Grégoire Lauvau

Blood monocytes: distinct subsets, how they relate to dendritic cells, and their possible roles in the regulation of T-cell responses

Monocytes can have important effects on the polarization and expansion of lymphocytes and may contribute to shaping primary and memory T‐cell responses in humans and mice. However, their precise contribution in terms of cellular subsets and the molecular mechanisms involved remains to be determined. Mouse monocytes originate from a bone marrow progenitor, the macrophage and DC precursor (MDP), which also gives rise to conventional dendritic cells through a separate differentiation pathway. Mouse monocytes may be grouped in different functional subsets. The CD115+ Gr1+ ‘inflammatory’ monocyte subset can give rise not only to immunostimulatory ‘TipDCs’ in infected mice but also to immunosuppressive ‘myeloid‐derived suppressor cells’ in tumor‐bearing mice. CD115+ Gr1+ monocytes can also contribute to the renewal of several resident subsets of macrophages and DCs, such as microglia and Langerhans cells, in inflammatory conditions. The CD115+ Gr1− ‘resident’ monocyte subset patrols blood vessels in the steady state and extravasates during infection with Listeria monocytogenes or in the healing myocardium. CD115+ Gr1− monocytes are responsible for an early and transient inflammatory burst during Lm infection, which may play a role in the recruitment of other effector cells and subsequently differentiate toward ‘M2’‐like macrophages that may be involved in wound healing. More research will no doubt confirm the existence of more functional subsets, the developmental relationship between mouse subsets as well as the correspondence between mouse subsets and human subsets of monocytes. We will discuss here the potential roles of monocytes in the immune response, the existence of functional subsets and their relationship with other myeloid cells, including dendritic cells.

CX3CR1+ CD115+ CD135+ common macrophage/DC precursors and the role of CX3CR1 in their response to inflammation.

CX3CR1 expression is associated with the commitment of CSF-1R+ myeloid precursors to the macrophage/dendritic cell (DC) lineage. However, the relationship of the CSF-1R+ CX3CR1+ macrophage/DC precursor (MDP) with other DC precursors and the role of CX3CR1 in macrophage and DC development remain unclear. We show that MDPs give rise to conventional DCs (cDCs), plasmacytoid DCs (PDCs), and monocytes, including Gr1+ inflammatory monocytes that differentiate into TipDCs during infection. CX3CR1 deficiency selectively impairs the recruitment of blood Gr1+ monocytes in the spleen after transfer and during acute Listeria monocytogenes infection but does not affect the development of monocytes, cDCs, and PDCs.

Distinct in vivo dendritic cell activation by live versus killed Listeria monocytogenes.

Immunization of mice with live or heat‐killed Listeria monocytogenes (HKLM) efficiently primes pathogen‐specific CD8+ T cells. T lymphocytes primed by HKLM, however, undergo attenuated proliferation and do not fully differentiate. Thus, only infection with live bacteria induces long‐term, CD8+ T cell‐mediated protective immunity. In this study we demonstrate that live and heat‐killed bacteria, while both associating with Mac‐3+CD11bhi cells, localize to distinct splenic areas following intravenous inoculation. While HKLM localize to the marginal zone and the splenic red pulp, live L. monocytogenes are carried to the T cell zone of splenic white pulp. Despite these differences, in vivo depletion of CD11c‐expressing cells prevents priming of naive T cells by either HKLM or live L. monocytogenes. Analysis of CD11chi dendritic cells (DC) reveals that infection with live L. monocytogenes induces higher levels of CD40, CD80 and CD86 expression than immunization with HKLM. Our results suggest that CD8+ T cell priming following HKLM immunization or live infection is mediated by DC and that the disparate outcomes of priming can be attributed to suboptimal conditioning of DC in the absence of live, cytosol‐invasive bacteria.

