Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening systemic hyper-inflammatory disorder. The mortality of HLH is higher in the elderly than in young adults. Senescence-accelerated mice (SAMP1/TA-1) exhibit characteristic accelerated aging after 30 weeks of age, and HLH-like features, including hematopoietic organ damage, are seen after lipopolysaccharide (LPS) treatment. Thus, SAMP1/TA-1 is a useful model of hematological pathophysiology in the elderly with HLH. In this study, dosing of SAMP1/TA-1 mice with LPS revealed that the suppression of myelopoiesis and B-lymphopoiesis was more severe in aged mice than in young mice. The bone marrow (BM) expression of genes encoding positive regulators of myelopoiesis (G-CSF, GM-CSF, and IL-6) and of those encoding negative regulators of B cell lymphopoiesis (TNF-α) increased in both groups, while the expression of genes encoding positive-regulators of B cell lymphopoiesis (IL-7, SDF-1, and SCF) decreased. The expression of the GM-CSF-encoding transcript was lower in aged mice than in young animals. The production of GM-CSF by cultured stromal cells after LPS treatment was also lower in aged mice than in young mice. The accumulation of the TNF-α-encoding transcript and the depletion of the IL-7-encoding transcript were prolonged in aged mice compared to young animals. LPS dosing led to a prolonged increase in the proportion of BM M1 macrophages in aged mice compared to young animals. The expression of the gene encoding p16INK4a and the proportion of β-galactosidase- and phosphorylated ribosomal protein S6-positive cells were increased in cultured stromal cells from aged mice compared to those from young animals, while the proportion of Ki67-positive cells was decreased in stromal cells from aged mice. Thus, age-related deterioration of stromal cells probably causes the suppression of hematopoiesis in aged mice. This age-related latent organ dysfunction may be exacerbated in elderly people with HLH, resulting in poor prognosis.

Lipopolysaccharide (LPS) treatment induced hemophagocytic lymphohistiocytosis in senescence-accelerated mice (SAMP1/TA-1), but not in senescence-resistant control mice (SAMR1). SAMP1/TA-1 treated with LPS exhibited functional impairment of the hematopoietic microenvironment, which disrupted the dynamics of hematopoiesis. Macrophages are a major component of the bone marrow (BM) hematopoietic microenvironment, which regulates hematopoiesis. Qualitative and quantitative changes in activated macrophages in LPS-treated SAMP1/TA-1 are thought to contribute to the functional deterioration of the hematopoietic microenvironment. Thus, we examined the polarization of pro-inflammatory (M1) and anti-inflammatory (M2) macrophages, and the dynamics of macrophage production in the BM of SAMP1/TA-1 and SAMR1 after LPS treatment. After LPS treatment, the proportions of M1 and M2 macrophages and the numbers of macrophage progenitor (CFU-M) cells increased in both SAMP1/TA-1 and SAMR1. However, compared to the SAMR1, the increase in the M1 macrophage proportion was prolonged, and the increase in the M2 macrophage proportion was delayed. The increase in the number of CFU-M cells was prolonged in SAMP1/TA-1 after LPS treatment. In addition, the levels of transcripts encoding an M1 macrophage-inducing cytokine (interferon-γ) and macrophage colony-stimulating factor were markedly increased, and the increases in the levels of transcripts encoding M2 macrophage-inducing cytokines (interleukin (IL)-4, IL-10, and IL-13) were delayed in SAMP1/TA-1 when compared to SAMR1. Our results suggest that LPS treatment led to the severely imbalanced polarization of activated M1/M2 macrophages accompanied by a prolonged increase in macrophage production in the BM of SAMP1/TA-1, which led to the impairment of the hematopoietic microenvironment, and disrupted the dynamics of hematopoiesis.

Neopterin is produced by monocytes and is a useful biomarker of inflammatory responses. We found that neopterin enhances granulopoiesis, but suppresses B-lymphopoiesis triggered by the positive-and negative regulations of cytokines produced by stromal cells in mice. Furthermore, neopterin suppressed the colony formation of mast cell progenitor (CFU-mast) from bone marrow cells in in vitro culture system. In this study, neopterin was also found to regulate the proliferation and differentiation of splenic CFU-mast in vitro as observed in that from bone marrow, which was confirmed in the mouse model of senescent stromal-cell impairment (SCI). In non-SCI mice (=less senescent stage of SCI mice), neopterin also decreased the number of colonies of interleukin-3 (IL-3)-dependent mast-cell progenitor cells (CFU-mast) from unfractionated spleen cells, but not that from the lineage-negative (fractionated) spleen cell population without stromal cells in a semisolid in vitro culture system. In contrast, in a case of SCI mice, the treatment with neopterin did not decrease the number of colonies of IL-3-dependent mast-cell progenitor cells (CFU-mast) from unfractionated spleen cells. These results suggest that, firstly, neopterin decrease the number of colonies of IL-3-dependent CFU-mast by stimulating splenic stromal cells, and secondly, such neopterin function becomes declined during senescence because of an impaired stromalcell function.