CD36 is differentially expressed by CD8+ splenic dendritic cells but is not required for cross-presentation in vivo

Efferocytosis in dendritic cells: an overlooked immunoregulatory process

Efferocytosis, the process of engulfing and removing apoptotic cells, plays an essential role in preserving tissue health and averting undue inflammation. While macrophages are primarily known for this task, dendritic cells (DCs) also play a significant role. This review delves into the unique contributions of various DC subsets to efferocytosis, highlighting the distinctions in how DCs and macrophages recognize and handle apoptotic cells. It further explores how efferocytosis influences DC maturation, thereby affecting immune tolerance. This underscores the pivotal role of DCs in orchestrating immune responses and sustaining immune equilibrium, providing new insights into their function in immune regulation.

Decoding immunogenic cell death from a dendritic cell perspective

Dendritic cells (DCs) are myeloid cells bridging the innate and adaptive immune system. By cross-presenting tumor-associated antigens (TAAs) liberated upon spontaneous or therapy-induced tumor cell death to T cells, DCs occupy a pivotal position in the cancer immunity cycle. Over the last decades, the mechanisms linking cancer cell death to DC maturation, have been the focus of intense research. Growing evidence supports the concept that the mere transfer of TAAs during the process of cell death is insufficient to drive immunogenic DC maturation unless this process is coupled with the release of immunomodulatory signals by dying cancer cells. Malignant cells succumbing to a regulated cell death variant called immunogenic cell death (ICD), foster a proficient interface with DCs, enabling their immunogenic maturation and engagement of adaptive immunity against cancer. This property relies on the ability of ICD to exhibit pathogen-mimicry hallmarks and orchestrate the emission of a spectrum of constitutively present or de novo-induced danger signals, collectively known as damage-associated molecular patterns (DAMPs). In this review, we discuss how DCs perceive and decode danger signals emanating from malignant cells undergoing ICD and provide an outlook of the major signaling and functional consequences of this interaction for DCs and antitumor immunity.

LXR signaling controls homeostatic dendritic cell maturation

Dendritic cells (DCs) mature in an immunogenic or tolerogenic manner depending on the context in which an antigen is perceived, preserving the balance between immunity and tolerance. Whereas the pathways driving immunogenic maturation in response to infectious insults are well-characterized, the signals that drive tolerogenic maturation during homeostasis are still poorly understood. We found that the engulfment of apoptotic cells triggered homeostatic maturation of type 1 conventional DCs (cDC1s) within the spleen. This maturation process could be mimicked by engulfment of empty, nonadjuvanted lipid nanoparticles (LNPs), was marked by intracellular accumulation of cholesterol, and was highly specific to cDC1s. Engulfment of either apoptotic cells or cholesterol-rich LNPs led to the activation of the liver X receptor (LXR) pathway, which promotes the efflux of cellular cholesterol, and repressed genes associated with immunogenic maturation. In contrast, simultaneous engagement of TLR3 to mimic viral infection via administration of poly(I:C)-adjuvanted LNPs repressed the LXR pathway, thus delaying cellular cholesterol efflux and inducing genes that promote T cell-mediated immunity. These data demonstrate that conserved cellular cholesterol efflux pathways are differentially regulated in tolerogenic versus immunogenic cDC1s and suggest that administration of nonadjuvanted cholesterol-rich LNPs may be an approach for inducing tolerogenic DC maturation.

Murine precursors to type 1 conventional dendritic cells induce tumor cytotoxicity and exhibit activated PD-1/PD-L1 pathway

The immediate precursor to murine type 1 conventional DCs (cDC1s) has recently been established and named “pre-cDC1s”. Mature CD8α+ cDC1s are recognized for suppressing graft-versus-host disease (GvHD) while promoting graft-versus-leukemia (GvL), however pre-cDC1s have not previously been investigated in the context of alloreactivity or anti-tumor responses. Characterization of pre-cDC1s, compared to CD8α+ cDC1s, found that a lower percentage of pre-cDC1s express PD-L1, yet express greater PD-L1 by MFI and a greater percent PIR-B, a GvHD-suppressing molecule. Functional assays were performed ex vivo following in vivo depletion of CD8α+ DCs to examine whether pre-cDC1s play a redundant role in alloreactivity. Proliferation assays revealed less allogeneic T-cell proliferation in the absence of CD8α+ cDC1s, with slightly greater CD8+ T-cell proliferation. Further, in the absence of CD8α+ cDC1s, stimulated CD8+ T-cells exhibited significantly less PD-1 expression compared to CD4+ T-cells, and alloreactive T-cell death was significantly lower, driven by reduced CD4+ T-cell death. Tumor-killing assays revealed that T-cells primed with CD8α-depleted DCs ex vivo induce greater killing of A20 B-cell leukemia cells, particularly when antigen (Ag) is limited. Bulk RNA sequencing revealed distinct transcriptional programs of these DCs, with pre-cDC1s exhibiting activated PD-1/PD-L1 signaling compared to CD8α+ cDC1s. These results indicate distinct T-cell-priming capabilities of murine pre-cDC1s compared to CD8α+ cDC1s ex vivo, with potentially clinically relevant implications in suppressing GvHD while promoting GvL responses, highlighting the need for greater investigation of murine pre-cDC1s.

RNA-Seq analysis of duck embryo fibroblast cells gene expression during duck Tembusu virus infection

Duck Tembusu virus (DTMUV), a member of the family Flaviviridae and an economically important pathogen with a broad host range, leads to markedly decreased egg production. However, the molecular mechanism underlying the host-DTMUV interaction remains unclear. Here, we performed high-throughput RNA sequencing (RNA-Seq) to study the dynamic changes in host gene expression at 12, 24, 36, 48 and 60 h post-infection (hpi) in duck embryo fibroblasts (DEF) infected with DTMUV. A total of 3129 differentially expressed genes (DEG) were identified after DTMUV infection. Gene Ontology (GO) category and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis revealed that these DEG were associated with multiple biological functions, including signal transduction, host immunity, virus infection, cell apoptosis, cell proliferation, and pathogenicity-related and metabolic process signaling pathways. This study analyzed viral infection and host immunity induced by DTMUV infection from a novel perspective, and the results provide valuable information regarding the mechanisms underlying host-DTMUV interactions, which will prove useful for the future development of antiviral drugs or vaccines for poultry, thus benefiting the entire poultry industry.

