Lack of PPAR? in myeloid cells confers resistance to Listeria monocytogenes infection.
ABSTRACT: The peroxisomal proliferator-activated receptor ? (PPAR?) is a nuclear receptor that controls inflammation and immunity. Innate immune defense against bacterial infection appears to be compromised by PPAR?. The relevance of PPAR? in myeloid cells, that organize anti-bacterial immunity, for the outcome of immune responses against intracellular bacteria such as Listeria monocytogenes in vivo is unknown. We found that Listeria monocytogenes infection of macrophages rapidly led to increased expression of PPAR?. This prompted us to investigate whether PPAR? in myeloid cells influences innate immunity against Listeria monocytogenes infection by using transgenic mice with myeloid-cell specific ablation of PPAR? (LysMCre×PPAR?(flox/flox)). Loss of PPAR? in myeloid cells results in enhanced innate immune defense against Listeria monocytogenes infection both, in vitro and in vivo. This increased resistance against infection was characterized by augmented levels of bactericidal factors and inflammatory cytokines: ROS, NO, IFN? TNF IL-6 and IL-12. Moreover, myeloid cell-specific loss of PPAR? enhanced chemokine and adhesion molecule expression leading to improved recruitment of inflammatory Ly6C(hi) monocytes to sites of infection. Importantly, increased resistance against Listeria infection in the absence of PPAR? was not accompanied by enhanced immunopathology. Our results elucidate a yet unknown regulatory network in myeloid cells that is governed by PPAR? and restrains both listeriocidal activity and recruitment of inflammatory monocytes during Listeria infection, which may contribute to bacterial immune escape. Pharmacological interference with PPAR? activity in myeloid cells might represent a novel strategy to overcome intracellular bacterial infection.
Project description:Neutrophils are the most abundant leukocytes in circulation and provide a primary innate immune defense function against bacterial pathogens before development of a specific immune response. These specialized phagocytes are short lived (12-24 hours) and continuously replenished from bone marrow. We found that if the host is overwhelmed by a high inoculum of Listeria monocytogenes, neutrophils are depleted despite high granulocyte-colony stimulating factor induction. In contrast to a low-dose innocuous L. monocytogenes infection, high-dose Listeria challenge blocks neutrophil recruitment to infectious abscesses and bacterial proliferation is not controlled, resulting in lethal outcomes. Administering synthetic TLR2-ligand or heat-killed bacteria during the innocuous L. monocytogenes infection reproduced these effects, once again leading to overwhelming bacterial propagation. The same stimuli also severely aggravated Salmonella typhimurium, Staphylococcus aureus, and Streptococcus pyogenes systemic infection. These data implicate systemic innate immune stimulation as a mechanism of bone marrow neutrophil exhaustion which negatively influences the outcome of bacterial infections.
Project description:Listeria monocytogenes is a Gram-positive intracellular bacterium that is acquired through tainted food and may lead to systemic infection and possible death. Despite the importance of the innate immune system in fighting L. monocytogenes infection, little is known about the role of complement and its activation products, including the potent C3a anaphylatoxin. In a model of systemic L. monocytogenes infection, we show that mice lacking the receptor for C3a (C3aR(-/-)) are significantly more sensitive to infection compared with wild-type mice, as demonstrated by decreased survival, increased bacterial burden, and increased damage to their livers and spleens. The inability of the C3aR(-/-) mice to clear the bacterial infection was not caused by defective macrophages or by a reduction in cytokines/chemokines known to be critical in the host response to L. monocytogenes, including IFN-? and TNF-?. Instead, TUNEL staining, together with Fas, active caspase-3, and Bcl-2 expression data, indicates that the increased susceptibility of C3aR(-/-) mice to L. monocytogenes infection was largely caused by increased L. monocytogenes-induced apoptosis of myeloid and lymphoid cells in the spleen that are required for ultimate clearance of L. monocytogenes, including neutrophils, macrophages, dendritic cells, and T cells. These findings reveal an unexpected function of C3a/C3aR signaling during the host immune response that suppresses Fas expression and caspase-3 activity while increasing Bcl-2 expression, thereby providing protection to both myeloid and lymphoid cells against L. monocytogenes-induced apoptosis.
