Dynamic imaging of the effector immune response to listeria infection in vivo

Long-term intravital subcellular imaging with confocal scanning light-field microscopy

Long-term observation of subcellular dynamics in living organisms is limited by background fluorescence originating from tissue scattering or dense labeling. Existing confocal approaches face an inevitable tradeoff among parallelization, resolution and phototoxicity. Here we present confocal scanning light-field microscopy (csLFM), which integrates axially elongated line-confocal illumination with the rolling shutter in scanning light-field microscopy (sLFM). csLFM enables high-fidelity, high-speed, three-dimensional (3D) imaging at near-diffraction-limit resolution with both optical sectioning and low phototoxicity. By simultaneous 3D excitation and detection, the excitation intensity can be reduced below 1 mW mm-2, with 15-fold higher signal-to-background ratio over sLFM. We imaged subcellular dynamics over 25,000 timeframes in optically challenging environments in different species, such as migrasome delivery in mouse spleen, retractosome generation in mouse liver and 3D voltage imaging in Drosophila. Moreover, csLFM facilitates high-fidelity, large-scale neural recording with reduced crosstalk, leading to high orientation selectivity to visual stimuli, similar to two-photon microscopy, which aids understanding of neural coding mechanisms.

New insights on extramedullary granulopoiesis and neutrophil heterogeneity in the spleen and its importance in disease

Neutrophils are traditionally viewed as uncomplicated exterminators that arrive quickly at sites of infection, kill pathogens, and then expire. However, recent studies employing modern transcriptomics coupled with novel imaging modalities have discovered that neutrophils exhibit significant heterogeneity within organs and have complex functional roles ranging from tissue homeostasis to cancer and chronic pathologies. This has revised the view that neutrophils are simplistic butchers, and there has been a resurgent interest in neutrophils. The spleen was described as a granulopoietic organ more than 4 decades ago, and studies indicate that neutrophils are briefly retained in the spleen before returning to circulation after proliferation. Transcriptomic studies have discovered that splenic neutrophils are heterogeneous and distinct compared with those in blood. This suggests that a unique hematopoietic niche exists in the splenic microenvironment, i.e., capable of programming neutrophils in the spleen. During severe systemic inflammation with an increased need of neutrophils, the spleen can adapt by producing neutrophils through emergency granulopoiesis. In this review, we describe the structure and microanatomy of the spleen and examine how cells within the splenic microenvironment help to regulate splenic granulopoiesis. A focus is placed on exploring the increase in splenic granulopoiesis to meet host needs during infection and inflammation. Emerging technologies such as single-cell RNA sequencing, which provide valuable insight into splenic neutrophil development and heterogeneity, are also discussed. Finally, we examine how tumors subvert this natural pathway in the spleen to generate granulocytic suppressor cells to promote tumor growth.

Listeria monocytogenes use multiple mechanisms to disseminate from the intestinal lamina propria to the mesenteric lymph nodes

Listeria monocytogenes are facultative intracellular bacterial pathogens that cause foodborne disease in humans. The bacteria can use the surface protein InlA to invade intestinal epithelial cells or transcytose across M cells in the gut, but it is not well understood how the bacteria traffic from the underlying lamina propria to the draining mesenteric lymph nodes (MLN). Previous studies indicated that L. monocytogenes associated with both monocytes and dendritic cells in the intestinal lamina propria. We show here that CCR2-/- mice had a significant reduction in Ly6Chi monocytes in the MLN but no change in bacterial burden following foodborne infection; thus, dissemination of L. monocytogenes associated with monocytes is not required for colonization of the MLN. To block CCR7-mediated trafficking of dendritic cells from the lamina propria, we treated mice with anti-VEGFR3 antibody (clone AFL4) prior to and during infection but did not see a change in dendritic numbers in the MLN as had been previously reported with other anti-VEGFR3-specific antibodies. However, increasing the number of circulating dendritic cells by treating mice with rFlt3L resulted in a significant increase in L. monocytogenes in the lymph nodes that drain the small intestine and the spleen. Whole-mount fluorescent microscopy of lymphatic vessels following ligated loop infection revealed both free-floating L. monocytogenes and cell-associated bacteria within lymphatic vessels. Together, these results suggest that L. monocytogenes can use multiple, redundant mechanisms to disseminate from the gut tissue to the MLN.

IMPORTANCE

Consumption of the foodborne bacterial pathogen Listeria monocytogenes results in a wide spectrum of human disease from mild self-limiting gastroenteritis to life-threatening infections of the bloodstream, brain, and placenta. It is not well understood how the bacteria migrate from the intestines to the draining mesenteric lymph nodes, which are thought to serve as the last barrier to prevent systemic infections. Results presented here reveal multiple redundant mechanisms L. monocytogenes can use to disseminate from the ileum or colon to the mesenteric lymph nodes.

Cellular mechanisms underlying beneficial versus detrimental effects of bacterial antitumor immunotherapy

Bacteria engineered to express tumor antigens as a cancer vaccine have yielded mixed results. Here, we utilized an attenuated strain of Listeria monocytogenes ( ΔactA, Lm ) that does not express tumor antigen to explore the immunological response to Listeria itself in the context of intravenous (IV), intratumoral (IT), or a combination of IV+IT administration into tumor-bearing mice. Unexpectedly, we found that Lm persisted in tumors of immune competent mice, regardless of the administration route. While IT Lm alone led to the recruitment of immunosuppressive immune cells that promoted tumor growth, IV Lm followed by IT Lm controlled tumor growth. IV Lm vaccination generated a pool of anti- Lm cytotoxic CD8 T cells that killed Lm -infected non-tumor cells to control tumor growth both indirectly, by limiting cancer cell proliferation, and directly, by enhancing tumor-specific T cell responses. Our findings reveal a differential impact of IT Lm administration on tumor progression that depends on the presence of anti- Lm CD8 T cells, which alone are sufficient to promote therapeutic efficacy.

The divergent outcome of IL-4Rα signalling on Foxp3 T regulatory cells in listeriosis and tuberculosis

Introduction

Forkhead box P3 (Foxp3) T regulatory cells are critical for maintaining self-tolerance, immune homeostasis, and regulating the immune system.

Methods

We investigated interleukin-4 receptor alpha (IL-4Rα) signalling on T regulatory cells (Tregs) during Listeria monocytogenes (L. monocytogenes) infection using a mouse model on a BALB/c background, specifically with IL-4Rα knockdown in Tregs (Foxp3creIL-4Rα-/lox).

Results

We showed an impairment of Treg responses, along with a decreased bacterial burden and diminished tissue pathology in the liver and spleen, which translated into better survival. Mechanistically, we observed an enhancement of the Th1 signature, characterised by increased expression of the T-bet transcription factor and a greater number of effector T cells producing IFN-γ, IL-2 following ex-vivo stimulation with heat-killed L. monocytogenes in Foxp3creIL-4Rα-/lox mice. Furthermore, CD8 T cells from Foxp3creIL-4Rα-/lox mice displayed increased cytotoxicity (Granzyme-B) with higher proliferation capacity (Ki-67), better survival (Bcl-2) with concomitant reduced apoptosis (activated caspase 3). In contrast to L. monocytogenes, Foxp3creIL-4Rα-/lox mice displayed similar bacterial burdens, lung pathology and survival during Mycobacterium tuberculosis (M. tuberculosis) infection, despite increased T cell numbers and IFN-γ, TNF and IL-17 production.

Conclusion

Our results demonstrated that the diminished IL-4Rα signalling on Foxp3+ T regulatory cells resulted in a loss of their functionality, leading to survival benefits in listeriosis but not in tuberculosis.

