dendritic cell genealogy: a new stem or just another branch?
TRANSCRIPT
NATURE IMMUNOLOGY VOLUME 8 NUMBER 11 NOVEMBER 2007 1199
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Dendritic cell genealogy: a new stem or just another branch?Miriam Merad & Florent Ginhoux
Two papers in this issue report the isolation and characterization of a common clonal bone marrow precursor of plasmacytoid and conventional dendritic cells.
Dendritic cells (DCs) are a heterogeneous population of rare hematopoietic cells
that are present in most tissues. DCs are organized as a specialized ‘network’ that is dedicated to shaping the immune response to peripheral antigens. Anatomically, DCs may be divided into those that reside in lymphoid tissues and those that are present in nonlym-phoid tissues. Lymphoid tissue DCs can be categorized into two groups: plasmacytoid DCs (pDCs), which produce type I interferon, and conventional DCs (cDCs), which include blood-derived CD8+ and CD8– DCs1. A third population of DCs is uniquely present in the lymph node, but absent from the spleen or thymus, and corresponds to migratory DCs that include Langerhans cells and interstitial DCs that migrate from peripheral tissues through the lymphatics1. Two new studies in this issue of Nature Immunology now report the identification of a bone marrow–derived clonogenic common DC precursor (CDP) that gives rise to pDCs and cDCs2,3.
DCs in lymphoid tissues are in a dynamic balance, with an estimated half-life of only a few days during steady-state conditions4. Although this rapid turnover mandates a con-tinuous replacement of DCs by a precursor population, the identities of hematogenous DC precursors that contribute to steady-state DC populations remain a subject of contro-versy, and attempts to identify committed DC precursors have led to a range of results. The common pDC and cDC precursor described
by Onai et al.2 and by Naik et al.3 in this issue is likely to resolve many of these long-stand-ing issues.
In isolating a common DC precursor, Onai et al.2 predicted, quite judiciously as it emerged, that such a cell would be present in the bone marrow in the steady state and would possess two key properties. The first property is that a DC precursor should have high expression of fms-like tyrosine kinase 3 (Flt3) and should respond to Flt3 ligand in vivo and in vitro. This prediction is based on several findings that indicate that the Flt3 signaling pathway is a key transcription fac-tor for DC development in lymphoid organs. Flt3 ligand as a single cytokine drives the dif-ferentiation of bone marrow progenitors into DCs and pDCs in vitro, Flt3 ligand–deficient mice have a strong reduction of the spleen, lymph node and thymic CD8+ and CD8– DC pool, and enforced gene expression of Flt3 in Flt3– hematopoietic progenitors can restore DC and pDC differentiation5. The second property is that a DC precursor should arise ‘downstream’ of the common lymphoid and myeloid progenitors and progressively become restricted to the DC lineage. This pre-diction is drawn from studies that show that the Flt3+ fraction of the common myeloid and lymphoid progenitors are both able to give rise to cDCs in vitro and in vivo6,7.
Using phenotypic expression and rigorous cloning experiments, Onai et al.2 identified, for the first time, a CDP in mouse bone mar-row that is able to give rise, at a single-cell level, to cDCs and pDCs, but not to other cell lineages, in vitro. Phenotypically, CDPs lack specific lineage markers (Lin–) and are nega-tive for CD11c and major histocompatibility complex class II (MHC-II), and they express
the receptor for the Flt3 ligand, the recep-tor for macrophage colony-stimulating fac-tor (M-CSFR) and the receptor for stem cell factor (c-Kit). After transfer to nonirradiated ‘steady-state’ mice, CDPs give rise exclusively to CD8+ DCs, CD8– DCs and pDCs in the spleen and in the lymph node. CDPs differen-tiate in vitro into DCs and pDCs in the pres-ence of Flt3 ligand alone and do not survive or differentiate in the presence of M-CSF, a cytokine that is essential for macrophage dif-ferentiation1. In addition, CDPs expand and differentiate into all DC subsets in response to Flt3 ligand injection in vivo.
Using an in vitro culture system in which purified Lin– bone marrow cells were incu-bated in the presence of a high dose of Flt3 ligand, Naik et al.3 identified a clonal DC precursor (pro-DC) that closely resembles the CDP phenotypically and functionally. Similar to the CDP, pro-DCs are defined phenotypically as Lin–CD11c–MHC-II–
Flt3+M-CSFR+c-Kit+. Pro-DCs give rise at a single-cell level to pDCs and cDCs in vitro, whereas pro-DCs injected into nonirradiated mice give rise to pDCs, CD8+ and CD8– DCs in the spleen and bone marrow. The pro-DCs, like the CDPs, retain a low macrophage dif-ferentiation potential in vitro (7% and 4%, respectively) but lack macrophage or other cell lineage differentiation potential in vivo.