A novel myeloid cell in murine spleen defined through gene profiling

A novel myeloid antigen presenting cell can be generated through in vitro haematopoiesis in long‐term splenic stromal cocultures. The in vivo equivalent subset was recently identified as phenotypically and functionally distinct from the spleen subsets of macrophages, conventional (c) dendritic cells (DC), resident monocytes, inflammatory monocytes and eosinophils. This novel subset which is myeloid on the basis of cell surface phenotype, but dendritic‐like on the basis of cell surface marker expression and antigen presenting function, has been tentatively labelled “L‐DC.” Transcriptome analysis has now been employed to determine the lineage relationship of this cell type with known splenic cDC and monocyte subsets. Principal components analysis showed separation of “L‐DC” and monocytes from cDC subsets in the second principal component. Hierarchical clustering then indicated a close lineage relationship between this novel subset, resident monocytes and inflammatory monocytes. Resident monocytes were the most closely aligned, with no genes specifically expressed by the novel subset. This subset, however, showed upregulation of genes reflecting both dendritic and myeloid lineages, with strong upregulation of several genes, particularly CD300e. While resident monocytes were found to be dependent on Toll‐like receptor signalling for development and were reduced in number in Myd88‐/‐ and Trif‐/‐ mutant mice, both the novel subset and inflammatory monocytes were unaffected. Here, we describe a novel myeloid cell type closely aligned with resident monocytes in terms of lineage but distinct in terms of development and functional capacity.

Autophagy protein ATG5 regulates CD36 expression and anti-tumor MHC class II antigen presentation in dendritic cells

Macroautophagy/autophagy has been implicated in cytoplasmic and viral antigen presentation on major histocompatibility complex (MHC) class II molecules. However, the role of autophagy in the presentation of phagocytized tumor-associated antigens in vivo remains unclear. Following the administration of apoptotic tumor cells and in vivo chemotherapy, mice with a dendritic cell-specific deletion of Atg5, a key autophagy gene, exhibit reduced CD4⁺ T-cell priming but not CD8⁺ cytotoxic T-cell priming. Interestingly, Atg5-deficient dendritic cells have an elevated expression of scavenger receptor CD36 and show excessive lipid accumulation. Atg5-deficient dendritic cells increased CD36-dependent phagocytosis of apoptotic tumor cells. CD36 blockade ameliorates elevated phagocytosis and increases CD4⁺ T-cell priming in dendritic cells; intratumoral CD36 blockade inhibits tumor growth. Our results demonstrate that Atg5 is required for proper antigen phagocytosis and presentation to MHC class II via modulation of CD36 in dendritic cells and may be a future therapeutic target for anti-tumor therapy.Abbreviations: APC: antigen-presenting cell; ATG: autophagy-related; BMDC: bone marrow-derived dendritic cell; BODIPY: 4,4-difluoro-1,3,5,7,8-pentamethyl-4-bora-3a,4a-diaza-s-indacene; CSFE: carboxyfluorescein diacetate succinimidyl ester; DAPI: 4',6-diamidino-2-phenylindole; IFNG/IFN-γ: interferon gamma; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MHC: major histocompatibility complex; NLDC: neonatal liver-derived dendritic cell; PDCD1/PD-1: programmed cell death 1; PI: propidium iodide; PtdIns3K: class III phosphatidylinositol 3-kinase; PtdIns3P: phosphatidylinositol 3-phosphate; SERPINB/OVA: serine (or cysteine) peptidase inhibitor, clade B; TIMD4/TIM-4: T cell immunoglobulin and mucin domain containing 4.

CD8α+ Dendritic Cells Dictate Leukemia-Specific CD8+ T Cell Fates

APCs are essential for the orchestration of antitumor T cell responses. Batf3-lineage CD8α⁺ and CD103⁺ dendritic cells (DCs), in particular, are required for the spontaneous initiation of CD8⁺ T cell priming against solid tumors. In contrast, little is known about the APCs that regulate CD8⁺ T cell responses against hematological malignancies. Using an unbiased approach, we aimed to characterize the APCs responsible for regulating CD8⁺ T cell responses in a syngeneic murine leukemia model. We show with single-cell resolution that CD8α⁺ DCs alone acquire and cross-present leukemia Ags in vivo, culminating in the induction of leukemia-specific CD8⁺ T cell tolerance. Furthermore, we demonstrate that the mere acquisition of leukemia cell cargo is associated with a unique transcriptional program that may be important in regulating tolerogenic CD8α⁺ DC functions in mice with leukemia. Finally, we show that systemic CD8α⁺ DC activation with a TLR3 agonist completely prevents their ability to generate leukemia-specific CD8⁺ T cell tolerance in vivo, resulting instead in the induction of potent antileukemia T cell immunity and prolonged survival of leukemia-bearing mice. Together, our data reveal that Batf3-lineage DCs imprint disparate CD8⁺ T cell fates in hosts with solid tumors versus systemic leukemia.

Transfer of Cell-Surface Antigens by Scavenger Receptor CD36 Promotes Thymic Regulatory T Cell Receptor Repertoire Development and Allo-tolerance

The development of T cell tolerance in the thymus requires the presentation of host proteins by multiple antigen-presenting cell (APC) types. However, the importance of transferring host antigens from transcription factor AIRE-dependent medullary thymic epithelial cells (mTECs) to bone marrow (BM) APCs is unknown. We report that antigen was primarily transferred from mTECs to CD8α⁺ dendritic cells (DCs) and showed that CD36, a scavenger receptor selectively expressed on CD8α⁺ DCs, mediated the transfer of cell-surface, but not cytoplasmic, antigens. The absence of CD8α⁺ DCs or CD36 altered thymic T cell selection, as evidenced by TCR repertoire analysis and the loss of allo-tolerance in murine allogeneic BM transplantation (allo-BMT) studies. Decreases in these DCs and CD36 expression in peripheral blood of human allo-BMT patients correlated with graft-versus-host disease. Our findings suggest that CD36 facilitates transfer of mTEC-derived cell-surface antigen on CD8α⁺ DCs to promote tolerance to host antigens during homeostasis and allo-BMT.

Dendritic cell subsets

Dendritic cells (DC) are professional antigen presenting cells comprising a variety of subsets, as either resident or migrating cells, in lymphoid and non-lymphoid organs. In the steady state DC continually process and present antigens on MHCI and MHCII, processes that are highly upregulated upon activation. By expressing differential sets of pattern recognition receptors different DC subsets are able to respond to a range of pathogenic and danger stimuli, enabling functional specialisation of the DC. The knowledge of functional specialisation of DC subsets is key to efficient priming of T cells, to the design of effective vaccine adjuvants and to understanding the role of different DC in health and disease. This review outlines mouse and human steady state DC subsets and key attributes that define their distinct functions.

