In This Issue

Surfactant protein D (SP-D) is a collagenous lectin secreted in the lungs and some mucosal epithelia that is an important component of innate immune defense. Mutations in this lectin are associated with chronic obstructive pulmonary disease (COPD) and colitis, but the molecular mechanisms of SP-D function under physiological or disease conditions are not well known. In this issue, Barrow et al. (p. 3317) discovered that SP-D was a binding target for osteoclast-associated receptor (OSCAR), an activating receptor for collagen expressed by osteoclasts and, in humans, some myeloid immune cells. Using a variety of approaches involving OSCAR fusion proteins and in vitro reporter assays, the authors determined that OSCAR was able to bind the collagenous region of both plate-bound SP-D and soluble SP-D found in human bronchoalveolar lavage fluid samples. Analysis of human lung sections from controls and those with COPD revealed that OSCAR was expressed intracellularly in alveolar macrophages in both groups and was able to colocalize with SP-D. OSCAR expression was also found on the cell surface of human CCR2⁺ inflammatory monocytes, which secreted TNF-α in an OSCAR-dependent fashion when stimulated with SP-D in vitro, and in lipid-laden macrophages infiltrating atherosclerotic plaques from patients with atherosclerosis. These findings suggest that the OSCAR–SP-D interaction may contribute to the pathogenesis of inflammatory diseases. Together, these results indicate that OSCAR is a binding partner for SP-D and that this interaction may be a viable therapeutic target for chronic inflammatory diseases.

Tumor immunotherapy using T cells expressing chimeric Ag receptors (CARs) targeted against tumor-associated Ags on cancerous cells has shown favorable results in clinical trials. However, using tumor-targeted CARs in other immune cell types involved in tumor immunosurveillance and defense, such as NK cells, remains an area of untapped therapeutic potential. To explore the tumor-eradicating abilities of CAR-expressing NK cells, Töpfer et al. (p. 3201) engineered a CAR construct containing a single chain Ab fragment specific for prostate stem cell Ag (PSCA) fused to DAP12, an intracellular signaling molecule downstream of NK cell activating receptors involved in anti-tumor responses. The authors expressed this construct in the YTS NK cell line and challenged them with tumor cells expressing PSCA in vitro. Compared with YTS cells transduced with a construct that contained only the DAP12 intracellular domain, YTS cells harboring the anti–PSCA-DAP12 CAR exhibited significantly enhanced cytotoxicity, increased phosphorylation of ZAP-70 (a signaling molecule downstream of DAP12 in NK cell activating pathways), and heightened release of IFN-γ. Over the course of 8 wk, periodic i.v. injection of YTS cells transduced with the anti–PSCA-DAP12 CAR into mice bearing PSCA-expressing tumors caused a statistically significant increase in the median lifespan of treated animals and near complete or complete tumor eradication in over half of these mice. Anti–PSCA-DAP12 CAR-expressing human primary NK cells also showed enhanced and specific cytotoxicity against PSCA positive tumor cells. These results suggest that DAP12 CAR expression in NK cells could be used to target these cells to specific tumor types in adoptive immunotherapy strategies.

Under steady state conditions, resident regulatory T cells (Tregs) help maintain tolerance in the eye. However, when inflammation is triggered by retinal Ag-specific peripheral T cells infiltrating the eye in diseases such as autoimmune uveitis, it is unclear how Tregs contribute to the resolution of the autoimmune response. Using an experimental autoimmune uveitis (EAU) model in which immunization with the retina protein interphotoreceptor retinoid-binding protein (IRBP) results in transient eye inflammation, Silver et al. (p. 3011) examined the roles that Tregs play in EAU resolution and maintenance of remission. They found that IRBP-specific Foxp3⁺ Tregs and effector CD4⁺ T cells accumulated in the eye during the course of EAU, whereas very few of these cells were found in the draining lymph nodes near the injection site. In vitro suppression assays revealed that Tregs from uveitic eyes were able to suppress proliferation and cytokine production in effector T cells derived from the same eyes. In vivo depletion of Tregs at the peak of disease or after EAU resolution suggested that Tregs were integral to both resolution and maintenance of remission in these mice. Together, these results indicate that, unlike in many experimental models of autoimmune inflammation, Tregs present in the eyes of EAU mice are functional and contribute to the resolution of EAU and re-establishment of tolerance in the eye.

