NEWS & VIEWS
doi:10.1038/nature13346
IMMUNO LO GY
When lymphocytes run out of steam The finding that absence of the enzyme CTPS1 underlies a form of human immunodeficiency highlights the role of metabolism in immune responses and suggests avenues for treating diseases such as leukaemia. A N D R É V E I L L E T T E & D O M I N I Q U E D AV I D S O N
L
ymphocytes are immune cells that play a crucial part in protection against microorganisms such as viruses and bacteria. The two main types of lymphocyte are T cells and B cells, which, in the presence of antigen molecules derived from microbes, undergo a series of molecular changes that induce a state of activation. This response is driven by antigen receptors on the cells’ surface, and leads to rapid cell proliferation and augmented immune protection (Fig. 1a). Proliferation under these conditions depends on metabolic adaptation, which allows immune cells to synthesize DNA and RNA molecules and the proteins needed for cell division1. In a paper published on Nature’s website today, Martin et al.2 link this metabolic requirement of proliferating lymphocytes to a newly described immune-deficiency disease. The authors describe children from several unrelated families who developed a severe immunodeficiency at birth or at a very young age. In some cases, two members of the same family had the deficiency. Typically, the patients exhibited severe and persistent infections with viruses such as Epstein–Barr, the cause of infectious mononucleosis, and varicella zoster, which causes chickenpox and herpes zoster (shingles). Severe infections from bacteria such as pneumococcus, a cause of pneumonia, were also noted. Several children underwent transplantation with haemato poietic stem cells (which can differentiate into all types of blood cell) to control the infections. Immunological investigations led the authors to propose that the patients might be suffering from an inherited immunodeficiency that compromises lymphocyte function. Sequencing of DNA from the affected children revealed that they all carried a mutation in CTPS1, the gene encoding the enzyme cytidine nucleotide triphosphate synthase 1 (CTPS1), that resulted in an absence of this enzyme in the patients’ lymphocytes. CTPS1 is one of two forms of CTP synthase enzymes produced in mammalian cells (the other
being CTPS2); both enzymes enable the production of cytidine nucleotide triphosphate (CTP), a nucleotide required for cellular DNA and RNA synthesis3. The authors show that normal lymphocytes express both CTPS1 and CTPS2: CTPS1 is present at low levels before lymphocyte activation and becomes markedly expressed in activated lymphocytes, whereas CTPS2 is already expressed at high levels in non-activated lymphocytes. Analyses of T and B cells from the CTPS1-deficient patients revealed that differentiation of the cells in the absence of activation by foreign microorganisms was largely unaffected by the mutation, and the immediate molecular changes triggered by antigenreceptor stimulation were mostly unaltered.
However, the cells’ capacity to synthesize DNA and proliferate following stimulation of the antigen receptor was severely compromised. Intracellular levels of CTP were also very low. These defects were reproduced when CTPS1 expression was artificially reduced in normal lymphocytes, or when 3-deazauridine, a pharmacological inhibitor of CTPS enzymes, was used to suppress their activity. Conversely, the defects were corrected when CTPS1 was introduced into cells of CTPS1-deficient patients by retrovirus-mediated gene transfer, or when CTP was added to the cells’ culture medium. These findings show that CTPS1 and its product, CTP, are required for lymphocytes to proliferate intensely during antigen-induced activation, further highlighting the importance of rapid metabolic adaptation for proper immunity. In the absence of CTPS1, antigenstimulated lymphocytes do not produce sufficient quantities of CTP, causing defects in DNA synthesis and cell proliferation (Fig. 1b). These effects explain in large part why CTPS1deficient children develop life-threatening viral and bacterial infections. In addition to identifying the genetic cause of a new immunodeficiency, Martin and colleagues’ results raise several prospects for future investigation. They indicate that even though CTPS2 is expressed in lymphocytes,
a Normal Antigen receptor
Antigen
Proliferation
Lymphocyte
Efficient immune response
↑ CTPS1 ↑ CTP
b CTPS1 deficiency
Defective proliferation
Defective immune response
No CTPS1 Unchanged CTP