IL-12p40/IL-10 Producing preCD8α/Clec9A+ Dendritic Cells Are Induced in Neonates upon Listeria monocytogenes Infection

Infection by Listeria monocytogenes (Lm) causes serious sepsis and meningitis leading to mortality in neonates. This work explored the ability of CD11c^high lineage DCs to induce CD8⁺ T-cell immune protection against Lm in mice before 7 days of life, a period symbolized by the absence of murine IL-12p70-producing CD11c^highCD8α⁺ dendritic cells (DCs). We characterized a dominant functional Batf3-dependent precursor of CD11c^high DCs that is Clec9A⁺CD205⁺CD24⁺ but CD8α^- at 3 days of life. After Lm-OVA infection, these pre-DCs that cross-present Ag display the unique ability to produce high levels of IL-12p40 (not IL-12p70 nor IL-23), which enhances OVA-specific CD8⁺ T cell response, and regulatory IL-10 that limits OVA-specific CD8⁺ T cell response. Targeting these neonatal pre-DCs for the first time with a single treatment of anti-Clec9A-OVA antibody in combination with a DC activating agent such as poly(I:C) increased the protection against later exposure to the Lm-OVA strain. Poly(I:C) was shown to induce IL-12p40 production, but not IL-10 by neonatal pre-DCs. In conclusion, we identified a new biologically active precursor of Clec9A⁺ CD8α^- DCs, endowed with regulatory properties in early life that represents a valuable target to augment memory responses to vaccines.

Lm is a gram-positive food-borne pathogen that is the ethiological agent of listeriosis, a worldwide disease reported most frequently in developed countries. It can cause spontaneous septic abortions, fatal meningitis or encephalitis in immunocompromised and pregnant individuals. The murine model of systemic Lm infection has been demonstrated as a useful model to understand host resistance to intracellular pathogens. Neonates are highly susceptible to infections such as Lm, and display low responses to vaccines requiring IFN-γ producing T cells. In the present study, we characterized in murine neonates a precursor of conventional dendritic cells that is able to produce IL-12p40 and IL-10 cytokines and to modulate the development of the adaptive immune response, more particularly the CD8⁺ T cell response upon exposure to Lm. By targeting Lm-associated antigens to these conventional dendritic cell precursors in neonates, we succeeded to confer a partial protection to a lethal dose of Lm at the adult stage. Our study provides new insights into our understanding of the innate immune response to infections in early life and will help to design new vaccine strategies in newborns.

Cross-Presentation of Cell-Associated Antigens by MHC Class I in Dendritic Cell Subsets

Dendritic cells (DCs) have the unique ability to pick up dead cells carrying antigens in tissue and migrate to the lymph nodes where they can cross-present cell-associated antigens by MHC class I to CD8⁺ T cells. There is strong in vivo evidence that the mouse XCR1⁺ DCs subset acts as a key player in this process. The intracellular processes underlying cross-presentation remain controversial and several pathways have been proposed. Indeed, a wide number of studies have addressed the cellular process of cross-presentation in vitro using a variety of sources of antigen and antigen-presenting cells. Here, we review the in vivo and in vitro evidence supporting the current mechanistic models and disscuss their physiological relevance to the cross-presentation of cell-associated antigens by DCs subsets.

Batf3-independent langerin- CX3CR1- CD8α+ splenic DCs represent a precursor for classical cross-presenting CD8α+ DCs

This study tests the hypothesis that CD8α(+) DCs in the spleen of mice contain an immature precursor for functionally mature, "classical" cross-presenting CD8α(+) DCs. The lymphoid tissues contain a network of phenotypically distinct DCs with unique roles in surveillance and immunity. Splenic CD8α(+) DCs have been shown to exhibit a heightened capacity for phagocytosis of cellular material, secretion of IL-12, and cross-priming of CD8(+) T cells. However, this population can be subdivided further on the basis of expression of both langerin/CD207 and CX(3)CR1. We therefore evaluated the functional capacities of these different subsets. The CX(3)CR1(+) CD8α(+) DC subset does not express langerin and does not exhibit the classical features above. The CX(3)CR1(-) CD8α(+) DC can be divided into langerin-positive and negative populations, both of which express DEC205, Clec9A, and high basal levels of CD86. However, the langerin(+) CX(3)CR1(-) CD8α(+) subset has a superior capacity for acquiring cellular material and producing IL-12 and is more susceptible to activation-induced cell death. Significantly, following purification and adoptive transfer into new hosts, the langerin(-) CX(3)CR1(-) CD8α(+) subset survives longer, up-regulates expression of langerin, and becomes more susceptible to activation-induced cell death. Last, in contrast to langerin(+) CX(3)CR1(-) CD8α(+), the langerin(-) CX(3)CR1(-) CD8α(+) are still present in Batf3(-/-) mice. We conclude that the classical attributes of CD8α(+) DC are confined primarily to the langerin(+) CX(3)CR1(-) CD8α(+) DC population and that the langerin(-) CX(3)CR1(-) subset represents a Batf3-independent precursor to this mature population.

The mononuclear phagocyte system in homeostasis and disease: a role for heme oxygenase-1

SIGNIFICANCE

Heme oxygenase-1 (HO-1) is a potential therapeutic target in many diseases, especially those mediated by oxidative stress and inflammation. HO-1 expression appears to regulate the homeostatic activity and distribution of mononuclear phagocytes (MP) in lymphoid tissue under physiological conditions. It also regulates the ability of MP to modulate the inflammatory response to tissue injury.

RECENT ADVANCES

The induction of HO-1 within MP-particularly macrophages and dendritic cells-modulates the effector functions that they acquire after activation. These effector functions include cytokine production, surface receptor expression, maturation state, and polarization toward a pro- or anti-inflammatory phenotype. The importance of HO-1 in MP is emphasized by their expression of specific receptors that primarily function to ingest heme-containing substrate and deliver it to HO-1.

CRITICAL ISSUES

MP are the first immunological responders to tissue damage. They critically affect the outcome of injury to many organ systems, yet few therapies are currently available to specifically target MP during disease pathogenesis. Elucidation of the role of HO-1 expression in MP may help to direct broadly applicable therapies to clinical use that are based on the immunomodulatory capabilities of HO-1.

FUTURE DIRECTIONS

Unraveling the complexities of HO-1 expression specifically within MP will more completely define how HO-1 provides cytoprotection in vivo. The use of models in which HO-1 expression is specifically modulated in bone marrow-derived cells will allow for a more complete characterization of its immunoregulatory properties.