Project description:Type I interferons (IFNs) induce a detrimental response during Listeria monocytogenes (L. monocytogenes) infection. We were interested in identifying mechanisms linking IFN signaling to negative host responses against L. monocytogenes infection. Herein, we found that infection of myeloid cells with L. monocytogenes led to a coordinated induction of type I IFNs and activation of the integrated stress response (ISR). Infected cells did not induce Xbp1 splicing or BiP upregulation, indicating that the unfolded protein response was not triggered. CHOP (Ddit3) gene expression was upregulated during the ISR activation induced by L. monocytogenes. Myeloid cells deficient in either type I IFN signaling or PKR activation had less upregulation of CHOP following infection. CHOP-deficient mice showed lower expression of innate immune cytokines and were more resistant than wild-type counterparts following L. monocytogenes infection. These findings indicate that L. monocytogenes infection induces type I IFNs, which activate the ISR through PKR, which contributes to a detrimental outcome in the infected host.
Project description:The type II interferon (IFNγ) promotes resistance to intracellular pathogens. Most immune and somatic cells also express the IFNγ receptor (IFNGR) and respond to IFNγ. While myeloid cell have been implicated as important targets of IFNγ, it remains unknown if IFNγ signaling to myeloid cell types suffices for resistance to infection. Here, we addressed this question by generating mice in which IFNGR1 is selectively expressed by myeloid cells. These "<i>MSGR1</i>" (myeloid selective IFNGR1) mice express an epitope-tagged <i>Ifngr1</i> transgene (fGR1) from the myeloid-specific <i>c-fms</i> promoter in a background lacking endogenous <i>Ifngr1</i>. IFNGR staining was selectively observed on myeloid cells in the MSGR1 mice and correlated with responsiveness of these cells to IFNγ. During systemic infection by the bacterium <i>Listeria monocytogenes</i>, activation marker staining was comparable on monocytes from MSGR1 and control B6 mice. Bacterial burdens and survival were also equivalent in MSGR1 and wildtype B6 animals at a timepoint when B6.<i>Ifngr1</i> <sup>-/-</sup> mice began to succumb. These data confirm that activation of inflammatory monocytes and neutrophils is a key mechanism by which IFNγ promotes innate anti-bacterial immunity and suggest that IFNγ targeting of myeloid cells is largely sufficient to mediate protection against systemic <i>L. monocytogenes</i>.
Project description:Scavenger receptors are part of a complex surveillance system expressed by host cells to efficiently orchestrate innate immune response against bacterial infections. Stabilin-1 (STAB-1) is a scavenger receptor involved in cell trafficking, inflammation, and cancer; however, its role in infection remains to be elucidated. <i>Listeria monocytogenes</i> (<i>Lm</i>) is a major intracellular human food-borne pathogen causing severe infections in susceptible hosts. Using a mouse model of infection, we demonstrate here that STAB-1 controls <i>Lm</i>-induced cytokine and chemokine production and immune cell accumulation in <i>Lm</i>-infected organs. We show that STAB-1 also regulates the recruitment of myeloid cells in response to <i>Lm</i> infection and contributes to clear circulating bacteria. In addition, whereas STAB-1 appears to promote bacterial uptake by macrophages, infection by pathogenic <i>Listeria</i> induces the down regulation of STAB-1 expression and its delocalization from the host cell membrane.We propose STAB-1 as a new SR involved in the control of <i>Lm</i> infection through the regulation of host defense mechanisms, a process that would be targeted by bacterial virulence factors to promote infection.
Project description:The vertebrate protein STING, an intracellular sensor of cyclic dinucleotides, is critical to the innate immune response and the induction of type I interferon during pathogenic infection. Here, we show that a STING ortholog (dmSTING) exists in Drosophila, which, similar to vertebrate STING, associates with cyclic dinucleotides to initiate an innate immune response. Following infection with Listeria monocytogenes, dmSTING activates an innate immune response via activation of the NF-?B transcription factor Relish, part of the immune deficiency (IMD) pathway. DmSTING-mediated activation of the immune response reduces the levels of Listeria-induced lethality and bacterial load in the host. Of significance, dmSTING triggers an innate immune response in the absence of a known functional cyclic guanosine monophosphate (GMP)-AMP synthase (cGAS) ortholog in the fly. Together, our results demonstrate that STING is an evolutionarily conserved antimicrobial effector between flies and mammals, and it comprises a key component of host defense against pathogenic infection in Drosophila.