Human adenosine deaminase 2 induces differentiation of monocytes into macrophages and stimulates proliferation of T helper cells and macrophages

ADAs play a pivotal role in regulating the level of adenosine, a signaling molecule controlling a variety of cellular responses by binding to and activating four ADRs. Two enzymes, ADA1 and ADA2, are known to possess ADA activity in humans. Although the structure of ADA1 and its role in lymphocytic activation have been known for a long time, the structure and function of ADA2, a member of ADGF, remain enigmatic. Here, we found that ADA2 is secreted by monocytes undergoing differentiation into macrophages or DCs and that it binds to the cell surface via proteoglycans and ADRs. We demonstrate that ADA1 and ADA2 increase the rate of proliferation of monocyte‐activated CD4+ T cells independently of their catalytic activity. We also show that ADA2 induces T cell‐dependent differentiation of monocytes into macrophages and stimulates macrophage proliferation. Our discovery of the growth factor‐like activity of ADA2 explains clinical observations and suggests that this enzyme could be used as a drug candidate to modulate the immune responses during inflammation and cancer.

A role for lipid bodies in the cross-presentation of phagocytosed antigens by MHC class I in dendritic cells

Dendritic cells (DCs) have the striking ability to cross-present exogenous antigens in association with major histocompatibility complex (MHC) class I to CD8(+) T cells. However, the intracellular pathways underlying cross-presentation remain ill defined. Current models involve cytosolic proteolysis of antigens by the proteasome and peptide import into endoplasmic reticulum (ER) or phagosomal lumen by the transporters associated with antigen processing (TAP1 and TAP2). Here, we show that DCs expressed an ER-resident 47 kDa immune-related GTPase, Igtp (Irgm3). Igtp resides on ER and lipid body (LB) membranes where it binds the LB coat component ADFP. Inactivation of genes encoding for either Igtp or ADFP led to defects in LB formation in DCs and severely impaired cross-presentation of phagocytosed antigens to CD8(+) T cells but not antigen presentation to CD4(+) T cells. We thus define a new role for LB organelles in regulating cross-presentation of exogenous antigens to CD8(+) T lymphocytes in DCs.

Memory CD8+ T cells mediate antibacterial immunity via CCL3 activation of TNF/ROI+ phagocytes

Cytolysis, interferon γ and tumor necrosis factor (TNF) α secretion are major effector mechanisms of memory CD8+ T cells that are believed to be required for immunological protection in vivo. By using mutants of the intracellular bacterium Listeria monocytogenes, we found that none of these effector activities is sufficient to protect against secondary infection with wild-type (WT) bacteria. We demonstrated that CCL3 derived from reactivated memory CD8+ T cells is required for efficient killing of WT bacteria. CCL3 induces a rapid TNF-α secretion by innate inflammatory mononuclear phagocytic cells (MPCs), which further promotes the production of radical oxygen intermediates (ROIs) by both MPCs and neutrophils. ROI generation is the final bactericidal mechanism involved in L. monocytogenes clearance. These results therefore uncover two levels of regulation of the antibacterial secondary protective response: (a) an antigen-dependent phase in which memory CD8+ T cells are reactivated and control the activation of the innate immune system, and (b) an antigen-independent phase in which the MPCs coordinate innate immunity and promote the bactericidal effector activities. In this context, CCL3-secreting memory CD8+ T cells are able to mediate “bystander” killing of an unrelated pathogen upon antigen-specific reactivation, a mechanism that may be important for the design of therapeutic vaccines.

TLR4 and Toll-IL-1 Receptor Domain-Containing Adapter-Inducing IFN-β, but Not MyD88, Regulate Escherichia coli-Induced Dendritic Cell Maturation and Apoptosis In Vivo

Dendritic cells (DC) are short-lived, professional APCs that play a central role in the generation of adaptive immune responses. Induction of efficient immune responses is dependent on how long DCs survive in the host. Therefore, the regulation of DC apoptosis in vivo during infection remains an important question that requires further investigation. The impact of Escherichia coli bacteremia on DCs has never been analyzed. We show here that i.v. or i.p. administration of live or heat-killed E. coli in mice induces splenic DC migration, maturation, and apoptosis. We further characterize which TLR and Toll-IL-1R (TIR)-containing adaptor molecules regulate these processes in vivo. In this model, DC maturation is impaired in TLR2−/−, TLR4−/− and TIR domain-containing adapter-inducing IFN-β (TRIF)−/− mice. In contrast, DC apoptosis is reduced only in TLR4−/− and TRIF−/− mice. As expected, DC apoptosis induced by the TLR4 ligand LPS is also abolished in these mice. Injection of the TLR9 ligand CpG-oligodeoxynucleotide (synthetic bacterial DNA) induces DC migration and maturation, but only modest DC apoptosis when compared with LPS and E. coli. Together, these results suggest that E. coli bacteremia directly impacts on DC maturation and survival in vivo through a TLR4-TRIF-dependent signaling pathway.

