Highlights d T cell maturation is impaired in retrogenic mice harboring a Lck Y192 mutation d Y19... more Highlights d T cell maturation is impaired in retrogenic mice harboring a Lck Y192 mutation d Y192 modification disrupts the ability of CD45 to associate with Lck and activate it d By disrupting Lck activation, the Y192 phosphosite can inhibit TCR signaling d A Zap70-dependent negative feedback loop is proposed to regulate active Lck levels
Proceedings of the National Academy of Sciences of the United States of America, May 2, 2006
CD45, a protein tyrosine phosphatase that regulates Src family kinases, is important for regulati... more CD45, a protein tyrosine phosphatase that regulates Src family kinases, is important for regulating T cell and B cell receptor signaling; however, little is known about how CD45 regulates immunoreceptor tyrosine-based activation motif (ITAM)-dependent natural killer (NK) cell receptor signaling and the resulting effector functions. NK cells from CD45-deficient mice are relatively competent for ITAM receptor-induced cell-mediated cytotoxicity, yet completely deficient for cytokine secretion after stimulation with ligands to or antibodies against NK1.1, CD16, Ly49H, Ly49D, and NKG2D. This deficiency in cytokine͞chemokine production occurs at the level of mRNA expression. After receptor engagement, extracellular signal-regulated kinase and c-Jun N-terminal kinase activation was markedly perturbed, whereas p38 activation was not substantially affected. The pattern and amounts of basal tyrosine phosphorylation were altered in freshly isolated NK cells and were surprisingly and markedly increased in IL-2-expanded NK cells from CD45؊/؊ mice. These findings indicate that CD45dependent regulation of ITAM-dependent signaling pathways is essential for NK cell-mediated cytokine production but not cytolytic activity.
Stimulation of antigen receptors in T and B cells leads to the activation of the Src and Syk fami... more Stimulation of antigen receptors in T and B cells leads to the activation of the Src and Syk families of protein tyrosine kinases (PTK). These PTKs subsequently phosphorylate numerous intracellular substrates, including the 95-kD protooncogene product Vav. Vav is essential for both T and B cell development and T and B cell antigen receptor-mediated signal transduction. After receptor ligation, Vav associates with phosphorylated Syk and ZAP-70 PTKs, an interaction that depends upon its SH2 domain. Here we demonstrate that a point mutation of tyrosine 315 (Y315F) in ZAP-70, a putative Vav SH2 domain binding site, eliminated the Vav-ZAP-70 interaction. Moreover, the Y315 mutation impaired the function of ZAP-70 in antigen receptor signaling. Surprisingly, this mutation also resulted in marked reduction in the tyrosine phosphorylation of ZAP-70, Vav, SLP-76, and Shc. These data demonstrate that the Vav binding site in ZAP-70 plays a critical role in antigen receptor-mediated signal transduction. ZAP-70 and Vav in Antigen Receptor Signaling
Initiation of T cell antigen receptor (TCR) signaling is dependent on Lck, a Src family kinase. T... more Initiation of T cell antigen receptor (TCR) signaling is dependent on Lck, a Src family kinase. The Src-like adaptor protein (SLAP) contains Src homology (SH)3 and SH2 domains, which are highly homologous to those of Lck and other Src family members. Because of the structural similarity between Lck and SLAP, we studied its potential role in TCR signaling. Here, we show that SLAP is expressed in T cells, and that when expressed in Jurkat T cells it can specifically inhibit TCR signaling leading to nuclear factor of activated T cells (NFAT)-, activator protein 1 (AP-1)-, and interleukin 2-dependent transcription. The SH3 and SH2 domains of SLAP are required for maximal attenuation of TCR signaling. This inhibitory activity can be bypassed by the combination of phorbol myristate acetate (PMA) and ionomycin, suggesting that SLAP acts proximally in the TCR signaling pathway. SLAP colocalizes with endosomes in Jurkat and in HeLa cells, and is insoluble in mild detergents. In stimulated Jurkat cells, SLAP associates with a molecular signaling complex containing CD3 , ZAP-70, SH2 domain-containing leukocyte protein of 76 kD (SLP-76), Vav, and possibly linker for activation of T cells (LAT). These results suggest that SLAP is a negative regulator of TCR signaling.
