Lymphatic Endothelial Cells Are Essential Components of the Subcapsular Sinus Macrophage Niche.

In lymph nodes, subcapsular sinus macrophages (SSMs) form an immunological barrier that monitors lymph drained from peripheral tissues. Upon infection, SSMs activate B and natural killer T (NKT) cells while secreting inflammatory mediators. Here, we investigated the mechanisms regulating development and homeostasis of SSMs. Embryonic SSMs originated from yolk sac hematopoiesis and were replaced by a postnatal wave of bone marrow (BM)-derived monocytes that proliferated to establish the adult SSM network. The SSM network self-maintained by proliferation with minimal BM contribution. Upon pathogen-induced transient deletion, BM-derived cells contributed to restoring the SSM network. Lymphatic endothelial cells (LECs) were the main source of CSF-1 within the lymph node and conditional deletion of Csf1 in adult LECs decreased the network of SSMs and medullary sinus macrophages (MSMs). Thus, SSMs have a dual hematopoietic origin, and LECs are essential to the niche supporting these macrophages.

T Cell Zone Resident Macrophages Silently Dispose of Apoptotic Cells in the Lymph Node.

In lymph nodes (LNs), dendritic cells (DCs) are thought to dispose of apoptotic cells, a function pertaining to macrophages in other tissues. We found that a population of CX3CR1+ MERTK+ cells located in the T cell zone of LNs, previously identified as DCs, are efferocytic macrophages. Lineage-tracing experiments and shield chimeras indicated that these T zone macrophages (TZM) are long-lived macrophages seeded in utero and slowly replaced by blood monocytes after birth. Imaging the LNs of mice in which TZM and DCs express different fluorescent proteins revealed that TZM-and not DCs-act as the only professional scavengers, clearing apoptotic cells in the LN T cell zone in a CX3CR1-dependent manner. Furthermore, similar to other macrophages, TZM appear inefficient in priming CD4 T cells. Thus, efferocytosis and T cell activation in the LN are uncoupled processes designated to macrophages and DCs, respectively, with implications to the maintenance of immune homeostasis.

Homeostatic NF-κB Signaling in Steady-State Migratory Dendritic Cells Regulates Immune Homeostasis and Tolerance.

Migratory non-lymphoid tissue dendritic cells (NLT-DCs) transport antigens to lymph nodes (LNs) and are required for protective immune responses in the context of inflammation and to promote tolerance to self-antigens in steady-state. However, the molecular mechanisms that elicit steady-state NLT-DC maturation and migration are unknown. By comparing the transcriptome of NLT-DCs in the skin with their migratory counterparts in draining LNs, we have identified a novel NF-κB-regulated gene network specific to migratory DCs. We show that targeted deletion of IKKß in DCs, a major activator of NF-κB, prevents NLT-DC accumulation in LNs and compromises regulatory T cell conversion in vivo. This was associated with impaired tolerance and autoimmunity. NF-κB is generally considered the prototypical pro-inflammatory transcription factor, but this study describes a role for NF-κB signaling in DCs for immune homeostasis and tolerance that could have implications in autoimmune diseases and immunity.

Functions of natural killer cells.

Natural killer (NK) cells are effector lymphocytes of the innate immune system that control several types of tumors and microbial infections by limiting their spread and subsequent tissue damage. Recent research highlights the fact that NK cells are also regulatory cells engaged in reciprocal interactions with dendritic cells, macrophages, T cells and endothelial cells. NK cells can thus limit or exacerbate immune responses. Although NK cells might appear to be redundant in several conditions of immune challenge in humans, NK cell manipulation seems to hold promise in efforts to improve hematopoietic and solid organ transplantation, promote antitumor immunotherapy and control inflammatory and autoimmune disorders.

Natural killer cell trafficking in vivo requires a dedicated sphingosine 1-phosphate receptor.

