Dendritic cell development: A personal historical perspective

Spatiotemporal Adaptations of Macrophage and Dendritic Cell Development and Function

Macrophages and conventional dendritic cells (cDCs) are distributed throughout the body, maintaining tissue homeostasis and tolerance to self and orchestrating innate and adaptive immunity against infection and cancer. As they complement each other, it is important to understand how they cooperate and the mechanisms that integrate their functions. Both are exposed to commensal microbes, pathogens, and other environmental challenges that differ widely among anatomical locations and over time. To adjust to these varying conditions, macrophages and cDCs acquire spatiotemporal adaptations (STAs) at different stages of their life cycle that determine how they respond to infection. The STAs acquired in response to previous infections can result in increased responsiveness to infection, termed training, or in reduced responses, termed paralysis, which in extreme cases can cause immunosuppression. Understanding the developmental stage and location where macrophages and cDCs acquire their STAs, and the molecular and cellular players involved in their induction, may afford opportunities to harness their beneficial outcomes and avoid or reverse their deleterious effects. Here we review our current understanding of macrophage and cDC development, life cycle, function, and STA acquisition before, during, and after infection. We propose a unified framework to explain how these two cell types adjust their activities to changing conditions over space and time to coordinate their immunosurveillance functions. Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

DEC-205 receptor-mediated long-circling nanoliposome as an antigen and Eucommia ulmoides polysaccharide delivery system enhances the immune response via facilitating dendritic cells maturation

DEC-205 receptor-mediated dendritic cells (DC) targeting nanoliposomes is a promising delivery system in eliciting an immune response against pathogens. When this delivery system carries both antigen and immunomodulator, it can effectively regulate the DC function as well as the initial T cell response. To maximize the desired therapeutic effects of Eucommia ulmoides Oliv. polysaccharides (EUPS), and induce an efficient humoral and cellular immune response against an antigen, we encapsulated the OVA and EUPS in long-circling nanoliposomes and conjugated it with anti-DEC-205 receptor antibody to obtain a DEC-205-targeted nanoliposomes (anti-DEC-205-EUPS-OVA-LPSM). The physicochemical properties and immune-modulating effects were investigated in vitro and in vivo by a series of the experiment to evaluate the targeting efficiency of anti-DEC-205-EUPS-OVA-LPSM. In vitro, anti-DEC-205-EUPS-OVA-LPSM (160 μg mL⁻¹) could enhance DCs proliferation and increase their phagocytic efficiency. In vivo anti-DEC-205-EUPS-OVA-LPSM remarkably promoted the OVA-specific IgG and IgG isotypes levels, enhanced the splenocyte proliferation, and induced the NK cell and CTL cytotoxicity. Besides, the anti-DEC-205-EUPS-OVA-LPSM enhanced the maturation of DCs. These findings suggest that the DEC-205 receptor antibody-conjugated EUPS nanoliposome can act as an efficient antigen delivery system to enhance the cellular and humoral immune response by promoting DC maturation. This indicates that the anti-DEC-205-EUPS-OVA-LPSM has significant potential as an immune-enhancing agent and antigen delivery system.

Antigen presentation in cancer: insights into tumour immunogenicity and immune evasion

Immune checkpoint blockade, which blocks inhibitory signals of T cell activation, has shown tremendous success in treating cancer, although success still remains limited to a fraction of patients. To date, clinically effective CD8⁺ T cell responses appear to target predominantly antigens derived from tumour-specific mutations that accumulate in cancer, also called neoantigens. Tumour antigens are displayed on the surface of cells by class I human leukocyte antigens (HLA-I). To elicit an effective antitumour response, antigen presentation has to be successful at two distinct events: first, cancer antigens have to be taken up by dendritic cells (DCs) and cross-presented for CD8⁺ T cell priming. Second, the antigens have to be directly presented by the tumour for recognition by primed CD8⁺ T cells and killing. Tumours exploit multiple escape mechanisms to evade immune recognition at both of these steps. Here, we review the tumour-derived factors modulating DC function, and we summarize evidence of immune evasion by means of quantitative modulation or qualitative alteration of the antigen repertoire presented on tumours. These mechanisms include modulation of antigen expression, HLA-I surface levels, alterations in the antigen processing and presentation machinery in tumour cells. Lastly, as complete abrogation of antigen presentation can lead to natural killer (NK) cell-mediated tumour killing, we also discuss how tumours can harbour antigen presentation defects and still evade NK cell recognition.

