Breaking Steroid Resistance: Effect of Vitamin D on IL-23

Chronic exposure to air pollution can lead to altered respiratory mucosal immune function and predisposition to or exacerbation of chronic obstructive pulmonary disease (COPD) or asthma. These diseases are characterized by genetic bias, airway inflammation and remodeling, and activation of innate and adaptive respiratory immune pathways. Glucocorticoids are typically used to treat inflammation and immune cell activation in asthma and COPD, but severe cases may be resistant to treatment. T helper 17 (Th17) cells and IL-17–related pathways have been linked to both air pollution–induced airway inflammation and glucocorticoid resistance. In this issue of the Journal, Mann and colleagues (pp. 355–366) describe the effects of the air pollutant known as urban particulate matter (UPM) and vitamin D on dendritic cell (DC)-induced stimulation of Th17.1 cells (1). They used an elegant myeloid DC-memory CD4⁺ T cell coculture system to investigate the in vitro mechanisms of UPM-driven IL-17A and IL-22 production, and to address whether glucocorticoid-resistant Th17.1 activation can be inhibited by vitamin D treatment (Figure 1).

Figure 1.

Urban particulate matter (UPM) primes dendritic cells to produce IL-23. In the presence of IL-23, T helper type 17 (Th17) cells upregulate Tbet, and in addition to IL-17A, they produce IFN-γ and granzyme B. RORγt, retinoid acid-related orphan receptor γ-t; Tbet, T-box expressed in T cells.

Exposure to UPM has been known to induce airway inflammation and impair lung function, and is implicated in the pathogenesis of asthma and COPD (2). In mouse models, functional changes in the airways after UPM inhalation were elicited by lymphocyte-derived cytokines (3). Indeed, T lymphocytes stimulated ex vivo with various types of UPM can produce IFN-γ, IL-13, IL-17A, and IL-22, suggesting that UPM can induce activation of Th1, Th2, Th17, and Th22 cells, respectively (4, 5). However, UPM was also shown to suppress Th1 responses, indicating that the composition of UPM can drive various types of T cell responses (6). Recent evidence also suggests that the classical T cell subtype paradigm is less rigid than once thought. In fact, T cells can be programmed to produce cytokines characteristic of more than one subset based on extrinsic and intrinsic signals (7, 8). In the present study, Mann and colleagues set out to clarify precisely how UPM alters the cytokine output of T cells, because of the potential of this material to induce a variety of T cell responses (1).

The authors found that UPM-primed DCs increased the proportion of memory CD4⁺ T cells with a “Th17.1-like” (9) proinflammatory phenotype. The importance of Th17.1 cells originally emerged in the context of autoimmune diseases. It was shown that in the presence of IL-23 and IL-12, Th17 cells decreased IL-17A and (like Th1 cells) increased IFN-γ production. Pathogenicity of these cells required IL-23 receptor activation and was abolished by blockade of IFN-γ, but not of IL-17A (10). Indeed, Th17 cells display a remarkable developmental plasticity; share functional characteristics with inducible regulatory T, Th1, and Th22 cells; and coexpress the corresponding lineage-specific transcription factor(s) or cytokine(s) (11). Because this plasticity determines whether Th17 cells will serve immune-protective or pathogenic functions, it poses a major concern. As patients suffering from COPD or asthma are increasingly being grouped into distinct subphenotypes and endotypes based on the mechanisms that drive their disease (12–15), the ability to clarify how Th17 cells function is becoming particularly critical.

In the current study, UPM-primed DCs induced CD4 T cells to produce the Th17-associated cytokines IL-17A, IL-17F, and IL-22, as well as IFN-γ, granulocyte-macrophage colony-stimulating factor, and granzyme B. A large proportion of the UPM-driven IL-17A⁺ cells coexpressed these cytokines, but not IL-10 or IL-13 (1). Notably, although IL-17A is a well-described proinflammatory and host-defense cytokine, IL-22 may have either pro- or antiinflammatory functions depending on the presence of IL-17A. Coactivation of IL-17R and IL-22R signaling pathways was suggested to synergize in the development of inflammation (16, 17). Further, although granzyme B is known as a cytotoxic T cell granule product and a mediator of cell death in target cells, it can also stimulate proinflammatory cytokines and play a role in asthma (18). Taken together, the results of Mann and colleagues show that UPM promotes a Th17 population with a potentially pathogenic phenotype.

