Human neutrophils ≠ murine neutrophils: Does it matter?

Human and murine neutrophils differ with respect to representation in blood, receptors, nuclear morphology, signaling pathways, granule proteins, NADPH oxidase regulation, magnitude of oxidant and hypochlorous acid production, and their repertoire of secreted molecules. These differences often matter and can undermine extrapolations from murine studies to clinical care, as illustrated by several failed therapeutic interventions based on mouse models. Likewise, coevolution of host and pathogen undercuts fidelity of murine models of neutrophil-predominant human infections. However, murine systems that accurately model the human condition can yield insights into human biology difficult to obtain otherwise. The challenge for investigators who employ murine systems is to distinguish models from pretenders and to know when the mouse provides biologically accurate insights. Testing with human neutrophils observations made in murine systems would provide a safeguard but is not always possible. At a minimum, studies that use exclusively murine neutrophils should have accurate titles supported by data and restrict conclusions to murine neutrophils and not encompass all neutrophils. For now, the integration of evidence from studies of neutrophil biology performed using valid murine models coupled with testing in vitro of human neutrophils combines the best of both approaches to elucidate the mysteries of human neutrophil biology.

Human neutrophils phagocytose and kill Acinetobacter baumannii and A. pittii

Acinetobacter baumannii is a common cause of health care associated infections worldwide. A. pittii is an opportunistic pathogen also frequently isolated from Acinetobacter infections other than those from A. baumannii. Knowledge of Acinetobacter virulence factors and their role in pathogenesis is scarce. Also, there are no detailed published reports on the interactions between A. pittii and human phagocytic cells. Using confocal laser and scanning electron microscopy, immunofluorescence, and live-cell imaging, our study shows that immediately after bacteria-cell contact, neutrophils rapidly and continuously engulf and kill bacteria during at least 4 hours of infection in vitro. After 3 h of infection, neutrophils start to release neutrophil extracellular traps (NETs) against Acinetobacter. DNA in NETs colocalizes well with human histone H3 and with the specific neutrophil elastase. We have observed that human neutrophils use large filopodia as cellular tentacles to sense local environment but also to detect and retain bacteria during phagocytosis. Furthermore, co-cultivation of neutrophils with human differentiated macrophages before infections shows that human neutrophils, but not macrophages, are key immune cells to control Acinetobacter. Although macrophages were largely activated by both bacterial species, they lack the phagocytic activity demonstrated by neutrophils.

How methicillin-resistant Staphylococcus aureus evade neutrophil killing

PURPOSE OF REVIEW

Methicillin-resistant strains of the important human pathogen Staphylococcus aureus pose a significant public health threat in the community, as they are easily transmitted, especially prone to cause invasive disease, and infect otherwise healthy individuals. The mechanistic basis for the ability of these organisms to evade the innate immune responses remains incompletely defined.

RECENT FINDINGS

The success of pathogens such as S. aureus rests, in part, on their capacity to overcome neutrophil-mediated host defense to establish infection and cause human disease. S. aureus has the potential to thwart effective neutrophil chemotaxis, and phagocytosis, and succeeds in evading killing by neutrophils. Furthermore, S. aureus surviving within neutrophils promotes neutrophil cytolysis, with release of host-derived molecules that promote local inflammation. Here, we provide a brief overview of our understanding of the mechanisms by which S. aureus - including methicillin-resistant S. aureus - avoids neutrophil-mediated host defense and causes disease.

SUMMARY

Understanding the molecular mechanisms by which S. aureus avoids neutrophil-mediated responses and initiates signaling cascades that culminate in neutrophil lysis will provide insights prerequisite to the development of novel targets for treating staphylococcal infections.