A Single Subset of Dendritic Cells Controls the Cytokine Bias of Natural Killer T Cell Responses to Diverse Glycolipid Antigens

Summary Many hematopoietic cell types express CD1d and are capable of presenting glycolipid antigens to invariant natural killer T cells (iNKT cells). However, the question of which cells are the principal presenters of glycolipid antigens in vivo remains controversial, and it has been suggested that this might vary depending on the structure of a particular glycolipid antigen. Here we have shown that a single type of cell, the CD8α+ DEC-205+ dendritic cell, was mainly responsible for capturing and presenting a variety of different glycolipid antigens, including multiple forms of α-galactosylceramide that stimulate widely divergent cytokine responses. After glycolipid presentation, these dendritic cells rapidly altered their expression of various costimulatory and coinhibitory molecules in a manner that was dependent on the structure of the antigen. These findings show flexibility in the outcome of two-way communication between CD8α+ dendritic cells and iNKT cells, providing a mechanism for biasing toward either proinflammatory or anti-inflammatory responses.

Quantifying Recruitment of Cytosolic Peptides for HLA Class I Presentation: Impact of TAP Transport

MHC class I ligands are recruited from the cytosolic peptide pool, whose size is likely to depend on the balance between peptide generation by the proteasome and peptide degradation by downstream peptidases. We asked what fraction of this pool is available for presentation, and how the size of this fraction is modulated by peptide affinity for the TAP transporters. A model epitope restricted by HLA-A2 and a series of epitope precursors with N-terminal extensions by single residues modifying TAP affinity were expressed in a system that allowed us to monitor and modulate cytosolic peptide copy numbers. We show that presentation varies strongly according to TAP affinities of the epitope precursors. The fraction of cytosolic peptides recruited for MHC presentation does not exceed 1% and is more than two logs lower for peptides with very low TAP affinities. Therefore, TAP affinity has a substantial impact on MHC class I Ag presentation.

CD4+CD25+ Tregs control the TRAIL-dependent cytotoxicity of tumor-infiltrating DCs in rodent models of colon cancer

Tumors that progress do so via their ability to escape the antitumor immune response through several mechanisms, including developing ways to induce the differentiation and/or recruitment of CD4(+)CD25(+) Tregs. The Tregs, in turn, inhibit the cytotoxic function of T cells and NK cells, but whether they have an effect on the cytotoxic function of tumor-infiltrating DCs (TIDCs) has not been determined. Here we have shown, in 2 rodent models of colon cancer, that CD4(+)CD25(+) Tregs inhibit the ability of CD11b(+) TIDCs to mediate TNF-related apoptosis-inducing ligand-induced (TRAIL-induced) tumor cell death. In both models of cancer, combination treatment with Mycobacterium bovis Bacillus Calmette-Guérin (BCG), which activates the innate immune system via TLR2, TLR4, and TLR9, and cyclophosphamide (CTX), which depletes Tregs, eradicated the tumors. Further analysis revealed that the treatment led to a marked increase in the number of CD11b(+) TIDCs that killed the tumor cells via a TRAIL-dependent mechanism. Furthermore, acquisition of TRAIL expression by the CD11b(+) TIDCs was induced by BCG and dependent on signaling through TLR2, TLR4, and TLR9. In vivo transfer of Tregs abrogated the ability of BCG to induce CD11b(+) TIDCs to express TRAIL and thereby nullified the efficacy of the CTX-BCG treatment. Our data have therefore delineated what we believe to be a novel mechanism by which Tregs inhibit the antitumor immune response.