Consistent with their function in immune surveillance, natural killer (NK) cells are distributed throughout lymphoid and nonlymphoid tissues. However, the mechanisms governing the steady-state trafficking of NK cells remain unknown. The lysophospholipid sphingosine 1-phosphate (S1P), by binding to its receptor S1P1, regulates the recirculation of T and B lymphocytes. In contrast, S1P5 is detected in the brain and regulates oligodendrocyte migration and survival in vitro. Here we show that S1P5 was also expressed in NK cells in mice and humans and that S1P5-deficient mice had aberrant NK cell homing during steady-state conditions. In addition, we found that S1P5 was required for the mobilization of NK cells to inflamed organs. Our data emphasize distinct mechanisms regulating the circulation of various lymphocyte subsets and raise the possibility that NK cell trafficking may be manipulated by therapies specifically targeting S1P5.

Lymph node macrophages: Scavengers, immune sentinels and trophic effectors.

Lymph nodes (LN) are secondary lymphoid organs dispersed throughout the body that filter lymph and assist the immune system in mounting immune responses. These functions are supported by a complex stromal microarchitecture composed of mesenchymal and vascular elements. Different subsets of macrophages (MΦ) reside in the LN and are endowed with immune and trophic functions. Here we review these different subsets with particular emphasis on the recently described T cell zone MΦ. We also address the potential crosstalk between LN stromal cells and MΦ, proposing that the former constitute niches for the latter by supplying factors required for their specification, survival and turnover. In turn, MΦ could inform their stromal partners about the immune status of the LN and orchestrate the remodelling of its microanatomy during immune responses.

Induction of B7-H6, a ligand for the natural killer cell-activating receptor NKp30, in inflammatory conditions.

B7-H6, a member of the B7 family of immunoreceptors, is as a cell-surface ligand for the NKp30-activating receptor expressed on natural killer cells. B7-H6 is not detected in normal human tissues at steady state but is expressed on tumor cells. However, whether B7-H6 can be expressed in other conditions remains unknown. We analyzed here the pathways that lead to the expression of B7-H6 in nontransformed cells. In vitro, B7-H6 was induced at the surface of CD14(+)CD16(+) proinflammatory monocytes and neutrophils upon stimulation by ligands of Toll-like receptors or proinflammatory cytokines such as interleukin-1ß and tumor necrosis factor α. In these conditions, a soluble form of B7-H6 (sB7-H6) was also produced by activated monocytes and neutrophils. In vivo, B7-H6 was expressed on circulating proinflammatory CD14(+)CD16(+) monocytes in a group of patients in sepsis conditions, and was linked to an increased mortality. sB7-H6 was selectively detected in the sera of patients with gram-negative sepsis and was associated with membrane vesicles that co-sedimented with the exosomal fraction. These findings reveal that B7-H6 is not only implicated in tumor immunosurveillance but also participates in the inflammatory response in infectious conditions.

Isolation and Characterization of Testis Macrophages Using Flow Cytometry.

Testis-resident macrophages are first responders of the innate immune system against pathogens. They also exert day-to-day functions that are poorly understood. To study testis macrophages, several techniques are used, among which we can find flow cytometry.Flow cytometry is a powerful tool that enables analysis of macrophages at a cellular as well as population level. To analyze testis macrophages using flow cytometry, a specific tissue processing is necessary to extract them. In this protocol, we explain how to extract and analyze the distinct macrophage populations.

B7-H6/NKp30 interaction: a mechanism of alerting NK cells against tumors.

Natural killer (NK) cells are lymphocytes of the innate immune system that sense target cells through a panel of activating and inhibitory receptors. Together with NKG2D, the natural cytotoxicity receptors (NCRs) are major activating receptors involved in tumor cell detection. Although numerous NKG2D ligands have been identified, characterization of the molecules interacting with the NCRs is still incomplete. The identification of B7-H6 as a counter structure of the NCR NKp30 shed light on the molecular basis of NK cell immunosurveillance. We review here the current knowledge on NKp30 and B7-H6, and we discuss their potential role in anti-tumor immunity.

Unveiling skin macrophage dynamics explains both tattoo persistence and strenuous removal.