terleukin-2 (IL-2), a key growth and differentiation factor * Department of Medicine for T cells ... more terleukin-2 (IL-2), a key growth and differentiation factor * Department of Medicine for T cells (Weiss and Littman, 1994). The transcription Department of Microbiology and Immunology factor AP-1 plays a critical role in the transactivation of Howard Hughes Medical Institute the IL-2 gene by binding to multiple regulatory elements University of California, San Francisco in the IL-2 promoter (Jain et al., 1995). In anergic T cells, San Francisco, California 94143 which fail to respond to antigen and produce IL-2, there † Max-Planck-Institute for Physiological is a specific block in AP-1-dependent transactivation and Clinical Research (Kang et al., 1992). Therefore, the regulation of the AP-1 W. G. Kerckhoff-Institute pathway may play an important role in coordinating im-Parkstrasse 1 mune responses. D-61231 Bad Nauheim AP-1 has been identified as a target of mitogen-Germany activated protein (MAP) kinases (Karin, 1995). Among ‡ Department of Pharmacology MAP kinases, extracellular signal-regulated kinases 1/2 University of Texas Southwestern Medical Center (ERK1/2) are activated following TCR engagement Dallas, Texas 75235 alone, whereas c-Jun N-terminal kinase (JNK) activation § Department of Molecular Genetics requires both TCR and CD28 stimulation (Su et al., 1994). Institute for Liver Research Recently, protein serine/threonine kinases related to Kansai Medical University germinal center kinase (GCK) have been implicated as Moriguchi 570-8506 upstream regulators of MAP kinase signaling pathways Japan (Kyriakis, 1999). The GCK family kinases (GCKs) include GCK (Katz et al., 1994), hematopoietic progenitor kinase-1 (HPK1) (Hu et al., 1996; Kiefer et al., 1996), Nck-Summary interacting kinase (NIK; also referred to as HGK) (Su et al., 1997; Yao et al., 1999), GCK-like kinase (GLK) (Diener The serine/threonine kinase HPK1 is a member of the et al., 1997), kinase homologous to SPS1/STE20 (KHS; germinal center kinase (GCK) family that has been also referred to as GCKR) (Shi and Kehrl, 1997; Tung implicated in the regulation of MAP kinase pathways. and Blenis, 1997), and Drosophila Misshapen (Su et al., Here, we demonstrate the involvement of HPK1 in anti-1998). All GCKs contain N-terminal STE20-like kinase gen receptor signaling. Engagement of the TCR or the domains and long C-terminal regulatory domains with BCR resulted in a marked induction of HPK1 catalytic proline-rich motifs. STE20 is thought to function downactivity. Subsequent analysis revealed that Src and stream of a G protein and upstream of the MAP kinase Syk/ZAP-70 tyrosine kinases and the adaptor proteins cascade in yeast (Herskowitz, 1995). Hence, STE20-like LAT, SLP-76, BLNK, Grb2, and Grap are involved in proteins may link receptor activation to the regulation HPK1 activation. Overexpression of HPK1 inhibited of MAP kinase modules. GCKs have been shown to be TCR activation of AP-1 and ERK2, whereas the kinasepotent activators of the JNK pathway, although they inactive mutant of HPK1 potentiated these responses. may well have additional biological functions. Neither form of HPK1 affected PMA or v-Ras activation Among GCKs, HPK1 is of particular interest because of AP-1 and ERK2. Thus, HPK1 is a negative regulator it is exclusively expressed in hematopoietic cells. Furof the TCR-induced AP-1 response pathway. thermore, HPK1 was shown to interact with Grb2 (growth factor receptor binding protein 2) in vitro and in a transfection model (Anafi et al., 1997). Grb2 is a
. At least one function of CD45 is to dephosphorylate the C-terminal site of negative regulatory ... more . At least one function of CD45 is to dephosphorylate the C-terminal site of negative regulatory tyrosine phosphorylation within src-family kinases (
Self-and non-self ligand discrimination is a core principle underlying T cell-mediated immunity. ... more Self-and non-self ligand discrimination is a core principle underlying T cell-mediated immunity. Mature ab T cells can respond to a foreign peptide ligand presented by major histocompatibility complex molecules (pMHCs) on antigen presenting cells, on a background of continuously sensed self-pMHCs. How ab T cells can properly balance high sensitivity and high specificity to foreign pMHCs, while surrounded by a sea of selfpeptide ligands is not well understood. Such discrimination cannot be explained solely by the affinity parameters of T cell antigen receptor (TCR) and pMHC interaction. In this review, we will discuss how T cell ligand discrimination may be molecularly defined by events downstream of the TCR-pMHC interaction. We will discuss new evidence in support of the kinetic proofreading model of TCR ligand discrimination, and in particular how the kinetics of specific phosphorylation sites within the adaptor protein linker for activation of T cells (LAT) determine the outcome of TCR signaling. In addition, we will discuss emerging data regarding how some kinases, including ZAP-70 and LCK, may possess scaffolding functions to more efficiently direct their kinase activities.