A LTB4/CD11b self-amplifying loop drives pyogranuloma formation in chronic granulomatous disease

Sterile pyogranulomas and heightened cytokine production are hyperinflammatory hallmarks of Chronic Granulomatous Disease (CGD). Using peritoneal cells of zymosan-treated CGD (gp91phox-/-) versus wild-type (WT) mice, an ex vivo system of pyogranuloma formation was developed to determine factors involved in and consequences of recruitment of neutrophils and monocyte-derived macrophages (MoMacs). Whereas WT cells failed to aggregate, CGD cells formed aggregates containing neutrophils initially, and MoMacs recruited secondarily. LTB4 was key, as antagonizing BLT1 blocked neutrophil aggregation, but acted only indirectly on MoMac recruitment. LTB4 upregulated CD11b expression on CGD neutrophils, and the absence/blockade of CD11b inhibited LTB4 production and cell aggregation. Neutrophil-dependent MoMac recruitment was independent of MoMac Nox2 status, BLT1, CCR1, CCR2, CCR5, CXCR2, and CXCR6. As proof of concept, CD11b-deficient CGD mice developed disrupted pyogranulomas with poorly organized neutrophils and diminished recruitment of MoMacs. Importantly, the disruption of cell aggregation and pyogranuloma formation markedly reduced proinflammatory cytokine production.

Crossing the Barrier: A Comparative Study of Listeria monocytogenes and Treponema pallidum in Placental Invasion

Vertically transmitted infections are a significant cause of fetal morbidity and mortality during pregnancy and pose substantial risks to fetal development. These infections are primarily transmitted to the fetus through two routes: (1) direct invasion and crossing the placenta which separates maternal and fetal circulation, or (2) ascending the maternal genitourinary tact and entering the uterus. Only two bacterial species are commonly found to cross the placenta and infect the fetus: Listeria monocytogenes and Treponema pallidum subsp. pallidum. L. monocytogenes is a Gram-positive, foodborne pathogen found in soil that acutely infects a wide variety of mammalian species. T. pallidum is a sexually transmitted spirochete that causes a chronic infection exclusively in humans. We briefly review the pathogenesis of these two very distinct bacteria that have managed to overcome the placental barrier and the role placental immunity plays in resisting infection. Both organisms share characteristics which contribute to their transplacental transmission. These include the ability to disseminate broadly within the host, evade immune phagocytosis, and the need for a strong T cell response for their elimination.

Molecular regulation of neutrophil swarming in health and disease: Lessons from the phagocyte oxidase

Neutrophil swarming is a complex coordinated process in which neutrophils sensing pathogen or damage signals are rapidly recruited to sites of infections or injuries. This process involves cooperation between neutrophils where autocrine and paracrine positive-feedback loops, mediated by receptor/ligand pairs including lipid chemoattractants and chemokines, amplify localized recruitment of neutrophils. This review will provide an overview of key pathways involved in neutrophil swarming and then discuss the cell intrinsic and systemic mechanisms by which NADPH oxidase 2 (NOX2) regulates swarming, including modulation of calcium signaling, inflammatory mediators, and the mobilization and production of neutrophils. We will also discuss mechanisms by which altered neutrophil swarming in disease may contribute to deficient control of infections and/or exuberant inflammation. Deeper understanding of underlying mechanisms controlling neutrophil swarming and how neutrophil cooperative behavior can be perturbed in the setting of disease may help to guide development of tools for diagnosis and precision medicine.

Bacteria-instructed B cells cross-prime naïve CD8+ T cells triggering effective cytotoxic responses

In addition to triggering humoral responses, conventional B cells have been described in vitro to cross-present exogenous antigens activating naïve CD8⁺ T cells. Nevertheless, the way B cells capture these exogenous antigens and the physiological roles of B cell-mediated cross-presentation remain poorly explored. Here, we show that B cells capture bacteria by trans-phagocytosis from previously infected dendritic cells (DC) when they are in close contact. Bacterial encounter "instructs" the B cells to acquire antigen cross-presentation abilities, in a process that involves autophagy. Bacteria-instructed B cells, henceforth referred to as BacB cells, rapidly degrade phagocytosed bacteria, process bacterial antigens and cross-prime naïve CD8⁺ T cells which differentiate into specific cytotoxic cells that efficiently control bacterial infections. Moreover, a proof-of-concept experiment shows that BacB cells that have captured bacteria expressing tumor antigens could be useful as novel cellular immunotherapies against cancer.

Chemo- and mechanosensing by dendritic cells facilitate antigen surveillance in the spleen

Spleen dendritic cells (DC) are critical for initiation of adaptive immune responses against blood-borne invaders. Key to DC function is their positioning at sites of pathogen entry, and their abilities to selectively capture foreign antigens and promptly engage T cells. Focusing on conventional DC2 (cDC2), we discuss the contribution of chemoattractant receptors (EBI2 or GPR183, S1PR1, and CCR7) and integrins to cDC2 positioning and function. We give particular attention to a newly identified role in cDC2 for adhesion G-protein coupled receptor E5 (Adgre5 or CD97) and its ligand CD55, detailing how this mechanosensing system contributes to splenic cDC2 positioning and homeostasis. Additional roles of CD97 in the immune system are reviewed. The ability of cDC2 to be activated by circulating missing self-CD47 cells and to integrate multiple red blood cell (RBC)-derived inputs is discussed. Finally, we describe the process of activated cDC2 migration to engage and prime helper T cells. Throughout the review, we consider the insights into cDC function in the spleen that have emerged from imaging studies.

In Vivo Motility Patterns Displayed by Immune Cells Under Inflammatory Conditions

The migration of immune cells plays a key role in inflammation. This is evident in the fact that inflammatory stimuli elicit a broad range of migration patterns in immune cells. Since these patterns are pivotal for initiating the immune response, their dysregulation is associated with life-threatening conditions including organ failure, chronic inflammation, autoimmunity, and cancer, amongst others. Over the last two decades, thanks to advancements in the intravital microscopy technology, it has become possible to visualize cell migration in living organisms with unprecedented resolution, helping to deconstruct hitherto unexplored aspects of the immune response associated with the dynamism of cells. However, a comprehensive classification of the main motility patterns of immune cells observed in vivo, along with their relevance to the inflammatory process, is still lacking. In this review we defined cell actions as motility patterns displayed by immune cells, which are associated with a specific role during the immune response. In this regard, we summarize the main actions performed by immune cells during intravital microscopy studies. For each of these actions, we provide a consensus name, a definition based on morphodynamic properties, and the biological contexts in which it was reported. Moreover, we provide an overview of the computational methods that were employed for the quantification, fostering an interdisciplinary approach to study the immune system from imaging data.

Imaging dendritic cell functions

Dendritic cells (DCs) are crucial for the appropriate initiation of adaptive immune responses. During inflammation, DCs capture antigens, mature, and migrate to lymphoid tissues to present foreign material to naïve T cells. These cells get activated and differentiate either into pathogen-specific cytotoxic CD8⁺ T cells that destroy infected cells or into CD4⁺ T helper cells that, among other effector functions, orchestrate antibody production by B cells. DC-mediated antigen presentation is equally important in non-inflammatory conditions. Here, DCs mediate induction of tolerance by presenting self-antigens or harmless environmental antigens and induce differentiation of regulatory T cells or inactivation of self-reactive immune cells. Detailed insights into the biology of DCs are, therefore, crucial for the development of novel vaccines as well as the prevention of autoimmune diseases. As in many other life science areas, our understanding of DC biology would be extremely restricted without bioimaging, a compilation of methods that visualize biological processes. Spatiotemporal tracking of DCs relies on various imaging tools, which not only enable insights into their positioning and migration within tissues or entire organs but also allow visualization of subcellular and molecular processes. This review aims to provide an overview of the imaging toolbox and to provide examples of diverse imaging techniques used to obtain fundamental insights into DC biology.

