The overall objectives of this study are to investigate the impact of inflammatory conditions on hematopoietic stem cell (HSC) maintenance and to elucidate the underlying mechanisms. HSCs are exposed to a variety of inflammatory conditions through life. How these conditions influence the integrity of HSCs is a fundamental issue of clinical importance but it is poorly understood. Equally unknown is the molecular regulation of HSC maintenance during inflammatory. In this context, our focus is on the role of basic helix-loop-helix (bHLH) proteins, which include transcription activators such as E2A proteins and their inhibitors including Id proteins. We and others have shown that these regulators are involved in normal hematopoiesis such as stem cell function and lineage specific differentiation. Recently, we have obtained evidence to suggest that signaling through Toll-like receptors (TLRs), which is closely linked to inflammation, causes down-regulation of E2A function by stimulating Id1 expression. Therefore, we hypothesize that inflammatory conditions causes down-regulation of E protein function, which disturbs the quiescence of long-term (LT)-HSC, leading to stem cell exhaustion over time.
To test this hypothesis, we induced chronic inflammation in wild type and Id1-/- mice by daily injection of 1 mg of LPS, i.p. for 30 days. Peripheral blood was collected on days 15 and 30 and levels of a panel of inflammatory cytokines were assayed using a Luminex multiplex kit. On day 15, dramatic increases were found in the levels of IL-10, IL-6, KC and TNFα but not IFN-γ, IL12-p70 and IL-1β. Interestingly, levels of IL-6 and TNFα were significantly lower in Id1-/- mice compared to wild type mice. By day 30 of LPS treatment, levels of these cytokines returned to the levels in animals without LPS injection. These results suggest that this chronic LPS treatment indeed elicited an inflammatory response that included transient elevation of inflammatory cytokines. Whether secretion of these cytokines has any direct effects on HSCs remains to be determined.
To measure HSC activity in these LPS-treated mice, we performed serial bone marrow transplant assays. Lin−Sca-1+c-kit+ (LSK) stem/progenitor cells were isolated from wild type or Id1-/- mice treated with or without LPS. These cells were transplanted into lethally irradiated CD45.1+ recipients along with equal numbers of YFP-expressing LSK as competitors. Six weeks later, cohorts of mice were sacrificed and bone marrow cells were collected. Pooled whole bone marrow cells within each cohort were injected into lethally irradiated secondary recipients. Secondary recipients were sacrificed 8 and 16 weeks post transplant. For assessment of primary and secondary engraftment, bone marrow cells were examined for expression of donor and lineage specific markers. Robust engraftment was observed in primary or secondary recipients. Donor derived cells were then gated for YFP− and YFP+ cells, which separate cells originated from tester and competitor LSK, respectively. While YFP− and YFP+ cells engrafted equivalently in primary recipients transplanted with cells treated with or without LPS, LPS treatment of wild type mice caused a great disparity in secondary recipients. In contrast, HSC in Id1-/- mice did not appear to be affected by the same treatment even though HSCs in Id1 deficient mice are normally lower in numbers and activities as we previously reported. These results suggest that chronic inflammation diminishes the LT-stem cell activity and this may involve the up-regulation of Id1 expression. To investigate the underlying mechanism, we performed label retaining assays to examine the quiescence of LT-HSCs. We found that BrdU-labeling in HSCs was 2-fold lower in mice treated with LPS compared to the untreated controls, suggesting that treatment with LPS promoted the cycling of HSCs, thus impairing their stem cell function.
Taken together, our study illustrates that chronic inflammation has a detrimental effect on LT-stem cell activity. Although HSCs have an enormous capability to repopulate the bone marrow by compensatory proliferation, pro-longed inflammation could eventually lead to stem cell exhaustion and seriously compromise hematopoiesis.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.