An AXL/LRP-1/RANBP9 complex mediates DC efferocytosis and antigen cross-presentation in vivo

The phagocytosis of apoptotic cells (ACs), or efferocytosis, by DCs is critical for self-tolerance and host defense. Although many efferocytosis-associated receptors have been described in vitro, the functionality of these receptors in vivo has not been explored in depth. Using a spleen efferocytosis assay and targeted genetic deletion in mice, we identified a multiprotein complex--composed of the receptor tyrosine kinase AXL, LDL receptor-related protein-1 (LRP-1), and RAN-binding protein 9 (RANBP9)--that mediates DC efferocytosis and antigen cross-presentation. We found that AXL bound ACs, but required LRP-1 to trigger internalization, in murine CD8α+ DCs and human-derived DCs. AXL and LRP-1 did not interact directly, but relied on RANBP9, which bound both AXL and LRP-1, to form the complex. In a coculture model of antigen presentation, the AXL/LRP-1/RANBP9 complex was used by DCs to cross-present AC-associated antigens to T cells. Furthermore, in a murine model of herpes simplex virus-1 infection, mice lacking DC-specific LRP-1, AXL, or RANBP9 had increased AC accumulation, defective viral antigen-specific CD8+ T cell activation, enhanced viral load, and decreased survival. The discovery of this multiprotein complex that mediates functionally important DC efferocytosis in vivo may have implications for future studies related to host defense and DC-based vaccines.

CD8α+ DCs can be induced in the absence of transcription factors Id2, Nfil3, and Batf3

Transcription factors Batf3, Id2, and Nfil3 are not essential for induced CD8α+ DC generation. Induced CD8α+ DCs can cross-present cellular antigens.

Transcriptional regulation of dendritic cell diversity

Dendritic cells (DCs) are specialized antigen presenting cells that are exquisitely adapted to sense pathogens and induce the development of adaptive immune responses. They form a complex network of phenotypically and functionally distinct subsets. Within this network, individual DC subsets display highly specific roles in local immunosurveillance, migration, and antigen presentation. This division of labor amongst DCs offers great potential to tune the immune response by harnessing subset-specific attributes of DCs in the clinical setting. Until recently, our understanding of DC subsets has been limited and paralleled by poor clinical translation and efficacy. We have now begun to unravel how different DC subsets develop within a complex multilayered system. These findings open up exciting possibilities for targeted manipulation of DC subsets. Furthermore, ground-breaking developments overcoming a major translational obstacle - identification of similar DC populations in mouse and man - now sets the stage for significant advances in the field. Here we explore the determinants that underpin cellular and transcriptional heterogeneity within the DC network, how these influence DC distribution and localization at steady-state, and the capacity of DCs to present antigens via direct or cross-presentation during pathogen infection.

Lymphoid organ-resident dendritic cells exhibit unique transcriptional fingerprints based on subset and site

Lymphoid organ-resident DC subsets are thought to play unique roles in determining the fate of T cell responses. Recent studies focusing on a single lymphoid organ identified molecular pathways that are differentially operative in each DC subset and led to the assumption that a given DC subset would more or less exhibit the same genomic and functional profiles throughout the body. Whether the local milieu in different anatomical sites can also influence the transcriptome of DC subsets has remained largely unexplored. Here, we interrogated the transcriptional relationships between lymphoid organ-resident DC subsets from spleen, gut- and skin-draining lymph nodes, and thymus of C57BL/6 mice. For this purpose, major resident DC subsets including CD4 and CD8 DCs were sorted at high purity and gene expression profiles were compared using microarray analysis. This investigation revealed that lymphoid organ-resident DC subsets exhibit divergent genomic programs across lymphoid organs. Interestingly, we also found that transcriptional and biochemical properties of a given DC subset can differ between lymphoid organs for lymphoid organ-resident DC subsets, but not plasmacytoid DCs, suggesting that determinants of the tissue milieu program resident DCs for essential site-specific functions.

Existence of CD8α-like dendritic cells with a conserved functional specialization and a common molecular signature in distant mammalian species

The mouse lymphoid organ-resident CD8alpha(+) dendritic cell (DC) subset is specialized in Ag presentation to CD8(+) T cells. Recent evidence shows that mouse nonlymphoid tissue CD103(+) DCs and human blood DC Ag 3(+) DCs share similarities with CD8alpha(+) DCs. We address here whether the organization of DC subsets is conserved across mammals in terms of gene expression signatures, phenotypic characteristics, and functional specialization, independently of the tissue of origin. We study the DC subsets that migrate from the skin in the ovine species that, like all domestic animals, belongs to the Laurasiatheria, a distinct phylogenetic clade from the supraprimates (human/mouse). We demonstrate that the minor sheep CD26(+) skin lymph DC subset shares significant transcriptomic similarities with mouse CD8alpha(+) and human blood DC Ag 3(+) DCs. This allowed the identification of a common set of phenotypic characteristics for CD8alpha-like DCs in the three mammalian species (i.e., SIRP(lo), CADM1(hi), CLEC9A(hi), CD205(hi), XCR1(hi)). Compared to CD26(-) DCs, the sheep CD26(+) DCs show 1) potent stimulation of allogeneic naive CD8(+) T cells with high selective induction of the Ifngamma and Il22 genes; 2) dominant efficacy in activating specific CD8(+) T cells against exogenous soluble Ag; and 3) selective expression of functional pathways associated with high capacity for Ag cross-presentation. Our results unravel a unifying definition of the CD8alpha(+)-like DCs across mammalian species and identify molecular candidates that could be used for the design of vaccines applying to mammals in general.

Dendritic cells in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) persists as a chronic inflammatory autoimmune disease and is characterized by the production of autoantibodies and immune complexes that affect multiple organs. The underlying mechanism that triggers and sustains disease are complex and involve certain susceptibility genes and environmental factors. There have been several immune mediators linked to SLE including cytokines and chemokines that have been reviewed elsewhere [ 1-3 ]. A number of articles have reviewed the role of B cells and T cells in SLE [ 4-10 ]. Here, we focus on the role of dendritic cells (DC) and innate immune factors that may regulate autoreactive B cells.

The synthetic TLR2 agonist BPPcysMPEG leads to efficient cross-priming against co-administered and linked antigens

The property of DC to generate effective CTL responses is influenced by TLR signaling. TLR ligands contain molecular signatures associated with pathogens, have an impact on the antigen processing and presentation by DC, and are being exploited as potential adjuvants. We hypothesized that the TLR2/6 heterodimer agonist S-[2,3-bispalmitoyiloxy-(2R)-propyl]-R-cysteinyl-amido-monomethoxyl polyethylene glycol (BPP), a synthetic derivative of the Mycoplasma macrophage activating lipopeptide-2, is a potent adjuvant for cross-priming against cellular antigens. Systemic administration of BPP-induced maturation of CD8alpha+ DC and CD8alpha- DC in the spleen and resulted in enhanced cross-presentation of intravenously co-administered antigen in mice. In addition, administration of BPP and cell-associated OVA generated an effective CTL response against OVA in vivo in a CD4+ T helper cell-dependent manner, but independent of IFN-alpha. Delivering antigenic peptides directly linked to BPP led to superior CTL immunity as compared to giving antigens and adjuvants admixed. In contrast to other TLR ligands, such as CpG, systemic activation of DC with BPP did not result in shut-down of antigen presentation by splenic DC subsets, although cross-priming against subsequently encountered antigens was reduced. Together, our data provide evidence that BPP is a potent stimulus to generate CTL via cross-priming.