T cell activation niches-Optimizing T cell effector function in inflamed and infected tissues

Successful immunity to infection, malignancy, and tissue damage requires the coordinated recruitment of numerous immune cell subsets to target tissues. Once within the target tissue, effector T cells rely on local chemotactic cues and structural cues from the tissue matrix to navigate the tissue, interact with antigen-presenting cells, and release effector cytokines. This highly dynamic process has been "caught on camera" in situ by intravital multiphoton imaging. Initial studies revealed a surprising randomness to the pattern of T cell migration through inflamed tissues, behavior thought to facilitate chance encounters with rare antigen-bearing cells. Subsequent tissue-wide visualization has uncovered a high degree of spatial preference when it comes to T cell activation. Here, we discuss the basic tenants of a successful effector T cell activation niche, taking cues from the dynamics of Tfh positioning in the lymph node germinal center. In peripheral tissues, steady-state microanatomical organization may direct the location of "pop-up" de novo activation niches, often observed as perivascular clusters, that support early effector T cell activation. These perivascular activation niches appear to be regulated by site-specific chemokines that coordinate the recruitment of dendritic cells and other innate cells for local T cell activation, survival, and optimized effector function.

Memory CD8+ T cells mediate early pathogen-specific protection via localized delivery of chemokines and IFNγ to clusters of monocytes

While cognate antigen drives clonal expansion of memory CD8+ T (CD8+ TM) cells to achieve sterilizing immunity in immunized hosts, not much is known on how cognate antigen contributes to early protection before clonal expansion occurs. Here, using distinct models of immunization, we establish that cognate antigen recognition by CD8+ TM cells on dendritic cells initiates their rapid and coordinated production of a burst of CCL3, CCL4, and XCL1 chemokines under the transcriptional control of interferon (IFN) regulatory factor 4. Using intravital microscopy imaging, we reveal that CD8+ TM cells undergo antigen-dependent arrest in splenic red pulp clusters of CCR2+Ly6C+ monocytes to which they deliver IFNγ and chemokines. IFNγ enables chemokine-induced microbicidal activities in monocytes for protection. Thus, rapid and effective CD8+ TM cell responses require spatially and temporally coordinated events that quickly restrict microbial pathogen growth through the local delivery of activating chemokines to CCR2+Ly6C+ monocytes.

Neutrophils self-limit swarming to contain bacterial growth in vivo

Neutrophils communicate with each other to form swarms in infected organs. Coordination of this population response is critical for the elimination of bacteria and fungi. Using transgenic mice, we found that neutrophils have evolved an intrinsic mechanism to self-limit swarming and avoid uncontrolled aggregation during inflammation. G protein-coupled receptor (GPCR) desensitization acts as a negative feedback control to stop migration of neutrophils when they sense high concentrations of self-secreted attractants that initially amplify swarming. Interference with this process allows neutrophils to scan larger tissue areas for microbes. Unexpectedly, this does not benefit bacterial clearance as containment of proliferating bacteria by neutrophil clusters becomes impeded. Our data reveal how autosignaling stops self-organized swarming behavior and how the finely tuned balance of neutrophil chemotaxis and arrest counteracts bacterial escape.

Lung Marginated and Splenic Murine Resident Neutrophils Constitute Pioneers in Tissue-Defense During Systemic E. coli Challenge

The rapid response of neutrophils throughout the body to a systemic challenge is a critical first step in resolution of bacterial infection such as Escherichia coli (E. coli). Here we delineated the dynamics of this response, revealing novel insights into the molecular mechanisms using lung and spleen intravital microscopy and 3D ex vivo culture of living precision cut splenic slices in combination with fluorescent labelling of endogenous leukocytes. Within seconds after challenge, intravascular marginated neutrophils and lung endothelial cells (ECs) work cooperatively to capture pathogens. Neutrophils retained on lung ECs slow their velocity and aggregate in clusters that enlarge as circulating neutrophils carrying E. coli stop within the microvasculature. The absolute number of splenic neutrophils does not change following challenge; however, neutrophils increase their velocity, migrate to the marginal zone (MZ) and form clusters. Irrespective of their location all neutrophils capturing heat-inactivated E. coli take on an activated phenotype showing increasing surface CD11b. At a molecular level we show that neutralization of ICAM-1 results in splenic neutrophil redistribution to the MZ under homeostasis. Following challenge, splenic levels of CXCL12 and ICAM-1 are reduced allowing neutrophils to migrate to the MZ in a CD29-integrin dependent manner, where the enlargement of splenic neutrophil clusters is CXCR2-CXCL2 dependent. We show directly molecular mechanisms that allow tissue resident neutrophils to provide the first lines of antimicrobial defense by capturing circulating E. coli and forming clusters both in the microvessels of the lung and in the parenchyma of the spleen.

Why is Listeria monocytogenes such a potent inducer of CD8+ T-cells?

Listeria monocytogenes is a rapidly growing, Gram-positive, facultative intracellular pathogen that has been used for over 5 decades as a model to study basic aspects of infection and immunity. In a murine intravenous infection model, immunisation with a sublethal infection of L. monocytogenes initially leads to rapid intracellular multiplication followed by clearance of the bacteria and ultimately culminates in the development of long-lived cell-mediated immunity (CMI) mediated by antigen-specific CD8+ cytotoxic T-cells. Importantly, effective immunisation requires live, replicating bacteria. In this review, we summarise the cell and immunobiology of L. monocytogenes infection and discuss aspects of its pathogenesis that we suspect lead to robust CMI. We suggest five specific features of L. monocytogenes infection that positively impact the development of CMI: (a) the bacteria have a predilection for professional antigen-presenting cells; (b) the bacteria escape from phagosomes, grow, and secrete antigens into the host cell cytosol; (c) bacterial-secreted proteins enter the major histocompatibility complex (MHC) class I pathway of antigen processing and presentation; (d) the bacteria do not induce rapid host cell death; and (e) cytosolic bacteria induce a cytokine response that favours CMI. Collectively, these features make L. monocytogenes an attractive vaccine vector for both infectious disease applications and cancer immunotherapy.

Doppler imaging detects bacterial infection of living tissue

Living 3D in vitro tissue cultures, grown from immortalized cell lines, act as living sentinels as pathogenic bacteria invade the tissue. The infection is reported through changes in the intracellular dynamics of the sentinel cells caused by the disruption of normal cellular function by the infecting bacteria. Here, the Doppler imaging of infected sentinels shows the dynamic characteristics of infections. Invasive Salmonella enterica serovar Enteritidis and Listeria monocytogenes penetrate through multicellular tumor spheroids, while non-invasive strains of Escherichia coli and Listeria innocua remain isolated outside the cells, generating different Doppler signatures. Phase distributions caused by intracellular transport display Lévy statistics, introducing a Lévy-alpha spectroscopy of bacterial invasion. Antibiotic treatment of infected spheroids, monitored through time-dependent Doppler shifts, can distinguish drug-resistant relative to non-resistant strains. This use of intracellular Doppler spectroscopy of living tissue sentinels opens a new class of microbial assay with potential importance for studying the emergence of antibiotic resistance.