rc-like adaptor protein (SLAP) down-regulates expression of the T cell receptor (TCR)-CD3 complex... more rc-like adaptor protein (SLAP) down-regulates expression of the T cell receptor (TCR)-CD3 complex during a specific stage of thymocyte development when the TCR repertoire is selected. Consequently, SLAP Ϫ / Ϫ thymocytes display alterations in thymocyte development. Here, we have studied the mechanism of SLAP function. We demonstrate that SLAP-deficient thymocytes have increased TCR chain expression as a result of a defect in TCR degradation. Failure to degrade S TCR leads to an increased pool of fully assembled TCR-CD3 complexes that are capable of recycling back to the cell surface. We also provide evidence that SLAP functions in a pathway that requires the phosphorylated TCR chain and the Src family kinase Lck, but not ZAP-70 ( -associated protein of 70 kD). These studies reveal a unique mechanism by which SLAP contributes to the regulation of TCR expression during a distinct stage of thymocyte development.
The Syk/ZAP-70 family of protein tyrosine kinases is indispensable for normal lymphoid developmen... more The Syk/ZAP-70 family of protein tyrosine kinases is indispensable for normal lymphoid development. Syk is necessary for the development of B cells and epithelial ␥␦ T cells, whereas ZAP-70 is essential for the normal development of T cells and TCR signaling. In this study, we show that although development of the ␣ lineage was arrested in the thymus, CD3-positive T cells, primarily of the ␥␦ lineage, were present in the lymph nodes of mice lacking ZAP-70. Moreover, in the absence of ZAP-70, dendritic epidermal T cells were fewer in number and of abnormal morphology, and intestinal intraepithelial lymphocytes, normally containing a large proportion of ␥␦ T cells, were markedly reduced. These data suggest that ␥␦ T cells show a variable dependence upon ZAP-70 for their development. Biochemical analyses of thymocytes revealed a lack of basal -chain tyrosine phosphorylation. However, several other substrates were inducibly tyrosine phosphorylated following TCR stimulation. Thus, TCR-mediated signaling in ZAP-70-deficient thymocytes is only partially impaired. These studies suggest that Syk compensates only partially for the loss of ZAP-70, and that there is an absolute requirement of ZAP-70 for ␣ T cells and epithelial ␥␦ T cells, but not for some ␥␦ T cells in peripheral lymphoid tissues.
