Type 2 immunity is critical for defense against cutaneous infections, but also underlies the development of allergic skin diseases. We report the identification in normal murine dermis of an abundant, phenotypically unique group 2 innate lymphoid cell (ILC2) subset that depends on interleukin 7 (IL-7) and constitutively produces IL-13. Intravital multiphoton microscopy revealed that dermal ILC2 specifically interact with mast cells, whose function was suppressed by IL-13. Treatment of Rag1−/− mice with IL-2 resulted in the expansion of activated, IL-5-producing dermal ILC2, leading to spontaneous dermatitis characterized by eosinophil infiltrate and activated mast cells. Our data show that ILC2 exhibit both pro- and anti-inflammatory properties and uncover a novel interactive pathway between two innate immune cell populations implicated in type 2 immunity and allergic diseases.

The dermis contains a novel population of γδT cells that are distinct from epidermal γδT cells and produce IL-17 in response to mycobacterial infection.

Transendothelial migration of neutrophils in post-capillary venules is a key event in the inflammatory response against pathogens and tissue damage. The precise regulation of this process is incompletely understood. We report that perivascular macrophages are critical for neutrophil migration into skin infected with the pathogen Staphylococcus aureus. Using multiphoton intravital microscopy we show that neutrophils extravasate from inflamed dermal venules in close proximity to perivascular macrophages, which are a major source of neutrophil chemoattractants. The virulence factor alpha-hemolysin lyses perivascular macrophages leading to decreased neutrophil transmigration. Our data illustrate a previously unrecognized role for perivascular macrophages in neutrophil recruitment to inflamed skin, and indicate that Staphylococcus aureus uses hemolysin-dependent killing of these cells as an immune evasion strategy.

Dendritic cells (DC), including those of the skin, act as sentinels for intruding microorganisms. In the epidermis, DC (termed Langerhans cells, LC) are sessile and screen their microenvironment through occasional movements of their dendrites. The spatio-temporal orchestration of antigen encounter by dermal DC (DDC) is not known. Since these cells are thought to be instrumental in the initiation of immune responses during infection, we investigated their behavior directly within their natural microenvironment using intravital two-photon microscopy. Surprisingly, we found that, under homeostatic conditions, DDC were highly motile, continuously crawling through the interstitial space in a Gαi protein-coupled receptor–dependent manner. However, within minutes after intradermal delivery of the protozoan parasite Leishmania major, DDC became immobile and incorporated multiple parasites into cytosolic vacuoles. Parasite uptake occurred through the extension of long, highly dynamic pseudopods capable of tracking and engulfing parasites. This was then followed by rapid dendrite retraction towards the cell body. DDC were proficient at discriminating between parasites and inert particles, and parasite uptake was independent of the presence of neutrophils. Together, our study has visualized the dynamics and microenvironmental context of parasite encounter by an innate immune cell subset during the initiation of the immune response. Our results uncover a unique migratory tissue surveillance program of DDC that ensures the rapid detection of pathogens.

Neutrophil granulocytes traffic into sites of organ injury in which they may not only participate in tissue repair and pathogen clearance but may also contribute to collateral cell damage through the release of noxious mediators. The dynamics and mechanisms of neutrophil migration in the extravascular space toward loci of tissue damage are not well understood. Here, we have used intravital multi-photon microscopy to dissect the behavior of neutrophils in response to tissue injury in the dermis of mice. We found that, following confined physical injury, initially rare scouting neutrophils migrated in a directional manner toward the damage focus. This was followed by the attraction of waves of additional neutrophils, and finally stabilization of the neutrophil cluster around the injury. Although neutrophil migration in the steady state and during the scouting phase depended on pertussis toxin-sensitive signals, the amplification phase was sensitive to interference with the cyclic adenosine diphosphate ribose pathway. We finally demonstrated that neutrophil scouts also transit through the non-inflamed dermis, suggesting immunosurveillance function by these cells. Together, our data unravel a three-step cascade of events that mediates the specific accumulation of neutrophils at sites of sterile tissue injury in the interstitial space.

Chemokines presented by the endothelium are critical for integrin-dependent adhesion and transendothelial migration of naive and memory lymphocytes. Here we found that effector lymphocytes of the type 1 helper T cell (T(H)1 cell) and type 1 cytotoxic T cell (T(C)1 cell) subtypes expressed adhesive integrins that bypassed chemokine signals and established firm arrests on variably inflamed endothelial barriers. Nevertheless, the transendothelial migration of these lymphocytes strictly depended on signals from guanine nucleotide-binding proteins of the G(i) type and was promoted by multiple endothelium-derived inflammatory chemokines, even without outer endothelial surface exposure. Instead, transendothelial migration-promoting endothelial chemokines were stored in vesicles docked on actin fibers beneath the plasma membranes and were locally released within tight lymphocyte-endothelial synapses. Thus, effector T lymphocytes can cross inflamed barriers through contact-guided consumption of intraendothelial chemokines without surface-deposited chemokines or extraendothelial chemokine gradients.

Defining the immune mechanisms underlying protective immunity to helminth infection remains an important challenge. Here we report that lung CD4 þ T cells and Group 2 innate lymphoid cells (ILC2s) work in concert to block Nippostrongylus brasiliensis (Nb) development in the parenchyma within 48 h in mice. Immune-damaged larvae have a striking morphological defect that is dependent on the expansion of IL-13-producing ILC2 and CD4 þ T cells, and the activation of M2 macrophages. This T-cell requirement can be bypassed by administration of IL-2 or IL-33, resulting in expansion of IL-13-producing ILC2s and larval killing. Depletion of ILC2s inhibits larval killing in IL-2-treated mice. Our results broaden understanding of ILC2's role in immunity to helminths by demonstrating that they not only act as alarmin sensors, but can also be sustained by CD4 þ T cells, ensuring both the prompt activation and the maintenance of IL-13-dependent M2 macrophage immunity in the lung.

The liver is positioned at the interface between two routes traversed by pathogens in disseminating infection. Whereas blood-borne pathogens are efficiently cleared in hepatic sinusoids by Kupffer cells (KCs), it is unknown how the liver prevents dissemination of peritoneal pathogens accessing its outer membrane. We report here that the hepatic capsule harbors a contiguous cellular network of liver-resident macrophages phenotypically distinct from KCs. These liver capsular macrophages (LCMs) were replenished in the steady state from blood monocytes, unlike KCs that are embryonically derived and self-renewing. LCM numbers increased after weaning in a microbiota-dependent process. LCMs sensed peritoneal bacteria and promoted neutrophil recruitment to the capsule, and their specific ablation resulted in decreased neutrophil recruitment and increased intrahepatic bacterial burden. Thus, the liver contains two separate and non-overlapping niches occupied by distinct resident macrophage populations mediating immunosurveillance at these two pathogen entry points to the liver.