Honggu Choi et al. use biodynamic Doppler imaging to monitor bacterial infection of 3D living tissue and describe changes in the intracellular motions of living host tissue induced by early-stage infection. This work demonstrates the potential for the clinical use of this method to test for antibiotic-resistant infections.

The Secretive Life of Neutrophils Revealed by Intravital Microscopy

Neutrophils are the most abundant circulating leukocyte within the blood stream and for many years the dogma has been that these cells migrate rapidly into tissues in response to injury or infection, forming the first line of host defense. While it has previously been documented that neutrophils marginate within the vascular beds of the lung and liver and are present in large numbers within the parenchyma of tissues, such as spleen, lymph nodes, and bone marrow (BM), the function of these tissue resident neutrophils under homeostasis, in response to pathogen invasion or injury has only recently been explored, revealing the unexpected role of these cells as immunoregulators or immune helpers and also unraveling their heterogeneity and plasticity. Neutrophils are highly motile cells and the use of intravital microscopy (IVM) to image cells within their environment with little manipulation has dramatically increased our understanding of the function, migratory behavior, and interaction of these short-lived cells with other innate and adaptive immune cells. Contrary to previous dogma, these studies have shown that marginated and tissue resident neutrophils are the first responders to pathogens and injury, critical in limiting the spread of infection and contributing to the orchestration of the subsequent immune response. The interplay of neutrophils, with other neutrophils, leukocytes, and stroma cells can also modulate and tune their early and late response in order to eradicate pathogens, minimize tissue damage, and, in certain circumstances, contribute to tissue repair. In this review, we will follow the extraordinary journey of neutrophils from their origin in the BM to their death, exploring their role as tissue resident cells in the lung, spleen, lymph nodes, and skin and outlining the importance of neutrophil subsets, their functions under homeostasis, and in response to infection. Finally, we will comment on how understanding these processes in greater detail at a molecular level can lead to development of new therapeutics.

TLR2 and endosomal TLR-mediated secretion of IL-10 and immune suppression in response to phagosome-confined Listeria monocytogenes

Listeria monocytogenes is a facultative intracellular bacterial pathogen that escapes from phagosomes and induces a robust adaptive immune response in mice, while mutants unable to escape phagosomes fail to induce a robust adaptive immune response and suppress the immunity to wildtype bacteria when co-administered. The capacity to suppress immunity can be reversed by blocking IL-10. In this study, we sought to understand the host receptors that lead to secretion of IL-10 in response to phagosome-confined L. monocytogenes (Δhly), with the ultimate goal of generating strains that fail to induce IL-10. We conducted a transposon screen to identify Δhly L. monocytogenes mutants that induced significantly more or less IL-10 secretion in bone marrow-derived macrophages (BMMs). A transposon insertion in lgt, which encodes phosphatidylglycerol-prolipoprotein diacylglyceryl transferase and is essential for the formation of lipoproteins, induced significantly reduced IL-10 secretion. Mutants with transposon insertions in pgdA and oatA, which encode peptidoglycan N-acetylglucosamine deacetylase and O-acetyltransferase, are sensitive to lysozyme and induced enhanced IL-10 secretion. A ΔhlyΔpgdAΔoatA strain was killed in BMMs and induced enhanced IL-10 secretion that was dependent on Unc93b1, a trafficking molecule required for signaling of nucleic acid-sensing TLRs. These data revealed that nucleic acids released by bacteriolysis triggered endosomal TLR-mediated IL-10 secretion. Secretion of IL-10 in response to infection with the parental strain was mostly TLR2-dependent, while IL-10 secretion in response to lysozyme-sensitive strains was dependent on TLR2 and Unc93b1. In mice, the IL-10 response to vacuole-confined L. monocytogenes was also dependent on TLR2 and Unc93b1. Co-administration of Δhly and ΔactA resulted in suppressed immunity in WT mice, but not in mice with mutations in Unc93b1. These data revealed that secretion of IL-10 in response to L. monocytogenes infection in vitro is mostly TLR2-dependent and immune suppression by phagosome-confined bacteria in vivo is mostly dependent on endosomal TLRs.

Neutrophil swarming delays the growth of clusters of pathogenic fungi

Neutrophils employ several mechanisms to restrict fungi, including the action of enzymes such as myeloperoxidase (MPO) or NADPH oxidase, and the release of neutrophil extracellular traps (NETs). Moreover, they cooperate, forming “swarms” to attack fungi that are larger than individual neutrophils. Here, we designed an assay for studying how these mechanisms work together and contribute to neutrophil's ability to contain clusters of live Candida. We find that neutrophil swarming over Candida clusters delays germination through the action of MPO and NADPH oxidase, and restricts fungal growth through NET release within the swarm. In comparison with neutrophils from healthy subjects, those from patients with chronic granulomatous disease produce larger swarms against Candida, but their release of NETs is delayed, resulting in impaired control of fungal growth. We also show that granulocyte colony-stimulating factors (GCSF and GM-CSF) enhance swarming and neutrophil ability to restrict fungal growth, even during treatment with chemical inhibitors that disrupt neutrophil function.

Neutrophils employ several mechanisms to control the growth of fungi, including enzymes, reactive oxygen species, extracellular traps, and formation of “swarms”. Here, Hopke et al. study how the different mechanisms work together, using an in vitro assay with human neutrophils and clusters of live Candida cells.

Structure and function of the immune system in the spleen

The spleen is the largest secondary lymphoid organ in the body and, as such, hosts a wide range of immunologic functions alongside its roles in hematopoiesis and red blood cell clearance. The physical organization of the spleen allows it to filter blood of pathogens and abnormal cells and facilitate low-probability interactions between antigen-presenting cells (APCs) and cognate lymphocytes. APCs specific to the spleen regulate the T and B cell response to these antigenic targets in the blood. This review will focus on cell types, cell organization, and immunologic functions specific to the spleen and how these affect initiation of adaptive immunity to systemic blood-borne antigens. Potential differences in structure and function between mouse and human spleen will also be discussed.

An Inducible Cre-lox System to Analyze the Role of LLO in Listeria monocytogenes Pathogenesis

Listeriolysin O (LLO) is a pore-forming cytolysin that allows Listeria monocytogenes to escape from phagocytic vacuoles and enter the host cell cytosol. LLO is expressed continuously during infection, but it has been a challenge to evaluate the importance of LLO secreted in the host cell cytosol because deletion of the gene encoding LLO (hly) prevents localization of L. monocytogenes to the cytosol. Here, we describe a L. monocytogenes strain (hlyfl) in which hly is flanked by loxP sites and Cre recombinase is under the transcriptional control of the L. monocytogenesactA promoter, which is highly induced in the host cell cytosol. In less than 2 h after infection of bone marrow-derived macrophages (BMMs), bacteria were 100% non-hemolytic. hlyfl grew intracellularly to levels 10-fold greater than wildtype L. monocytogenes and was less cytotoxic. In an intravenous mouse model, 90% of bacteria were non-hemolytic within three hours in the spleen and eight hours in the liver. The loss of LLO led to a 2-log virulence defect in the spleen and a 4-log virulence defect in the liver compared to WT L. monocytogenes. Thus, the production of LLO in the cytosol has significant impact on the pathogenicity of L. monocytogenes.