Here we describe a new mouse model that exploits the pattern of expression of the high-affinity IgG receptor (CD64) and allows diphtheria toxin (DT)-mediated ablation of tissue-resident macrophages and monocyte-derived cells. We found that the myeloid cells of the ear skin dermis are dominated by DT-sensitive, melanin-laden cells that have been missed in previous studies and correspond to macrophages that have ingested melanosomes from neighboring melanocytes. Those cells have been referred to as melanophages in humans. We also identified melanophages in melanocytic melanoma. Benefiting of our knowledge on melanophage dynamics, we determined the identity, origin, and dynamics of the skin myeloid cells that capture and retain tattoo pigment particles. We showed that they are exclusively made of dermal macrophages. Using the possibility to delete them, we further demonstrated that tattoo pigment particles can undergo successive cycles of capture-release-recapture without any tattoo vanishing. Therefore, congruent with dermal macrophage dynamics, long-term tattoo persistence likely relies on macrophage renewal rather than on macrophage longevity.

Complement factor P is a ligand for the natural killer cell-activating receptor NKp46.

Innate lymphoid cells (ILCs) are involved in immune responses to microbes and various stressed cells, such as tumor cells. They include group 1 [such as natural killer (NK) cells and ILC1], group 2, and group 3 ILCs. Besides their capacity to respond to cytokines, ILCs detect their targets through a series of cell surface-activating receptors recognizing microbial and nonmicrobial ligands. The nature of some of these ligands remains unclear, limiting our understanding of ILC biology. We focused on NKp46, which is highly conserved in mammals and expressed by all mature NK cells and subsets of ILC1 and ILC3. We show here that NKp46 binds to a soluble plasma glycoprotein, the complement factor P (CFP; properdin), the only known positive regulator of the alternative complement pathway. Consistent with the selective predisposition of patients lacking CFP to lethal Neisseria meningitidis (Nm) infections, NKp46 and group 1 ILCs bearing this receptor were found to be required for mice to survive Nm infection. Moreover, the beneficial effects of CFP treatment for Nm infection were dependent on NKp46 and group 1 NKp46+ ILCs. Thus, group 1 NKp46+ ILCs interact with the complement pathway, via NKp46, revealing a cross-talk between two partners of innate immunity in the response to an invasive bacterial infection.

[NK cells and innate immunity to malaria]. / Cellules natural killer et immunité innée contre le paludisme.

Innate immune response against Plasmodium falciparum (Pf), a causative agent of human malaria, is the result of several thousand years of co-evolution between the parasite and his host. An early IFN-gamma production during infection is associated with a better evolution of the disease. Natural killer (NK) cells are among the first cells in peripheral blood to produce IFN-gamma in response to Pf-infected erythrocytes (Pf-E). NK cells are found in blood, in secondary lymphoid organs as well as in peripheral non-lymphoid tissues. They participate in host innate responses that occur upon viral and intracytoplasmic bacterial infections, but also during the course of tumor development and allogeneic transplantation. These lymphocytes are not only important players of innate effector responses, but also participate in the initiation and development of adaptive immune responses. In addition, direct sensing of Pf infection by NK cells induces their production of the proinflammatory chemokine IL-8, suggesting a role for NK cells in the recruitment and the activation of other cells during malaria infection. Several other cell subsets are involved in the innate immune response to Pf. Dendritic cells, macrophages, gamma delta T cells, NKT cells are able to sense the presence of the parasite. Along this line, the presence of IL-12 is necessary to NK cell IFN-gamma production and a functional cooperation takes place between macrophages and NK cells in the context of this parasitic infection. In particular, IL-18 produced by macrophages is a key factor for this NK response. However, the molecular basis of Pf-E recognition by NK cells as well as the functional role of NK cell responses during the course of the disease remain to be adressed.

The B7 family member B7-H6 is a tumor cell ligand for the activating natural killer cell receptor NKp30 in humans.