So how does this DC precursor measure up to previously described candidates? del Hoyo et al.8 were the first to suggest the concept of a proliferative common cDC-pDC precursor, showing that blood-derived Lin–CD11c+MHC-II– progenitors differenti-ate into spleen CD8+, CD8– DCs and pDCs, but not into macrophages, after transfer to irradiated mice. These cells were later found
The authors are at the Mount Sinai School of
Medicine, Department of Gene and Cell Medicine,
1425 Madison Avenue, New York, New York
10029, USA.
e-mail: [email protected].
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to include many DX5+CD11c+ natural killer cells and likely represent a heterogeneous population with some CDP potential. A CD11c+MHC-II– (pre-cDC) proliferative precursor with a high clonal efficiency in mouse bone marrow and lymphoid tissues that is able to give rise to CD8+ and CD8– DCs but not to pDCs or macrophages in vivo was identified by two separate groups9,10. This population likely represents the prog-eny of the CDP, a hypothesis that could be tested in vivo.
Another result that must be integrated into this picture is the recently described macro-
phage DC precursor (MDP). The MDP was isolated by its expression of the fractalkine receptor CX3CR1, a dispensable molecule for DC differentiation11. In contrast to CDPs and pro-DCs, the MDP gives rise to cDCs and macrophages but not to pDCs in vitro. After transfer into nonirradiated mice, MDPs give rise to spleen cDCs and macrophages, but not to pDCs. MDP progeny have not been exam-ined in the lymph node. MDPs have high expression of CX3CR1 and M-CSFR; they also express Flt3 mRNA, whereas the expres-sion of Flt3 protein was not reported. MDPs do not survive in response to Flt3 ligand,
but do survive in the presence of M-CSF as a single cytokine. In addition, MDPs do not differentiate into DCs in response to Flt3 ligand, but differentiate into DCs in response to granulocyte-macrophage colony-stimulat-ing factor (GM-CSF) and into macrophages in response to M-CSF in vitro. Each of these attributes is shared by circulating monocytes. Monocytes have high expression of CX3CR1 and M-CSFR. They also express Flt3, but the monocyte frequency is not reduced in Flt3 ligand–deficient mice12. Monocytes dif-ferentiate in vitro into DCs in the presence of GM-CSF and into macrophages in the
1200 VOLUME 8 NUMBER 11 NOVEMBER 2007 NATURE IMMUNOLOGY
Bone marrow Blood
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Figure 1 DC ontogeny in mice. A hypothetical view of pDC and cDC differentiation pathways in mice. The Flt3+ fraction, but not the Flt3– fraction, of the common myeloid precursor (CMP) and the common lymphoid precursor (CLP) give rise to DCs in mice and humans6,7. A clonal cDC-pDC precursor, CDP, was recently identified in mouse bone marrow as being Lin–Flt3+ M-CSFR+c-Kitint (refs. 2,3). The CDPs likely derive from Flt3+ CMPs and Flt3+ CLPs, although this remains to be tested in vivo. CDPs are shown in the two papers discussed here to give rise to pDCs and cDCs. It is likely that, before differentiating into cDCs, CDPs give rise to MHC II+CD11c– cDC precursors (pre-cDC) that further differentiate into CD8+ and CD8– DCs9,10 after three to four cell cycles. MDPs give rise to blood monocytes, cDCs and macrophages, but not to pDCs11,16. We propose that MDP-DC progeny develop mainly through a monocytoid pathway. In inflamed conditions, monocytes differentiate into inflammatory splenic DCs11. CDP contribution to DC homeostasis in nonlymphoid tissues has not been tested. Monocytes give rise to DCs in the gut and the lung16 in the steady state. Monocytes differentiate into epidermal Langerhans cells in inflamed skin14, whereas Langerhans cells are maintained by local hematopoietic precursors in the steady state15.
Kim
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presence of M-CSF1. Therefore, it is difficult to exclude the possibility that MDPs represent an early monocyte precursor and share the monocyte-derived DC differentiation pro-gram. This point was dismissed in the MDP study11, as adoptive transfer experiments showed that the yield of monocyte-derived DCs was negligible compared with that of MDP-derived DCs, leading to the concept that MDP-derived DCs arise independently of monocytes. We would argue that mono-cytes, in contrast to MDPs, are more differ-entiated cells and may have a poor survival rate during adoptive transfer manipulation in the steady state, which might explain why monocyte-derived DCs are best seen in the presence of inflammatory signals (discussed below).
Therefore, the studies by Onai et al.2 and Naik et al.3 do not support the tentative theory that the MDP is a major pathway for DC development in lymphoid organs in vivo. However, it remains possible that both MDPs, through their monocyte progeny, and CDPs contribute to DC homeostasis (Fig. 1). So what would be the advantage of having two separate DC differentiation pathways? One advantage would be to allow increased numbers of DCs to be deployed quickly to communicate potential dangers and to ini-tiate an immune response. This hypoth-esis is supported by data showing that DCs, although confined to Flt3+ progenitors, can
differentiate along the myeloid and lymphoid lineage, a flexibility that is almost unique to DCs among hematopoietic cells6,7,13. The hypothesis is also supported by earlier studies that suggest that DCs are clearly derived from monocytes1 in experimental inflammatory conditions and which establish the concept that DC homeostasis is critically dependent on conditions of quiescence or inflammation. Results from our laboratory have established that monocytes repopulate Langerhans cells in inflamed skin14, whereas Langerhans cells are maintained by local hematopoietic pre-cursors in the steady state15 (Fig. 1). The factors that govern this difference are largely unknown, but it is interesting to note that GM-CSF has a critical role in monocyte dif-ferentiation into splenic DCs in injured mice, but not in the steady state1.
