The possible immunomodulatory role of polymorphonuclear leukocytes (PMN) in CD4⁺ T lymphocyte differentiation in mice was examined by studying the effect of transient depletion of PMN during the early phase after Leishmania major delivery. A single injection of the PMN-depleting NIMP-R14 mAb 6 h before infection with L. major prevented the early burst of IL-4 mRNA transcription otherwise occurring in the draining lymph node of susceptible BALB/c mice. Since this early burst of IL-4 mRNA transcripts had previously been shown to instruct Th2 differentiation in mice from this strain, we examined the effect of PMN depletion on Th subset differentiation at later time points after infection. The transient depletion of PMN in BALB/c mice was sufficient to inhibit Th2 cell development otherwise occurring after L. major infection. Decreased Th2 responses were paralleled with partial resolution of the footpad lesions induced by L. major. Furthermore, draining lymph node-derived CD4⁺ T cells from PMN-depleted mice remained responsive to IL-12 after L. major infection, unlike those of infected BALB/c mice receiving control Ab. PMN depletion had no effect when the NIMP-R14 mAb was injected 24 h postinfection. The protective effect of PMN depletion was shown to be IL-12 dependent, as concomitant neutralization of IL-12 reversed the protective effect of PMN depletion. These results suggest a role for an early wave of PMN in the development of the Th2 response characteristic of mice susceptible to infection with L. major.

Significance: Group 3 innate lymphoid cells (ILC3s) play decisive roles in mammalian physiology including tissue repair, lymphoid tissue development, and immune regulation. So far, the functions of ILC3s in the adult immune system have been mainly linked to their capacity to release cytokines in response to microbial or inflammatory signals. The results presented here show that activated ILC3s can alter the outcome of adaptive immune responses by directly stimulating CD4 ⁺ T cells. Indeed, IL-1β–activated ILC3s express costimulatory molecules and induce cognate CD4 ⁺ T-cell responses. More importantly, antigen-driven T- and B-cell responses are impaired in vivo when this cellular interaction is disrupted. Overall, our data show that peripheral ILC3s play a yet unappreciated role in T-cell–mediated immunity.

Neutrophils are rapidly and massively recruited to sites of microbial infection, where they can influence the recruitment of dendritic cells. Here, we have analyzed the role of neutrophil released chemokines in the early recruitment of dendritic cells (DCs) in an experimental model of Leishmania major infection. We show in vitro, as well as during infection, that the parasite induced the expression of CCL3 selectively in neutrophils from L. major resistant mice. Neutrophil-secreted CCL3 was critical in chemotaxis of immature DCs, an effect lost upon CCL3 neutralisation. Depletion of neutrophils prior to infection, as well as pharmacological or genetic inhibition of CCL3, resulted in a significant decrease in DC recruitment at the site of parasite inoculation. Decreased DC recruitment in CCL3^−/− mice was corrected by the transfer of wild type neutrophils at the time of infection. The early release of CCL3 by neutrophils was further shown to have a transient impact on the development of a protective immune response. Altogether, we identified a novel role for neutrophil-secreted CCL3 in the first wave of DC recruitment to the site of infection with L. major, suggesting that the selective release of neutrophil-secreted chemokines may regulate the development of immune response to pathogens. When infectious agents enter our body, neutrophils are the first cells recruited to the scene. In addition to their capacity to kill microbes, neutrophils secrete molecules that attract other cells also involved in immune defense, such as dendritic cells (DCs). Here, we investigate the secretion of DC-attracting chemokines by neutrophils after inoculation of mice with Leishmania major, a protozoan parasite that can cause long-lasting, skin ulcers in man. Following parasite inoculation, most inbred strains of mice (e.g.C57BL/6) develop self-resolving lesions, while in a few strains (e.g. BALB/c) lesions fail to heal. We report here that in “healer” C57BL/6 mice, higher numbers of DCs were attracted at the site of infection than in “non-healer” BALB/c mice. DC recruitment correlated with secretion by neutrophils of the chemokine CCL3, as indeed a markedly decreased DC recruitment was observed in C57BL/6 mice depleted of neutrophils or deprived of the capacity to produce CCL3. DC recruitment was restored upon transfer of normal neutrophils to CCL3 deficient mice. Our results reveal a crucial role for neutrophil-secreted CCL3 in early recruitment of DCs in L. major-infected “healer” mice, and suggest that the type of chemokine secreted by neutrophils will have consequences in the launching of pathogen-specific immune response.

