LXR signaling controls homeostatic dendritic cell maturation

Dendritic cells (DCs) mature in an immunogenic or tolerogenic manner depending on the context in which an antigen is perceived, preserving the balance between immunity and tolerance. Whereas the pathways driving immunogenic maturation in response to infectious insults are well-characterized, the signals that drive tolerogenic maturation during homeostasis are still poorly understood. We found that the engulfment of apoptotic cells triggered homeostatic maturation of type 1 conventional DCs (cDC1s) within the spleen. This maturation process could be mimicked by engulfment of empty, nonadjuvanted lipid nanoparticles (LNPs), was marked by intracellular accumulation of cholesterol, and was highly specific to cDC1s. Engulfment of either apoptotic cells or cholesterol-rich LNPs led to the activation of the liver X receptor (LXR) pathway, which promotes the efflux of cellular cholesterol, and repressed genes associated with immunogenic maturation. In contrast, simultaneous engagement of TLR3 to mimic viral infection via administration of poly(I:C)-adjuvanted LNPs repressed the LXR pathway, thus delaying cellular cholesterol efflux and inducing genes that promote T cell-mediated immunity. These data demonstrate that conserved cellular cholesterol efflux pathways are differentially regulated in tolerogenic versus immunogenic cDC1s and suggest that administration of nonadjuvanted cholesterol-rich LNPs may be an approach for inducing tolerogenic DC maturation.

Impaired Cutaneous Immune Responses in Thy-1-Deficient Mice

Thy-1 is a cell surface glycoprotein expressed mainly on brain and lymphoid tissue. Although the functions of Thy-1 are incompletely understood, evidence exists that Thy-1 participates in T cell activation. To examine the functional role of Thy-1 in cutaneous immune responses in vivo, Thy-1 gene-targeted mice (Thy-1−/−) and wild-type mice (Thy-1+/+) were immunized with the hapten oxazolone. After challenge with oxazolone, contact hypersensitivity responses in Thy-1−/− mice were reduced by 25% compared with Thy-1+/+ mice. Likewise, irritant dermatitis induced by croton oil was also decreased. In addition, Thy-1−/− mice showed a significantly reduced delayed-type hypersensitivity response after injection of allogeneic spleen cells into the hind footpads of allosensitized animals when compared with Thy-1+/+ mice. Moreover, proliferative responses to immobilized anti-CD3 were decreased in peripheral Thy-1−/− lymphocytes; this decrease was associated with a significantly reduced intracellular Ca2+ influx and protein tyrosine phosphorylation, indicating impairment of early lymphocyte activation. In contrast, the T cell proliferation induced by mitogens was normal, suggesting that Thy-1 expression weakly contributes to TCR-mediated T cell activation. Epidermal Langerhans cells and bone marrow-derived dendritic cells from Thy-1−/− mice exhibited a normal expression of costimulatory surface molecules as well as an unaltered ability to stimulate allogeneic T cells. Taken together, these findings demonstrate that a lack of Thy-1 expression does not generally compromise the immune system; however, Thy-1 expression may be involved in the fine-tuning of T cell-mediated immune responses.

Fetal Mouse NK Cell Clones Are Deficient in Ly49 Expression, Share a Common Broad Lytic Specificity, and Undergo Continuous and Extensive Diversification In Vitro

NK cells obtained by exposing mouse fetal thymocytes to appropriate combinations of IL-4, IL-2, and PMA are phenotypically indistinguishable from cultured adult splenic NK cells with the exception that they generally lack measurable expression of all of the inhibitory Ly49 molecules that can currently be detected with Abs (Ly49A, -C, -G, and -I) and of the activating molecule Ly49D. Despite this deficiency, fetal NK cells have a similar specificity to Ly49-expressing adult splenic NK cells. Individual fetal NK cell clones display an essentially invariant and broad specificity similar to that of polyclonal populations of fetal or adult NK cells, although significant differences in the fine specificity of clones can occasionally be detected. Most remarkably, cloned fetal NK cell lines display heterogeneous expression of a restricted set of surface molecules that includes 10A7, Ly6C, 3C2, CD8, certain isoforms of CD45, and also, occasionally, Ly49 molecules. This heterogeneity is not related to the cell cycle or activation status of the cells, and micromanipulation recloning demonstrates unambiguously that it is not due to a lack of a single cell origin. Diversity is generated rapidly and the capacity for diversification appears to persist indefinitely in vitro. The expression of individual variable Ags is independent and stochastic, resulting in fetal NK “clones” being potentially composed of hundreds of phenotypically distinct cells. We hypothesize that fetal NK cells behave as progenitor cells that are undergoing a process of rapid, extensive, and continuous diversification and that are individually capable of generating and regenerating a complex NK cell repertoire.