Williams et al., 1997). Avidity depends directly University of California at San Francisco on bot... more Williams et al., 1997). Avidity depends directly University of California at San Francisco on both the affinity of TCR for the MHC-self-peptide San Francisco, California 94143 complexes and on the number of such interactions. Thus, according to this model, the level of TCR surface expression is of critical importance for the generation Summary of thymocyte repertoire. Positive selection occurs within the population of DP In this report, we show that the Src-like adaptor prothymocytes that expresses a low level of TCR relative tein (SLAP) plays an important role in thymocyte develto more mature thymocytes and T cells (Shortman et opment. SLAP expression is developmentally regual., 1991). This low level of TCR expression has been lated; it is low in CD4 Ϫ CD8 Ϫ thymocytes, it peaks in proposed to be regulated by posttranscriptional mechathe CD4 ؉ CD8 ؉ subset, and it decreases to low levels nisms (Bonifacino et al., 1990; Kearse et al., 1994; Main more mature cells. Disruption of the SLAP gene guire et al., 1990). Several reports have shown that cerleads to a marked upregulation of TCR and CD5 extain experimental conditions, as well as some genetic pression at the CD4 ؉ CD8 ؉ stage. The absence of SLAP deficiencies, can interfere with this regulation. For examwas also developmentally significant because it enple, injection of anti-CD4 mAb (Bonifacino et al., 1990; hanced positive selection in mice expressing the McCarthy et al., 1988) prolonged incubation of thymo-DO11.10 transgenic T cell receptor. Moreover, SLAP cytes in single-cell suspension (Marrack et al., 1988), deletion at least partially rescued the development of and disruption of MHC class II (Gosgrove et al., 1991) ZAP-70-deficient thymocytes. These results demonor Lck genes (Molina et al., 1992) all resulted in upregulastrate that SLAP participates in a novel mechanism tion of the TCR on DP thymocytes. Lck, a member of of TCR downregulation at the CD4 ؉ CD8 ؉ stage and the Src family of protein tyrosine kinases, has been reregulates positive selection. ported to participate in induction of TCR downregulation (D'Oro et al., 1997). On the basis of these observations,
394. However, it has not been clearly demonstrated that Stanford University School of Medicine ph... more 394. However, it has not been clearly demonstrated that Stanford University School of Medicine physiological antigens mediate lck autophosphorylation Stanford, California 94305 (Luo and Sefton, 1990). Nonetheless, lck, CD4, and CD3 3 Department of Medicine and the can form a trimolecular complex (Rudd et al., 1991), Howard Hughes Medical Institute bringing lck into proximity with CD3 ITAMs to initiate
Highlights d T cell maturation is impaired in retrogenic mice harboring a Lck Y192 mutation d Y19... more Highlights d T cell maturation is impaired in retrogenic mice harboring a Lck Y192 mutation d Y192 modification disrupts the ability of CD45 to associate with Lck and activate it d By disrupting Lck activation, the Y192 phosphosite can inhibit TCR signaling d A Zap70-dependent negative feedback loop is proposed to regulate active Lck levels
Proceedings of the National Academy of Sciences of the United States of America, May 2, 2006
CD45, a protein tyrosine phosphatase that regulates Src family kinases, is important for regulati... more CD45, a protein tyrosine phosphatase that regulates Src family kinases, is important for regulating T cell and B cell receptor signaling; however, little is known about how CD45 regulates immunoreceptor tyrosine-based activation motif (ITAM)-dependent natural killer (NK) cell receptor signaling and the resulting effector functions. NK cells from CD45-deficient mice are relatively competent for ITAM receptor-induced cell-mediated cytotoxicity, yet completely deficient for cytokine secretion after stimulation with ligands to or antibodies against NK1.1, CD16, Ly49H, Ly49D, and NKG2D. This deficiency in cytokine͞chemokine production occurs at the level of mRNA expression. After receptor engagement, extracellular signal-regulated kinase and c-Jun N-terminal kinase activation was markedly perturbed, whereas p38 activation was not substantially affected. The pattern and amounts of basal tyrosine phosphorylation were altered in freshly isolated NK cells and were surprisingly and markedly increased in IL-2-expanded NK cells from CD45؊/؊ mice. These findings indicate that CD45dependent regulation of ITAM-dependent signaling pathways is essential for NK cell-mediated cytokine production but not cytolytic activity.
