IL-23 and IL-17 have a multi-faceted largely negative role in fungal infection

Abstract

The role of IL-23 and IL-17 in the response to fungal infection has been the focus of recent reports. In this issue of the European Journal of Immunology there is an article that reports an important role for IL-23 and IL-17 in limiting fungal control, promoting neutrophillic inflammation and regulating the killing activity of neutrophils. In the fungal model it appears that IL-23 and IL-17 are counter-productive for protection.

See accompanying article: http://dx.doi.org/10.1002/eji200737409

Abbreviations:

IL-17ARKO:

IL-17A receptor knock out

Th17:

IL-17 producing T cells

Host pathogen interactions have evolved over time to benefit both parties. Although complex, our understanding of the directive role of pattern recognition receptors in the development of cellular responses has begun to clarify the factors mediating this interaction. Two cytokines that have garnered a lot of interest in this respect are IL-23 and IL-17, particularly recently with respect to fungal infection. Candida albicans is a dimorphic fungal pathogen that, when in yeast form, activates human dendritic cells (DC) to produce IL-12 and when in the hyphal form promotes IL-23; this differential induction corresponds with the generation of an IL-17 response in naïve human T cells by the hyphae, but not by yeast 1. In mice, the induction of IL-17-producing T cells (Th17) occurs upon C. albicans infection and this is dependent upon the CARD adaptor protein likely as a result of C. albicans β-glucans binding to the dectin-1 pattern recognition receptor 2. In agreement with other studies 3, the generation of Th17 cells by dectin-1 ligation is dependent upon the presence of TGF-β-producing Treg cells 2. In human studies, memory T cells responding to C. albicans express IL-17 and the mucosal homing chemokine receptors CCR6 and CCR4, suggesting that both polarization and homing capacity of cells can be dependent upon the interaction between the pathogen and the DC 1, 4. While these recent studies have identified the mechanism whereby DC are able to promote Th17 cells in response to C. albicans they did not address the role of IL-17 and IL-23 in protection against fungal pathogens. It is this role that has been addressed in an article in this issue of the European Journal of Immunology by Zelante et al.5.

While IL-23 and IL-17 have been associated with the development of inappropriate autoimmune inflammation they have also been implicated in protection against bacterial infections 6. A protective role for IL-17 in systemic C. albicans infection has been demonstrated by the increased susceptibility of IL-17A receptor (IL-17ARKO) mice which is associated with reduced infiltration of neutrophils into infected organs 7. IL-17 is a potent recruiter of neutrophils to the lung 8 and with IL-23 regulates neutrophil homeostasis 9. Using two models of fungal infection; one mediated by C. albicansvia the intragastric route and, one mediated by Aspergillus fumigatusvia the intranasal route, the current study by Zelante et al.[5] highlights a regulatory role of IL-23 and IL-17 in limiting the protective IFN-γ/IL-12 response. In this study, [5] delivery of the pathogen was via a mucosal surface in both cases, which is likely to be important as the pattern recognition receptors and cytokine milieu in these locations will impact generation of cellular responses in a manner quite distinct from the locations encountered by a systemically delivered pathogen 4.

The regulatory activity of IL-23 and IL-17 reported by Zelante et al. 5, covers several aspects of the immune response. Fig. 1 illustrates how the absence of IL-12p35, IL-12p40 and IL-23p19 impact the cytokine milieu, pathogen burden and inflammation occurring upon fungal infection. In IL-12p40KO mice there is a modest increase in pathogen burden in the C. albicans infected mice at days three and ten post-infection, whereas, in the A. fumigatus infected mice pathogen burden is reduced at three days post-infection, a similar dichotomy was seen in the absence of IL-12p35. In contrast, IL-23KO mice exhibited reduced pathogen burden in both infections. The absence of IL-23 increased IL-12p70 in the mucosal site three days post-infection while the absence of IL-12p35 increased IL-23. Importantly, while IL-12p40KO mice exhibited minimal inflammation one week post-infection, IL-12p35KO mice had pyogranulomatous inflammation and IL-23KO mice had increased mononuclear inflammation. Survival of IL-12p35KO infected mice was reduced in the C. albicans model and was reduced in A. fumigatus model upon mild immunosuppression. These data suggest that an IL-12p40-dependent, IL-12p35-regulated cytokine (i.e., IL-23) promotes pathogen growth, neutrophillic inflammation and regulates the IL-12p70 response to mucosal fungal infection.

Figure 1. The balance between protective and damaging cellular responses to fungal infection is a delicate one. The figure represents the consequences of mucosal infection with either C. albicans or A. fumigatus in terms of both immediate and T cell derived cytokine response, pathogen burden and the nature of the inflammatory response. Four conditions are shown: the infected intact animal (Intact); IL-23p19KO or anti-IL-23 (or anti-IL-17) treated infected animal (no IL-23); IL-12p35KO infected animal (no IL-12p70) and an IL-12p40KO infected animal (no IL-12p40).

