Unravelling human hematopoietic progenitor cell diversity through association with intrinsic regulatory factors

Hematopoietic stem and progenitor cell (HSPC) transplantation is an essential therapy for hematological conditions, but finer definitions of human HSPC subsets with associated function could enable better tuning of grafts and more routine, lower-risk application. To deeply phenotype HSPCs, following a screen of 328 antigens, we quantified 41 surface proteins and functional regulators on millions of CD34+ and CD34- cells, spanning four primary human hematopoietic tissues: bone marrow, mobilized peripheral blood, cord blood, and fetal liver. We propose more granular definitions of HSPC subsets and provide new, detailed differentiation trajectories of erythroid and myeloid lineages. These aspects of our revised human hematopoietic model were validated with corresponding epigenetic analysis and in vitro clonal differentiation assays. Overall, we demonstrate the utility of using molecular regulators as surrogates for cellular identity and functional potential, providing a framework for description, prospective isolation, and cross-tissue comparison of HSPCs in humans.

Microglia phagocytose oligodendrocyte progenitor cells and synapses during early postnatal development: implications for white versus gray matter maturation

Emerging roles for microglia in modifying normal brain development continue to provide new perspectives on the functions of this resident immune cell in the brain. While the molecular underpinnings driving microglia's position in regulating developmental programs remain largely an unchartered territory, innate immune signaling lies at the forefront. At least three innate immune receptors expressed on microglia-fractalkine, complement, and triggering receptor expressed on microglia (TREM2)-modulate developmental synaptic pruning to refine brain circuitry. Our laboratory recently published that microglia with a unique amoeboid morphology invade the corpus callosum and engulf oligodendrocyte progenitor cells (OPCs) during early postnatal development before myelination in a fractalkine receptor (CX3CR1)-dependent manner to modulate ensheathment of axons. Amoeboid microglia are observed in the corpus callosum but not cerebral cortex, and lose their amoeboid shape at the commencement of myelination assuming a resting phenotype. Furthermore, OPCs contacted or engulfed by microglia do not express markers of cell death suggesting a novel homeostatic mechanism facilitating an appropriate OPC:axon ratio for proper myelin ensheathment. The unique morphology of microglia and the restricted window for phagocytic engulfment of OPCs suggest a critical period for OPC engulfment important for action potential propagation during development when activity-dependent mechanisms regulate synaptic pruning. In this review, we summarize the role of activity-dependent mechanisms in sculpting brain circuitry, how myelin ensheathment influences action potential propagation, the spatial and temporal relationship of microglia-dependent elimination of OPCs and synapses, and implications for the synergistic role of microglial phagocytosis in shaping the architecture for neuronal function.

Dietary Fatty Acids in Postprandial Triglyceride-Rich Lipoproteins Modulate Human Monocyte-Derived Dendritic Cell Maturation and Activation

Dietary fatty acids have been demonstrated to modulate systemic inflammation and induce the postprandial inflammatory response of circulating immune cells. We hypothesized that postprandial triglyceride-rich lipoproteins (TRLs) may have acute effects on immunometabolic homeostasis by modulating dendritic cells (DCs), sentinels of the immunity that link innate and adaptive immune systems. In healthy volunteers, saturated fatty acid (SFA)-enriched meal raised serum levels of granulocyte/macrophage colony-stimulating factor GM-CSF (SFAs > monounsaturated fatty acids (MUFAs) = polyunsaturated fatty acids (PUFAs)) in the postprandial period. Autologous TRL-SFAs upregulated the gene expression of DC maturation (CD123 and CCR7) and DC pro-inflammatory activation (CD80 and CD86) genes while downregulating tolerogenic genes (PD-L1 and PD-L2) in human monocyte-derived DCs (moDCs). These effects were reversed with oleic acid-enriched TRLs. Moreover, postprandial SFAs raised IL-12p70 levels, while TRL-MUFAs and TRL-PUFAs increased IL-10 levels in serum of healthy volunteers and in the medium of TRL-treated moDCs. In conclusion, postprandial TRLs are metabolic entities with DC-related tolerogenic activity, and this function is linked to the type of dietary fat in the meal. This study shows that the intake of meals enriched in MUFAs from olive oil, when compared with meals enriched in SFAs, prevents the postprandial production and priming of circulating pro-inflammatory DCs, and promotes tolerogenic response in healthy subjects. However, functional assays with moDCs generated in the presence of different fatty acids and T cells could increase the knowledge of postprandial TRLs’ effects on DC differentiation and function.

