TREM-2 and Dendritic Cells in the Pathogenesis of Allergic Airway Inflammation
Asthma is a chronic disorder of the conducting airways that involves the interplay of multiple genetic and environmental factors. It is characterized by reversible airway obstruction, cellular infiltration, airway inflammation and airway remodeling. Dendritic cells (DCs) are professional antigen presenting cells that initiate the asthmatic inflammatory response by capturing inhaled antigens and migrating to the lymph nodes to mediate T-helper cell responses. Lung DCs consist of a heterogenous population of cells that express CD11c and are further divided into two major subsets based on the expression of CD11b and CD103. Both subsets have been shown to drive pro- or anti-inflammatory responses based on stimulus and cytokine environment.|Dendritic cells express several pattern recognition receptors (PRRs) that are critical for their function as sentinels of the immune system. DC maturation can be induced by several signaling pathways, such as TLR, Fc receptors and DAP-12 mediated -signaling, based on the activating signal. The recently discovered triggering receptor expressed on myeloid cells (TREM)-2 belongs to a family of cell surface receptors that mediate signaling via associations with the DAP-12 adaptor protein. TREM-2 is expressed on DCs, microglia and macrophages and was originally described as an anti-inflammatory mediator. However, recent studies have shown that the TREM-2/DAP-12 pathway can provide both anti-inflammatory and pro-inflammatory signals based on the micro-environment. Although TREM-2 has been shown to play a role in several inflammatory diseases, its expression in the airways and role in allergic airway inflammation is yet to be elucidated. Given that TREM-2 has been shown to drive inflammation in other disease models, we hypothesized that TREM-2 might play a role in the onset and progression of allergic airway inflammation.|Sensitization and challenge with ovalbumin was shown to reproduce hallmark features of allergic airway inflammation. Analysis of the CD11c+MHC-IIhi subset of cells in the lungs revealed three populations of cells; the previously described CD11b+CD103- (CD11bhi) and CD11blo/-CD103+ (CD103+) as well as a third subset of cell that expressed both CD11b+CD103+. Further analysis of each subset showed that they were all bona fide mature DCs capable of migrating to the lymph nodes given the expression of DC-specific transcription factors (Zbtb46, IRF-4, IRF-8 and BATF-3) as well as high expression of costimulatory molecule CD86 and chemokine receptor CCR-7.|Whole lungs from OVA-sensitized and challenged mice were then examined for TREM-2 expression. As seen with other inflammatory conditions, TREM-2 was found to be upregulated in the airways of mice exposed to OVA when compared to the control groups. In steady state, TREM-2 was expressed on all subsets of DCs identified in the lungs confirming that DCs do express TREM-2. OVA-sensitization and challenge led to upregulation of TREM-2 expression on the three subsets of CD11c+ cells in both the lungs and mediastinal lymph nodes, suggesting a potential role of TREM-2 in driving allergic airway inflammation mediated by DC subsets.|To further investigate the functional responses associated with increased TREM-2 expression on the DCs from OVA-sensitized and challenged mice, lung and lymph nodes were examined for expression of T-helper cell cytokines and transcription factors. It was found that there was significant increased mRNA expression of Th2 and Th17 cytokines in the lungs and lymph nodes. Analysis of transcription factor expression revealed higher expression of GATA-3 compared to T-bet and RORγt compared to Foxp3 suggesting a Th2 and Th17 skewed response. This suggests some division of labor among the DC subsets identified. Several studies have shown that CD11b+ DCs secrete a host of proinflammatory cytokines and play a role in priming and re-stimulating effector CD4+ T-cells driving Th2 responses. CD103+ DCs have also been shown to prime Th1 as well as Th17 responses.|Given that there was increased TREM-2 expression on DC subsets isolated from OVA-sensitized and challenged mice and CCR-7 is critical for dendritic cell migration to the lymph modes, the TREM-2+ and TREM-2- cells from the three DC populations were further analyzed for expression of costimulatory molecules and CCR-7. TREM-2+ DCs in the lungs and mediastinal lymph nodes isolated from OVA-sensitized and challenged mice had greater expression of both CD86 and CCR-7 when compared to their TREM-2 negative counterparts, suggesting a potential role of TREM-2 in maturation and/or migration of DC subsets.|Finally, to determine the functional role of increased TREM-2 expression on DCs, RNA interference was employed to silence protein expression in vitro. Transfection with TREM-2 siRNA was successful at decreasing both protein and mRNA expression of TREM-2 on isolated cells. To determine the role of TREM-2 in maturation and/or migration, transfected cells were stimulated with OVA, isolated and analyzed for expression of co-stimulatory molecules and CCR-7. Cells transfected with siRNA against TREM-2 showed decreased mRNA expression of CD86 and CCR-7 after antigen induction. Analysis of expression of cell surface markers revealed that silencing of TREM-2 resulted in significantly decreased expression CCR-7 on the transfected cells. These results suggest that TREM-2 may play a role in both maturation and migration.|Collectively, results from our studies show that TREM-2 is upregulated in the lungs and lymph nodes of OVA-sensitized and challenged mice. TREM-2 is expressed on DC subsets with higher expression on DCs isolated from the mediastinal lymph nodes. This increased expression is associated with primarily Th2 responses in the lung and lymph nodes. Silencing of TREM-2 decreased antigen-induced mRNA and protein expression of CD86 and CCR-7 in vitro. These finding highlight a potential role of TREM-2 in the survival, migration and partial maturation of DCs in allergic airway inflammation. The receptor might well be a novel target for therapeutic intervention.
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