Stimulation of antigen receptors in T and B cells leads to the activation of the Src and Syk fami... more Stimulation of antigen receptors in T and B cells leads to the activation of the Src and Syk families of protein tyrosine kinases (PTK). These PTKs subsequently phosphorylate numerous intracellular substrates, including the 95-kD protooncogene product Vav. Vav is essential for both T and B cell development and T and B cell antigen receptor-mediated signal transduction. After receptor ligation, Vav associates with phosphorylated Syk and ZAP-70 PTKs, an interaction that depends upon its SH2 domain. Here we demonstrate that a point mutation of tyrosine 315 (Y315F) in ZAP-70, a putative Vav SH2 domain binding site, eliminated the Vav-ZAP-70 interaction. Moreover, the Y315 mutation impaired the function of ZAP-70 in antigen receptor signaling. Surprisingly, this mutation also resulted in marked reduction in the tyrosine phosphorylation of ZAP-70, Vav, SLP-76, and Shc. These data demonstrate that the Vav binding site in ZAP-70 plays a critical role in antigen receptor-mediated signal transduction. ZAP-70 and Vav in Antigen Receptor Signaling
Initiation of T cell antigen receptor (TCR) signaling is dependent on Lck, a Src family kinase. T... more Initiation of T cell antigen receptor (TCR) signaling is dependent on Lck, a Src family kinase. The Src-like adaptor protein (SLAP) contains Src homology (SH)3 and SH2 domains, which are highly homologous to those of Lck and other Src family members. Because of the structural similarity between Lck and SLAP, we studied its potential role in TCR signaling. Here, we show that SLAP is expressed in T cells, and that when expressed in Jurkat T cells it can specifically inhibit TCR signaling leading to nuclear factor of activated T cells (NFAT)-, activator protein 1 (AP-1)-, and interleukin 2-dependent transcription. The SH3 and SH2 domains of SLAP are required for maximal attenuation of TCR signaling. This inhibitory activity can be bypassed by the combination of phorbol myristate acetate (PMA) and ionomycin, suggesting that SLAP acts proximally in the TCR signaling pathway. SLAP colocalizes with endosomes in Jurkat and in HeLa cells, and is insoluble in mild detergents. In stimulated Jurkat cells, SLAP associates with a molecular signaling complex containing CD3 , ZAP-70, SH2 domain-containing leukocyte protein of 76 kD (SLP-76), Vav, and possibly linker for activation of T cells (LAT). These results suggest that SLAP is a negative regulator of TCR signaling.
terleukin-2 (IL-2), a key growth and differentiation factor * Department of Medicine for T cells ... more terleukin-2 (IL-2), a key growth and differentiation factor * Department of Medicine for T cells (Weiss and Littman, 1994). The transcription Department of Microbiology and Immunology factor AP-1 plays a critical role in the transactivation of Howard Hughes Medical Institute the IL-2 gene by binding to multiple regulatory elements University of California, San Francisco in the IL-2 promoter (Jain et al., 1995). In anergic T cells, San Francisco, California 94143 which fail to respond to antigen and produce IL-2, there † Max-Planck-Institute for Physiological is a specific block in AP-1-dependent transactivation and Clinical Research (Kang et al., 1992). Therefore, the regulation of the AP-1 W. G. Kerckhoff-Institute pathway may play an important role in coordinating im-Parkstrasse 1 mune responses. D-61231 Bad Nauheim AP-1 has been identified as a target of mitogen-Germany activated protein (MAP) kinases (Karin, 1995). Among ‡ Department of Pharmacology MAP kinases, extracellular signal-regulated kinases 1/2 University of Texas Southwestern Medical Center (ERK1/2) are activated following TCR engagement Dallas, Texas 75235 alone, whereas c-Jun N-terminal kinase (JNK) activation § Department of Molecular Genetics requires both TCR and CD28 stimulation (Su et al., 1994). Institute for Liver Research Recently, protein serine/threonine kinases related to Kansai Medical University germinal center kinase (GCK) have been implicated as Moriguchi 570-8506 upstream regulators of MAP kinase signaling pathways Japan (Kyriakis, 1999). The GCK family kinases (GCKs) include GCK (Katz et al., 1994), hematopoietic progenitor kinase-1 (HPK1) (Hu et al., 1996; Kiefer et al., 1996), Nck-Summary interacting