Figure 1 | Regulation of lymphocyte activation by CTPS1. a, In normal lymphocytes (T and B cells), stimulation of the cells’ antigen receptor triggers a series of molecular changes that induce the cells to proliferate, fuelling the immune response. Martin et al.2 show that these events include an increase in levels of the enzyme CTPS1 and its product, CTP, which supports the increased DNA synthesis required for cell proliferation. b, In lymphocytes from CTPS1-deficient humans, stimulation by antigen results in some, but not all, of the molecular changes associated with lymphocyte activation. In particular, there is no increase in CTP levels in the activated cells, resulting in compromised DNA synthesis, reduced lymphocyte proliferation and an impaired immune response. | NAT U R E | 1
© 2014 Macmillan Publishers Limited. All rights reserved
RESEARCH NEWS & VIEWS it cannot replace CTPS1 when the latter is deficient. One possible explanation for this is that CTPS1 is much more active than CTPS2 in lymphocytes, possibly owing to differences in intrinsic activity or abundance, or to modifications such as phosphorylation or co-factor binding that could influence the enzymes’ activity. For instance, in mammalian cells, CTPS1, but not CTPS2, can be regulated by phosphorylation on certain amino-acid residues4,5. Alternatively, because it has been reported that CTPS1 and CTPS2 can be localized in both the cytoplasm and the nucleus6, it is possible that CTPS1 accumulates in a locale that is especially important for generating the CTP needed for intense DNA synthesis. Moreover, these enzymes form tetramers and can exist as large filamentous structures7,8; whether differences in these arrangements exist between CTPS1 and CTPS2 remains to be clarified. Most studies of CTPS enzymes have focused on their capacity to promote DNA and RNA synthesis. However, investigations of the two yeast homologues of CTPS enzymes, URA7 and URA8, indicated that these enzymes are also needed for the synthesis of phospholipids9. It is possible that the use of CTP during the synthesis of membrane
phospholipids is needed for the interaction of signalling molecules with the inner leaflet of the cell membrane. If such an activity exists for mammalian CTPS enzymes, defects in membrane-driven signalling could contribute to the lymphocyte dysfunctions observed in CTPS1-deficient patients. This activity could also explain Martin and colleagues’ observation that CTPS1-deficient human T cells have reduced activation of the enzyme Erk kinase and reduced expression of the signaltransmission proteins CD25 and CD69 following antigen-receptor stimulation. These molecular events occur at early stages in cell activation, before the initiation of DNA syn thesis, and are crucial for productive lymphocyte activation. The data also raise the provocative possi bility that pharmacological inhibitors of CTPS1 could be useful tools for treating human diseases associated with excessive or uncontrolled lymphocyte proliferation, such as transplant rejection, graft-versus-host disease and some forms of cancers such as leukaemia and lymphoma. In keeping with the latter idea, the CTPS inhibitor 3-deazauridine has already been shown to display some therapeutic efficacy against leukaemic cells in vitro, although it probably also inhibited
2 | NAT U R E |
© 2014 Macmillan Publishers Limited. All rights reserved
targets other than CTPS in these cells10. The development of more-specific inhibitors of CTPS1 will aid the further investigation of this possible therapeutic avenue. ■ André Veillette and Dominique Davidson are in the Laboratory of Molecular Oncology, Institut de recherches cliniques de Montréal, Montreal, Quebec H2W1R7, Canada. A.V. is also in the Department of Medicine, University of Montreal and the Department of Medicine, McGill University, Montreal. e-mail: [email protected] 1. Pearce, E. L., Poffenberger, M. C., Chang, C.-H. & Jones, R. G. Science 342, 1242454 (2013). 2. Martin, E. et al. Nature http://dx.doi.org/10.1038/ nature13386 (2014). 3. Evans, D. R. & Guy, H. I. J. Biol. Chem. 279, 33035–33038 (2004). 4. Higgins, M. J., Graves, P. R. & Graves, L. M. J. Biol. Chem. 282, 29493–29503 (2007). 5. Kassel, K. M., Au, D. R., Higgins, M. J., Hines, M. & Graves, L. M. J. Biol. Chem. 285, 33727–33736 (2010). 6. Gou, K.-M., Chang, C.-C., Shen, Q.-J., Sung, L.-Y. & Liu, J.-L. Exp. Cell Res. 323, 242–253 (2014). 7. Levitzki, A. & Koshland, D. E. Jr Biochemistry 11, 247–253 (1972). 8. Liu, J.-L. BioEssays 33, 159–164 (2011). 9. Chang, Y.-F. & Carman, G. M. Prog. Lipid Res. 47, 333–339 (2008). 10. McPartland, R. P., Wang, M. C., Bloch, A. & Weinfeld, H. Cancer Res. 34, 3107–3111 (1974).
