Characterization of plasmacytoid dendritic cells in bone marrow of pig-tailed macaques

Dendritic Cell Responses and Function in Malaria

Malaria remains a serious threat to global health. Sustained malaria control and, eventually, eradication will only be achieved with a broadly effective malaria vaccine. Yet a fundamental lack of knowledge about how antimalarial immunity is acquired has hindered vaccine development efforts to date. Understanding how malaria-causing parasites modulate the host immune system, specifically dendritic cells (DCs), key initiators of adaptive and vaccine antigen-based immune responses, is vital for effective vaccine design. This review comprehensively summarizes how exposure to Plasmodium spp. impacts human DC function in vivo and in vitro. We have highlighted the heterogeneity of the data observed in these studies, compared and critiqued the models used to generate our current understanding of DC function in malaria, and examined the mechanisms by which Plasmodium spp. mediate these effects. This review highlights potential research directions which could lead to improved efficacy of existing vaccines, and outlines novel targets for next-generation vaccine strategies to target malaria.

Hematopoietic and nonhematopoietic cells promote Type I interferon- and TLR7-dependent monocytosis during low-dose LCMV infection

Release of inflammatory monocytes from the bone marrow (BM) into the blood is an important physiological response to infection, but the mechanisms regulating this phenomenon during viral infection are not completely defined. Here, we show that low-dose infection with lymphocytic choriomeningitis virus (LCMV) caused rapid, transient inflammatory monocytosis that required type I interferon (IFN) and Toll-like receptor (TLR) 7 signaling. Type I IFN and TLR7 signals were critical for induction of IFN-stimulated gene expression and CCR2 ligand upregulation in the BM microenvironment in response to LCMV infection. Experiments utilizing BM chimeric mice demonstrated that type I IFN and TLR7 signaling on either hematopoietic or nonhematopoietic cells was sufficient to initiate monocytosis in response to LCMV infection. BM plasmacytoid dendritic cells (pDCs) generated type I IFN directly ex vivo, suggesting that pDCs are a hematopoietic contributor of type I IFN in the BM early during LCMV infection. Overall, we describe novel roles for type I IFN and TLR7 signaling in nonhematopoietic cells and BM pDCs in directing IFN-stimulated gene and CCR2 ligand expression in the BM to initiate an increase in blood inflammatory monocytes during viral infection.

Plasmacytoid dendritic cell dynamics tune interferon-alfa production in SIV-infected cynomolgus macaques

IFN-I production is a characteristic of HIV/SIV primary infections. However, acute IFN-I plasma concentrations rapidly decline thereafter. Plasmacytoid dendritic cells (pDC) are key players in this production but primary infection is associated with decreased responsiveness of pDC to TLR 7 and 9 triggering. IFNα production during primary SIV infection contrasts with increased pDC death, renewal and dysfunction. We investigated the contribution of pDC dynamics to both acute IFNα production and the rapid return of IFNα concentrations to pre-infection levels during acute-to-chronic transition. Nine cynomolgus macaques were infected with SIVmac251 and IFNα-producing cells were quantified and characterized. The plasma IFN-I peak was temporally associated with the presence of IFNα(+) pDC in tissues but IFN-I production was not detectable during the acute-to-chronic transition despite persistent immune activation. No IFNα(+) cells other than pDC were detected by intracellular staining. Blood-pDC and peripheral lymph node-pDC both lost IFNα(-) production ability in parallel. In blood, this phenomenon correlated with an increase in the counts of Ki67(+)-pDC precursors with no IFNα production ability. In tissues, it was associated with increase of both activated pDC and KI67(+)-pDC precursors, none of these being IFNα(+) in vivo. Our findings also indicate that activation/death-driven pDC renewal rapidly blunts acute IFNα production in vivo: pDC sub-populations with no IFNα-production ability rapidly increase and shrinkage of IFNα production thus involves both early pDC exhaustion, and increase of pDC precursors.

All-trans-retinoic acid imprints expression of the gut-homing marker α4β7 while suppressing lymph node homing of dendritic cells

Tissue-directed trafficking of dendritic cells (DCs) as natural adjuvants and/or direct vaccine carriers is highly attractive for the next generation of vaccines and immunotherapeutics. Since these types of studies would undoubtedly be first conducted using nonhuman primate models, we evaluated the ability of all-trans-retinoic acid (ATRA) to induce gut-homing α4β7 expression on rhesus macaque plasmacytoid and myeloid DCs (pDCs and mDCs, respectively). Induction of α4β7 occurred in both a time-dependent and a dose-dependent manner with up to 8-fold increases for mDCs and 2-fold increases for pDCs compared to medium controls. ATRA treatment was also specific in inducing α4β7 expression, but not expression of another mucosal trafficking receptor, CCR9. Unexpectedly, upregulation of α4β7 was associated with a concomitant downregulation of CD62L, a marker of lymph node homing, indicating an overall shift in the trafficking repertoire. These same phenomena occurred with ATRA treatment of human and chimpanzee DCs, suggesting a conserved mechanism among primates. Collectively, these data serve as a first evaluation for ex vivo modification of primate DC homing patterns that could later be used in reinfusion studies for the purposes of immunotherapeutics or mucosa-directed vaccines.

