Innate IFNs and Plasmacytoid Dendritic Cells Constrain Th2 Cytokine Responses to Rhinovirus: A Regulatory Mechanism with Relevance to Asthma

Human rhinoviruses (RV) cause only minor illness in healthy individuals, but can have deleterious consequences in people with asthma. This study sought to examine normal homeostatic mechanisms regulating adaptive immunity to RV in healthy humans, focusing on effects of IFN-αβ and plasmacytoid dendritic cells (pDC) on Th2 immune responses. PBMC were isolated from 27 healthy individuals and cultured with RV16 for up to 5 d. In some experiments, IFN-αβ was neutralized using a decoy receptor that blocks IFN signaling, whereas specific dendritic cell subsets were depleted from cultures with immune-magnetic beads. RV16 induced robust expression of IFN-α, IFN-β, multiple IFN-stimulated genes, and T cell-polarizing factors within the first 24 h. At 5 d, the production of memory T cell-derived IFN-γ, IL-10, and IL-13, but not IL-17A, was significantly elevated. Neutralizing the effects of type-I IFN with the decoy receptor B18R led to a significant increase in IL-13 synthesis, but had no effect on IFN-γ synthesis. Depletion of pDC from RV-stimulated cultures markedly inhibited IFN-α secretion, and led to a significant increase in expression and production of the Th2 cytokines IL-5 (p = 0.02), IL-9 (p < 0.01), and IL-13 (p < 0.01), but had no effect on IFN-γ synthesis. Depletion of CD1c+ dendritic cells did not alter cytokine synthesis. In healthy humans, pDC and the IFN-αβ they secrete selectively constrain Th2 cytokine synthesis following RV exposure in vitro. This important regulatory mechanism may be lost in asthma; deficient IFN-αβ synthesis and/or pDC dysfunction have the potential to contribute to asthma exacerbations during RV infections. Respiratory viruses are associated with the majority of severe asthma exacerbations, with human rhinovirus (RV) infections being the most common viruses identified in children and adults (1–3). Asthmatics do not appear to have more frequent viral infections than healthy individuals, but instead suffer more persistent and severe lower respiratory tract symptoms (4). It has long been a puzzle why a commonly innocuous mucosal virus such as RV should cause such adverse clinical effects in asthma, although an increasing body of evidence points to a dysregulated immune or inflammatory response to respiratory viruses (5, 6). Some reports indicate that airway epithelial cells from asthmatics have a reduced capacity for synthesis of innate IFN following RV exposure in vitro, including IFN-α, IFN-β, and IFN-λ, compared with normal airway epithelial cells from healthy people (7, 8), although these findings have been disputed by others (9, 10). The dysregulated antiviral immune response in asthma has also been demonstrated in migratory populations of immune cells derived from the bone marrow. RV-stimulated alveolar macrophages from asthmatics produce less IFN-λ than alveolar macrophages from healthy individuals (7). PBMC from asthmatic children and adults secrete less IFN-α following in vitro exposure to viruses (11, 12), and this is associated with reduced function of TLR7, the receptor for viral ssRNA (13). Plasmacytoid dendritic cells (pDC) are a potent source of type-I IFN synthesis during virus infections (14), and numerical changes in circulating pDC have been linked both to asthma development in young children (15) and to established asthma in adults (16). The function of pDC may also be abnormal in asthma, with reports demonstrating that pDC from allergic asthmatics are less able to synthesize IFN-α in response to influenza A (17) or TLR9 activation (18) than pDC from healthy subjects. In addition to these facets of innate immune function, adaptive immunity to RV also appears abnormal in asthma. Several groups have reported that RV-stimulated PBMC from asthmatics exhibit greater synthesis of IL-5, IL-13, and IL-10 and reduced synthesis of IFN-γ (19–22). Cytokine dysregulation in circulating cells has been shown to be proportional to asthma severity, to the degree of bronchial hyperresponsiveness, and to the extent of viral shedding following experimental human RV infection in vivo, providing strong circumstantial evidence that these alterations in immune function are clinically relevant (19, 21, 22). The normal homeostatic mechanisms regulating adaptive immune responses to RV in humans are not well understood. Type-I IFNs (IFN-αβ) play a well-established role in early innate immune defense against a wide range of viral pathogens (23). Experimental evidence also suggests that IFN-αβ can modulate adaptive immunity to many viruses, supporting the polarization and effector function of Th1 cells (24) and Th17 cells (25), while inhibiting Th2 commitment and secretion of potentially maladaptive Th2 cytokines (26, 27). The extent to which these observations hold true in relation to RV has not been studied, but may be highly relevant to the pathophysiology of asthma where IFN-αβ synthesis appears to be deficient, and where a Th2 or Th17 response to the virus might worsen eosinophilic or neutrophilic airway inflammation, respectively. In the current study, we characterized the innate immune response to RV in detail, in particular the type-I IFNs and associated IFN-stimulated genes, and the adaptive immune responses to RV, focusing on the secretion of Th1, Th2, Th17 cytokines, and the regulatory cytokine IL-10. Our experiments then focused on the extent to which type-I IFNs regulated Th2 adaptive immune responses to RV using rIFN-β and the type-I IFN receptor antagonist B18R. As pDC are a highly potent source of IFN-αβ, we also examined whether pDC modulate the adaptive immune responses to RV. Our results indicate that, in healthy individuals, IFN-αβ and pDC play critical roles in selectively constraining maladaptive Th2 cytokine responses to RV, but have little, if any, effects on synthesis of IFN-γ or IL-10 protein production or IL-17a gene expression.