Dendritic cells (DCs) can translate innate immune stimuli into signals that modulate T cell differentiation into pro- or anti-inflammatory phenotypes in diseases such as experimental autoimmune encephalomyelitis (EAE). Having seen that, in DCs, activity of β-catenin, a transcriptional cofactor in the canonical wnt signaling pathway, led to the suppression of inflammatory responses, Suryawanshi et al. (p. 3295) hypothesized that wnt signaling in DCs could modulate EAE pathogenesis. Mice with DC-specific deletion of β-catenin or of the wnt coreceptors low-density lipoprotein receptor-related protein (LRP)5 and LRP6 developed exacerbated EAE relative to wild-type (WT) mice. Augmented disease in both sets of mice was associated with increased CNS infiltration by CD4⁺ and CD8⁺ T cells producing proinflammatory cytokines and decreased infiltration by IL-10–producing regulatory T cells. LRP5/6 signaling in DCs, leading to β-catenin activation, was found to be important for the control of proinflammatory cytokines during the induction and effector phases of EAE, resulting in inhibition of Th1 and Th17 cell differentiation. Treatment of WT mice with a β-catenin agonist effectively ameliorated EAE when given either before or 10 d after disease induction, and could reverse the augmented disease observed in mice with DC-specific deletion of LRP5 and 6. The canonical wnt signaling pathway in DCs, acting through LRP5/6 and β-catenin, therefore controls neuroinflammation in EAE and could represent a therapeutic target for patients with multiple sclerosis.

Activation of TLRs or of cytosolic receptors can lead to type I IFN production through multiple mechanisms, and how bacterial infections can trigger a type I IFN response is far from clear. In this issue, Storek et al. (p. 3236) used the intracellular bacterium Francisella novicida to investigate IFN induction in macrophages, which is known to be dependent on the endoplasmic reticulum membrane protein stimulator of IFN genes (STING). The authors have now identified the DNA sensors cyclic GMP-AMP synthase (cGAS) and Ifi204 as important for this process, using siRNA-mediated knockdown in bone marrow-derived macrophages and generation of functional knockout macrophage cell lines using the CRISPR/Cas9 system. cGAS and Ifi204 were both necessary to cooperatively respond to dsDNA and induce type I IFN through the activity of STING following F. novicida infection. Activation of the AIM2 inflammasome, leading to inflammatory cytokine production and cell death, is key to protection against F. novicida infection and required STING and cGAS. However, increased survival following F. novicida infection of mice deficient in either STING or the downstream transcription factor IRF3, relative to wild-type mice, indicated that the IFN response mediated via STING and IRF3 activity was actually harmful to the host. Taken together, these data reveal the involvement of dsDNA sensors cGAS and Ifi204 in the induction of type I IFN during F. novicida infection and emphasize the complexity of the type I IFN response in antimicrobial defense.

MAPK signaling is key to a wide variety of immune responses and is regulated by members of the MAPK phosphatase (MKP) protein family. To investigate the roles of MKP7 in T cell responses in vivo, Zhang et al. (p. 3088) generated Mkp7-deficient (Mkp7^−/−) mice; however, germline deletion of Mkp7 was found to confer embryonic lethality. Therefore, the authors transferred wild-type (WT) or Mkp7^−/− fetal liver cells into irradiated WT recipients to generate chimeras either sufficient in or lacking MKP7 in the hematopoietic compartment and saw normal T cell development in both sets of chimeras. Relative to WT cells, Mkp7^−/− CD4⁺ T cells upregulated IL-2 and proliferated to a greater degree following TCR stimulation, and although their differentiation into Th1 and Th2 phenotypes was not affected, Th17 differentiation was impaired. Activation of the MAPK ERK promotes IL-2 expression and inhibits Th17 differentiation, and normal MKP7 activity was found to relieve this inhibition by dephosphorylating ERK. Mkp7^−/− chimeras were less susceptible than WT chimeras to experimental autoimmune encephalomyelitis, and this protection was associated with reduced IL-17A–producing cells and increased IFN-γ–producing cells in the CNS. MKP7 also promotes T follicular helper (Tfh) cell differentiation, as evidenced by reduced percentages of Tfh cells in the lymph nodes of Mkp7^−/− versus WT chimeras following immunization with keyhole limpet hemocyanin. This study reveals non-redundant roles for MKP7 in T cell activation and the selective regulation of Th17 and Tfh cell differentiation.

Extracellular vesicles (EVs) produced by bacteria have the potential to stimulate both innate and adaptive immune responses and have been shown to induce neutrophilic pulmonary inflammation, which is involved in the pathogenesis of chronic obstructive pulmonary disease (COPD). To better understand the involvement of these vesicles in lung inflammation, Kim et al. (p. 3361) analyzed the effects of inhalation of E. coli–derived EVs in mice. E. coli EVs were found to be present in indoor dust and enhanced cellular infiltration and inflammatory cytokine production in the lungs following intranasal administration. Repeated inhalation of E. coli EVs resulted in infiltration of neutrophils into the lungs and emphysema, accompanied by the production of IL-17A and IFN-γ by T cells in the lung. IL-17A was particularly important for emphysema development in these mice, whereas IFN-γ was produced downstream of IL-17A. EV-induced emphysema was associated with enhancement of elastase activity and upregulation of lung MMP-9 expression, which, along with the early phase of lung inflammation and emphysema, were dependent on IL-17A and TNF-α. An important role was also identified for LPS/TLR4 signaling in E. coli EV uptake by macrophages and resultant cytokine production. This study shows that E. coli EVs found in dust can cause IL-17A-dependent neutrophilic lung inflammation and emphysema, suggesting that EVs could serve as a target for the treatment of COPD.