All hematopoietic and immune cells are continuously generated by hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) through highly organized process of stepwise lineage commitment. In the steady state, HSCs are mostly quiescent, while HPCs are actively proliferating and contributing to daily hematopoiesis. In response to hematopoietic challenges, e.g., life-threatening blood loss, infection, and inflammation, HSCs can be activated to proliferate and engage in blood formation. The HSC activation induced by hematopoietic demand is mediated by direct or indirect sensing mechanisms involving pattern recognition receptors or cytokine/chemokine receptors. In contrast to the hematopoietic challenges with obvious clinical symptoms, how the aging process, which involves low-grade chronic inflammation, impacts hematopoiesis remains undefined. Herein, we summarize recent findings pertaining to functional alternations of hematopoiesis, HSCs, and the bone marrow (BM) microenvironment during the processes of aging and inflammation and highlight some common cellular and molecular changes during the processes that influence hematopoiesis and its cells of origin, HSCs and HPCs, as well as the BM microenvironment. We also discuss how age-dependent alterations of the immune system lead to subclinical inflammatory states and how inflammatory signaling might be involved in hematopoietic aging. Our aim is to present evidence supporting the concept of “Inflamm-Aging,” or inflammation-associated aging of hematopoiesis.

Bacterial infection leads to consumption of short-lived innate immune effector cells, which then need to be replenished from hematopoietic stem and progenitor cells (HSPCs). HSPCs express pattern recognition receptors, such as Toll-like receptors (TLRs), and ligation of these receptors induces HSPC mobilization, cytokine production, and myeloid differentiation. The underlying mechanisms involved in pathogen signal transduction in HSCs and the resulting biological consequences remain poorly defined. Here, we show that in vivo lipopolysaccharide (LPS) application induces proliferation of dormant HSCs directly via TLR4 and that sustained LPS exposure impairs HSC self-renewal and competitive repopulation activity. This process is mediated via TLR4-TRIF-ROS-p38, but not MyD88 signaling, and can be inhibited pharmacologically without preventing emergency granulopoiesis. Live Salmonella Typhimurium infection similarly induces proliferative stress in HSCs, in part via TLR4-TRIF signals. Thus, while direct TLR4 activation in HSCs might be beneficial for controlling systemic infection, prolonged TLR4 signaling has detrimental effects and may contribute to inflammation-associated HSPC dysfunction.

O ver the last decades, incrementally improved xenograft mouse models, supporting the engraftment and development of a human hemato-lymphoid system, have been developed and now represent an important research tool in the field. The most significant contributions made by means of humanized mice are the identification of normal and leukemic hematopoietic stem cells, the characterization of the human hematopoietic hierarchy, and their use as preclinical therapy models for malignant hematopoietic disorders. Successful xenotransplantation depends on three major factors: tolerance by the mouse host, correct spatial location, and appropriately cross-reactive support and interaction factors such as cytokines and major histocompatibility complex molecules. Each of these can be modified. Experimental approaches include the genetic modification of mice to faithfully express human support factors as non-cross-reactive cytokines, to create free niche space, the co-transplantation of human mesenchymal stem cells, the implantation of humanized ossicles or other stroma, and the implantation of human thymic tissue. Besides the source of hematopoietic cells, the conditioning regimen and the route of transplantation also significantly affect human hematopoietic development in vivo. We review here the achievements, most recent developments, and the remaining challenges in the generation of pre-clinicallypredictive systems for human hematology and immunology, closely resembling the human situation in a xenogeneic mouse environment. Humanized hemato-lymphoid system mice