kinase (NIK; also referred to as HGK) (Su et al., 1997; Yao et al., 1999), GCK-like kinase (GLK) (Diener The serine/threonine kinase HPK1 is a member of the et al., 1997), kinase homologous to SPS1/STE20 (KHS; germinal center kinase (GCK) family that has been also referred to as GCKR) (Shi and Kehrl, 1997; Tung implicated in the regulation of MAP kinase pathways. and Blenis, 1997), and Drosophila Misshapen (Su et al., Here, we demonstrate the involvement of HPK1 in anti-1998). All GCKs contain N-terminal STE20-like kinase gen receptor signaling. Engagement of the TCR or the domains and long C-terminal regulatory domains with BCR resulted in a marked induction of HPK1 catalytic proline-rich motifs. STE20 is thought to function downactivity. Subsequent analysis revealed that Src and stream of a G protein and upstream of the MAP kinase Syk/ZAP-70 tyrosine kinases and the adaptor proteins cascade in yeast (Herskowitz, 1995). Hence, STE20-like LAT, SLP-76, BLNK, Grb2, and Grap are involved in proteins may link receptor activation to the regulation HPK1 activation. Overexpression of HPK1 inhibited of MAP kinase modules. GCKs have been shown to be TCR activation of AP-1 and ERK2, whereas the kinasepotent activators of the JNK pathway, although they inactive mutant of HPK1 potentiated these responses. may well have additional biological functions. Neither form of HPK1 affected PMA or v-Ras activation Among GCKs, HPK1 is of particular interest because of AP-1 and ERK2. Thus, HPK1 is a negative regulator it is exclusively expressed in hematopoietic cells. Furof the TCR-induced AP-1 response pathway. thermore, HPK1 was shown to interact with Grb2 (growth factor receptor binding protein 2) in vitro and in a transfection model (Anafi et al., 1997). Grb2 is a
. At least one function of CD45 is to dephosphorylate the C-terminal site of negative regulatory ... more . At least one function of CD45 is to dephosphorylate the C-terminal site of negative regulatory tyrosine phosphorylation within src-family kinases (
Self-and non-self ligand discrimination is a core principle underlying T cell-mediated immunity. ... more Self-and non-self ligand discrimination is a core principle underlying T cell-mediated immunity. Mature ab T cells can respond to a foreign peptide ligand presented by major histocompatibility complex molecules (pMHCs) on antigen presenting cells, on a background of continuously sensed self-pMHCs. How ab T cells can properly balance high sensitivity and high specificity to foreign pMHCs, while surrounded by a sea of selfpeptide ligands is not well understood. Such discrimination cannot be explained solely by the affinity parameters of T cell antigen receptor (TCR) and pMHC interaction. In this review, we will discuss how T cell ligand discrimination may be molecularly defined by events downstream of the TCR-pMHC interaction. We will discuss new evidence in support of the kinetic proofreading model of TCR ligand discrimination, and in particular how the kinetics of specific phosphorylation sites within the adaptor protein linker for activation of T cells (LAT) determine the outcome of TCR signaling. In addition, we will discuss emerging data regarding how some kinases, including ZAP-70 and LCK, may possess scaffolding functions to more efficiently direct their kinase activities.
rc-like adaptor protein (SLAP) down-regulates expression of the T cell receptor (TCR)-CD3 complex... more rc-like adaptor protein (SLAP) down-regulates expression of the T cell receptor (TCR)-CD3 complex during a specific stage of thymocyte development when the TCR repertoire is selected. Consequently, SLAP Ϫ / Ϫ thymocytes display alterations in thymocyte development. Here, we have studied the mechanism of SLAP function. We demonstrate that SLAP-deficient thymocytes have increased TCR chain expression as a result of a defect in TCR degradation. Failure to degrade S TCR leads to an increased pool of fully assembled TCR-CD3 complexes that are capable of recycling back to the cell surface. We also provide evidence that SLAP functions in a pathway that requires the phosphorylated TCR chain and the Src family kinase Lck, but not ZAP-70 ( -associated protein of 70 kD). These studies reveal a unique mechanism by which SLAP contributes to the regulation of TCR expression during a distinct stage of thymocyte development.
