Neutrophils are potent immune effectors against bacterial infections. Macrophages are important in infections as effectors and regulators, but their exact roles, phenotypic characterization and their relation to neutrophils is incompletely understood. Here we report in a model of bacterial urinary tract infection, one of the most prevalent bacterial infections that tissue-resident Ly6C− macrophages recruited circulating neutrophils and inflammatory Ly6C+ macrophages through chemokines. Neutrophils were primarily recruited through ligands of the chemokine receptor CXCR2, in particular by CXCL1 and less by macrophage migration inhibitory factor (MIF), but not through CXCL5 and CXCL2. Neutrophils, but not Ly6C+ macrophages, cleared the bacteria by phagocytosis. Ly6C+ macrophages instead performed a regulatory function: in response to the infection, they produced the cytokine tumor necrosis factor (TNF), which in turn caused the resident macrophages to secrete CXCL2. This chemokine induced the secretion of matrix metalloproteinase-9 (MMP-9) in neutrophils and allowed these cells to degrade the uroepithelial basement membrane, in order to enter the uroepithelium, the mucosal interface from where the bacteria invade the bladder. Thus, the phagocyte response against bacteria is a highly coordinated event, in which Ly6C− macrophages act as sentinels and Ly6C+ macrophages as innate helper cells. In analogy with T helper cells (Th), we propose to name these helper macrophages (Ph) as they provide a second signal on whether to unleash the principal effector phagocytes, the neutrophils. This cellular triage may prevent ‘false-positive’ immune responses. The role of TNF as innate ‘licensing’ factor contributes to its central role in antibacterial immunity.

Localized abdominal surgery can lead to disruption of motility in the entire gastrointestinal tract (postoperative ileus). Intestinal macrophages produce mediators that paralyze myocytes, but it is unclear how the macrophages are activated, especially those in unmanipulated intestinal areas. Here we show that intestinal surgery activates intestinal CD103(+)CD11b(+) dendritic cells (DCs) to produce interleukin-12 (IL-12). This promotes interferon-γ (IFN-γ) secretion by CCR9(+) memory T helper type 1 (T(H)1) cells which activates the macrophages. IL-12 also caused some T(H)1 cells to migrate from surgically manipulated sites through the bloodstream to unmanipulated intestinal areas where they induced ileus. Preventing T cell migration with the drug FTY720 or inhibition of IL-12, T-bet (T(H)1-specific T box transcription factor) or IFN-γ prevented postoperative ileus. CCR9(+) T(H)1 memory cells were detected in the venous blood of subjects 1 h after abdominal surgery. These findings indicate that postoperative ileus is a T(H)1 immune-mediated disease and identify potential targets for disease monitoring and therapy.

Summary The important role of macrophages in host defense against a variety of pathogens has long been recognized and has been documented and reviewed in numerous publications. Recently, it has become clear that tissue macrophages are not entirely derived from monocytes, as has been assumed for a long time, but rather show an ontogenetic dichotomy in most tissues: while part of the tissue macrophages are derived from monocytes, a major subset is prenatally seeded from the yolk sac. The latter subset shows a remarkable longevity and is maintained by self‐renewal in the adult animal. This paradigm shift poses interesting questions: are these two macrophage subsets functionally equivalent cells that are recruited into the tissue at different development stages, or are both macrophage subsets discrete cell types with distinct functions, which have to exist side by side? Is the functional specialization that can be observed in most macrophages due to their lineage or due to their anatomical niche? This review will give an overview about what we know of macrophage ontogeny and will discuss the influence of the macrophage lineage and location on their functional specialization.

A main function of splenic red pulp macrophages is the degradation of damaged or aged erythrocytes. Here we show that these macrophages accumulate ferrimagnetic iron oxides that render them intrinsically superparamagnetic. Consequently, these cells routinely contaminate splenic cell isolates obtained with the use of MCS, a technique that has been widely used in immunological research for decades. These contaminations can profoundly alter experimental results. In mice deficient for the transcription factor SpiC, which lack red pulp macrophages, liver Kupffer cells take over the task of erythrocyte degradation and become superparamagnetic. We describe a simple additional magnetic separation step that avoids this problem and substantially improves purity of magnetic cell isolates from the spleen.

Dendritic cells (DCs) are the main inducers and regulators of cytotoxic T lymphocyte (CTL) responses against viruses and tumors. One checkpoint to avoid misguided CTL activation, which might damage healthy cells of the body, is the necessity for multiple activation signals, involving both antigenic as well as additional signals that reflect the presence of pathogens. DCs provide both signals when activated by ligands of pattern recognition receptors and “licensed” by helper lymphocytes. Recently, it has been established that such T cell licensing can be facilitated by CD4+ T helper cells (“classical licensing”) or by natural killer T cells (“alternative licensing”). Licensing regulates the DC/CTL cross-talk at multiple layers. Direct recruitment of CTLs through chemokines released by licensed DCs has recently emerged as a common theme and has a crucial impact on the efficiency of CTL responses. Here, we discuss recent advances in our understanding of DC licensing for cross-priming and implications for the temporal and spatial regulation underlying this process. Future vaccination strategies will benefit from a deeper insight into the mechanisms that govern CTL activation.