Neutrophils Enhance Their Own Influx to Sites of Bacterial Infection via Endosomal TLR-Dependent Cxcl2 Production

The influx of neutrophils to infection sites is a fundamental step in host defenses against the frequent human pathogen group B Streptococcus (GBS) and other extracellular bacteria. Using a mouse model of GBS-induced peritonitis, we show in this study that the chemokines Cxcl1 and Cxcl2 play distinctive roles in enhancing the recruitment and the antibacterial activities of neutrophils in a manner that is linked to differences in the cellular sources of these mediators. Cell depletion experiments demonstrated that neutrophils make a significant contribution to the in vivo production of Cxcl2 but not Cxcl1. In vitro, neutrophils responded weakly to LPS but released high levels of Cxcl2 after stimulation with GBS or other bacteria. Neutrophil-derived Cxcl2 acted in an autocrinous manner to increase its own production and to enhance antibacterial activities, including the release of oxygen radicals. In both neutrophils and macrophages, the production of Cxcl1/2 largely required the presence of functional UNC93B1, a chaperone protein involved in signaling by endosomal TLRs. Moreover, the phenotype of UNC93B1-defective phagocytes could be recapitulated by the simultaneous absence of TLR7, 9, and 13 but not by the absence of individual TLRs. Collectively, our data show that neutrophils recognize Gram-positive and Gram-negative bacteria by means of multiple phagosomal TLRs, resulting in de novo synthesis of Cxcl2, amplification of neutrophil recruitment, and potentiation of their antibacterial activities. These data may be useful to devise alternative therapeutic strategies aimed at enhancing the recruitment and the functional activities of polymorphonuclear leukocytes during infections caused by antibiotic-resistant bacteria.

From the Cradle to the Grave of an Infection: Host-Pathogen Interaction Visualized by Intravital Microscopy

During infections, interactions between host immune cells and the pathogen occur in distinct anatomical locations and along defined time scales. This can best be assessed in the physiological context of an infection in the living tissue. Consequently, intravital imaging has enabled us to dissect the critical phases and events throughout an infection in real time in living tissues. Specifically, advances in visualizing specific cell types and individual pathogens permitted tracking the early events of tissue invasion of the pathogen, cellular interactions involved in the induction of the immune response as well the events implicated in clearance of the infection. In this respect, two vantage points have evolved since the initial employment of this technique in the field of infection biology. On the one hand, strategies acquired by the pathogen to establish within the host and circumvent or evade the immune defenses have been elucidated. On the other hand, analyzing infections from the immune system's perspective has led to insights into the dynamic cellular interactions that are involved in the initial recognition of the pathogen, immune induction as well as effector function delivery and immunopathology. Furthermore, an increasing interest in probing functional parameters in vivo has emerged, such as the analysis of pathogen reactivity to stress conditions imposed by the host organism in order to mediate clearance upon pathogen encounter. Here, we give an overview on recent intravital microscopy findings of host-pathogen interactions along the course of an infection, from both the immune system's and pathogen's perspectives. We also discuss recent developments and future perspectives in extracting intravital information beyond the localization of pathogens and their interaction with immune cells. Such reporter systems on the pathogen's physiological state and immune cell functions may prove useful in dissecting the functional dynamics of host-pathogen interactions. © 2019 The Authors. Cytometry Part A published by Wiley Periodicals, Inc. on behalf of International Society for Advancement of Cytometry.

Innate and Adaptive Immune Responses during Listeria monocytogenes Infection

It could be argued that we understand the immune response to infection with Listeria monocytogenes better than the immunity elicited by any other bacteria. L. monocytogenes are Gram-positive bacteria that are genetically tractable and easy to cultivate in vitro, and the mouse model of intravenous (i.v.) inoculation is highly reproducible. For these reasons, immunologists frequently use the mouse model of systemic listeriosis to dissect the mechanisms used by mammalian hosts to recognize and respond to infection. This article provides an overview of what we have learned over the past few decades and is divided into three sections: "Innate Immunity" describes how the host initially detects the presence of L. monocytogenes and characterizes the soluble and cellular responses that occur during the first few days postinfection; "Adaptive Immunity" discusses the exquisitely specific T cell response that mediates complete clearance of infection and immunological memory; "Use of Attenuated Listeria as a Vaccine Vector" highlights the ways that investigators have exploited our extensive knowledge of anti-Listeria immunity to develop cancer therapeutics.

Imaging the neutrophil: Intravital microscopy provides a dynamic view of neutrophil functions in host immunity

Neutrophils are the first cellular responders of the immune system. They employ their impressive arsenal of microbicidal molecules to provide rapid and efficient defense against pathogens. However, the role of neutrophils extends far beyond microbial destruction to include tissue repair and remodeling, provision of signals to the adaptive immune system and body homeostasis. Intravital imaging has allowed the visualization of neutrophils in their native environment in both health and disease and provided crucial insights into their mechanisms of action. In the last few years the power of intravital imaging has been considerably extended by the introduction of photoconvertible proteins and intracellular signaling reporter mice. This review will highlight recent advances in our understanding of neutrophil biology based on the use of intravital microscopy to visualize their modus operandi in vivo including migration in and out of inflamed tissues, host-pathogen interactions and cell fate.

Distinct behavior of myelomonocytic cells and CD8 T cells underlies the hepatic response to Listeria monocytogenes

Background: The immune response to Listeria monocytogenes (LM) is characterized by formation of leukocyte rich foci of infection in liver and spleen.  Although much has been gained in our understanding of immune response through the study of LM, little is known about spatio-temporal regulation of immune response to Listeria in liver. Methods: We utilize a combination of molecular, genetic and intravital microscopic approaches to gain insight into the dynamics of foci and leukocyte behavior during hepatic Listeriosis.  Results: LM foci efficiently exclude blood flow, indicating the presence of a barrier separating the foci and healthy tissue.  Despite this barrier, sinusoidal myelomonocytic cells readily enter or transiently interact with cells at the edge of foci of infection.  Next, utilizing L9.6 transgenic CD8 ⁺ T cells specific for an endogenously processed LM antigen, p60 217-225, along with LM deficient in this epitope, we define the role of TCR in T cell migratory behavior in infected liver.  Surprisingly, T cell behavior varies with micro-anatomic locale.  Near foci, non-specific adhesion mechanisms dominate lymphocyte behavior.  Antigen specific effects on motility became detectable only distal to foci.  Conclusions: These data suggest that LM antigens act in a paracrine manner to mediate protection from Listeriosis in the liver.

Inflammation induces two types of inflammatory dendritic cells in inflamed lymph nodes

The spatiotemporal regulation of immune cells in lymph nodes (LNs) is crucial for mounting protective T-cell responses, which are orchestrated by dendritic cells (DCs). However, it is unclear how the DC subsets are altered by the inflammatory milieu of LNs. Here, we show that the inflamed LNs of Listeria-infected mice are characterized by the clustering of neutrophils and monocytes and IFN-γ production. Significantly, the early inflammatory responses are coupled with the differentiation of not one, but two types of CD64⁺CD11c⁺MHCII⁺ inflammatory DCs. Through the assessment of chemokine receptor dependency, gene expression profiles, growth factor requirements and DC-specific lineage mapping, we herein unveil a novel inflammatory DC population (we termed ‘CD64⁺ cDCs’) that arises from conventional DCs (cDCs), distinguishable from CD64⁺ monocyte-derived DCs (moDCs) in inflamed LNs. We determined that Listeria-induced type I IFN is a critical inflammatory cue for the development of CD64⁺ cDCs but not CD64⁺ moDCs. Importantly, CD64⁺ cDCs displayed a higher potential to activate T cells than CD64⁺ moDCs, whereas the latter showed more robust expression of inflammatory genes. Although CD64⁺ and CD64⁻ cDCs were able to cross-present soluble antigens at a high dose to CD8⁺ T cells, CD64⁺ cDCs concentrated and cross-presented a minute amount of soluble antigens delivered via CD64 (FcγRI) as immune complexes. These findings reveal the role of early inflammatory responses in driving the differentiation of two inflammatory DC subsets empowered with distinct competencies.