HLA-DR Class II expression on myeloid and lymphoid cells in relation to HLA-DRB1 as a genetic risk factor for visceral leishmaniasis

Genetic variation at HLA-DRB1 is a risk factor for visceral leishmaniasis (VL) caused by Leishmania donovani. The single nucleotide polymorphism rs9271252 upstream of the DRB1 gene provides a perfect tag for protective versus risk HLA-DRB1 four-digit alleles. In addition to the traditional role of the membrane-distal region of HLA class II molecules in antigen presentation and CD4 T-cell activation, the membrane-proximal region mediates 'non-traditional' multi-functional activation, differentiation, or death signals, including in DR-expressing T cells. To understand how HLA-DR contributes to disease pathogenesis, we examined expression at the protein level in circulating myeloid (CD14⁺ , CD16⁺ ) and lymphoid (CD4⁺ , CD8⁺ , CD19⁺ ) cells of VL patients (pre- and post-treatment) compared with endemic healthy controls (EHC). Although DR expression is reduced in circulating myeloid cells in active disease relative to EHC and post-treatment groups, expression is enhanced on CD4⁺  DR⁺ and CD8⁺  DR⁺ T cells consistent with T-cell activation. Cells of all myeloid and lymphoid populations from active cases were refractory to stimulation of DR expression with interferon-γ (IFN-γ). In contrast, all populations except CD19⁺ B cells from healthy blood bank controls showed enhanced DR expression following IFN-γ stimulation. The rs9271252 genotype did not impact significantly on IFN-γ-activated DR expression in myeloid, B or CD8⁺ T cells, but CD4⁺ T cells from healthy individuals homozygous for the risk allele were particularly refractory to activated DR expression. Further analysis of DR expression on subsets of CD4⁺ T cells regulating VL disease could uncover additional ways in which pleiotropy at HLA DRB1 contributes to disease pathogenesis.

Signature Channels of Excitability no More: L-Type Channels in Immune Cells

Although the concept of Ca(2+) as a universal messenger is well established, it was assumed that the regulatory mechanisms of Ca(2+)-signaling were divided along the line of electric excitability. Recent advances in molecular biology and genomics have, however, provided evidence that non-excitable cells such as immunocytes also express a wide and diverse pool of ion channels that does not differ as significantly from that of excitable cells as originally assumed. Ion channels and transporters are involved in virtually all aspects of immune response regulation, from cell differentiation and development to activation, and effector functions such as migration, antibody-secretion, phagosomal maturation, or vesicular delivery of bactericidal agents. This comprises TRP channel family members, voltage- and Ca(2+)-gated K(+)- and Na(+)-channels, as well as unexpectedly, components of the CaV1-subfamily of voltage-gated L-type Ca(2+)-channels, originally thought to be signature molecules of excitability. This article provides an overview of recent observations made in the field of CaV1 L-type channel function in the immune context, as well as presents results we obtained studying these channels in B-lymphocytes.

Calcineurin/NFAT signalling inhibits myeloid haematopoiesis

Nuclear factor of activated T cells (NFAT) comprises a family of transcription factors that regulate T cell development, activation and differentiation. NFAT signalling can also mediate granulocyte and dendritic cell (DC) activation, but it is unknown whether NFAT influences their development from progenitors. Here, we report a novel role for calcineurin/NFAT signalling as a negative regulator of myeloid haematopoiesis. Reconstituting lethally irradiated mice with haematopoietic stem cells expressing an NFAT-inhibitory peptide resulted in enhanced development of the myeloid compartment. Culturing bone marrow cells in media supplemented with Flt3-L in the presence of the calcineurin/NFAT inhibitor Cyclosporin A increased numbers of differentiated DC. Global gene expression analysis of untreated DC and NFAT-inhibited DC revealed differential expression of transcripts that regulate cell cycle and apoptosis. In conclusion, these results provide evidence that calcineurin/NFAT signalling negatively regulates myeloid lineage development. The finding that inhibition of NFAT enhances myeloid development provides a novel insight into understanding how the treatment with drugs targeting calcineurin/NFAT signalling influence the homeostasis of the innate immune system.