The exact factors that control the development of Th17.1 cells in vivo are not well understood. Several cytokines, including IL-1β, IL-12, and IL-23, have been implicated in this process (19). Engagement of Th17 cells with IL-12 and IL-23 promotes the activation of transcription factor Tbet (Tbx21, T-box expressed in T cells), which in turn regulates the expression of IFN-γ and related chemokine genes (CXCL9, CXCL10, and CXCL11), leading to the Th17.1 phenotype (20, 21). In addition to Th1 and Th17 cytokines, the pathogenicity of IL-23–driven Th17 cells is dependent on granulocyte-macrophage colony-stimulating factor (22). Further, although high IL-23R expression characterizes pathogenic Th17 cells in mice, human proinflammatory Th17.1 cells (identified as CCR6⁺CXCR3^hiCCR4^loCCR10⁻CD161⁺, and transiently c-Kit⁺) express multidrug resistance type 1 (MDR1) (23). This is of importance because MDR1⁺ proinflammatory Th17 cells are resistant to several glucocorticoids. In the study by Mann and colleagues, UPM pretreatment of CD1c⁺ DCs increased IL-12/23p40 and expression of IL-17a and MDR1 mRNA after 48 hours in coculture supernatants. A neutralizing antibody specific for IL-23p19 (thus inhibiting IL-23, but not IL-12) significantly reduced this effect, but not expression of IL-10. At the protein level, although anti–IL-23p19 had no effect on UPM-driven cell division, it did significantly and specifically reduce the frequency of IL-17A⁺IFN-γ⁺ cells (1).

Previous studies also characterized how antigen, in combination with inhaled UPM, can alter the cytokine output of T cells in mouse models, and how the T cell response in turn affects inflammation and lung function (24, 25). Unraveling the in vivo significance of Th17.1 cells in mouse models and studies in humans should be an exciting and significant future extension of the study by Mann and colleagues.

Mann and colleagues found that 1,25(OH)2D3 counteracted the UPM-driven DC maturation and inhibited the frequency of IL-17A⁺IFN-γ⁺ cells, most prominently when DCs were cotreated with the corticosteroid dexamethasone, while maintaining antiinflammatory IL-10 synthesis. These data indicate that UPM might promote an inflammatory milieu in part by inducing an IL-23–driven proinflammatory Th17 response. Restoring vitamin D sufficiency may counteract these UPM-driven effects without obliterating important homeostatic immune functions.

The finding by Mann and colleagues that the UPM-induced Th17.1-like response was significantly attenuated when myeloid DCs were cultured with a sufficient concentration of vitamin D adds to our current understanding of how vitamin D receptor (VDR) signaling suppresses proinflammatory T cell responses and promotes tolerogenic ones. Their demonstration that vitamin D suppressed the proliferation of IL-17A⁺IFN-γ⁺ CD4⁺ T cells in an IL-23–dependent manner is novel and parallels the recent finding of Konya and colleagues (26) that vitamin D treatment downregulated IL-23R expression in mucosal ILC3s from patients with inflammatory bowel disease. VDR signaling has been implicated in regulating the expression of both pro- and antiinflammatory cytokines. However, these recent findings support a possible therapeutic role for vitamin D supplementation in asthmatic patients with a neutrophilic endotype. In conjunction with evidence regarding racial disparities in the prevalence of vitamin D deficiency (27), they also provide an explanation for the observation of Th17-cytokine responses associated with asthma severity specifically in African-Americans in an inner-city population of pediatric patients with refractory asthma (28, 29). Researchers have observed the greatest prevalence of vitamin D deficiency in African-American populations (>80%), which also suggests the viability of vitamin D supplementation as a low-cost intervention to improve public health. The synergism between vitamin D and dexamethasone in this study corroborates previous findings (30). Nanzer and colleagues showed the effects of vitamin D3 on IL-17A production in patients with severe asthma (31). Corticosteroid treatment in patients decreases the bioavailability of vitamin D by inducing the expression of CYP24A1 hydroxylase, and also by regulating the transcription of the VDR (32), suggesting the need for elevated vitamin D intake in patients on corticosteroid therapy.