The CD8+ dendritic cell subset

Mouse lymphoid tissues contain a subset of dendritic cells (DCs) expressing CD8 alpha together with a pattern of other surface molecules that distinguishes them from other DCs. These molecules include particular Toll-like receptor and C-type lectin pattern recognition receptors. A similar DC subset, although lacking CD8 expression, exists in humans. The mouse CD8(+) DCs are non-migrating resident DCs derived from a precursor, distinct from monocytes, that continuously seeds the lymphoid organs from bone marrow. They differ in several key functions from their CD8(-) DC neighbors. They efficiently cross-present exogenous cell-bound and soluble antigens on major histocompatibility complex class I. On activation, they are major producers of interleukin-12 and stimulate inflammatory responses. In steady state, they have immune regulatory properties and help maintain tolerance to self-tissues. During infection with intracellular pathogens, they become major presenters of pathogen antigens, promoting CD8(+) T-cell responses to the invading pathogens. Targeting vaccine antigens to the CD8(+) DCs has proved an effective way to induce cytotoxic T lymphocytes and antibody responses.

Differential expression of pathogen-recognition molecules between dendritic cell subsets revealed by plasma membrane proteomic analysis

Dendritic cells (DC) are comprised of several subsets with distinct functions, differing in their capacity to respond to pathogen products. To gain novel insights into their pathogen specificity, we compared the protein composition of the plasma membrane of CD8+ and CD8- DC, directly isolated from mouse spleens. Differences in protein expression were determined using semi-quantitative high-resolution mass spectrometry of label-free plasma membrane-enriched fractions. Our comparative proteomic analysis detected over 1500 proteins, revealing broad differences in expression of pathogen receptors, adhesion molecules and T-cell regulatory molecules. Many of these findings were validated using flow cytometry and Western Blot analysis of integral and luminal surface-associated membrane proteins. This analysis provides major advantages over genomic approaches as it directly measures protein expression at a particular location. Our study highlights the diversity of surface protein expression amongst components of the DC network.

A new triggering receptor expressed on myeloid cells (Trem) family member, Trem-like 4, binds to dead cells and is a DNAX activation protein 12-linked marker for subsets of mouse macrophages and dendritic cells

Dendritic cells (DCs) are professional APCs that can control immune responses against self and altered self, typically foreign, determinants. DCs can be divided into several subsets, including CD8alpha(+) and CD8alpha(-) DCs. These subsets possess specific functions. For example, mouse splenic CD8alpha(+), but not CD8alpha(-) DCs selectively take up dying cells and cross-present cell-associated Ags to naive T cells. In this study, we identified genes that were more expressed in CD8alpha(+) than CD8alpha(-) DCs by microarray analysis. Only one of these genes, when the extracellular domains were linked to human IgG Fc domain, could bind to late apoptotic or necrotic cells. This gene was a new member of the triggering receptor expressed on myeloid cells (Trem) family, Trem-like 4 (Treml4). Treml4 mRNA and protein, the latter detected with a new mAb, were predominantly expressed in spleen. Treml4, like other Trem family members, could associate with the adaptor molecule DNAX activation protein 12 kDa, but neither DNAX activation protein 10 kDa nor FcRgamma. Consistent with the microarray data, we confirmed that Treml4 protein was more expressed on CD8alpha(+) than CD8alpha(-) DCs, and we also found that Treml4 was expressed at high levels on splenic macrophages in spleen, particularly red pulp and marginal metallophilic macrophages. In addition, Treml4 expression on DCs was not changed after maturation induced by TLR ligands. Thus, Treml4 is a new Trem family molecule that is abundantly expressed on CD8alpha(+) DCs and subsets of splenic resident macrophages, and can recognize dead cells by different types of phagocytes in spleen.

Tim-3 mediates phagocytosis of apoptotic cells and cross-presentation

Phagocytes such as macrophages and dendritic cells (DCs) engulf apoptotic cells to maintain peripheral immune tolerance. However, the mechanism for the recognition of dying cells by phagocytes is not fully understood. Here, we demonstrate that T-cell immunoglobulin mucin-3 (Tim-3) recognizes apoptotic cells through the FG loop in the IgV domain, and is crucial for clearance of apoptotic cells by phagocytes. Whereas Tim-4 is highly expressed on peritoneal resident macrophages, Tim-3 is expressed on peritoneal exudate macrophages, monocytes, and splenic DCs, indicating distinct Tim-mediated phagocytic pathways used by different phagocytes. Furthermore, phagocytosis of apoptotic cells by CD8(+) DCs is inhibited by anti-Tim-3 mAb, resulting in a reduced cross-presentation of dying cell-associated antigens in vitro and in vivo. Administration of anti-Tim-3 as well as anti-Tim-4 mAb induces autoantibody production. These results indicate a crucial role for Tim-3 in phagocytosis of apoptotic cells and cross-presentation, which may be linked to peripheral tolerance.

A Plasmodium yoelii soluble factor inhibits the phenotypic maturation of dendritic cells

Background

Infection with the protozoan parasite Plasmodium is the cause of malaria. Plasmodium infects host erythrocytes causing the pathology of the disease. Plasmodium-infected erythrocytes can modulate the maturation of dendritic cells (DCs) and alter their capacity to activate T cells.

Methods

Mice infected with Plasmodium yoelii and isolated P. yoelii-infected erythrocytes were used to study their effect on the maturation of mouse dendritic cells.

Results

DCs are not able to mature in response to LPS injection during the late stage of P. yoelii infection in mice, indicating impaired functionality of these cells in vivo. P. yoelii- infected erythrocytes inhibit the maturation of DCs in vitro in a dose-dependent manner, which is consistent with the inhibition found during late infection when parasite burden is highest. The inhibition of DC maturation and the cytokine secretion profile of DCs are modulated by soluble factors released by P. yoelii-infected erythrocytes. A small, heat-stable, non-hydrophobic molecule of P. yoelii-infected erythrocytes rapidly inhibits the LPS induced phenotypic maturation of DCs in a reversible manner.

Conclusion

These findings add evidence to the malaria associated immune suppression in vivo and in vitro and provide insight into the nature and mechanism of the Plasmodium factor(s) responsible for altering DC functions.