Single-cell analyses reveal the clonal and molecular aetiology of Flt3L-induced emergency dendritic cell development

Regulation of haematopoietic stem and progenitor cell (HSPC) fate is crucial during homeostasis and under stress conditions. Here we examine the aetiology of the Flt3 ligand (Flt3L)-mediated increase of type 1 conventional dendritic cells (cDC1s). Using cellular barcoding we demonstrate this occurs through selective clonal expansion of HSPCs that are primed to produce cDC1s and not through activation of cDC1 fate by other HSPCs. In particular, multi/oligo-potent clones selectively amplify their cDC1 output, without compromising the production of other lineages, via a process we term tuning. We then develop Divi-Seq to simultaneously profile the division history, surface phenotype and transcriptome of individual HSPCs. We discover that Flt3L-responsive HSPCs maintain a proliferative 'early progenitor'-like state, leading to the selective expansion of multiple transitional cDC1-primed progenitor stages that are marked by Irf8 expression. These findings define the mechanistic action of Flt3L through clonal tuning, which has important implications for other models of 'emergency' haematopoiesis.

Dendritic Cells Revisited

Dendritic cells (DCs) possess the ability to integrate information about their environment and communicate it to other leukocytes, shaping adaptive and innate immunity. Over the years, a variety of cell types have been called DCs on the basis of phenotypic and functional attributes. Here, we refocus attention on conventional DCs (cDCs), a discrete cell lineage by ontogenetic and gene expression criteria that best corresponds to the cells originally described in the 1970s. We summarize current knowledge of mouse and human cDC subsets and describe their hematopoietic development and their phenotypic and functional attributes. We hope that our effort to review the basic features of cDC biology and distinguish cDCs from related cell types brings to the fore the remarkable properties of this cell type while shedding some light on the seemingly inordinate complexity of the DC field.

The mechanisms of hydroxychloroquine in rheumatoid arthritis treatment: Inhibition of dendritic cell functions via Toll like receptor 9 signaling

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HCQ efficiently inhibited DC phenotypic and functional maturation stimulated by serum from RA patients.

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HCQ prevented progression of arthritis by inhibiting DC maturation and migration from peripheral blood to LNs.

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HCQ inhibited CpG ODN 1826-activated BMDC maturation and migration.

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The effect of HCQ on DCs was related to the block in TLR9 signaling.

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The development of arthritis was impaired in TLR9^−/− mice.

Hydroxychloroquine (HCQ) is one of the most commonly prescribed immune-suppressants in treating rheumatoid arthritis (RA). Our previous research showed that HCQ suppressed RA development by inhibiting T follicular helper (Tfh) cells directly. Dendritic cells (DCs) serve as the link between innate and acquired immunity. Whether HCQ suppressed Tfh cell through DCs was not clear. In current study, we found that HCQ efficiently inhibited CD86, chemokine (C-X-C motif) receptor 4 (CXCR4) expression and interferon-α (IFN-α) secretion of healthy donor derived purified DCs stimulated by RA patient serum. To mimic RA, collagen-induced arthritis (CIA) mouse model was used and treated with HCQ daily for fifty-four days prior to sacrifice. We found HCQ inhibited DC maturation and migration to lymph nodes (LNs), manifested as down-regulated expression of CD40, CD80, CD86, MHCII (I-A^q) on LN DCs. In addition, HCQ reduced the level of chemokine receptor 7 (CCR7) and L-selectin on peripheral blood DCs and diminished percentage of LN DCs. Of note, HCQ only inhibited CpG ODN 1826-induced IL-12 secretion by bone marrow DCs (BMDCs) stimulated by various toll like receptor (TLR) agonists. Mechanistically, HCQ down-regulated the expression of TLR9 not only in healthy donor PBMC-derived DCs stimulated by RA patient serum, but also in LN DCs of CIA mice and CpG-activated BMDCs. Furthermore, arthritis scores in TLR9^−/− mice were much lower than that in wild type mice with impaired maturity and migration capability of DCs. Collectively, activation of DCs contributes to the pathogenesis of RA and HCQ shows protective effects on RA by inhibition of DC activation via blocking TLR9.

Dendritic cell development at a clonal level within a revised 'continuous' model of haematopoiesis

Understanding development of the dendritic cell (DC) subtypes continues to evolve. The origin and relationship of conventional DC type 1 (cDC1), cDC type 2 (cDC2) and plasmacytoid DCs (pDCs) to each other, and in relation to classic myeloid and lymphoid cells, has had a long and controversial history and is still not fully resolved. This review summarises the technological developments and findings that have been achieved at a clonal level, and how that has enhanced our knowledge of the process. It summarises the single cell lineage tracing technologies that have emerged, their application in in vitro and in vivo studies, in both mouse and human settings, and places the findings in a wider context of understanding haematopoiesis at a single cell or clonal level. In particular, it addresses the fate heterogeneity observed in many phenotypically defined progenitor subsets and how these findings have led to a departure from the classic ball-and-stick models of haematopoiesis to the emerging continuous model. Prior contradictions in DC development may be reconciled if they are framed within this revised model, where commitment to a lineage or cell type does not occur in an all-or-nothing process in defined progenitors but rather can occur at many stages of haematopoiesis in a dynamic process.