B-cell development in the bone marrow comprises proliferative and resting phases in different niches. We asked whether B-cell metabolism relates to these changes. Compared to pro B and small pre B cells, large pre B cells revealed the highest glucose uptake and ROS but not mitochondrial mass, whereas small pre B cells exhibited the lowest mitochondrial membrane potential. Small pre B cells from Rag1;33.C9 μ heavy chain knock-in mice revealed decreased glycolysis (ECAR) and mitochondrial spare capacity compared to pro B cells from Rag1 mice. We were interested in the step regulating this metabolic switch from pro to pre B cells and uncovered that Swiprosin-2/EFhd1, a Ca-binding protein of the inner mitochondrial membrane involved in Ca-induced mitoflashes, is expressed in pro B cells, but downregulated by surface pre B-cell receptor expression. Knockdown and knockout of EFhd1 in 38B9 pro B cells decreased the oxidative phosphorylation/glycolysis (OCR/ECAR) ratio by increasing glycolysis, glycolytic capacity and reserve. Prolonged expression of EFhd1 in EFhd1 transgenic mice beyond the pro B cell stage increased expression of the mitochondrial co-activator PGC-1α in primary pre B cells, but reduced mitochondrial ATP production at the pro to pre B cell transition in IL-7 cultures. Transgenic EFhd1 expression caused a B-cell intrinsic developmental disadvantage for pro and pre B cells. Hence, coordinated expression of EFhd1 in pro B cells and by the pre BCR regulates metabolic changes and pro/pre B-cell development.

Swiprosin-1/EFhd2 (EFhd2) is a cytoskeletal Ca2+ sensor protein strongly expressed in the brain. It has been shown to interact with mutant tau, which can promote neurodegeneration, but nothing is known about the physiological function of EFhd2 in the nervous system. To elucidate this question, we analyzed EFhd2−/−/lacZ reporter mice and showed that lacZ was strongly expressed in the cortex, the dentate gyrus, the CA1 and CA2 regions of the hippocampus, the thalamus, and the olfactory bulb. Immunohistochemistry and western blotting confirmed this pattern and revealed expression of EFhd2 during neuronal maturation. In cortical neurons, EFhd2 was detected in neurites marked by MAP2 and co-localized with pre- and post-synaptic markers. Approximately one third of EFhd2 associated with a biochemically isolated synaptosome preparation. There, EFhd2 was mostly confined to the cytosolic and plasma membrane fractions. Both synaptic endocytosis and exocytosis in primary hippocampal EFhd2−/− neurons were unaltered but transport of synaptophysin-GFP containing vesicles was enhanced in EFhd2−/− primary hippocampal neurons, and notably, EFhd2 inhibited kinesin mediated microtubule gliding. Therefore, we found that EFhd2 is a neuronal protein that interferes with kinesin-mediated transport.

Although the molecular pathways that cause acute myeloid leukemia (AML) are increasingly well understood, the pathogenesis of peripheral blood cytopenia, a major cause of AML mortality, remains obscure. A prevailing assumption states that AML spatially displaces nonleukemic hematopoiesis from the bone marrow. However, examining an initial cohort of 223 AML patients, we found no correlation between bone marrow blast content and cytopenia, questioning the displacement theory. Measuring serum concentration of thrombopoietin (TPO), a key regulator of hematopoietic stem cells and megakaryocytes, revealed loss of physiologic negative correlation with platelet count in AML cases with blasts expressing MPL, the thrombopoietin (scavenging) receptor. Mechanistic studies demonstrated that MPL blasts could indeed clear TPO, likely therefore leading to insufficient cytokine levels for nonleukemic hematopoiesis. Microarray analysis in an independent multicenter study cohort of 437 AML cases validated MPL expression as a central predictor of thrombocytopenia and neutropenia in AML. Moreover, t(8;21) AML cases demonstrated the highest average MPL expression and lowest average platelet and absolute neutrophil counts among subgroups. Our work thus explains the pathophysiology of peripheral blood cytopenia in a relevant number of AML cases.

Summary The generation of humanized ectopic ossicles (hOss) in mice has been proposed as an advanced translational and fundamental model to study the human hematopoietic system. The approach relies on the presence of human bone marrow-derived mesenchymal stromal cells (hMSCs) supporting the engraftment of transplanted human hematopoietic stem and progenitor cells (HSPCs). However, the functional distribution of hMSCs within the humanized microenvironment remains to be investigated. Here, we combined genetic tools and quantitative confocal microscopy to engineer and subsequently analyze hMSCs′ fate and distribution in hOss. Implanted hMSCs reconstituted a humanized environment including osteocytes, osteoblasts, adipocytes, and stromal cells associated with vessels. By imaging full hOss, we identified rare physical interactions between hMSCs and human CD45+/CD34+/CD90+ cells, supporting a functional contact-triggered regulatory role of hMSCs. Our study highlights the importance of compiling quantitative information from humanized organs, to decode the interactions between the hematopoietic and the stromal compartments.

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