Redundancy renders the glycoprotein 96 receptor scavenger receptor A dispensable for cross priming in vivo

CD8(+) T cells (T(CD8+)) differentiate into effector cells following recognition of specific peptide-major histocompatibility complex (MHC) class I complexes (pMHC-I) on the surface of professional APCs (pAPCs), such as dendritic cells. Antigenic pMHC-I can be generated from two spatially distinct sources. The direct presentation pathway involves generation of peptide from protein substrate synthesized within the cell that is presenting the pMHC-I. Alternatively, the cross presentation pathway involves presentation of antigen that is not synthesized within the presenting cell, but is derived from exogenous proteins synthesized within other donor cells. The mechanisms by which cross presentation of exogenous antigens occur in vivo remain controversial. The C-type lectin scavenger receptor A (SR-A) has been implicated in a number of potential cross presentation pathways, including the presentation of peptide bound to heat shock proteins, such as glycoprotein 96 (gp96), and the transfer of pMHC-I from a donor cell to the pAPC. We demonstrate here that initiation of T(CD8+) responses is normal in mice lacking SR-A, and that the redundancy of ligand binding exhibited by the SR family is likely to be an important mechanism that ensures cross presentation in vivo. These observations emphasize the requirement to target multiple receptors and antigen-processing pathways during the rational design of vaccines aimed at eliciting protective T(CD8+).

Activation of plasmacytoid dendritic cells

Four years after the discovery of mouse plasmacytoid dendritic cells (pDC), pDC are still very much an 'enigmatic' cell type. It is clear that pDC are potent producers of type I IFN in response to viral, bacterial and even mammalian nucleotides. The role that they play in vivo before and after activation is still under scrutiny. This review concentrates on the pathways to activation of pDC, examining the activating ligands, receptors and signalling molecules that are known to be involved, and the relevance of these activation pathways to human disease.

Dendritic cell subsets and toll-like receptors

Toll-like receptors exist as highly conserved pathogen sensors throughout the animal kingdom and they represent a key family of molecules bridging the ancient innate and adaptive immune systems. The first molecules of adaptive immunity appeared in the cartilaginous fishes and, with these, major histocompatibility proteins and cells expressing these molecules, and thus, by definition, the advent of antigen-presenting cells and the "professional" antigen-presenting cells, the dendritic cells. Dendritic cells themselves are highly specialized subsets of cells with the major roles of antigen presentation and stimulation of lymphocytes. The dendritic cell functions of inducing immunity are regulated by their own activation status, which is governed by their encounter with pathogen-associated molecular patterns that signal through pattern recognition receptors, including Toll-like receptors, expressed at the surface and within the cytoplasm and endosomal membranes of dendritic cells. Thus although dendritic cells play a crucial role in the induction of adaptive immunity, the adaptive response is itself initiated at the level of ancient receptors of the innate immune system. A further degree in the complexity of dendritic cell activation is established by the fact that not all dendritic cells are equal. Dendritic cells exist as multiple subsets that vary in location, function, and phenotype. Distinct dendritic cell subsets display great variation in the type of Toll-like receptors expressed and consequently variation in the type of pathogens sensed and the subsequent type of immune responses initiated.

A role for protein misfolding in immunogenicity of biopharmaceuticals

For largely unknown reasons, biopharmaceuticals evoke potentially harmful antibody formation. Such antibodies can inhibit drug efficacy and, when directed against endogenous proteins, cause life-threatening complications. Insight into the mechanisms by which biopharmaceuticals break tolerance and induce an immune response will contribute to finding solutions to prevent this adverse effect. Using a transgenic mouse model, we here demonstrate that protein misfolding, detected with the use of tissue-type plasminogen activator and thioflavin T, markers of amyloid-like properties, results in breaking of tolerance. In wild-type mice, misfolding enhances protein immunogenicity. Several commercially available biopharmaceutical products were found to contain misfolded proteins. In some cases, the level of misfolded protein was found to increase upon storage under conditions prescribed by the manufacturer. Our results indicate that misfolding of therapeutic proteins is an immunogenic signal and a risk factor for immunogenicity. These findings offer novel possibilities to detect immunogenic protein entities with tPA and reduce immunogenicity of biopharmaceuticals.

Dendritic cells: translating innate to adaptive immunity

The innate immune system provides many ways to quickly resist infection. The two best-studied defenses in dendritic cells (DCs) are the production of protective cytokines-like interleukin (IL)-12 and type I interferons-and the activation and expansion of innate lymphocytes. IL-12 and type I interferons influence distinct steps in the adaptive immune response of lymphocytes, including the polarization of T-helper type 1 (Th1) CD4+ T cells, the development of cytolytic T cells and memory, and the antibody response. DCs have many other innate features that do not by themselves provide innate resistance but are critical for the induction of adaptive immunity. We have emphasized three intricate and innate properties of DCs that account for their sentinel and sensor roles in the immune system: (1) special mechanisms for antigen capture and processing, (2) the capacity to migrate to defined sites in lymphoid organs, especially the T cell areas, to initiate immunity, and (3) their rapid differentiation or maturation in response to a variety of stimuli ranging from Toll-like receptor (TLR) ligands to many other nonmicrobial factors such as cytokines, innate lymphocytes, and immune complexes. The combination of innate defenses and innate physiological properties allows DCs to serve as a major link between innate and adaptive immunity. DCs and their subsets contribute to many subjects that are ripe for study including memory, B cell responses, mucosal immunity, tolerance, and vaccine design. DC biology should continue to be helpful in understanding pathogenesis and protection in the setting of prevalent clinical problems.

RUNX3 negatively regulates CD36 expression in myeloid cell lines

CD36 is a member of the scavenger receptor type B family implicated in the binding of lipoproteins, phosphatidylserine, thrombospondin-1, and the uptake of long-chain fatty acids. On mononuclear phagocytes, recognition of apoptotic cells by CD36 contributes to peripheral tolerance and prevention of autoimmunity by impairing dendritic cell (DC) maturation. Besides, CD36 acts as a coreceptor with TLR2/6 for sensing microbial diacylglycerides, and its deficiency leads to increased susceptibility to Staphylococcus aureus infections. The RUNX3 transcription factor participates in reprogramming DC transcription after pathogen recognition, and its defective expression leads to abnormally accelerated DC maturation. We present evidence that CD36 expression is negatively regulated by the RUNX3 transcription factor during myeloid cell differentiation and activation. In molecular terms, RUNX3 impairs the activity of the proximal regulatory region of the CD36 gene in myeloid cells through in vitro recognition of two functional RUNX-binding elements. Moreover, RUNX3 occupies the CD36 gene proximal regulatory region in vivo, and its overexpression in myeloid cells results in drastically diminished CD36 expression. The down-regulation of CD36 expression by RUNX3 implies that this transcription factor could impair harmful autoimmune responses by contributing to the loss of pathogen- and apoptotic cell-recognition capabilities by mature DCs.

