Neutrophils are specialized innate cells that require constant replenishment from proliferative bone marrow (BM) precursors as a result of their short half-life. Although it is established that neutrophils are derived from the granulocyte-macrophage progenitor (GMP), the differentiation pathways from GMP to functional mature neutrophils are poorly defined. Using mass cytometry (CyTOF) and cell-cycle-based analysis, we identified three neutrophil subsets within the BM: a committed proliferative neutrophil precursor (preNeu) which differentiates into non-proliferating immature neutrophils and mature neutrophils. Transcriptomic profiling and functional analysis revealed that preNeu require the C/EBPε transcription factor for their generation from the GMP, and their proliferative program is substituted by a gain of migratory and effector function as they mature. preNeus expand under microbial and tumoral stress, and immature neutrophils are recruited to the periphery of tumor-bearing mice. In summary, our study identifies specialized BM granulocytic populations that ensure supply under homeostasis and stress responses.

In recent years, various intervention strategies have reduced malaria morbidity and mortality, but further improvements probably depend upon development of a broadly protective vaccine. To better understand immune requirement for protection, we examined liver-stage immunity after vaccination with irradiated sporozoites, an effective though logistically difficult vaccine. We identified a population of memory CD8 T cells that expressed the gene signature of tissue-resident memory T (Trm) cells and remained permanently within the liver, where they patrolled the sinusoids. Exploring the requirements for liver Trm cell induction, we showed that by combining dendritic cell-targeted priming with liver inflammation and antigen recognition on hepatocytes, high frequencies of Trm cells could be induced and these cells were essential for protection against malaria sporozoite challenge. Our study highlights the immune potential of liver Trm cells and provides approaches for their selective transfer, expansion, or depletion, which may be harnessed to control liver infections or autoimmunity.

Immune protection relies on the capacity of neutrophils to infiltrate challenged tissues. Naive tissues, in contrast, are believed to remain free of these cells and protected from their toxic cargo. Here, we show that neutrophils are endowed with the capacity to infiltrate multiple tissues in the steady-state, a process that follows tissue-specific dynamics. By focusing in two particular tissues, the intestine and the lungs, we find that neutrophils infiltrating the intestine are engulfed by resident macrophages, resulting in repression of Il23 transcription, reduced G-CSF in plasma, and reinforced activity of distant bone marrow niches. In contrast, diurnal accumulation of neutrophils within the pulmonary vasculature influenced circadian transcription in the lungs. Neutrophil-influenced transcripts in this organ were associated with carcinogenesis and migration. Consistently, we found that neutrophils dictated the diurnal patterns of lung invasion by melanoma cells. Homeostatic infiltration of tissues unveils a facet of neutrophil biology that supports organ function, but can also instigate pathological states.

In contrast with many capillary beds, the glomerulus readily supports leukocyte recruitment. However, little is known regarding the actions of leukocytes following their recruitment to glomeruli. We used multiphoton confocal microscopy to examine leukocyte behavior in the glomerular microvasculature. In normal glomeruli, neutrophils and monocytes were retained in capillaries for several minutes, remaining static or migrating intravascularly. Induction of glomerular inflammation resulted in an increase in the duration of retention of static and migratory leukocytes. In response to immune complex deposition, both static and migratory neutrophils generated oxidants in inflamed glomeruli via a Mac-1-dependent mechanism. Our results describe a new paradigm for glomerular inflammation, suggesting that the major effect of acute inflammation is to increase the duration of leukocyte retention in the glomerulus. Moreover, these findings describe a previously unknown form of multicellular intravascular patrolling that involves both monocytes and neutrophils, which may underlie the susceptibility of the glomerulus to inflammation.

The CXCR4 antagonist plerixafor augments frequency of circulating neutrophils via release from the lung and prevents neutrophil homing to the bone marrow.

Lymph nodes (LNs) are constructed of intricate networks of endothelial and mesenchymal stromal cells. How these lymphoid stromal cells (LSCs) regulate lymphoid tissue remodeling and contribute to immune responses remains poorly understood. We performed a comprehensive functional and transcriptional analysis of LSC responses to skin viral infection and found that LSC subsets responded robustly, with different kinetics for distinct pathogens. Recruitment of cells to inflamed LNs induced LSC expansion, while B cells sustained stromal responses in an antigen-independent manner. Infection induced rapid transcriptional responses in LSCs. This transcriptional program was transient, returning to homeostasis within 1 month of infection, yet expanded fibroblastic reticular cell networks persisted for more than 3 months after infection, and this altered LN composition reduced the magnitude of LSC responses to subsequent heterologous infection. Our results reveal the complexity of LSC responses during infection and suggest that amplified networks of LN stromal cells support successive immune responses.

Nonclassical monocytes undergo intravascular patrolling in blood vessels, positioning them ideally to coordinate responses to inflammatory stimuli. Under some circumstances, the actions of monocytes have been shown to involve promotion of neutrophil recruitment. However, the mechanisms whereby patrolling monocytes control the actions of neutrophils in the circulation are unclear. Here, we examined the contributions of monocytes to antibody-and neutrophildependent inflammation in a model of in situ immune complexmediated glomerulonephritis. Multiphoton and spinning disk confocal intravital microscopy revealed that monocytes patrol both uninflamed and inflamed glomeruli using β 2 and α 4 integrins and CX 3 CR1. Monocyte depletion reduced glomerular injury, demonstrating that these cells promote inappropriate inflammation in this setting. Monocyte depletion also resulted in reductions in neutrophil recruitment and dwell time in glomerular capillaries and in reactive oxygen species (ROS) generation by neutrophils, suggesting a role for cross-talk between monocytes and neutrophils in induction of glomerulonephritis. Consistent with this hypothesis, patrolling monocytes and neutrophils underwent prolonged interactions in glomerular capillaries, with the duration of these interactions increasing during inflammation. Moreover, neutrophils that interacted with monocytes showed increased retention and a greater propensity for ROS generation in the glomerulus. Also, renal patrolling monocytes, but not neutrophils, produced TNF during inflammation, and TNF inhibition reduced neutrophil dwell time and ROS production, as well as renal injury. These findings show that monocytes and neutrophils undergo interactions within the glomerular microvasculature. Moreover, evidence indicates that, in response to an inflammatory stimulus, these interactions allow monocytes to promote neutrophil recruitment and activation within the glomerular microvasculature, leading to neutrophil-dependent tissue injury.monocyte-neutrophil interaction | inflammation | glomerulonephritis | reactive oxygen species | tumor necrosis factor N eutrophil recruitment is a crucial event in the response to tissue injury and host defense against pathogens. However, if dysregulated, it can also cause significant tissue injury. Inappropriate recruitment and activation of neutrophils are recognized as major contributors to organ damage in numerous inflammatory diseases, including inflammatory arthritis, acute respiratory distress syndrome, systemic lupus erythematosus, and acute glomerulonephritis (1-4). Although our understanding of the molecular basis of neutrophil recruitment is well-developed, less is known about the interactions of neutrophils with other circulating immune cells during the development of the innate inflammatory response. However, over the last decade, evidence has emerged that neutrophil recruitment and function during inflammation can be influenced by another circulating immune cell-the monocyte (5-10).Monocytes exist in two major populations,...

CXCR4 identifies an immobilized BM precursor (i.e., transitional premonocyte [TpMo]) that proliferates and replenishes mature Ly6Chi monocytes in mice and humans. Upon entering the circulation, CXCR4 governs monocyte margination in the lung vasculature.