promoting proliferation [17]. Where does PKCδδ come in? Breeding of the PKCδδ knockout mutation into doubletransgenic IgHEL/mHEL mice had no overt effect: as usual, potentially auto-reactive HEL-responsive B cells were deleted in the bone marrow. However, introduction of the mutation into IgHEL/sHEL animals showed a striking influence: HEL-reactive B cells emerged into the periphery as usual, but they were no longer tolerant to HEL, no longer anergic. These animals had significant titers of anti-HEL antibodies in the serum, and their B cells resembled those of non-transgenic or
IgHEL mice in many features of their behavior, culminating in a normal response to HEL. Thus, chronic exposure of PKCδδ-deficient B cells to self-antigen stimulation failed to induce tolerance, via anergy, as it does for wild-type B cells; in marked contrast, acute exposure to the same antigen was perfectly capable of eliciting immunity. The search is now on for the molecular basis of the B-lymphocyte abnormalities exhibited by PKCδδ-deficient mice. One group [3] observed an augmentation of interleukin-6 production upon engagement of surface Ig (and the costimulatory molecule CD40), attributed to an increase in the transcription factor NF-IL6. This seems an attractive explanation for at least some of the B-cell aberrancies because mice lacking PKCδδ and transgenic mice overexpressing IL-6 exhibit a number of similarities. The other group [2] focused on the tranκB, no doubt prompted by a previscription factor NF-κ ous report [15] that this factor is not activated in anergic B cells, neither ex vivo nor after cognate antigen stimulation in vitro. This lack of activity is due κB, a reflection to reduced nuclear translocation of NF-κ of inefficient degradation of an inhibitor that sequesκB. Indeed, the non-anergic B ters it in the cytoplasm, Iκ cells from IgHEL/sHEL mice lacking PKCδδ showed κB activation, greater than that of anergic enhanced NF-κ B cells from double-transgenic animals expressing PKCδδ, both with and without exogenous HEL stimulation, and even greater than that of non-anergic B cells from IgHEL animals, whether or not they expressed κB was PKCδδ. Rather surprisingly, degradation of Iκ not diminished in parallel with enhanced activation of
Dispatch R556
κB, suggesting that there might be an alternative NF-κ κB route to boost activation. The differences in NF-κ behavior in the presence and absence of PKCδδ were a property of the anergic state, as they were not observed with B cells from either non-transgenic or IgHEL mice. These new findings pointing to a ‘braking’ role for PKCδδ during anergy induction in B cells are of significant interest from a number of perspectives. Firstly, they pose again the question, in a perfectly matched setting, of how the same signal transducer can have divergent effects in two cell types: increased proliferation [3], normal death [2,3] in B cells; and normal proliferation, increased death in smooth muscle cells [10]. The answer must await painstaking dissection of the sequence of events downstream of PKCδδ activation in the two cell types. Secondly, these studies raise the issue of what transducer might have an analogous function in T cells, which also undergo anergy induction when confronted with a self-antigen under the appropriate circumstances. In fact, there are similarities in the activation state of anergic T and B cells, κB notably normal NFAT [15,18] but defective NF-κ [15,19] activation. No T-cell abnormalities were θ has a observed in the PKCδδ-deficient mice. PKCθ number of features in common with PKCδδ, such as not requiring Ca2+ for activation and perhaps being more dependent on phosphatidylinositol 3-kinase than DAG θ as an upstream regulator [8,20]. The results on PKCθ knockout mice argue for this isoform having a positive role in the T-cell response, however, being essential for activation of mature T cells through the antigen receptor [6]. It is imperative now to look elsewhere. Finally, one cannot help but evoke the potential of PKCδδ-specific modulators to prevent or reverse Bcell-mediated autoimmune diseases, in particular systemic lupus erythematosus. References 1. Liu, W.S. and Heckman, C.A. (1998). The sevenfold way of PKC regulation. Cell. Signal. 