There are three major dendritic cell (DC) subsets in both humans and mice, that is, plasmacytoid DCs and two types of conventional DCs (cDCs), cDC1s and cDC2s. cDC2s are important for polarizing CD4⁺ naive T cells into different subsets, including Th1, Th2, Th17, Th22, and regulatory T cells. In mice, cDC2s can be further divided into phenotypically and functionally distinct subgroups. However, subsets of human cDC2s have not been reported. In the present study, we showed that human blood CD1c⁺ cDCs (cDC2s) can be further separated into two subpopulations according to their CD5 expression status. Comparative transcriptome analyses showed that the CD5^high DCs expressed higher levels of cDC2-specific genes, including IFN regulatory factor 4, which is essential for the cDC2 development and its migration to lymph nodes. In contrast, CD5^low DCs preferentially expressed monocyte-related genes, including the lineage-specific transcription factor MAFB. Furthermore, compared with the CD5^low subpopulation, the CD5^high subpopulation showed stronger migration toward CCL21 and overrepresentation among migratory DCs in lymph nodes. Additionally, the CD5^high DCs induced naive T cell proliferation more potently than did the CD5^low DCs. Moreover, CD5^high DCs induced higher levels of IL-10–, IL-22–, and IL-4–producing T cell formation, whereas CD5^low DCs induced higher levels of IFN-γ–producing T cell formation. Thus, we show that human blood CD1c⁺ cDC2s encompass two subsets that differ significantly in phenotype, that is, gene expression and functions. We propose that these two subsets of human cDC2s could potentially play contrasting roles in immunity or tolerance.

The role of plasmacytoid dendritic cells (pDC) in human immunodeficiency virus type 1 (HIV-1) infection and pathogenesis remains unclear. HIV-1 infection in the humanized mouse model leads to persistent HIV-1 infection and immunopathogenesis, including type I interferons (IFN-I) induction, immune-activation and depletion of human leukocytes, including CD4 T cells. We developed a monoclonal antibody that specifically depletes human pDC in all lymphoid organs in humanized mice. When pDC were depleted prior to HIV-1 infection, the induction of IFN-I and interferon-stimulated genes (ISGs) were abolished during acute HIV-1 infection with either a highly pathogenic CCR5/CXCR4-dual tropic HIV-1 or a standard CCR5-tropic HIV-1 isolate. Consistent with the anti-viral role of IFN-I, HIV-1 replication was significantly up-regulated in pDC-depleted mice. Interestingly, the cell death induced by the highly pathogenic HIV-1 isolate was severely reduced in pDC-depleted mice. During chronic HIV-1 infection, depletion of pDC also severely reduced the induction of IFN-I and ISGs, associated with elevated HIV-1 replication. Surprisingly, HIV-1 induced depletion of human immune cells including T cells in lymphoid organs, but not the blood, was reduced in spite of the increased viral replication. The increased cell number in lymphoid organs was associated with a reduced level of HIV-induced cell death in human leukocytes including CD4 T cells. We conclude that pDC play opposing roles in suppressing HIV-1 replication and in promoting HIV-1 induced immunopathogenesis. These findings suggest that pDC-depletion and IFN-I blockade will provide novel strategies for treating those HIV-1 immune non-responsive patients with persistent immune activation despite effective anti-retrovirus treatment. Persistent expression of IFN-I is correlated with disease progression in HIV-1 infected humans or SIV-infected monkeys. Thus, persistent pDC activation has been implicated in contributing to AIDS pathogenesis. To define the role of pDC in HIV-1 infection and immunopathogenesis in vivo, we developed a monoclonal antibody that specifically and efficiently depletes human pDC in all lymphoid organs in humanized mice. We discover that pDC are the critical IFN-I producer cells in response to acute HIV-1 infection, because depletion of pDC completely abolished induction of IFN-I or ISG by HIV-1 infection, correlated with elevated level of HIV-1 replication. When pDC were depleted during chronic HIV-1 infection in humanized mice, pDC were still the major IFN-I producing cells in vivo, which contributed to HIV-1 suppression. Despite of higher level of viral replication in pDC-depleted mice, we found that HIV-induced depletion of human T cells and leukocytes was significantly reduced in lymphoid organs, correlated with reduced cell death induction by HIV-1 infection. Our findings demonstrate that pDC play two opposing roles in HIV-1 pathogenesis: they produce IFN-I to suppress HIV-1 replication and induce death of human immune cells to contribute to HIV-induced T cell depletion and immunopathogenesis.

Scavenger receptor class B, member 2 (SCARB2) is essential for endosome biogenesis and reorganization and serves as a receptor for both β-glucocerebrosidase and enterovirus 71. However, little is known about its function in innate immune cells. In this study, we show that, among human peripheral blood cells, SCARB2 is most highly expressed in plasmacytoid dendritic cells (pDCs), and its expression is further upregulated by CpG oligodeoxynucleotide stimulation. Knockdown of SCARB2 in pDC cell line GEN2.2 dramatically reduces CpG-induced type I IFN production. Detailed studies reveal that SCARB2 localizes in late endosome/lysosome of pDCs, and knockdown of SCARB2 does not affect CpG oligodeoxynucleotide uptake but results in the retention of TLR9 in the endoplasmic reticulum and an impaired nuclear translocation of IFN regulatory factor 7. The IFN-I production by TLR7 ligand stimulation is also impaired by SCARB2 knockdown. However, SCARB2 is not essential for influenza virus or HSV-induced IFN-I production. These findings suggest that SCARB2 regulates TLR9-dependent IFN-I production of pDCs by mediating endosomal translocation of TLR9 and nuclear translocation of IFN regulatory factor 7.