CD34-derived dendritic cells transfected ex vivo with HIV-Gag mRNA induce polyfunctional T-cell responses in nonhuman primates

The pivotal role of DCs in initiating immune responses led to their use as vaccine vectors. However, the relationship between DC subsets involved in antigen presentation and the type of elicited immune responses underlined the need for the characterization of the DCs generated in vitro. The phenotypes of tissue-derived APCs from a cynomolgus macaque model for human vaccine development were compared with ex vivo-derived DCs. Monocyte/macrophages predominated in bone marrow (BM) and blood. Myeloid DCs (mDCs) were present in all tested tissues and were more highly represented than plasmacytoid DCs (pDCs). As in human skin, Langerhans cells (LCs) resided exclusively in the macaque epidermis, expressing CD11c, high levels of CD1a and langerin (CD207). Most DC subsets were endowed with tissue-specific combinations of PRRs. DCs generated from CD34(+) BM cells (CD34-DCs) were heterogeneous in phenotype. CD34-DCs shared properties (differentiation and PRR) of dermal and epidermal DCs. After injection into macaques, CD34-DCs expressing HIV-Gag induced Gag-specific CD4(+) and CD8(+) T cells producing IFN-γ, TNF-α, MIP-1β, or IL-2. In high responding animals, the numbers of polyfunctional CD8(+) T cells increased with the number of booster injections. This DC-based vaccine strategy elicited immune responses relevant to the DC subsets generated in vitro.

Non-human primate dendritic cells

Non-human primates (NHP) are essential translational models for biomedical research. Dendritic cells (DC) are a group of antigen presenting cells (APC) that play pivotal roles in the immunobiology of health and disease and are attractive cells for adoptive immunotherapy to stimulate and suppress immunity. DC have been studied extensively in humans and mice but until recently, have not been well characterized in NHP. This review considers the available data about DC across a range of NHP species and summarizes the understanding of in vitro-propagated DC and in vivo-isolated DC, which is now established. It is clear that although NHP DC exist within the paradigm of human DC, there are important functional and phenotypic differences when compared with human DC subsets. These differences need to be taken into account when designing preclinical, translational studies of DC therapy using NHP models.

In acute pathogenic SIV infection plasmacytoid dendritic cells are depleted from blood and lymph nodes despite mobilization

Background  Plasmacytoid dendritic cells (pDC) are depleted from blood of individuals with HIV infection associated with progression to disease. It has been postulated but not proven that pDC accumulate in lymph nodes and induce sustained immune activation characteristic of disease.

Methods  The dynamics of the pDC response to acute pathogenic SIV infection of rhesus macaques were studied using methods to track recently divided cells.

Results  pDC were lost from blood and lymph nodes in acute SIV infection despite rapid mobilization and recruitment. pDC had a low frequency of infection, were uniformly activated and had increased levels of apoptosis, while maintaining normal function.

Conclusions  pDC mobilization into blood and lymph nodes in acute SIV infection does not keep pace with excessive pDC loss through activation and apoptosis. The depletion of pDC from lymphoid tissues in acutely infected rhesus macaques does not support a pathogenic role for pDC in disease.

Systemic dendritic cell mobilization associated with administration of FLT3 ligand to SIV- and SHIV-infected macaques

Reports indicate that myeloid and plasmacytoid dendritic cells (mDCs and pDCs), which are key effector cells in host innate immune responses, can be infected with HIV-1 and are reduced in number and function during the chronic phase of HIV disease. Furthermore, it was recently demonstrated that a sustained loss of mDCs and pDCs occurs in SIV-infected macaques. Since loss of functional DC populations might impair innate immune responses to opportunistic microorganisms and neoplastic cells, we explored whether inoculation of naive and SIV- or SHIV-infected pigtailed macaques with the hematopoietic cytokine FLT3-ligand (FLT3-L) would expand the number of mDCs and pDCs in vivo. After the macaques received supraphysiologic doses of FLT3-L, mDCs, pDCs, and monocytes increased up to 45-fold in blood, lymph nodes, and bone marrow (BM), with DC expansion in the BM preceding mobilization in blood and lymphoid tissues. FLT3-L also increased serum levels of IL-12, at least transiently, and elicited higher surface expression of HLA-DR and the activation markers CD25 and CD69 on NK and T cells. During and after treatment of infected animals, APCs increased in number and were activated; however, CD4(+) T cell numbers, virion RNA, and anti-SIV/SHIV antibody titers remained relatively stable, suggesting that FLT3-L might be a safe modality to expand DC populations and provide therapeutic benefit during chronic lentivirus infections.

Rapid influx and death of plasmacytoid dendritic cells in lymph nodes mediate depletion in acute simian immunodeficiency virus infection

Plasmacytoid dendritic cells (pDC) are essential innate immune system cells that are lost from the circulation in human immunodeficiency virus (HIV)-infected individuals associated with CD4(+) T cell decline and disease progression. pDC depletion is thought to be caused by migration to tissues or cell death, although few studies have addressed this directly. We used precise methods of enumeration and in vivo labeling with 5-bromo-2'-deoxyuridine to track recently divided pDC in blood and tissue compartments of monkeys with acute pathogenic simian immunodeficiency virus (SIV) infection. We show that pDC are lost from blood and peripheral lymph nodes within 14 days of infection, despite a normal frequency of pDC in bone marrow. Paradoxically, pDC loss masked a highly dynamic response characterized by rapid pDC mobilization into blood and a 10- to 20-fold increase in recruitment to lymph nodes relative to uninfected animals. Within lymph nodes, pDC had increased levels of apoptosis and necrosis, were uniformly activated, and were infected at frequencies similar to CD4(+) T cells. Nevertheless, remaining pDC had essentially normal functional responses to stimulation through Toll-like receptor 7, with half of lymph node pDC producing both TNF-alpha and IFN-alpha. These findings reveal that cell migration and death both contribute to pDC depletion in acute SIV infection. We propose that the rapid recruitment of pDC to inflamed lymph nodes in lentivirus infection has a pathologic consequence, bringing cells into close contact with virus, virus-infected cells, and pro-apoptotic factors leading to pDC death.