doi:10.1038/nature13346
IMMUNO LO GY
When lymphocytes run out of steam The finding that absence of the enzyme CTPS1 underlies a form of human immunodeficiency highlights the role of metabolism in immune responses and suggests avenues for treating diseases such as leukaemia. A N D R É V E I L L E T T E & D O M I N I Q U E D AV I D S O N
L
ymphocytes are immune cells that play a crucial part in protection against microorganisms such as viruses and bacteria. The two main types of lymphocyte are T cells and B cells, which, in the presence of antigen molecules derived from microbes, undergo a series of molecular changes that induce a state of activation. This response is driven by antigen receptors on the cells’ surface, and leads to rapid cell proliferation and augmented immune protection (Fig. 1a). Proliferation under these conditions depends on metabolic adaptation, which allows immune cells to synthesize DNA and RNA molecules and the proteins needed for cell division1. In a paper published on Nature’s website today, Martin et al.2 link this metabolic requirement of proliferating lymphocytes to a newly described immune-deficiency disease. The authors describe children from several unrelated families who developed a severe immunodeficiency at birth or at a very young age. In some cases, two members of the same family had the deficiency. Typically, the patients exhibited severe and persistent infections with viruses such as Epstein–Barr, the cause of infectious mononucleosis, and varicella zoster, which causes chickenpox and herpes zoster (shingles). Severe infections from bacteria such as pneumococcus, a cause of pneumonia, were also noted. Several children underwent transplantation with haemato poietic stem cells (which can differentiate into all types of blood cell) to control the infections. Immunological investigations led the authors to propose that the patients might be suffering from an inherited immunodeficiency that compromises lymphocyte function. Sequencing of DNA from the affected children revealed that they all carried a mutation in CTPS1, the gene encoding the enzyme cytidine nucleotide triphosphate synthase 1 (CTPS1), that resulted in an absence of this enzyme in the patients’ lymphocytes. CTPS1 is one of two forms of CTP synthase enzymes produced in mammalian cells (the other
being CTPS2); both enzymes enable the production of cytidine nucleotide triphosphate (CTP), a nucleotide required for cellular DNA and RNA synthesis3. The authors show that normal lymphocytes express both CTPS1 and CTPS2: CTPS1 is present at low levels before lymphocyte activation and becomes markedly expressed in activated lymphocytes, whereas CTPS2 is already expressed at high levels in non-activated lymphocytes. Analyses of T and B cells from the CTPS1-deficient patients revealed that differentiation of the cells in the absence of activation by foreign microorganisms was largely unaffected by the mutation, and the immediate molecular changes triggered by antigenreceptor stimulation were mostly unaltered.
However, the cells’ capacity to synthesize DNA and proliferate following stimulation of the antigen receptor was severely compromised. Intracellular levels of CTP were also very low. These defects were reproduced when CTPS1 expression was artificially reduced in normal lymphocytes, or when 3-deazauridine, a pharmacological inhibitor of CTPS enzymes, was used to suppress their activity. Conversely, the defects were corrected when CTPS1 was introduced into cells of CTPS1-deficient patients by retrovirus-mediated gene transfer, or when CTP was added to the cells’ culture medium. These findings show that CTPS1 and its product, CTP, are required for lymphocytes to proliferate intensely during antigen-induced activation, further highlighting the importance of rapid metabolic adaptation for proper immunity. In the absence of CTPS1, antigenstimulated lymphocytes do not produce sufficient quantities of CTP, causing defects in DNA synthesis and cell proliferation (Fig. 1b). These effects explain in large part why CTPS1deficient children develop life-threatening viral and bacterial infections. In addition to identifying the genetic cause of a new immunodeficiency, Martin and colleagues’ results raise several prospects for future investigation. They indicate that even though CTPS2 is expressed in lymphocytes,
a Normal Antigen receptor
Antigen
Proliferation
Lymphocyte
Efficient immune response
↑ CTPS1 ↑ CTP
b CTPS1 deficiency
Defective proliferation
Defective immune response
No CTPS1 Unchanged CTP