Cancer development is often associated with the lack of specific and efficient recognition of tumor cells by the immune system. Natural killer (NK) cells are lymphocytes of the innate immune system that participate in the elimination of tumors. We report the identification of a tumor cell surface molecule that binds NKp30, a human receptor which triggers antitumor NK cell cytotoxicity and cytokine secretion. This previously unannotated gene belongs to the B7 family and, hence, was designated B7-H6. B7-H6 triggers NKp30-mediated activation of human NK cells. B7-H6 was not detected in normal human tissues but was expressed on human tumor cells, emphasizing that the expression of stress-induced self-molecules associated with cell transformation serves as a mode of cell recognition in innate immunity.

Expression of the CD85j (leukocyte Ig-like receptor 1, Ig-like transcript 2) receptor for class I major histocompatibility complex molecules in idiopathic inflammatory myopathies.

OBJECTIVE: Expression of class I major histocompatibility complex (MHC) molecules on the surface of muscle cells is a biologic feature of idiopathic inflammatory myopathies (IIM). Class I MHC-transgenic mouse models support a causative role for class I MHC expression by muscle cells in the pathogenesis of IIM. The muscle lesions are characterized by leukocyte infiltration. We undertook this study to analyze the expression in muscle lesions of various class I MHC-specific receptors on leukocytes and natural killer (NK) cells. METHODS: We generated a panel of cell transfectants to control the immunofluorescence analysis of class I MHC receptor expression. We then analyzed the expression of CD158 (killer cell Ig-like receptors [KIRs]) and CD85j (leukocyte Ig-like receptor 1, Ig-like transcript 2) on muscle sections prepared from 14 patients with IIM (5 with dermatomyositis [DM], 5 with polymyositis [PM], and 4 with sporadic inclusion body myositis [IBM]). RESULTS: We could not detect the presence of NK cells in inflammatory lesions. However, the class I MHC receptor CD85j, but no KIRs, was expressed by inflammatory cells infiltrating muscle lesions in IIM. CONCLUSION: CD85j is expressed in PM and sporadic IBM at the sites of partial invasion and in DM in perivascular inflammation, paving the way for dissecting the role of CD85j in the pathogenesis of inflammatory myopathies.

Dissection of the role of PfEMP1 and ICAM-1 in the sensing of Plasmodium-falciparum-infected erythrocytes by natural killer cells.

BACKGROUND: Host innate immunity contributes to malaria clinical outcome by providing protective inflammatory cytokines such as interferon-gamma, and by shaping the adaptive immune response. Plasmodium falciparum (Pf) is the etiologic agent of the most severe forms of human malaria. Natural Killer (NK) cells are lymphocytes of the innate immune system that are the first effectors to produce interferon-gamma in response to Pf. However, the molecular bases of Pf-NK cell recognition events are unknown. Our study focuses on the role of Pf erythrocyte membrane protein 1 (PfEMP1), a major Pf virulence factor. PfEMP1 is expressed on parasitized-erythrocytes and participates to vascular obstruction through the binding to several host receptors. PfEMP1 is also a pivotal target for host antibody response to Pf infection. METHODOLOGY/PRINCIPAL FINDINGS: Using genetically-engineered parasite mutant strains, a human genetic deficiency, and blocking antibodies, we identified two receptor-ligand pairs involved in two uncoupled events occurring during the sensing of Pf infection by NK cells. First, PfEMP1 interaction with one of its host receptor, chondroitin sulfate A, mediates the cytoadhesion of Pf-infected erythrocytes to human NK cell lines, but is not required for primary NK cell activation. Second, intercellular adhesion molecule-1 (ICAM-1), another host receptor for PfEMP1, is mandatory for NK cell interferon-gamma response. In this case, ICAM-1 acts via its engagement with its host ligand, LFA-1, and not with PfEMP1, consistent with the obligatory cross-talk of NK cells with macrophages for their production of interferon-gamma. CONCLUSION/SIGNIFICANCE: PfEMP1-independent but ICAM-1/LFA-1-dependent events occurring during NK cell activation by Pf highlight the fundamental role of cellular cooperation during innate immune response to malaria.