The mannose receptor mediates uptake of soluble but not of cell-associated antigen for cross-presentation

The mannose receptor (MR) has been implicated in the recognition and clearance of microorganisms and serum glycoproteins. Its endocytic function has been studied extensively using macrophages, although it is expressed by a variety of cell types, including dendritic cells (DC). In this study, we investigated its role in Ag presentation by DC using MR-/- mice. Uptake of the model Ag, soluble OVA, by bone marrow-derived DC and in vitro activation of OVA-specific CD8 T cells (OT-I cells) strictly depended on the MR. In vivo, MR deficiency impaired endocytosis of soluble OVA by DC and concomitant OT-I cell activation. No alterations in the DC subtype composition in MR-/- mice were accountable. Uptake of cell-associated OVA was unaffected by MR deficiency, resulting in unchanged activation of OT-I cells. These findings demonstrate that DC use the MR for endocytosis of a particular Ag type intended for cross-presentation.

The apoptotic-cell receptor CR3, but not alphavbeta5, is a regulator of human dendritic-cell immunostimulatory function

Dendritic cells (DCs) that capture apoptotic cells (ACs) in the steady state mediate peripheral tolerance to self-antigens. ACs are recognized by an array of receptors on DCs, the redundancy of which is not completely defined. We made use of an AC surrogate system to address the individual roles of the alphavbeta5 and complement receptors (CRs) in the phagocytosis and induction of immunity. CR3 and CR4, while substantially less efficient than alphavbeta5 in internalizing ACs, initiate signals that render DCs tolerogenic. Responding T cells show impaired proliferation and IFNgamma production and subsequently die by apoptosis. While tolerogenic DCs are not induced via alphavbeta5, coligation of CR3 and alphavbeta5 maintains the DC's tolerogenic profile. This immunomodulatory role, however, is countered by a significant inflammatory stimulus such as bacterial infection. Overall, our data suggest that under steady-state conditions, signaling via CRs predominates to render DCs tolerogenic.

Malaria pigment paralyzes dendritic cells

The capacity of malarial infection to suppress the patient's immune responses both to the parasite and to other antigens has long puzzled researchers. A prime suspect, the parasite-produced pigment hemozoin, has now been clearly shown to mediate immunosuppression by inhibiting dendritic cell activity.

The delivery of an antigen from the endocytic compartment into the cytosol for cross-presentation is restricted to early immature dendritic cells

Dendritic cells (DCs) are the only antigen-presenting cell population having a cross-presentation capacity. For cross-presentation, however, the intracellular antigen-processing pathway and its regulatory mechanism have not been defined. Here we report the differences in cross-presentation ability among murine bone marrow-derived immature DC, early immature day8-DC and late immature day10-DC, and fully mature day10 + lipopolysaccharide DC. Day8-DCs and day10-DCs show an immature phenotypic profile but are different in morphology. Day8-DCs can internalize an abundant volume of exogenous soluble ovalbumin (OVA) and result in cross-presentation. In contrast, day10-DCs are not able to cross-present, although they maintain efficient macropinocytosis. Exogenously internalized OVA antigens are stored in the endocytic compartments. The endocytic compartments are temporarily maintained at mildly acidic pH in day8-DCs and are rapidly acidified in day10-DCs after uptake of antigens. We show that OVA antigens accumulated in the endocytic compartments move into the cytosol in day8-DCs but do not in day10-DCs. NH(4)Cl-treatment, which neutralizes the acidic endocytic compartments and/or delays endosomal maturation, restores day10-DCs for transport the stored OVA antigens from the endocytic compartments into the cytosol. Diphenyleneiodonium chloride-treatment, which acidifies the endocytic compartments, decreases an amount of transported OVA antigen into the cytosol in day8-DCs. These data indicate that only the early immature stage of DC interferes with endosomal maturation, even after uptake of exogenous antigens, and then transports the antigens into the cytosol.

Apoptotic cells at the crossroads of tolerance and immunity

Clearance of apoptotic cells by phagocytes can result in either anti-inflammatory and immunosuppressive effects or prostimulatory consequences through presentation of cell-associated antigens to T cells. The differences in outcome are due to the conditions under which apoptosis is induced, the type of phagocytic cell, the nature of the receptors involved in apoptotic cell capture, and the milieu in which phagocytosis of apoptotic cells takes place. Preferential ligation of specific receptors on professional antigen-presenting cells (dendritic cells) has been proposed to induce potentially tolerogenic signals. On the other hand, dendritic cells can efficiently process and present antigens from pathogen-infected apoptotic cells to T cells. In this review, we discuss how apoptotic cells manipulate immunity through interactions with dendritic cells.

IPC: professional type 1 interferon-producing cells and plasmacytoid dendritic cell precursors

Type 1 interferon-(alpha, beta, omega)-producing cells (IPCs), also known as plasmacytoid dendritic cell precursors (pDCs), represent 0.2%-0.8% of peripheral blood mononuclear cells in both humans and mice. IPCs display plasma cell morphology, selectively express Toll-like receptor (TLR)-7 and TLR9, and are specialized in rapidly secreting massive amounts of type 1 interferon following viral stimulation. IPCs can promote the function of natural killer cells, B cells, T cells, and myeloid DCs through type 1 interferons during an antiviral immune response. At a later stage of viral infection, IPCs differentiate into a unique type of mature dendritic cell, which directly regulates the function of T cells and thus links innate and adaptive immune responses. After more than two decades of effort by researchers, IPCs finally claim their place in the hematopoietic chart as the most important cell type in antiviral innate immunity. Understanding IPC biology holds future promise for developing cures for infectious diseases, cancer, and autoimmune diseases.

Vaccines that facilitate antigen entry into dendritic cells

Although vaccines have been highly successful in preventing and treating many infectious diseases (including smallpox, polio and diphtheria) diseases prevalent in the developing world such as malaria and HIV, that suppress the host immune system, require new, multiple strategies that will be defined by our growing understanding of specific immune activation. The definition of adjuvants, previously thought of as any substance that enhanced the immunogenicity of antigen, could now include soluble mediators and antigenic carriers that interact with surface molecules present on DC (e.g. LPS, Flt3L, heat shock protein) particulate antigens which are taken up by mechanisms available to APC but not other cell types (e.g. immunostimulatory complexes, latex, polystyrene particles) and viral/bacterial vectors that infect antigen presenting cells (e.g. vaccinia, lentivirus, adenovirus). These approaches, summarized herein, have shown potential in vaccinating against disease in animal models, and in some cases in humans. Of these, particle-antigen conjugates provide rapid formulation of the vaccine, easy storage and wide application, with both carrier and adjuvant functions that activate DC. Combined vaccines of the future could use adjuvants such as virus-like particles and particles targeted towards a predominant cellular type or immune response, with target cell activation enhanced by growth factors or maturation signals prior to, or during immunization. Collectively, these new additions to adjuvant technology provide opportunities for more specific immune regulation than previously available.

