Mice repopulated with human hematopoietic cells are a powerful tool for the study of human hematopoiesis and immune function in vivo. However, existing humanized mouse models are unable to support development of human innate immune cells, including myeloid cells and NK cells. Here we describe a mouse strain, called MI(S)TRG, in which human versions of four genes encoding cytokines important for innate immune cell development are knocked in to their respective mouse loci. The human cytokines support the development and function of monocytes/macrophages and natural killer cells derived from human fetal liver or adult CD34+ progenitor cells injected into the mice. Human macrophages infiltrated a human tumor xenograft in MI(S)TRG mice in a manner resembling that observed in tumors obtained from human patients. This humanized mouse model may be used to model the human immune system in scenarios of health and pathology, and may enable evaluation of therapeutic candidates in an in vivo setting relevant to human physiology.

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.

Neutropenia is probably the strongest known predisposition to infection with otherwise harmless environmental or microbiota-derived species. Because initial swarming of neutrophils at the site of infection occurs within minutes, rather than the hours required to induce “emergency granulopoiesis,” the relevance of having high numbers of these cells available at any one time is obvious. We observed that germ-free (GF) animals show delayed clearance of an apathogenic bacterium after systemic challenge. In this article, we show that the size of the bone marrow myeloid cell pool correlates strongly with the complexity of the intestinal microbiota. The effect of colonization can be recapitulated by transferring sterile heat-treated serum from colonized mice into GF wild-type mice. TLR signaling was essential for microbiota-driven myelopoiesis, as microbiota colonization or transferring serum from colonized animals had no effect in GF MyD88−/−TICAM1−/− mice. Amplification of myelopoiesis occurred in the absence of microbiota-specific IgG production. Thus, very low concentrations of microbial Ags and TLR ligands, well below the threshold required for induction of adaptive immunity, sets the bone marrow myeloid cell pool size. Coevolution of mammals with their microbiota has probably led to a reliance on microbiota-derived signals to provide tonic stimulation to the systemic innate immune system and to maintain vigilance to infection. This suggests that microbiota changes observed in dysbiosis, obesity, or antibiotic therapy may affect the cross talk between hematopoiesis and the microbiota, potentially exacerbating inflammatory or infectious states in the host.

SHP-2 is a cytoplasmic protein tyrosine phosphatase (PTP) that contains two

No abstract

The life span of intestinal epithelial cells (IECs) is short (3–5 days), and its regulation is thought to be important for homeostasis of the intestinal epithelium. We have now investigated the role of commensal bacteria in regulation of IEC turnover in the small intestine. The proliferative activity of IECs in intestinal crypts as well as the migration of these cells along the crypt-villus axis were markedly attenuated both in germ-free mice and in specific pathogen–free (SPF) mice treated with a mixture of antibiotics, with antibiotics selective for Gram-positive bacteria being most effective in this regard. Oral administration of chloroform-treated feces of SPF mice to germ-free mice resulted in a marked increase in IEC turnover, suggesting that spore-forming Gram-positive bacteria contribute to this effect. Oral administration of short-chain fatty acids (SCFAs) as bacterial fermentation products also restored the turnover of IECs in antibiotic-treated SPF mice as well as promoted the development of intestinal organoids in vitro. Antibiotic treatment reduced the phosphorylation levels of ERK, ribosomal protein S6, and STAT3 in IECs of SPF mice. Our results thus suggest that Gram-positive commensal bacteria are a major determinant of IEC turnover, and that their stimulatory effect is mediated by SCFAs.