Neutrophils Cause an Intravascular Traffic Jam

Neutrophil swarming is defined by large numbers of cells simultaneously and rapidly migrating to a site of injury or infection. In this issue of Cell Host & Microbe, Lee et al. (2018) demonstrate that intravascular swarming of neutrophils occurs in response to Candida albicans infection and causes vascular occlusion and pathological sequelae.

Complement Receptors in Myeloid Cell Adhesion and Phagocytosis

Myeloid cells make extensive use of the complement system in the context of recruitment, phagocytosis, and other effector functions. There are several types of complement receptors on myeloid cells, including G protein-coupled receptors for localizing the source of complement activation, and three sets of type I transmembrane proteins that link complement to phagocytosis: complement receptor 1, having an extracellular domain with tandem complement regulatory repeats; complement receptors 3 and 4, which are integrin family receptors comprising heterodimers of type I transmembrane subunits; and VSIG4, a member of the Ig superfamily. This review will focus on the role of the different classes of complement receptors and how their activities are integrated in the setting of immune tolerance and inflammatory responses.

Limited Colonization Undermined by Inadequate Early Immune Responses Defines the Dynamics of Decidual Listeriosis

The bacterial pathogen Listeria monocytogenes 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 L. monocytogenes infection in primary human decidual organ cultures and in the murine decidua in vivo 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 L. monocytogenes in the spleen, as well as a decrease in the tissue density of inflammatory Ly6C^hi monocytes in vivo These results suggest that the infectivity of the decidua is not the result of an enhanced recruitment of L. monocytogenes to the gestational uterus but rather is due to compromised local innate cellular immune responses.

Neutrophil swarming: an essential process of the neutrophil tissue response

Neutrophil infiltration into inflamed and infected tissues is a fundamental process of the innate immune response. While neutrophil interactions with the blood vessel wall have been intensely studied over the last decades, neutrophil dynamics beyond the vasculature have for a long time remained poorly investigated. Recent intravital microscopy studies of neutrophil populations directly at the site of tissue damage or microbial invasion have changed our perspective on neutrophil responses within tissues. Swarm-like migration patterns of neutrophils, referred to as 'neutrophil swarming', have been detected in diverse tissues under conditions of sterile inflammation and infection with various pathogens, including bacteria, fungi, and parasites. Current work has begun to unravel the molecular pathways choreographing the sequential phases of highly coordinated chemotaxis followed by neutrophil accumulation and the formation of substantial neutrophil clusters. It is now clear that intercellular communication among neutrophils amplifies their recruitment in a feed-forward manner, which provides them with a level of self-organization during neutrophil swarming. This review will summarize recent developments and current concepts on neutrophil swarming, an important process of the neutrophil tissue response with a critical role in maintaining the balance between host protection and inflammation-driven tissue destruction.

Deep insight into neutrophil trafficking in various organs

Neutrophils are professional phagocytes that constitute the first line of defense in humans. The primary function of neutrophils is to eliminate invading pathogens through oxidative and nonoxidative mechanisms. Because neutrophils rapidly migrate into inflammatory foci via diapedesis and chemotaxis, neutrophil recruitment has long been considered a hallmark of inflammation. Recent advances in intravital microscopic technologies using animal model systems have enabled researchers to directly visualize neutrophil trafficking. Consequently, the specific mechanisms of neutrophil transmigration have been identified, and even the reverse migration of neutrophils can be verified visually. Moreover, the detailed phenomena of neutrophil infiltration into various organs, such as the liver, lymphoid organs, and CNS have been identified. This progress in the study of neutrophil migration from the blood vessels to organs results in a deeper understanding of these immune cells' motility and morphology, which are closely related to the spatiotemporal regulation of the overall immune response. In this review, we discuss our current understanding of neutrophil trafficking in various organs.

Deciphering the landscape of host barriers to Listeria monocytogenes infection

Listeria monocytogenes is a common food-borne pathogen that can disseminate from the intestine and infect multiple organs. Here, we used sequence tag-based analysis of microbial populations (STAMP) to investigate Lmonocytogenes population dynamics during infection. We created a genetically barcoded library of murinized Lmonocytogenes and then used deep sequencing to track the pathogen's dissemination routes and quantify its founding population (N_b) sizes in different organs. We found that the pathogen disseminates from the gastrointestinal tract to distal sites through multiple independent routes and that N_b sizes vary greatly among tissues, indicative of diverse host barriers to infection. Unexpectedly, comparative analyses of sequence tags revealed that fecally excreted organisms are largely derived from the very small number of L. monocytogenes cells that colonize the gallbladder. Immune depletion studies suggest that distinct innate immune cells restrict the pathogen's capacity to establish replicative niches in the spleen and liver. Finally, studies in germ-free mice suggest that the microbiota plays a critical role in the development of the splenic, but not the hepatic, barriers that prevent L. monocytogenes from seeding these organs. Collectively, these observations illustrate the potency of the STAMP approach to decipher the impact of host factors on population dynamics of pathogens during infection.

Tuning Collective Cell Migration by Cell-Cell Junction Regulation

Collective cell migration critically depends on cell-cell interactions coupled to a dynamic actin cytoskeleton. Important cell-cell adhesion receptor systems implicated in controlling collective movements include cadherins, immunoglobulin superfamily members (L1CAM, NCAM, ALCAM), Ephrin/Eph receptors, Slit/Robo, connexins and integrins, and an adaptive array of intracellular adapter and signaling proteins. Depending on molecular composition and signaling context, cell-cell junctions adapt their shape and stability, and this gradual junction plasticity enables different types of collective cell movements such as epithelial sheet and cluster migration, branching morphogenesis and sprouting, collective network migration, as well as coordinated individual-cell migration and streaming. Thereby, plasticity of cell-cell junction composition and turnover defines the type of collective movements in epithelial, mesenchymal, neuronal, and immune cells, and defines migration coordination, anchorage, and cell dissociation. We here review cell-cell adhesion systems and their functions in different types of collective cell migration as key regulators of collective plasticity.

Neutrophil migration in inflammation: intercellular signal relay and crosstalk

Neutrophils are innate effector cells armed with a potent machinery to combat damage and infection within tissues. Their ability to rapidly respond to danger signals and mobilise is crucial to their role. After extravasation, neutrophil populations often exhibit swarming behaviour. Swarming occurs in distinct phases and is coordinated via inter-neutrophil signal relay in the form of small molecule mediators. Neutrophils also engage in multi-dimensional crosstalk with tissue-resident cells and incoming leukocytes in the inflammatory milieu. The complexity of neutrophil crosstalk with other innate immune cells mirrors that of the adaptive immune system, with rudimentary features of 'priming' and 'licensing'. We review recent findings relating to the migration and intercellular crosstalks of neutrophils in the initiation and resolution of inflammation.

Combination of Mass Cytometry and Imaging Analysis Reveals Origin, Location, and Functional Repopulation of Liver Myeloid Cells in Mice

BACKGROUND & AIMS

Resident macrophages are derived from yolk sac precursors and seed the liver during embryogenesis. Native cells may be replaced by bone marrow precursors during extensive injuries, irradiation, and infections. We investigated the liver populations of myeloid immune cells and their location, as well as the dynamics of phagocyte repopulation after full depletion. The effects on liver function due to the substitution of original phagocytes by bone marrow-derived surrogates were also examined.