Triggering receptor expressed on myeloid cells-1 (TREM-1) is a potent amplifier of pro-inflammatory innate immune reactions. While TREM-1-amplified responses likely aid an improved detection and elimination of pathogens, excessive production of cytokines and oxygen radicals can also severely harm the host. Studies addressing the pathogenic role of TREM-1 during endotoxin-induced shock or microbial sepsis have so far mostly relied on the administration of TREM-1 fusion proteins or peptides representing part of the extracellular domain of TREM-1. However, binding of these agents to the yet unidentified TREM-1 ligand could also impact signaling through alternative receptors. More importantly, controversial results have been obtained regarding the requirement of TREM-1 for microbial control. To unambiguously investigate the role of TREM-1 in homeostasis and disease, we have generated mice deficient in Trem1. Trem1^−/− mice are viable, fertile and show no altered hematopoietic compartment. In CD4⁺ T cell- and dextran sodium sulfate-induced models of colitis, Trem1^−/− mice displayed significantly attenuated disease that was associated with reduced inflammatory infiltrates and diminished expression of pro-inflammatory cytokines. Trem1^−/− mice also exhibited reduced neutrophilic infiltration and decreased lesion size upon infection with Leishmania major. Furthermore, reduced morbidity was observed for influenza virus-infected Trem1^−/− mice. Importantly, while immune-associated pathologies were significantly reduced, Trem1^−/− mice were equally capable of controlling infections with L. major, influenza virus, but also Legionella pneumophila as Trem1^+/+ controls. Our results not only demonstrate an unanticipated pathogenic impact of TREM-1 during a viral and parasitic infection, but also indicate that therapeutic blocking of TREM-1 in distinct inflammatory disorders holds considerable promise by blunting excessive inflammation while preserving the capacity for microbial control. Triggering receptor expressed on myeloid cells-1 (TREM-1) is an immune receptor expressed by myeloid cells that has the capacity to augment pro-inflammatory responses in the context of a microbial infection. While a TREM-1-amplified response likely serves the efficient clearance of pathogens, it also bears the potential to cause substantial tissue damage or even death. Hence, TREM-1 appears a possible therapeutic target for tempering deleterious host-pathogen interactions. However, in models of bacterial sepsis controversial findings have been obtained regarding the requirement of TREM-1 for bacterial control - depending on the overall degree of the TREM-1 blockade that was achieved. In order to conclusively investigate harmful versus essential functions of TREM-1 in vivo, we have generated mice deficient in Trem1. Trem1^−/− mice were subjected to experimentally-induced intestinal inflammation (as a model of a non-infectious, yet microbial-driven disease) and also analysed following infections with Leishmania major, influenza virus and Legionella pneumophila. Across all models analysed, Trem1^−/− mice showed substantially reduced immune-associated disease. We thus describe a previously unanticipated pathogenic role for TREM-1 also during a parasitic and viral infection. Importantly, our data suggest that in certain diseases microbial control can be achieved in the context of blunted inflammation in the absence of TREM-1.

Fibroblast-like cells of secondary lymphoid organs (SLO) are important for tissue architecture. In addition, they regulate lymphocyte compartmentalization through the secretion of chemokines, and participate in the orchestration of appropriate cell–cell interactions required for adaptive immunity. Here, we provide data demonstrating the functional importance of SLO fibroblasts during Notch-mediated lineage specification and immune response. Genetic ablation of the Notch ligand Delta-like (DL)1 identified splenic fibroblasts rather than hematopoietic or endothelial cells as niche cells, allowing Notch 2–driven differentiation of marginal zone B cells and of Esam⁺ dendritic cells. Moreover, conditional inactivation of DL4 in lymph node fibroblasts resulted in impaired follicular helper T cell differentiation and, consequently, in reduced numbers of germinal center B cells and absence of high-affinity antibodies. Our data demonstrate previously unknown roles for DL ligand-expressing fibroblasts in SLO niches as drivers of multiple Notch-mediated immune differentiation processes.