The Syk/ZAP-70 family of protein tyrosine kinases is indispensable for normal lymphoid developmen... more The Syk/ZAP-70 family of protein tyrosine kinases is indispensable for normal lymphoid development. Syk is necessary for the development of B cells and epithelial ␥␦ T cells, whereas ZAP-70 is essential for the normal development of T cells and TCR signaling. In this study, we show that although development of the ␣ lineage was arrested in the thymus, CD3-positive T cells, primarily of the ␥␦ lineage, were present in the lymph nodes of mice lacking ZAP-70. Moreover, in the absence of ZAP-70, dendritic epidermal T cells were fewer in number and of abnormal morphology, and intestinal intraepithelial lymphocytes, normally containing a large proportion of ␥␦ T cells, were markedly reduced. These data suggest that ␥␦ T cells show a variable dependence upon ZAP-70 for their development. Biochemical analyses of thymocytes revealed a lack of basal -chain tyrosine phosphorylation. However, several other substrates were inducibly tyrosine phosphorylated following TCR stimulation. Thus, TCR-mediated signaling in ZAP-70-deficient thymocytes is only partially impaired. These studies suggest that Syk compensates only partially for the loss of ZAP-70, and that there is an absolute requirement of ZAP-70 for ␣ T cells and epithelial ␥␦ T cells, but not for some ␥␦ T cells in peripheral lymphoid tissues.
Williams et al., 1997). Avidity depends directly University of California at San Francisco on bot... more Williams et al., 1997). Avidity depends directly University of California at San Francisco on both the affinity of TCR for the MHC-self-peptide San Francisco, California 94143 complexes and on the number of such interactions. Thus, according to this model, the level of TCR surface expression is of critical importance for the generation Summary of thymocyte repertoire. Positive selection occurs within the population of DP In this report, we show that the Src-like adaptor prothymocytes that expresses a low level of TCR relative tein (SLAP) plays an important role in thymocyte develto more mature thymocytes and T cells (Shortman et opment. SLAP expression is developmentally regual., 1991). This low level of TCR expression has been lated; it is low in CD4 Ϫ CD8 Ϫ thymocytes, it peaks in proposed to be regulated by posttranscriptional mechathe CD4 ؉ CD8 ؉ subset, and it decreases to low levels nisms (Bonifacino et al., 1990; Kearse et al., 1994; Main more mature cells. Disruption of the SLAP gene guire et al., 1990). Several reports have shown that cerleads to a marked upregulation of TCR and CD5 extain experimental conditions, as well as some genetic pression at the CD4 ؉ CD8 ؉ stage. The absence of SLAP deficiencies, can interfere with this regulation. For examwas also developmentally significant because it enple, injection of anti-CD4 mAb (Bonifacino et al., 1990; hanced positive selection in mice expressing the McCarthy et al., 1988) prolonged incubation of thymo-DO11.10 transgenic T cell receptor. Moreover, SLAP cytes in single-cell suspension (Marrack et al., 1988), deletion at least partially rescued the development of and disruption of MHC class II (Gosgrove et al., 1991) ZAP-70-deficient thymocytes. These results demonor Lck genes (Molina et al., 1992) all resulted in upregulastrate that SLAP participates in a novel mechanism tion of the TCR on DP thymocytes. Lck, a member of of TCR downregulation at the CD4 ؉ CD8 ؉ stage and the Src family of protein tyrosine kinases, has been reregulates positive selection. ported to participate in induction of TCR downregulation (D'Oro et al., 1997). On the basis of these observations,
394. However, it has not been clearly demonstrated that Stanford University School of Medicine ph... more 394. However, it has not been clearly demonstrated that Stanford University School of Medicine physiological antigens mediate lck autophosphorylation Stanford, California 94305 (Luo and Sefton, 1990). Nonetheless, lck, CD4, and CD3 3 Department of Medicine and the can form a trimolecular complex (Rudd et al., 1991), Howard Hughes Medical Institute bringing lck into proximity with CD3 ITAMs to initiate
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