No abstract

Although microbial infections can alter steady-state hematopoiesis, the mechanisms that drive such changes are not well understood. We addressed a role for IFN-γ signaling in infection-induced bone marrow suppression and anemia in a murine model of human monocytic ehrlichiosis, an emerging tick-borne disease. Within the bone marrow of Ehrlichia muris-infected C57BL/6 mice, we observed a reduction in myeloid progenitor cells, as defined both phenotypically and functionally. Infected mice exhibited a concomitant increase in developing myeloid cells within the bone marrow, an increase in the frequency of circulating monocytes, and an increase in splenic myeloid cells. The infection-induced changes in progenitor cell phenotype were critically dependent on IFN-γ, but not IFN-α, signaling. In mice deficient in the IFN-γ signaling pathway, we observed an increase in myeloid progenitor cells and CDllbloGr1lo promyelocytic cells within the bone marrow, as well as reduced frequencies of mature granulocytes and monocytes. Furthermore, E. muris-infected IFN-γR–deficient mice did not exhibit anemia or an increase in circulating monocytes, and they succumbed to infection. Gene transcription studies revealed that IFN-γR–deficient CDllbloGr1lo promyelocytes from E. muris-infected mice exhibited significantly reduced expression of irf-1 and irf-8, both key transcription factors that regulate the differentiation of granulocytes and monocytes. Finally, using mixed bone marrow chimeric mice, we show that IFN-γ–dependent infection-induced myelopoiesis occurs via the direct effect of the cytokine on developing myeloid cells. We propose that, in addition to its many other known roles, IFN-γ acts to control infection by directly promoting the differentiation of myeloid cells that contribute to host defense.

Mechanisms underlying lymphocyte lineage stability and plasticity remain elusive. Recent work indicates that innate lymphoid cells (ILC) possess substantial plasticity. Whereas natural ILC2 (nILC2) produce type-2 cytokines, “plastic” inflammatory ILC2 (iILC2) can co-produce both type-2 cytokines and also the ILC3-characteristic cytokine IL-17. Mechanisms that elicit this lineage plasticity, and the importance in health and disease, remain unclear. Here we show that iILC2 are potent inducers of airway inflammation in response to acute house dust mite challenge. We find that Notch signaling induces lineage plasticity of mature ILC2 and drives the conversion of nILC2 into iILC2. Acute blockade of Notch signaling abolished functional iILC2, but not nILC2, in vivo. Exposure of isolated nILC2 to Notch ligands induced Rorc expression and elicited dual IL-13/IL-17 production, converting nILC2 into iILC2. Together these results reveal a novel role for Notch signaling in eliciting ILC2 plasticity and driving the emergence of highly pro-inflammatory innate lymphocytes.

IgM responses are well known to occur early postinfection and tend to be short-lived, which has suggested that this Ig does not significantly contribute to long-term immunity. In this study, we demonstrate that chronic infection with the intracellular bacterium Ehrlichia muris elicits a protective, long-term IgM response. Moreover, we identified a population of CD138highIgMhigh B cells responsible for Ag-specific IgM production in the bone marrow. The IgM-secreting cells, which exhibited characteristics of both plasmablasts and plasma cells, contributed to protection against fatal ehrlichial challenge. Mice deficient in activation-induced cytidine deaminase, which produce only IgM, were protected against fatal ehrlichial challenge infection. The IgM-secreting cells that we have identified were maintained in the bone marrow in the absence of chronic infection, as antibiotic-treated mice remained protected against challenge infection. Our studies identify a cell population that is responsible for the IgM production in the bone marrow, and they highlight a novel role for IgM in the maintenance of long-term immunity during intracellular bacterial infection.

Inhalation of Francisella tularensis (Ft) causes acute and fatal pneumonia. The lung cytokine milieu favors exponential Ft replication, but the mechanisms underlying acute pathogenesis and death remain unknown. Evaluation of the sequential and systemic host immune response in pulmonary tularemia reveals that in contrast to overwhelming bacterial burden or cytokine production, an overt innate cellular response to Ft drives tissue pathology and host mortality. Lethal infection with Ft elicits medullary and extra-medullary myelopoiesis supporting recruitment of large numbers of immature myeloid cells and MDSC to the lungs. These cells fail to mature and die, leading to subsequent necrotic lung damage, loss of pulmonary function, and host death that is partially dependent upon immature Ly6G+ cells. Acceleration of this process may account for the rapid lethality seen with Ft SchuS4. In contrast, during sub-lethal infection with Ft LVS the pulmonary cellular response is characterized by a predominance of mature neutrophils and monocytes required for protection, suggesting a required threshold for lethal bacterial infection. Further, eliciting a mature phagocyte response provides transient, but dramatic, innate protection against Ft SchuS4. This study reveals that the nature of the myeloid cell response may be the primary determinant of host mortality versus survival following Francisella infection.