Selective Rac1 inhibition in dendritic cells diminishes apoptotic cell uptake and cross-presentation in vivo

To better understand the influence of cytoskeletal regulation on dendritic cell (DC) function in vivo, the Rho guanosine triphosphatase (GTPase) Rac1 was selectively inhibited in DCs in transgenic (Tg) mice. Although transgene expression did not interfere with the migratory capacities of DC in vivo, a decreased uptake of fluorescent probes was observed. Interestingly, the absence of full Rac1 function most strongly affected the development and function of CD8+ DCs. Apoptotic cell uptake was severely reduced in Tg mice, impairing subsequent DC-mediated cross-presentation and priming of bacteria-specific T-cell responses. These findings highlight a special role for Rac1 in the capacity of CD8+ DCs to endocytose apoptotic cells and prime T cells via cross-presentation.

Pathways for antigen cross presentation

Dendritic cells (DCs) have the unique ability to capture cellular tissue antigens, and to present them on MHC class I molecules to antigen-specific CD8(+) T lymphocytes after migration to the draining lymph nodes. This process, called "cross presentation" can lead either to the tolerization or activation of antigen-specific CD8(+) T cells. Antigen capture is believed to occur by phagocytosis of antigen-bearing dead cells. Recent studies suggest that the antigen transferred from the phagocytosed cell to the DC during cross presentation is a proteasome substrate, rather than a proteasomal degradation product. In most cases, the formation of the peptide-MHC class I complexes in DCs requires the export of protein antigens from phagosomes to the cytosol, where they undergo proteasomal degradation. The resulting peptides are then translocated by TAP to the lumen of a cross presentation-loading compartment, for association to MHC class I under the control of chaperones and oxido-reductases. This loading compartment may be either the endoplasmic reticulum (ER) or a mix phagosome-ER compartment. MHC class I egress from the loading compartment to cell surface remains to be analyzed.

Stromal Cells Direct Local Differentiation of Regulatory Dendritic Cells

CD11c(hi) dendritic cells (DC) play an essential role during the initiation of cell-mediated immunity. Recently, CD11c(lo)CD45RB(hi) DC with regulatory properties have been described. However, the origins of regulatory DC are poorly understood. Here, we show that spleen-derived stromal cells promote selective development of CD11c(lo)CD45RB(+) IL-10-producing regulatory DC from lineage-negative c-kit(+) progenitor cells. These DC have the capacity to suppress T cell responses and induce IL-10-producing regulatory T cells in vitro and to induce antigen-specific tolerance in vivo. Furthermore, stromal cells from mice infected with Leishmania donovani more effectively supported differentiation of these highly potent regulatory DC. The ability of tissue stromal cells to direct the development of DC with a regulatory phenotype thus provides a new mechanism for local immune regulation.

Cross-presentation: dendritic cells and macrophages bite off more than they can chew!

As immunologists, our knowledge of the molecular mechanisms which underlie the presentation of antigens derived from extracellular or 'exogenous' sources to CD8 cytotoxic lymphocytes (CTL) has been limited. This process, termed 'cross-presentation', has been linked to the elicitation of protective CTL responses against tumours and may be extremely important in generating immune responses against clinically relevant pathogens that do not infect tissues of haemopoietic origin. It is now known that cross-presentation of exogenous antigens on major histocompatibility complex (MHC) class I occurs through several distinct cellular pathways. In this review we outline and discuss some recent advances in our understanding of these pathways.

Relationships among murine CD11c(high) dendritic cell subsets as revealed by baseline gene expression patterns

The functional relationships and properties of different subtypes of dendritic cells (DC) remain largely undefined. To better characterize these cells, we used global gene analysis to determine gene expression patterns among murine CD11c(high) DC subsets. CD4(+), CD8alpha(+), and CD8alpha(-) CD4(-) (double negative (DN)) DC were purified from spleens of normal C57/BL6 mice and analyzed using Affymetrix microarrays. The CD4(+) and CD8alpha(+) DC subsets showed distinct basal expression profiles differing by >200 individual genes. These included known DC subset markers as well as previously unrecognized, differentially expressed CD Ags such as CD1d, CD5, CD22, and CD72. Flow cytometric analysis confirmed differential expression in nine of nine cases, thereby validating the microarray analysis. Interestingly, the microarray expression profiles for DN cells strongly resembled those of CD4(+) DC, differing from them by <25 genes. This suggests that CD4(+) and DN DC are closely related phylogenetically, whereas CD8alpha(+) DC represent a more distant lineage, supporting the historical distinction between CD8alpha(+) and CD8alpha(-) DC. However, staining patterns revealed that in contrast to CD4(+) DC, the DN subset is heterogeneous and comprises at least two subpopulations. Gene Ontology and literature mining analyses of genes expressed differentially among DC subsets indicated strong associations with immune response parameters as well as cell differentiation and signaling. Such associations offer clues to possible unique functions of the CD11c(high) DC subsets that to date have been difficult to define as rigid distinctions.

Phenotype and function of rat dendritic cell subsets

Dendritic cells (DC) comprise phenotypically-distinct subsets that sub-serve distinct functions in immune induction. Understanding the biology of DC subsets in vivo is crucial for the understanding of immune regulation and its perturbations in disease. This review focuses on the phenotype and functions of rat DC subsets and compares these with subsets identified in other species. Our research has concentrated on DC migrating in lymph. DC migrate constitutively from peripheral tissues to draining nodes, probably to induce/maintain tolerance to self- or harmless foreign antigens. After removal of mesenteric lymph nodes (MLN) in the rat, healing of afferent and efferent lymphatics permits migrating intestinal DC (iLDC) to be collected from the thoracic duct. We have shown that iLDC consist of least two subsets that differ in phenotype, in situ distribution and function. CD4+/SIRPalpha+ iLDC are highly immunostimulatory, but are excluded from T cell areas of MLN. In contrast, CD4-/SIRPalpha- iLDC are less potent stimulators of T cells, but carry material from apoptotic enterocytes to T cell areas of MLN. Similar subsets exist in both lymph nodes and spleen. It has been shown that phenotypically-similar subsets migrate in skin-draining lymph in cattle and sheep. We and others have shown that splenic CD4-/SIRPalpha- DC can phagocytose allogeneic cells in vitro, are poor stimulators of CD8+ T cells, and can lyse NK-sensitive target cells. Although some of our data suggest that rat CD4-/SIRPalpha- DC may equate to murine CD8+ DC, there is at present insufficient evidence to be confident of this.