Delineating the role for LIF in monocyte development and function
Introduction
Leukemia inhibitory factor (LIF) is a member of the interleukin-6 (IL-6) cytokine family. LIF binds its unique receptor subunit LIFR, to elicit intracellular signaling cascades that mediate its activity. LIF is a pleiotropic cytokine, broadly expressed across tissues with many well-established roles; nonetheless, the role for LIF in hematopoiesis remains elusive.
Methods
Our group and others have observed that LIF is expressed in the bone marrow (BM) in homeostasis and inflammation. Additionally, our group identified monocytes and plasmacytoid dendritic cells (pDCs) as the major LIFR+ subsets in the BM, suggesting these populations respond to LIF to mediate immune responses. To determine the function of LIF signaling in these subsets, we generated CD11c-cre+ Lifrf/f (LIFRΔ/Δ) mice.
Results
Immune profiling of PBS- and lipopolysaccharide (LPS)-treated CD11c-cre+ (control mice, WT) and LIFRΔ/Δ mice showed that LIFRΔ/Δ mice have a defect in upregulating circulating CD11b+ Ly6C+ monocytes in response to LPS. Additionally, we observed that circulating amounts of IL-1β were reduced in LIFRΔ/Δ mice after LPS challenge, compared to controls. Monocytes are a major source of IL-1β following LPS treatment. Thus, we investigated LIFR control of monocytes by performing RNA-sequencing of FACS-purified BM monocytes (CD45+ CD11b+ Ly6C+ cells) derived from PBS- or LPS-challenged WT or LIFRΔ/Δ mice. LPS-stimulated monocytes from LIFRΔ/Δ mice show compromised function, including suppressed inflammatory signaling, interferon responses, TNF-α signaling via NF-κB, along with increased activation of proliferation pathways. We also observed suppressed induction of costimulatory molecules (Cd40, Cd86), antigen-presentation molecules (Cd74, H2-Aa), and pro-inflammatory chemokines (Cxcl10, Cxcl11) in LIFRΔ/Δ monocytes after LPS stimulation. These data suggest LIFR signaling in BM monocytes is necessary for their appropriate function and pro-inflammatory response to TLR ligands. Since inflammatory factors regulate BM hematopoietic stem and progenitor cells (HSPCs), we next evaluated HSPCs in WT and LIFRΔ/Δ mice. These studies revealed BM LSKs (Lin- Sca-1+ cKit+ cells) in LIFRΔ/Δ mice are significantly impaired in their response to LPS, which drives accumulation of LSKs in WT mice. Moreover, absolute numbers and frequencies of long term (LT)-Hematopoietic stem cells (HSCs), Multipotent Progenitor (MPP)-myeloid and MPP-megakaryocyte/erythroid subsets are also suppressed in LPS-challenged LIFRΔ/Δ mice versus controls. These results indicate LIFR signaling in CD11c+ cells is important to regulate appropriate responses of HSPCs to LPS.
Conclusion
The development from HSCs to MPPs and specific lineages is tightly controlled. Importantly, dysregulation of these processes can result in susceptibility to infections or development of bone marrow stem cell and progenitor associated diseases e.g., Myelodysplastic Syndrome (MDS) and Acute Myeloid Leukemia (AML). Cytokines and chemokines play a pivotal role in these processes, influencing the development of BM HSCs into terminally differentiated immune cell subsets and regulating response of these populations to pathogens and inflammation. Our studies investigating LIF function will provide better understanding of HSC regulation and hematopoietic-associated cancers.