METHODS

We collected and analyzed liver tissues from C57BL/6 (control), LysM-EGFP, B6 ACTb-EGFP, CCR2^-/-, CD11c-EYFP, CD11c-EYFP-DTR, germ-free mice, CX3CR1^gfp/gfp, CX3CR1^gpf/wt, and CX3CR1-DTR-EYFP. Liver nonparenchymal cells were immunophenotyped using mass cytometry and gene expression analyses. Kupffer and dendritic cells were depleted from mice by administration of clodronate, and their location and phenotype were examined using intravital microscopy and time-of-flight mass cytometry. Mice were given acetaminophen gavage or intravenous injections of fluorescently labeled Escherichia coli, blood samples were collected and analyzed, and liver function was evaluated. We assessed cytokine profiles of liver tissues using a multiplexed array.

RESULTS

Using mass cytometry and gene expression analyses, we identified 2 populations of hepatic macrophages and 2 populations of monocytes. We also identified 4 populations of dendritic cells and 1 population of basophils. After selective depletion of liver phagocytes, intravascular myeloid precursors began to differentiate into macrophages and dendritic cells; dendritic cells migrated out of sinusoids, after a delay, via the chemokine CX3CL1. The cell distribution returned to normal in 2 weeks, but the repopulated livers were unable to fully respond to drug-induced injury or clear bacteria for at least 1 month. This defect was associated with increased levels of inflammatory cytokines, and dexamethasone accelerated the repopulation of liver phagocytes.

CONCLUSIONS

In studies of hepatic phagocyte depletion in mice, we found that myeloid precursors can differentiate into liver macrophages and dendritic cells, which each localize to distinct tissue compartments. During replenishment, macrophages acquire the ability to respond appropriately to hepatic injury and to remove bacteria from the blood stream.

IAP antagonists induce anti-tumor immunity in multiple myeloma

The cellular inhibitors of apoptosis (cIAP) 1 and 2 are amplified in about 3% of cancers and have been identified in multiple malignancies as being potential therapeutic targets as a result of their role in the evasion of apoptosis. Consequently, small-molecule IAP antagonists, such as LCL161, have entered clinical trials for their ability to induce tumor necrosis factor (TNF)-mediated apoptosis of cancer cells. However, cIAP1 and cIAP2 are recurrently homozygously deleted in multiple myeloma (MM), resulting in constitutive activation of the noncanonical nuclear factor (NF)-κB pathway. To our surprise, we observed robust in vivo anti-myeloma activity of LCL161 in a transgenic myeloma mouse model and in patients with relapsed-refractory MM, where the addition of cyclophosphamide resulted in a median progression-free-survival of 10 months. This effect was not a result of direct induction of tumor cell death, but rather of upregulation of tumor-cell-autonomous type I interferon (IFN) signaling and a strong inflammatory response that resulted in the activation of macrophages and dendritic cells, leading to phagocytosis of tumor cells. Treatment of a MM mouse model with LCL161 established long-term anti-tumor protection and induced regression in a fraction of the mice. Notably, combination of LCL161 with the immune-checkpoint inhibitor anti-PD1 was curative in all of the treated mice.

Analysis of the Rab GTPase Interactome in Dendritic Cells Reveals Anti-microbial Functions of the Rab32 Complex in Bacterial Containment

Dendritic cells (DCs) orchestrate complex membrane trafficking through an interconnected transportation network linked together by Rab GTPases. Through a tandem affinity purification strategy and mass spectrometry, we depicted an interactomic landscape of major members of the mammalian Rab GTPase family. When complemented with imaging tools, this proteomic analysis provided a global view of intracellular membrane organization. Driven by this analysis, we investigated dynamic changes to the Rab32 subnetwork in DCs induced by L. monocytogenes infection and uncovered an essential role of this subnetwork in controlling the intracellular proliferation of L. monocytogenes. Mechanistically, Rab32 formed a persistent complex with two interacting proteins, PHB and PHB2, to encompass bacteria both during early phagosome formation and after L. monocytogenes escaped the original containment vacuole. Collectively, we have provided a functional compartmentalization overview and an organizational framework of intracellular Rab-mediated vesicle trafficking that can serve as a resource for future investigations.

Recent advances in microscopic techniques for visualizing leukocytes in vivo

Leukocytes are inherently motile and interactive cells. Recent advances in intravital microscopy approaches have enabled a new vista of their behavior within intact tissues in real time. This brief review summarizes the developments enabling the tracking of immune responses in vivo.

Reverse Migration of Neutrophils: Where, When, How, and Why?

Neutrophil migration to injured and pathogen-infected tissues is a fundamental component of innate immunity. An array of cellular and molecular events mediate this response to collectively guide neutrophils out of the vasculature and towards the core of the ensuing inflammatory reaction where they exert effector functions. Advances in imaging modalities have revealed that neutrophils can also exhibit motility away from sites of inflammation and injury, although it is unclear under what circumstances this reverse migration is a physiological protective response, and when it has pathophysiological relevance. Here we review different types of neutrophil reverse migration and discuss the current understanding of the associated mechanisms. In this context we propose clarifications to the existing terminology used to describe the many facets of neutrophil reverse migration.

Listeria monocytogenes and the Inflammasome: From Cytosolic Bacteriolysis to Tumor Immunotherapy

Inflammasomes are cytosolic innate immune surveillance systems that recognize a variety of danger signals, including those from pathogens. Listeria monocytogenes is a Gram-positive intracellular bacterium evolved to live within the harsh environment of the host cytosol. Further, L. monocytogenes can activate a robust cell-mediated immune response that is being harnessed as an immunotherapeutic platform. Access to the cytosol is critical for both causing disease and inducing a protective immune response, and it is hypothesized that the cytosolic innate immune system, including the inflammasome, is critical for both host protection and induction of long-term immunity. L. monocytogenes can activate a variety of inflammasomes via its pore-forming toxin listeriolysin-O, flagellin, or DNA released through bacteriolysis; however, inflammasome activation attenuates L. monocytogenes, and as such, L. monocytogenes has evolved a variety of ways to limit inflammasome activation. Surprisingly, inflammasome activation also impairs the host cell-mediated immune response. Thus, understanding how L. monocytogenes activates or avoids detection by the inflammasome is critical to understand the pathogenesis of L. monocytogenes and improve the cell-mediated immune response generated to L. monocytogenes for more effective immunotherapies.

Macrophages mediate flagellin induced inflammasome activation and host defense in zebrafish

The inflammasome is an innate immune complex whose rapid inflammatory outputs play a critical role in controlling infection; however, the host cells that mediate inflammasome responses in vivo are not well defined. Using zebrafish larvae, we examined the cellular immune responses to inflammasome activation during infection. We compared the host responses with two Listeria monocytogenes strains: wild type and Lm-pyro, a strain engineered to activate the inflammasome via ectopic expression of flagellin. Infection with Lm-pyro led to activation of the inflammasome, macrophage pyroptosis and ultimately attenuation of virulence. Depletion of caspase A, the zebrafish caspase-1 homolog, restored Lm-pyro virulence. Inflammasome activation specifically recruited macrophages to infection sites, whereas neutrophils were equally recruited to wild type and Lm-pyro infections. Similar to caspase A depletion, macrophage deficiency rescued Lm-pyro virulence to wild-type levels, while defective neutrophils had no specific effect. Neutrophils were, however, important for general clearance of L. monocytogenes, as both wild type and Lm-pyro were more virulent in larvae with defective neutrophils. This study characterizes a novel model for inflammasome studies in an intact host, establishes the importance of macrophages during inflammasome responses and adds importance to the role of neutrophils in controlling L. monocytogenes infections.

