Rate of replenishment and microenvironment contribute to the sexually dimorphic phenotype and function of peritoneal macrophages.

Macrophages reside in the body cavities where they maintain serosal homeostasis and provide immune surveillance. Peritoneal macrophages are implicated in the etiology of pathologies including peritonitis, endometriosis, and metastatic cancer; thus, understanding the factors that govern their behavior is vital. Using a combination of fate mapping techniques, we have investigated the impact of sex and age on murine peritoneal macrophage differentiation, turnover, and function. We demonstrate that the sexually dimorphic replenishment of peritoneal macrophages from the bone marrow, which is high in males and very low in females, is driven by changes in the local microenvironment that arise upon sexual maturation. Population and single-cell RNA sequencing revealed marked dimorphisms in gene expression between male and female peritoneal macrophages that was, in part, explained by differences in composition of these populations. By estimating the time of residency of different subsets within the cavity and assessing development of dimorphisms with age and in monocytopenic Ccr2 -/- mice, we demonstrate that key sex-dependent features of peritoneal macrophages are a function of the differential rate of replenishment from the bone marrow, whereas others are reliant on local microenvironment signals. We demonstrate that the dimorphic turnover of peritoneal macrophages contributes to differences in the ability to protect against pneumococcal peritonitis between the sexes. These data highlight the importance of considering both sex and age in susceptibility to inflammatory and infectious diseases.

Hypoxia determines survival outcomes of bacterial infection through HIF-1alpha dependent re-programming of leukocyte metabolism.

Hypoxia and bacterial infection frequently co-exist, in both acute and chronic clinical settings, and typically result in adverse clinical outcomes. To ameliorate this morbidity, we investigated the interaction between hypoxia and the host response. In the context of acute hypoxia, both S. aureus and S. pneumoniae infections rapidly induced progressive neutrophil mediated morbidity and mortality, with associated hypothermia and cardiovascular compromise. Preconditioning animals through longer exposures to hypoxia, prior to infection, prevented these pathophysiological responses and profoundly dampened the transcriptome of circulating leukocytes. Specifically, perturbation of HIF pathway and glycolysis genes by hypoxic preconditioning was associated with reduced leukocyte glucose utilisation, resulting in systemic rescue from a global negative energy state and myocardial protection. Thus we demonstrate that hypoxia preconditions the innate immune response and determines survival outcomes following bacterial infection through suppression of HIF-1α and neutrophil metabolism. The therapeutic implications of this work are that in the context of systemic or tissue hypoxia therapies that target the host response could improve infection associated morbidity and mortality.

Characterization of the survival effect of tumour necrosis factor-alpha in human neutrophils.

Granulocyte apoptosis has been proposed as a fundamental, injury-limiting granulocyte-clearance mechanism. As such, inhibition of this process may prevent the resolution of inflammation. Our previous studies have shown that TNFalpha (tumour necrosis factor-alpha) has a bi-modal influence on the rate of constitutive neutrophil apoptosis in vitro, causing early acceleration and late inhibition of this process. The pro-apoptotic effect is uniquely TNFR1 (TNF receptor 1) and TNFR2-dependent and the latter survival process is mediated via phosphoinositide 3-kinase and NF-kappaB (nuclear factor-kappaB) activation. In the present study, we show that, in contrast with GM-CSF (granulocyte/macrophage colony-stimulating factor), the delayed addition (i.e. at 6 h) of TNFalpha increases its survival effect despite substantial loss of neutrophil TNFR1 and TNFR2 at that time. This paradox was resolved using PBMC (peripheral blood mononuclear cell)-deplete and 5% PBMC-replete neutrophil cultures, where the enhanced survival effect observed after delayed TNFalpha addition was shown to be PBMC-dependent. TNFR2-blocking antibodies had no effect on the late survival effect of TNFalpha, implying a TNFR1-dependent process. Finally, I-kappaBalpha (inhibitory kappaB-alpha) and NF-kappaB time-course studies demonstrated that the survival effects of both GM-CSF and TNFalpha could be explained by maintenance of functional NF-kappaB.

What can we learn from highly purified neutrophils?

Neutrophil purification has traditionally been performed by centrifugation of leucocytes through density gradients. These reliable methods produce populations that are typically >95% pure neutrophils, and have allowed the widespread study of the function of these cells. Our recent work has suggested that residual monocytes may play a more important role than has been previously realized, and suggest that for some functional experiments, further purification of cells is required to understand fully the neutrophil phenotype.

Hypoxic regulation of neutrophil apoptosis role: of reactive oxygen intermediates in constitutive and tumor necrosis factor alpha-induced cell death.

Activation of the NADPH oxidase system to generate reactive oxygen species (ROS) plays a key role in bacterial killing by human neutrophils. However, the involvement of such radicals in spontaneous and TNFalpha-driven neutrophil apoptosis remains uncertain. While incubation of cells under anoxic conditions attenuated the pro-apoptotic effect of TNFalpha, full activation of the respiratory burst using PAF followed by fMLP, or the addition of physiologically relevant concentrations of H(2)O(2), had no effect on the rate of apoptosis. Furthermore, the phosphoinositide 3-kinase inhibitor, LY294002, which abolishes receptor-mediated activation of the NADPH oxidase, and five discrete anti-oxidants all failed to affect apoptotic thresholds. Thus ROS do not appear to modulate constitutive apoptosis in neutrophils or appear sufficient to mediate the pro-apoptotic effect of TNFalpha.