Combined Host-Pathogen Fate Mapping to Investigate Lung Macrophages in Viral Infection.

Macrophage identity, as defined by epigenetic, transcriptional, proteomic, and functional programs, is greatly impacted by cues originating from the microenvironment. As a consequence, immunophenotyping based on surface marker expression is established and reliable in homeostatic conditions, whereas environmental challenges, in particular infections, severely hamper the determination of identity states. This has become more evident with recent discoveries that macrophage-inherent plasticity may go beyond limits of lineage-defining immunophenotypes. Therefore, transgenic fate mapping tools, such as the phage-derived loxP-cre-system, are essential for the analysis of macrophage adaptation in the tissue under extreme environmental conditions, for example, upon encounter with pathogens. In this chapter, we describe an advanced application of the loxP-cre-system during infection. Here, the host encodes a cell type-specific cre-recombinase, while the pathogen harbors a STOP-floxed fluorescent reporter gene. As an instructive example for the versatility of the system, we demonstrate that alveolar macrophages are predominantly targeted after respiratory tract infection with mouse cytomegalovirus (MCMV). Combined host-pathogen fate mapping not only enables to distinguish between infected and non-infected (bystander) macrophages but also spurs exploration of phenotypic adaptation and tracing of cellular localization in the context of MCMV infection. Moreover, we provide a gating strategy for resolving the diversity of pulmonary immune cell populations.

Metabolic rewiring tunes dermal macrophages in staphylococcal skin infection.

The skin needs to balance tolerance of colonizing microflora with rapid detection of potential pathogens. Flexible response mechanisms would seem most suitable to accommodate the dynamic challenges of effective antimicrobial defense and restoration of tissue homeostasis. Here, we dissected macrophage-intrinsic mechanisms and microenvironmental cues that tune macrophage signaling in localized skin infection with the colonizing and opportunistic pathogen Staphylococcus aureus. Early in skin infection, the cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) produced by γδ T cells and hypoxic conditions within the dermal microenvironment diverted macrophages away from a homeostatic M-CSF- and hypoxia-inducible factor 1α (HIF-1α)-dependent program. This allowed macrophages to be metabolically rewired for maximal inflammatory activity, which requires expression of Irg1 and generation of itaconate, but not HIF-1α. This multifactorial macrophage rewiring program was required for both the timely clearance of bacteria and for the provision of local immune memory. These findings indicate that immunometabolic conditioning allows dermal macrophages to cycle between antimicrobial activity and protection against secondary infections.

Cytomegaloviruses and Macrophages-Friends and Foes From Early on?

Starting at birth, newborn infants are exposed to numerous microorganisms. Adaptation of the innate immune system to them is a delicate process, with potentially advantageous and harmful implications for health development. Cytomegaloviruses (CMVs) are highly adapted to their specific mammalian hosts, with which they share millions of years of co-evolution. Throughout the history of mankind, human CMV has infected most infants in the first months of life without overt implications for health. Thus, CMV infections are intertwined with normal immune development. Nonetheless, CMV has retained substantial pathogenicity following infection in utero or in situations of immunosuppression, leading to pathology in virtually any organ and particularly the central nervous system (CNS). CMVs enter the host through mucosal interfaces of the gastrointestinal and respiratory tract, where macrophages (MACs) are the most abundant immune cell type. Tissue MACs and their potential progenitors, monocytes, are established target cells of CMVs. Recently, several discoveries have revolutionized our understanding on the pre- and postnatal development and site-specific adaptation of tissue MACs. In this review, we explore experimental evidences and concepts on how CMV infections may impact on MAC development and activation as part of host-virus co-adaptation.

Macrophages Are a Potent Source of Streptococcus-Induced IFN-ß.

IFN-ß essentially modulates the host response against mucocutaneous colonizers and potential pathogens, such as group B Streptococcus (GBS). It has been reported that the dominant signaling cascade driving IFN-ß in macrophages (MΦ) in streptococcal infection is the cGAS-STING pathway, whereas conventional dendritic cells (DC) exploit endosomal recognition by intracellular TLRs. In this study, we revisited this issue by precisely monitoring the phenotypic dynamics in mixed mouse MΦ/DC cultures with GM-CSF, which requires snapshot definition of cellular identities. We identified four mononuclear phagocyte populations, of which two were transcriptionally and morphologically distinct MΦ-DC-like subsets, and two were transitional types. Notably, GBS induced a TLR7-dependent IFN-ß signal only in MΦ-like but not in DC-like cells. IFN-ß induction did not require live bacteria (i.e., the formation of cytolytic toxins), which are essential for IFN-ß induction via cGAS-STING. In contrast to IFN-ß, GBS induced TNF-α independently of TLR7. Subsequent to the interaction with streptococci, MΦ changed their immunophenotype and gained some typical DC markers and DC-like morphology. In summary, we identify IFN-ß formation as part of the antistreptococcal repertoire of GM-CSF differentiated MΦ in vitro and in vivo and delineate their plasticity.

A Subset of Skin Macrophages Contributes to the Surveillance and Regeneration of Local Nerves.

The skin comprises tissue macrophages as the most abundant resident immune cell type. Their diverse tasks including resistance against invading pathogens, attraction of bypassing immune cells from vessels, and tissue repair require dynamic specification. Here, we delineated the postnatal development of dermal macrophages and their differentiation into subsets by adapting single-cell transcriptomics, fate mapping, and imaging. Thereby we identified a phenotypically and transcriptionally distinct subset of prenatally seeded dermal macrophages that self-maintained with very low postnatal exchange by hematopoietic stem cells. These macrophages specifically interacted with sensory nerves and surveilled and trimmed the myelin sheath. Overall, resident dermal macrophages contributed to axon sprouting after mechanical injury. In summary, our data show long-lasting functional specification of macrophages in the dermis that is driven by stepwise adaptation to guiding structures and ensures codevelopment of ontogenetically distinct cells within the same compartment.