Project description:Memory ?? T cells are important for the clearance of Listeria monocytogenes infection in the intestinal mucosa. However, the mechanisms by which memory ?? T cells provide protection against secondary oral infection are poorly understood. Here we used a recombinant strain of L. monocytogenes that efficiently invades the intestinal epithelium to show that V?4(+) memory ?? T cells represent a resident memory (Trm) population in the mesenteric lymph nodes (MLNs). The ?? Trm exhibited a remarkably static pattern of migration that radically changed following secondary oral L. monocytogenes infection. The ?? Trms produced IL-17A early after rechallenge and formed organized clusters with myeloid cells surrounding L. monocytogenes replication foci only after a secondary oral infection. Antibody blocking studies showed that in addition to IL-17A, the chemokine receptor C-X-C chemokine receptor 3 (CXCR3) is also important to enable the local redistribution of ?? Trm cells and myeloid cells specifically near the sites of L. monocytogenes replication within the MLN to restrict bacterial growth and spread. Our findings support a role for ?? Trms in orchestrating protective immune responses against intestinal pathogens.
Project description:Immunity against infection with Listeria monocytogenes is not achieved from innate immune stimulation by contact with killed but requires viable Listeria gaining access to the cytosol of infected cells. It has remained ill-defined how such immune sensing of live Listeria occurs. Here, we report that efficient cytosolic immune sensing requires access of nucleic acids derived from live Listeria to the cytoplasm of infected cells. We found that Listeria released nucleic acids and that such secreted bacterial RNA/DNA was recognized by the cytosolic sensors RIG-I, MDA5 and STING thereby triggering interferon ? production. Secreted Listeria nucleic acids also caused RIG-I-dependent IL-1?-production and inflammasome activation. The signalling molecule CARD9 contributed to IL-1? production in response to secreted nucleic acids. In conclusion, cytosolic recognition of secreted bacterial nucleic acids by RIG-I provides a mechanistic explanation for efficient induction of immunity by live bacteria.
Project description:Listeria monocytogenes is a facultative intracellular pathogen that invades both phagocytic and non-phagocytic cells. Recent studies have shown that L. monocytogenes infection activates the autophagy pathway. However, the innate immune receptors involved and the downstream signaling pathways remain unknown. Here, we show that macrophages deficient in the TLR2 and NOD/RIP2 pathway display defective autophagy induction in response to L. monocytogenes. Inefficient autophagy in Tlr2(-/-) and Nod2(-/-) macrophages led to a defect in bacteria colocalization with the autophagosomal marker GFP-LC3. Consequently, macrophages lacking TLR2 and NOD2 were found to be more susceptible to L. monocytogenes infection, as were the Rip2(-/-) mice. Tlr2(-/-) and Nod2(-/-) cells showed perturbed NF-?B and ERK signaling. However, autophagy against L. monocytogenes was dependent selectively on the ERK pathway. In agreement, wild-type cells treated with a pharmacological inhibitor of ERK or ERK-deficient cells displayed inefficient autophagy activation in response to L. monocytogenes. Accordingly, fewer bacteria were targeted to the autophagosomes and, consequently, higher bacterial growth was observed in cells deficient in the ERK signaling pathway. These findings thus demonstrate that TLR2 and NOD proteins, acting via the downstream ERK pathway, are crucial to autophagy activation and provide a mechanistic link between innate immune receptors and induction of autophagy against cytoplasm-invading microbes, such as L. monocytogenes.
Project description:The bacterial pathogen <i>Listeria monocytogenes</i> causes foodborne systemic disease in pregnant women, which can lead to preterm labor, stillbirth, or severe neonatal disease. Colonization of the maternal decidua appears to be an initial step in the maternal component of the disease as well as bacterial transmission to the placenta and fetus. Host-pathogen interactions in the decidua during this early stage of infection remain poorly understood. Here, we assessed the dynamics of <i>L. monocytogenes</i> infection in primary human decidual organ cultures and in the murine decidua <i>in vivo</i> A high inoculum was necessary to infect both human and mouse deciduas, and the data support the existence of a barrier to initial colonization of the murine decidua. If successful, however, colonization in both species was followed by significant bacterial expansion associated with an inability of the decidua to mount appropriate innate cellular immune responses. The innate immune deficits included the failure of bacterial foci to attract macrophages and NK cells, cell types known to be important for early defenses against <i>L. monocytogenes</i> in the spleen, as well as a decrease in the tissue density of inflammatory Ly6C<sup>hi</sup> monocytes <i>in vivo</i> These results suggest that the infectivity of the decidua is not the result of an enhanced recruitment of <i>L. monocytogenes</i> to the gestational uterus but rather is due to compromised local innate cellular immune responses.