Natural killer cells and malaria.

Malaria, caused by the infection with parasites of the germs Plasmodium, is one of the three most important infectious diseases worldwide, along with tuberculosis and infection with human immunodeficiency virus. Natural killer (NK) cells are lymphocytes classically involved in the early defense against viral infections and intracytoplasmic bacterial infections and are also implicated during the course of tumor development and allogeneic transplantation. These cells display important cytotoxic activity and produce high levels of proinflammatory cytokines. In both mouse and human models of malaria, NK cells appear to be a major source of interferon-gamma during the early phase of infection. In humans, indirect signaling through monocytes/macrophages required to optimally stimulate NK cell activity. However, the in vivo functions of NK cells during malaria are still enigmatic, and many issues remain to be dissected, such as the molecular basis of the direct recognition of iRBCs by NK cells.

Natural killer cell and macrophage cooperation in MyD88-dependent innate responses to Plasmodium falciparum.

IFN-gamma secretion by natural killer (NK) cells is pivotal to several tumor and viral immune responses, during which NK and dendritic cells cooperation is required. We show here that macrophages are mandatory for NK cell IFN-gamma secretion in response to erythrocytes infected with Plasmodium falciparum (Pf), a causative agent of human malaria. In addition, direct sensing of Pf infection by NK cells induces their production of the proinflammatory chemokine CXCL8, without triggering their granule-mediated cytolytic programs. Despite their reported role in Pf recognition, Toll-like receptor (TLR) 2, TLR9, and TLR11 are individually dispensable for NK cell activation induced by Pf-infected erythrocytes. However, IL-18R expression on NK cells, IL-18 production by macrophages, and MyD88 on both cell types are essential components of this previously undescribed pathway of NK cell activation in response to a parasite infection.

Frontline: Peripheral priming of alloreactive T cells by the direct pathway of allorecognition.

Recent studies, though controversial, have suggested that secondary lymphoid organs may not constitute an essential site for the initiation of immune responses to transplant antigens. However, this issue has never been examined in the context of direct and indirect allorecognition. Here, we characterized immune responses arising in draining lymph nodes and skin allografts, in a murine model based on a single T cell clonotype where these two pathways can be independently studied. In this model, graft rejection by the direct or the indirect pathway occurred with similar kinetics, although initiation of the alloreactive responses was clearly different. During indirect responses, expansion and activation of alloreactive T cells were first observed in draining lymph nodes, at day 7 post-transplant, and graft-infiltrating T cells were observed later, at day 11. In striking contrast, directly activated alloreactive T cells were detected at an early stage inside the graft, and only later in the draining lymph nodes, after skin allograft rejection was almost completed. These results suggest that sensitization of naive T cells through the direct pathway could take place outside secondary lymphoid organs.

Alloantibody production is regulated by CD4+ T cells' alloreactive pathway, rather than precursor frequency or Th1/Th2 differentiation.

Although CD4(+) T cells play an important role in the regulation of allograft rejection, the exact mechanisms by which they operate and the actual contribution of direct and indirect alloreactivity pathways remain to be fully characterized. Previous studies have established a possible relationship between the indirect alloreactivity pathway and antibody production, but interpretation of these results have been complicated by shortcomings inherent to the models used in these studies. To address this issue, we have developed a model based on TCR transgenic mice derived from a CD4(+) T-cell clone which recognize specific alloantigens by both alloreactivity pathways. Skin allografts on alphabeta T-cell deficient mice adoptively transferred with transgenic CD4(+) T cells were rejected without significant delay between the two alloreactivity pathways. No IgG alloantibody was produced following allograft rejection by the direct alloreactivity pathway alone. Importantly, production of antibodies against alloantigens of the direct pathway was shown to require help from CD4(+) T cells activated by the indirect pathway. These results indicate that the events leading to the initiation of immune responses responsible for graft rejection are clearly dependent on the population of antigen-presenting cells involved in T- and B-lymphocyte activation.