Hongbo Luo

One of the major players in innate immune system is neutrophil which is the most abundant cell type among circulating white blood cells and constitutes the first line of host defense against invading bacteria and other pathogens. In response to inflammatory stimuli, neutrophils migrate from the blood to infected tissues, where they protect their host by engulfing, killing, and digesting invading bacterial and fungal pathogens. Conversely, excessive neutrophil accumulation or hyper-responsiveness of neutrophils can be detrimental to the system. Hence, the production, recruitment, and response of neutrophils to inflammatory stimuli need to be well controlled. The long-term goal of our research is to elucidate the molecular mechanisms that control the production, trafficking, function, and fate of neutrophils during infection and inflammation. We are also interested in the potential role of neutrophils in regulating bone marrow hematopoiesis and tumorigenesis.

Currently, we are particularly interested in signaling pathways mediated by inositol phospholipid PtdIns(3,4,5)P3, inositol phosphates (e.g. InsP4 and InsP7), reactive oxygen species (ROS), and neutrophil serine proteinases (elastase, proteinase 3, and cathepsin G). We also identified gasdermin family proteins, particularly GSDMD and GSDME, as key cellular factors controlling neutrophil death and function. We utilize a wide variety of approaches ranging from basic molecular, cellular, and biochemical methods to automatic high-resolution imaging, scRNA-seq, high throughput chemical genetic screening, and animal inflammation models to dissect these pathways. The ultimate goal is to develop more efficient and effective therapies for treating various infectious and inflammatory diseases.

The full neutrophil heterogeneity and differentiation landscape remains incompletely characterized. Recently, we profiled >25,000 differentiating and mature mouse neutrophils using single-cell RNA sequencing to provide a comprehensive transcriptional landscape of neutrophil maturation, function, and fate decision in their steady state and during bacterial infection. Eight neutrophil populations were defined by distinct molecular signatures. The three mature peripheral blood neutrophil subsets arise from distinct maturing bone marrow neutrophil subsets. Driven by both known and uncharacterized transcription factors, neutrophils gradually acquire microbicidal capability as they traverse the transcriptional landscape, representing an evolved mechanism for fine-tuned regulation of an effective but balanced neutrophil response. Bacterial infection reprograms the genetic architecture of neutrophil populations, alters dynamic transition between each subpopulation, and primes neutrophils for augmented functionality without affecting overall heterogeneity. In summary, these data establish a reference model and general framework for studying neutrophil-related disease mechanisms, biomarkers, and therapeutic targets at single-cell resolution. We are presently investigating how neutrophil heterogeneity is regulated and contributes to host immune response in health and disease.