DC turnover in tissues is less dynamic than it is in lymphoid organs (M.B. Bogunovic, F.G. and M.M., unpublished data), suggest-ing that DC homeostasis may be regulated differently in the periphery than it is in the spleen and lymph node. None of the studies described here have examined the potential of their precursor candidates to give rise to interstitial and epithelial-associated DCs. Recent results suggest that monocytes can also contribute to the steady-state homeosta-sis of DCs in the colon and the lung but not in the spleen16. These results are consistent with data from our laboratory showing that
M-CSF has a critical role in DC development in the gut and skin, but not in the spleen, in the steady state (F.G. and M.M., unpub-lished data). Whether these results suggest a separate ontogeny for DCs in lymphoid and nonlymphoid tissues in the steady state or whether they only reflect the limitations of an experimental system dealing with very low cell numbers is unclear. To circumvent this, we will have to move away from monocyte adoptive transfer experiments and develop genetic tracing lineage tools that are rel-evant for DC and monocyte differentiation programs.
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2. Onai, N. et al. Nat. Immunol. 8, 1207–1216 (2007)3. Naik, S.H. et al. Nat. Immunol. 8, 1217–1226
(2007)4. Liu, K. et al. Nat. Immunol. 8, 578–583 (2007).5. Onai, N., Obata-Onai, A., Schmid, M.A. & Manz, M.G.
Ann. NY Acad. Sci. published online 14 March 2007 (doi: 10.1196/annals.1392.015)
6. D’Amico, A. & Wu, L. J. Exp. Med. 198, 293–303 (2003).
7. Karsunky, H., Merad, M., Cozzio, A., Weissman, I.L. & Manz, M.G. J. Exp. Med. 198, 305–313 (2003).
8. del Hoyo, G.M. et al. Nature 415, 1043–1047 (2002)
9. Diao, J. et al. J. Immunol. 176, 7196–7206 (2006). 10. Naik, S.H. et al. Nat. Immunol. 7, 663–671 (2006). 11. Fogg, D.K. et al. Science 311, 83–87 (2006). 12. McKenna, H.J. et al. Blood 95, 3489–3497 (2000). 13. Karsunky, H. et al. Exp. Hematol. 33, 173–181
(2005). 14. Ginhoux, F. et al. Nat. Immunol. 7, 265–273 (2006). 15. Merad, M. et al. Nat. Immunol. 3, 1135–1141
(2002). 16. Varol, C. et al. J. Exp. Med. 204, 171–180 (2007).
NATURE IMMUNOLOGY VOLUME 8 NUMBER 11 NOVEMBER 2007 1201
Inhibitory receptors: whose side are they on?Alison Crawford & E John Wherry
A new study demonstrates involvement of the inhibitory receptor CTLA-4 in T cell exhaustion during infection with human immunodeficiency virus, adding complexity and diversity to the inhibitory pathways regulating T cell responses during chronic viral infections in humans.
Alison Crawford and E. John Wherry are with
the Immunology Program, The Wistar Institute,
Philadelphia, Pennsylvania 19104, USA
e-mail: [email protected].
The immune system has evolved to provide specific protection from a wide range of
pathogens. The tremendous diversity of the adaptive immune system, however, must be balanced by mechanisms that limit self-reac-tivity. The deletion of self-reactive T cells and B cells removes many potentially autoreactive specificities, but peripheral negative regula-tory pathways are also crucial for suppressing potentially pathogenic lymphocytes that escape
central tolerance. Successful pathogens have developed many strategies for evading immu-nity, including the ability to exploit negative regulatory pathways and facilitate pathogen persistence. Studies have demonstrated the importance of negative regulatory pathways during chronic infections in humans. The com-plexity and diversity of the regulatory pathways suggest that therapeutic interventions could be specifically tailored to augment some types of immune responses during chronic infection, but the precise negative regulatory pathways exploited by viral pathogens remain incom-pletely defined. A new study by Kaufmann et al. in this issue of Nature Immunology addresses
such issues and reports a chief function for the inhibitory receptor CTLA-4 during chronic infection with human immunodeficiency virus (HIV) in humans1.
T cell exhaustion is a common occurrence during chronic viral infection. Defects in the production of cytokines, including inter-feron-γ, tumor necrosis factor and interleukin 2 (IL-2), and in cytotoxicity and proliferative capacity underlie the poor control of viral rep-lication noted during chronic infection with lymphocytic choriomeningitis virus in mice and infection with HIV and hepatitis C virus in humans2. However, the persistence of func-tionally exhausted T cells during these chronic
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