10, 529–542. 2. Mecklenbrauker, I., Saijo, K., Zheng, N.Y., Leitges, M. and Tarakhovsky, A. (2002). Protein kinase Cdelta controls self-antigeninduced B-cell tolerance. Nature 416, 860–865. 3. Miyamoto, A., Nakayama, K., Imaki, H., Hirose, S., Jiang, Y., Abe, M., Tsukiyama, T., Nagahama, H., Ohno, S., Hatakeyama, S. et al. (2002). Increased proliferation of B cells and auto-immunity in mice lacking protein kinase Cdelta. Nature 416, 865–869. 4. Ohkusu, K., Du, J., Isobe, K.I., Yi, H., Akhand, A.A., Kato, M., Suzuki, H., Hidaka, H. and Nakashima, I. (1997). Protein kinase C alphamediated chronic signal transduction for immunosenescence. J. Immunol. 159, 2082–2084. 5. Leitges, M., Schmedt, C., Guinamard, R., Davoust, J., Schaal, S., Stabel, S. and Tarakhovsky, A. (1996). Immunodeficiency in protein kinase c beta-deficient mice. Science 273, 788–791. 6. Sun, Z., Arendt, C.W., Ellmeier, W., Schaeffer, E.M., Sunshine, M.J., Gandhi, L., Annes, J., Petrzilka, D., Kupfer, A., Schwartzberg, P.L. et al. (2000). PKC-theta is required for TCR-induced NF-kappaB activation in mature but not immature T lymphocytes. Nature 404, 402–407. 7. Gschwendt, M. (1999). Protein kinase C delta. Eur. J. Biochem. 259, 555–564. 8. Popoff, I.J. and Deans, J.P. (1999). Activation and tyrosine phosphorylation of protein kinase C delta in response to B cell antigen receptor stimulation. Mol. Immunol. 36, 1005–1016. 9. Barbazuk, S.M. and Gold, M.R. (1999). Protein kinase C-delta is a target of B-cell antigen receptor signaling. Immunol. Lett. 69, 259–267. 10. Leitges, M., Mayr, M., Braun, U., Mayr, U., Li, C., Pfister, G., Ghaffari-Tabrizi, N., Baier, G., Hu, Y. and Xu, Q. (2001). Exacerbated vein graft arteriosclerosis in protein kinase Cdelta-null mice. J. Clin. Invest. 108, 1505–1512.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
Goodnow, C.C., Crosbie, J., Adelstein, S., Lavoie, T.B., Smith-Gill, S.J., Brink, R.A., Pritchard-Briscoe, H., Wotherspoon, J.S., Loblay, R.H., Raphael, K. et al. (1988). Altered immunoglobulin expression and functional silencing of self-reactive B lymphocytes in transgenic mice. Nature 334, 676–682. Hartley, S.B., Crosbie, J., Brink, R., Kantor, A.B., Basten, A. and Goodnow, C.C. (1991). Elimination from peripheral lymphoid tissues of self-reactive B lymphocytes recognizing membrane-bound antigens. Nature 353, 765–769. Cooke, M.P., Heath, A.W., Shokat, K.M., Zeng, Y., Finkelman, F.D., Linsley, P.S., Howard, M. and Goodnow, C.C. (1994). Immunoglobulin signal transduction guides the specificity of B cell-T cell interactions and is blocked in tolerant self-reactive B cells. J. Exp. Med. 179, 425–438. Cyster, J.G. and Goodnow, C.C. (1995). Antigen-induced exclusion from follicles and anergy are separate and complementary processes that influence peripheral B cell fate. Immunity 3, 691–701. Healy, J.I., Dolmetsch, R.E., Timmerman, L.A., Cyster, J.G., Thomas, M.L., Crabtree, G.R., Lewis, R.S. and Goodnow, C.C. (1997). Different nuclear signals are activated by the B cell receptor during positive versus negative signaling. Immunity 6, 419–428. Dolmetsch, R.E., Lewis, R.S., Goodnow, C.C. and Healy, J.I. (1997). Differential activation of transcription factors induced by Ca2+ response amplitude and duration. Nature 386, 855–858. Glynne, R., Akkaraju, S., Healy, J.I., Rayner, J., Goodnow, C.C. and Mack, D.H. (2000). How self-tolerance and the immunosuppressive drug FK506 prevent B-cell mitogenesis. Nature 403, 672–676. Li, W., Whaley, C.D., Mondino, A. and Mueller, D.L. (1996). Blocked signal transduction to the ERK and JNK protein kinases in anergic CD4+ T cells. Science 271, 1272–1276. Sundstedt, A., Sigvardsson, M., Leanderson, T., Hedlund, G., Kalland, T. and Dohlsten, M. (1996). In vivo anergized CD4+ T cells express perturbed AP-1 and NF-kappa B transcription factors. Proc Natl. Acad. Sci U.S.A. 93, 979–984. Villalba, M., Bi, K., Hu, J., Altman, Y., Bushway, P., Reits, E., Neefjes, J., Baier, G., Abraham, R.T. and Altman, A. (2002). Translocation of PKC[theta] in T cells is mediated by a nonconventional, PI3-K- and Vav-dependent pathway, but does not absolutely require phospholipase C. J. Cell Biol. 157, 253–263.