Recognition and removal of apoptotic cells by professional phagocytes, including dendritic cells and macrophages, preserves immune self-tolerance and prevents chronic inflammation and autoimmune pathologies1,2. The diverse array of phagocytes that reside within different tissues, combined with the necessarily prompt nature of apoptotic cell clearance, makes it difficult to study this process in situ. The full spectrum of functions executed by tissue-resident phagocytes in response to homeostatic apoptosis, therefore, remains unclear. Here we show that mouse apoptotic intestinal epithelial cells (IECs), which undergo continuous renewal to maintain optimal barrier and absorptive functions3, are not merely extruded to maintain homeostatic cell numbers4, but are also sampled by a single subset of dendritic cells and two macrophage subsets within a well-characterized network of phagocytes in the small intestinal lamina propria5,6. Characterization of the transcriptome within each subset before and after in situ sampling of apoptotic IECs revealed gene expression signatures unique to each phagocyte, including macrophage-specific lipid metabolism and amino acid catabolism, and a dendritic-cell-specific program of regulatory CD4+ T-cell activation. A common ‘suppression of inflammation’ signature was noted, although the specific genes and pathways involved varied amongst dendritic cells and macrophages, reflecting specialized functions. Apoptotic IECs were trafficked to mesenteric lymph nodes exclusively by the dendritic cell subset and served as critical determinants for the induction of tolerogenic regulatory CD4+ T-cell differentiation. Several of the genes that were differentially expressed by phagocytes bearing apoptotic IECs overlapped with susceptibility genes for inflammatory bowel disease7. Collectively, these findings provide new insights into the consequences of apoptotic cell sampling, advance our understanding of how homeostasis is maintained within the mucosa and set the stage for development of novel therapeutics to alleviate chronic inflammatory diseases such as inflammatory bowel disease.

Significance Precise regulation of chemokine signal is critical for directional migration of cells. In the study of complex cell behavior, it remains difficult to manipulate chemokine activity at precise times and places within living animals, and it is not possible to study different chemokine effects on defined cell types over a range of timescales. Furthermore, a given chemokine can activate multiple chemokine receptors and vice versa. Here we developed a photoactivatable chemokine receptor that can induce highly specific chemokine signals and guide cell migration toward the light stimulation. This work will advance our understanding of the cell migration process with a number of previously unidentified findings. Clinically, our photoactivatable chemokine receptor approach may have broad applications for adoptive cell transfer therapy.

In the event of a deliberate or accidental radiological emergency, the skin would likely receive substantial ionizing radiation (IR) poisoning which could negatively impact cellular proliferation, communication, and immune-regulation within the cutaneous microenvironment. Indeed, as we have previously shown, local IR exposure to the murine ear causes a reduction of two types of cutaneous dendritic cells (cDC), including interstitial DC (iDC) of the dermis and Langerhans cells (LC) of the epidermis, in a dose and time dependent manner. These APCs are critical regulators of skin homeostasis, immuno-surveillance, and the induction of T and B cell-mediated immunity as previously demonstrated using conditional cDC knockout mice. To mimic a radiological emergency, we developed a murine model of sub-lethal total body irradiation (TBI). Our data would suggest that TBI results in the reduction of cDC from the murine ear that was not due to a systemic response to IR as a loss was not observed in shielded ears. We further determined that this reduction was due, in part, to the up-regulation of the chemoattractant CCL21 on lymphatic vessels as well as CCR7 expressed on cDC. Migration as a potential mechanism was confirmed using CCR7−/− mice where cDC were not depleted following TBI. Finally, we demonstrated that the loss of cDC following TBI results in an impaired contact hypersensitivity (CHS) response to hapten by using a modified CHS protocol. Taken together, these data suggest that IR exposure may result in diminished immuno-surveillance in the skin, which could render the host more susceptible to pathogens.

We describe an imaging assay that monitors the migration of two unique subsets of immune dendritic cells (DC), interstitial dendritic cells (iDC) and Langerhans cells (LC), found in the dermal and epidermal layers of skin, respectively. Using this assay, we study responses of these cells to ionizing radiation. Results obtained using whole-mount histology and fluorescence microscopy suggest that ionizing radiation triggered the migration of both major histocompatibility complex (MHC) class II + iDC and Langerin + LC in a dose-and time-dependent manner. Migration appeared to be limited by local administration of recombinant IL-12, a potent immunostimulatory cytokine known to induce DNA repair. Those findings were extended to an in vivo model by injecting fluorescently conjugated anti-MHC class II antibodies intradermally into the ears of live, anesthetized mice and visualizing the DC population in the same ear before and after radiation exposure using confocal microscopy.

Apoptosis is a programmed form of cell death whereby characteristic internal cellular dismantling is accompanied by the preservation of plasma membrane integrity. Maintaining this order during apoptosis prevents the release of cellular contents and ensures a noninflammatory death. Here, we consider examples of apoptosis in different contexts and discuss how the same form of cell death could have different immunological consequences. Multiple parameters such as cell death as a result of microbial infection, the nature of the inflammatory microenvironment, the type of responding phagocytic cells and the genetic background of the host organism all differentially influence the immunological consequences of apoptosis.