How hematopoietic stem cells (HSCs) respond to inflammatory signals during infections is not well understood. Our studies have used a murine model of ehrlichiosis, an emerging tick-born disease, to address how infection impacts hematopoietic function. Infection of C57BL/6 mice with the intracellular bacterium, Ehrlichia muris, results in anemia and thrombocytopenia, similar to what is observed in human ehrlichiosis patients. In the mouse, infection promotes myelopoiesis, a process that is critically dependent on interferon gamma (IFNγ) signaling. In the present study, we demonstrate that E. muris infection also drives the transient proliferation and expansion of bone marrow Lin-negative Sca-1+ cKit+ (LSK) cells, a population of progenitor cells that contains HSCs. Expansion of the LSK population in the bone marrow was associated with a loss of dormant, long-term repopulating HSCs, reduced engraftment, and a bias towards myeloid lineage differentiation within that population. The reduced engraftment and myeloid bias of the infection-induced LSK cells was transient, and was most pronounced on day 8 post-infection. The infection-induced changes were accompanied by an expansion of more differentiated multipotent progenitor cells, and required IFNγ signaling. Thus, in response to inflammatory signals elicited during acute infection, HSCs can undergo a rapid, IFNγ-dependent, transient shift from dormancy to activity, ostensibly, to provide the host with additional or better-armed innate cells for host defense. Similar changes in hematopoietic function likely underlie many different infections of public health importance.

Bone marrow (BM) resident macrophages (Mϕs) regulate hematopoietic stem cell (HSC) mobilization, however their impact on HSC function has not been investigated. We demonstrate that depletion of BM resident Mϕs increases HSC proliferation as well as the pool of quiescent HSCs. At the same time, during bacterial infection where BM resident Mϕs are selectively increased we observe a decrease in HSC numbers. Moreover, strategies that deplete or reduce Mϕs during infection prevent HSC loss and rescue HSC function. We previously found that the transient loss of HSCs during infection is interferon-gamma (IFNγ)-dependent. We now demonstrate that IFNγ signaling specifically in Mϕs is critical for both the diminished HSC pool and maintenance of BM resident Mϕs during infection. In addition to the IFNγ-dependent loss of BM HSC and progenitor cells (HSPCs) during infection, IFNγ reduced circulating HSPC numbers. Importantly, under infection conditions AMD3100 or G-CSF-induced stem cell mobilization was impaired. Taken together our data show that IFNγ acts on Mϕs, which are a negative regulator of the HSC pool, to drive the loss in BM and peripheral HSCs during infection. Our findings demonstrate that modulating BM resident Mϕ numbers can impact HSC function in vivo, which may be therapeutically useful for hematologic conditions and refinement of HSC transplantation protocols.

Germinal centers (GCs) are specialized microenvironments in secondary lymphoid organs that facilitate the development of high-affinity, isotype-switched Abs, and immunological memory; consequently, many infections require GC-derived IgG for pathogen clearance. Although Ehrlichia muris infection elicits a robust expansion of splenic, IgM-secreting plasmablasts, we detected only very low frequencies of isotype-switched IgG-secreting cells in mouse spleens, until at least 3 wk postinfection. Instead, Ag-specific IgG was produced in lymph nodes, where it required CD4 T cell help. Consistent with these findings, organized GCs and phenotypically defined splenic GC B cells were found in lymph nodes, but not spleens. Ehrlichial infection also inhibited spleen IgG responses against a coadministered T cell-dependent Ag, hapten 4-hydroxy-3-nitrophenyl acetyl (NP)-conjugated chicken γ globulin in alum. NP-specific B cells failed to undergo expansion and differentiation into GC B cells in the spleen, Ab titers were reduced, and splenic IgG production was inhibited nearly 10-fold when the Ag was administered during infection. Our data provide a mechanism whereby an intracellular bacterial infection can compromise local immunity to coinfecting pathogens or antigenic challenge.