In the eye of the neutrophil swarm-navigation signals that bring neutrophils together in inflamed and infected tissues

Neutrophils are sentinel cells that express in higher vertebrates >30 chemokine and chemoattractant receptors to sense and quickly react to tissue damage signals. Intravital microscopy studies in mouse models of wounding, inflammation, and infection have revealed that neutrophils form cell swarms at local sites of tissue injury and cell death. This swarming response is choreographed by chemokines, lipids, and other chemoattractants, controlling sequential phases of highly coordinated chemotaxis, intercellular signal relay, and cluster formation among neutrophils. This review will give a brief overview about the basic principles and key molecules that have led to the refined multistep model of how neutrophils come together to isolate sites of tissue injury and microbial invasion from healthy tissue. Whereas auto- and paracrine signaling among neutrophils during later phases of swarming can provide a level of self-organization for robust navigation in diverse inflammatory settings, guidance factors from primary tissue lesions, resident bystander cells, and dying cells regulate the initial phases of the swarming response. This review will discuss how the specific environmental context and mixture of attractants at the locally inflamed site can lead to variants of the multistep attraction model and influence the extent of neutrophil swarming, ranging from accumulations of only few individual cells to the aggregation of several hundreds of neutrophils, as found in abscesses. Given the critical roles of neutrophils in both host protection and tissue destruction, novel insights on neutrophil swarming might provide useful for the therapeutic modulation of neutrophil-dependent inflammatory processes.

Visualization of granzyme B-expressing CD8 T cells during primary and secondary immune responses to Listeria monocytogenes

CD8 T cells contribute to long-term protection against Listeria monocytogenes infection by differentiating into memory T cells. These rapidly respond to antigen or inflammation upon secondary infection. In this study we used CD8 T cells from OT1 mice and CD4 T cells from OT2 mice expressing a fluorescent chimeric granzyme (GZMB-Tom) protein to monitor the primary response to infection with ovalbumin-expressing L. monocytogenes (Lm-OVA). We show that, unlike poorly responding CD4 T cells, CD8 T cells readily proliferated and expressed high levels of GZMB-Tom as early as 2 days after infection. FACS analysis showed GZMB-Tom expression in undivided CD8 T cells, with its level increasing over one to four divisions. OT1 T cells were visualized in the T-cell zone by confocal microscopy. This showed GZMB-Tom-containing granules oriented towards MHCII-positive cells. Twenty hours later, most OT1 T cells had divided but their level of GZMB-Tom expression was reduced. Recently divided OT1 cells failed to express GZMB-Tom. Fourteen hours after secondary infection, GZMB-Tom was re-expressed in memory OT1 T cells responding either to Lm-OVA or L. monocytogenes. Differences in the activation phenotype and in the splenic distribution of OT1 T cells were observed, depending on the challenge. Notably, OTI T cells with polarized granules were only observed after challenge with cognate antigen. This work showed that the GZMB-Tom knock-in mice in which GZMB-Tom faithfully reproduced GZMB expression, provide useful tools to dissect mechanisms leading to the development of anti-bacterial effector and memory CD8 T cells and reactivation of the memory response to cognate antigen or inflammatory signals.

In vivo approaches reveal a key role for DCs in CD4+ T cell activation and parasite clearance during the acute phase of experimental blood-stage malaria

Dendritic cells (DCs) are phagocytes that are highly specialized for antigen presentation. Heterogeneous populations of macrophages and DCs form a phagocyte network inside the red pulp (RP) of the spleen, which is a major site for the control of blood-borne infections such as malaria. However, the dynamics of splenic DCs during Plasmodium infections are poorly understood, limiting our knowledge regarding their protective role in malaria. Here, we used in vivo experimental approaches that enabled us to deplete or visualize DCs in order to clarify these issues. To elucidate the roles of DCs and marginal zone macrophages in the protection against blood-stage malaria, we infected DTx (diphtheria toxin)-treated C57BL/6.CD11c-DTR mice, as well as C57BL/6 mice treated with low doses of clodronate liposomes (ClLip), with Plasmodium chabaudi AS (Pc) parasites. The first evidence suggesting that DCs could contribute directly to parasite clearance was an early effect of the DTx treatment, but not of the ClLip treatment, in parasitemia control. DCs were also required for CD4+ T cell responses during infection. The phagocytosis of infected red blood cells (iRBCs) by splenic DCs was analyzed by confocal intravital microscopy, as well as by flow cytometry and immunofluorescence, at three distinct phases of Pc malaria: at the first encounter, at pre-crisis concomitant with parasitemia growth and at crisis when the parasitemia decline coincides with spleen closure. In vivo and ex vivo imaging of the spleen revealed that DCs actively phagocytize iRBCs and interact with CD4+ T cells both in T cell-rich areas and in the RP. Subcapsular RP DCs were highly efficient in the recognition and capture of iRBCs during pre-crisis, while complete DC maturation was only achieved during crisis. These findings indicate that, beyond their classical role in antigen presentation, DCs also contribute to the direct elimination of iRBCs during acute Plasmodium infection.

T cells kill bacteria captured by transinfection from dendritic cells and confer protection in mice

Dendritic cells (DCs) phagocytose, process, and present bacterial antigens to T lymphocytes to trigger adaptive immunity. In vivo, bacteria can also be found inside T lymphocytes. However, T cells are refractory to direct bacterial infection, leaving the mechanisms by which bacteria invade T cells unclear. We show that T cells take up bacteria from infected DCs by the process of transinfection, which requires direct contact between the two cells and is enhanced by antigen recognition. Prior to transfer, bacteria localize to the immunological synapse, an intimate DC/T cell contact structure that activates T cells. Strikingly, T cells efficiently eliminate the transinfecting bacteria within the first hours after infection. Transinfected T cells produced high levels of proinflammatory cytokines and were able to protect mice from bacterial challenge following adoptive transfer. Thus, T lymphocytes can capture and kill bacteria in a manner reminiscent of innate immunity.

CD45 ligation expands Tregs by promoting interactions with DCs

Regulatory T cells (Tregs), which express CD4 and FOXP3, are critical for modulating the immune response and promoting immune tolerance. Consequently, methods to expand Tregs for therapeutic use are of great interest. While transfer of Tregs after massive ex vivo expansion can be achieved, in vivo expansion of Tregs would be more practical. Here, we demonstrate that targeting the CD45 tyrosine phosphatase with a tolerogenic anti-CD45RB mAb acutely increases Treg numbers in WT mice, even in absence of exogenous antigen. Treg expansion occurred through substantial augmentation of homeostatic proliferation in the preexisting Treg population. Moreover, anti-CD45RB specifically increased Treg proliferation in response to cognate antigen. Compared with conventional T cells, Tregs differentially regulate their conjugation with DCs. Therefore, we determined whether CD45 ligation could alter interactions between Tregs and DCs. Live imaging showed that CD45 ligation specifically reduced Treg motility in an integrin-dependent manner, resulting in enhanced interactions between Tregs and DCs in vivo. Increased conjugate formation, in turn, augmented nuclear translocation of nuclear factor of activated T cells (NFAT) and Treg proliferation. Together, these results demonstrate that Treg peripheral homeostasis can be specifically modulated in vivo to promote Treg expansion and tolerance by increasing conjugation between Tregs and DCs.