IntroductionThe lifespan of circulating red blood cells (RBCs) is approximately 120 and 40 days in humans and mice, respectively, and is determined by their production in bone marrow (BM) and their clearance from the peripheral circulation, predominantly in the spleen and liver. [1][2][3] The production of RBCs is controlled by the primary erythropoietic regulator erythropoietin, 2,4 whereas clearance of old RBCs by the spleen is achieved mostly as a result of their specific recognition and phagocytosis by splenic macrophages. 5,6 The precise molecular mechanism by which splenic macrophages recognize senescent RBCs for phagocytosis is largely unknown, however. [1][2][3] Src homology 2 domain-containing protein tyrosine phosphatase substrate-1 (SHPS-1), 7,8 also known as signal-regulatory protein ␣, 9 brain immunoglobulin (Ig)-like molecule with tyrosinebased activation motifs, 10 and p84 neural adhesion molecule, 11 is a transmembrane protein that is especially abundant in macrophages. [12][13][14] The putative extracellular region of SHPS-1 comprises 3 Ig-like domains, and its cytoplasmic region contains 4 tyrosine phosphorylation sites that mediate the binding of Src homology 2 domain-containing protein tyrosine phosphatases designated SHP-1 and SHP-2. 7,9 Tyrosine phosphorylation of SHPS-1 is regulated by various growth factors and cytokines as well as by integrinmediated cell adhesion to extracellular matrix proteins. 7,9,[15][16][17][18] SHPS-1 thus functions as a docking protein to recruit and activate SHP-1 or SHP-2 at the cell membrane in response to extracellular stimuli. In macrophages, tyrosine-phosphorylated SHPS-1 binds SHP-1, 12,19,20 which is implicated in negative regulation of the functions of a variety of hematopoietic cells. [21][22][23] The complex of SHPS-1 and SHP-1 is thus thought to regulate macrophage functions in a negative manner.CD47 is a ligand for the extracellular region of SHPS-1. 24,25 This protein, which was originally identified in association with ␣v␤3 integrin, is also a member of the Ig superfamily, possessing an Ig-V-like extracellular domain, 5 putative membrane-spanning segments, and a short cytoplasmic tail. 26 CD47 and SHPS-1 constitute a cell-cell communication system (the CD47-SHPS-1 system) that plays important roles in a variety of cellular processes including cell migration, 27,28 adhesion of B cells, 29 and T-cell activation. 14,30 In addition, the CD47-SHPS-1 system is implicated in negative regulation of phagocytosis by macrophages. CD47 is highly expressed on the surface of RBCs, where it associates with the Rh protein complex instead of with integrins. 31 The rate of clearance of CD47-deficient RBCs from the bloodstream was found to be markedly increased compared with that of wild-type (WT) cells. 6,32 Furthermore, the phagocytosis of CD47-deficient RBCs by splenic or BM-derived macrophages was greatly enhanced Supported by a Grant-in-Aid for Scientific Research on Priority Areas Cancer; a Grant-in-Aid for Scientific Research (B); a Grant-in-Aid for Young Scie...

The molecular basis for regulation of dendritic cell (DC) development and homeostasis remains unclear. Signal regulatory protein α (SIRPα), an immunoglobulin superfamily protein that is predominantly expressed in DCs, mediates cell-cell signaling by interacting with CD47, another immunoglobulin superfamily protein. We now show that the number of CD11c(high) DCs (conventional DCs, or cDCs), in particular, that of CD8-CD4+ (CD4+) cDCs, is selectively reduced in secondary lymphoid tissues of mice expressing a mutant form of SIRPα that lacks the cytoplasmic region. We also found that SIRPα is required intrinsically within cDCs or DC precursors for the homeostasis of splenic CD4+ cDCs. Differentiation of bone marrow cells from SIRPα mutant mice into DCs induced by either macrophage-granulocyte colony-stimulating factor or Flt3 ligand in vitro was not impaired. Although the accumulation of the immediate precursors of cDCs in the spleen was also not impaired, the half-life of newly generated splenic CD4+ cDCs was markedly reduced in SIRPα mutant mice. Both hematopoietic and nonhematopoietic CD47 was found to be required for the homeostasis of CD4+ cDCs and CD8-CD4- (double negative) cDCs in the spleen. SIRPα as well as its ligand, CD47, are thus important for the homeostasis of CD4+ cDCs or double negative cDCs in lymphoid tissues.