Figure 1 | Regulation of lymphocyte activation by CTPS1. a, In normal lymphocytes (T and B cells), stimulation of the cells’ antigen receptor triggers a series of molecular changes that induce the cells to proliferate, fuelling the immune response. Martin et al.2 show that these events include an increase in levels of the enzyme CTPS1 and its product, CTP, which supports the increased DNA synthesis required for cell proliferation. b, In lymphocytes from CTPS1-deficient humans, stimulation by antigen results in some, but not all, of the molecular changes associated with lymphocyte activation. In particular, there is no increase in CTP levels in the activated cells, resulting in compromised DNA synthesis, reduced lymphocyte proliferation and an impaired immune response. | NAT U R E | 1
© 2014 Macmillan Publishers Limited. All rights reserved
RESEARCH NEWS & VIEWS it cannot replace CTPS1 when the latter is deficient. One possible explanation for this is that CTPS1 is much more active than CTPS2 in lymphocytes, possibly owing to differences in intrinsic activity or abundance, or to modifications such as phosphorylation or co-factor binding that could influence the enzymes’ activity. For instance, in mammalian cells, CTPS1, but not CTPS2, can be regulated by phosphorylation on certain amino-acid residues4,5. Alternatively, because it has been reported that CTPS1 and CTPS2 can be localized in both the cytoplasm and the nucleus6, it is possible that CTPS1 accumulates in a locale that is especially important for generating the CTP needed for intense DNA synthesis. Moreover, these enzymes form tetramers and can exist as large filamentous structures7,8; whether differences in these arrangements exist between CTPS1 and CTPS2 remains to be clarified. Most studies of CTPS enzymes have focused on their capacity to promote DNA and RNA synthesis. However, investigations of the two yeast homologues of CTPS enzymes, URA7 and URA8, indicated that these enzymes are also needed for the synthesis of phospholipids9. It is possible that the use of CTP during the synthesis of membrane
phospholipids is needed for the interaction of signalling molecules with the inner leaflet of the cell membrane. If such an activity exists for mammalian CTPS enzymes, defects in membrane-driven signalling could contribute to the lymphocyte dysfunctions observed in CTPS1-deficient patients. This activity could also explain Martin and colleagues’ observation that CTPS1-deficient human T cells have reduced activation of the enzyme Erk kinase and reduced expression of the signaltransmission proteins CD25 and CD69 following antigen-receptor stimulation. These molecular events occur at early stages in cell activation, before the initiation of DNA syn thesis, and are crucial for productive lymphocyte activation. The data also raise the provocative possi bility that pharmacological inhibitors of CTPS1 could be useful tools for treating human diseases associated with excessive or uncontrolled lymphocyte proliferation, such as transplant rejection, graft-versus-host disease and some forms of cancers such as leukaemia and lymphoma. In keeping with the latter idea, the CTPS inhibitor 3-deazauridine has already been shown to display some therapeutic efficacy against leukaemic cells in vitro, although it probably also inhibited
2 | NAT U R E |
© 2014 Macmillan Publishers Limited. All rights reserved
targets other than CTPS in these cells10. The development of more-specific inhibitors of CTPS1 will aid the further investigation of this possible therapeutic avenue. ■ André Veillette and Dominique Davidson are in the Laboratory of Molecular Oncology, Institut de recherches cliniques de Montréal, Montreal, Quebec H2W1R7, Canada. A.V. is also in the Department of Medicine, University of Montreal and the Department of Medicine, McGill University, Montreal. e-mail: [email protected] 1. Pearce, E. L., Poffenberger, M. C., Chang, C.-H. & Jones, R. G. Science 342, 1242454 (2013). 2. Martin, E. et al. Nature http://dx.doi.org/10.1038/ nature13386 (2014). 3. Evans, D. R. & Guy, H. I. J. Biol. Chem. 279, 33035–33038 (2004). 4. Higgins, M. J., Graves, P. R. & Graves, L. M. J. Biol. Chem. 282, 29493–29503 (2007). 5. Kassel, K. M., Au, D. R., Higgins, M. J., Hines, M. & Graves, L. M. J. Biol. Chem. 285, 33727–33736 (2010). 6. Gou, K.-M., Chang, C.-C., Shen, Q.-J., Sung, L.-Y. & Liu, J.-L. Exp. Cell Res. 323, 242–253 (2014). 7. Levitzki, A. & Koshland, D. E. Jr Biochemistry 11, 247–253 (1972). 8. Liu, J.-L. BioEssays 33, 159–164 (2011). 9. Chang, Y.-F. & Carman, G. M. Prog. Lipid Res. 47, 333–339 (2008). 10. McPartland, R. P., Wang, M. C., Bloch, A. & Weinfeld, H. Cancer Res. 34, 3107–3111 (1974).