To determine the causal link between the absence of IL-23 and the increased protection against fungal infection, both C. albicans- and A. fumigatus-infected B6 mice were treated with blocking antibody [5]. Anti-IL-23 treatment reduced pathogen burden and reduced polymorphonuclear inflammation while anti-IL-17 had similar effects on inflammation but reduced pathogen burden only in the A. fumigatus model. When gene-deficient C. albicans-infected mice were given anti-IL-23 the protective effect was apparent in the absence of IL-4, IL-12p35 and IFN-γ/IL-12p35 jointly; however, when no IFN-γ was present, the absence of IL-23 was not protective. These data directly support a regulatory role for IL-23 (and IL-17) in the protective response to fungal infection 5.

Zelante et al.5 also investigated the regulatory role of IL-23 and IL-17 in the generation of antigen-specific IFN-γ and IL-17-producing cells following fungal infection. They found that three days post-infection, the draining lymph nodes (DLN) expressed higher levels of IL-23R in the absence of IL-12p35 and higher levels of IL-12Rβ2R in the absence of IL-23p19. At one week post-infection, the cells from the DLN of infected IL-12p35KO mice had a higher number of IL-17-producing and IL-4-producing T cells whereas, the IL-23KO mice had a higher number of IFN-γ-producing cells. These data suggest that the regulatory role of IL-23 in fungal infection extends to the development of antigen-specific T cell populations. As confirmation of a direct causal link between early IL-23 induction and reduced IFN-γ-producing T cells, intact infected mice treated with anti-IL-23 or anti-IL-17 had reduced numbers of IL-17-producing T cells but increased numbers of IFN-γ-producing T cells. Thus, IL-23 was able to regulate pathogen burden, T cell polarization and inflammation in response to fungal infection of the mucosa.

Having identified an important regulatory role for IL-23 in limiting the protective IL-12, IFN-γ and mononuclear response to fungal infection the Zelante et al.5 investigated the induction of IL-23 by APC. Using conventional DC generated from bone marrow and cultured with GM-CSF and IL-4 or CD11c+ cells isolated from spleens, the authors found that exposure to either the yeast or hyphal form of C. albicans induced rapid but short lived IL-23 production, which is dependent upon TLR2/TR4 and MyD88 but not TRIF. Comparably, flt3-induced DC made a sustained IL-12p70 and IL-10 response to both forms but little IL-23. These data are in contrast to the human monocyte-derived DC model wherein only the hyphae induced IL-23 1. This may reflect the source and differentiation state of the DC or represent a difference between human and mouse cells. That morphologic form may be important in the mouse model is, however, suggested by the fact that when A. fumigatus spores are delivered directly to the lung the inflammatory response is dependent upon the germination-dependent expression of fungal β-glucans that bind to the dectin-1 receptor 10, 11. Importantly, Zelante et al. [5] show that during fungal activation, IL-12 and IL-23 cross-regulate each other and that therefore, the relative levels of these cytokines will be the key to the outcome of infection.

An important question arising from the work reported here by Zelante et al.5 is why the neutrophillic inflammation observed in the fungally infected IL-12p35KO mice was not protective? Particularly as the increased susceptibility of IL-17ARKO mice in the systemic fungal challenge was associated with reduced neutrophil mobilization 7. In addressing this question the Zelante et al.5 have highlighted an important regulatory functions of IL-23 and IL-17. They show that neutrophils from IL-23KO mice exhibit improved killing against both C. albicans and A. fumigatusin vitro, whereas, neutrophils from IL-12p35KO mice exhibit reduced killing. Furthermore, when IL-23 or IL-17 was added to intact neutrophils, killing was reduced suggesting a direct activity of these cytokines on the functional profile of neutrophils. In this regard, Zelante et al.5 show that increased killing activity of neutrophils exposed to IFN-γ can be overcome by either IL-23 or IL-17 and that this corresponds with ablation of IFN-γ-induced IDO expression. In addition, both IL-23 and IL-17 promote expression of matrix metalloproteinase-9 (MMP-9) function while IL-17 alone promotes myeloperoxidase (MPO) activity. Zelante et al.5 also mention that addition of IL-23 or IL-17 resulted in reduced apoptosis, which has recently been reported for IL-17 12. These changes in neutrophil function and survival resulting from exposure to IL-23 or IL-17 may reconcile the data shown here with that from the IL-17ARKO mouse 7. Specifically, the expression of IDO by neutrophils in response to IFN-γ appears protective and limits the inflammatory response 13. In contrast, the inhibition of IDO by IL-23 or IL-17 reduces the protective efficacy of the neutrophils and, by inducing MMP and MPO, promotes inflammation and tissue damage. Thus while neutrophils need to be present to control infection they also in turn need to be correctly activated to mediate protection.

The host/pathogen interface for fungal infections is clearly complex and depends upon the morphologic state of the fungus, the route of infection, the activation state of DC, the cytokine milieu and the functional activity of the inflammatory cells recruited to the site. This interaction progresses rapidly via the innate response and is then modulated by the acquired response. The paper by Zelante et al.5 highlights a key role for IL-23 and IL-17 in regulating both the innate and acquired response to fungal infections.

Acknowledgements

AMC is funded by the Trudeau Institute and NIH grants AI46530, AI067723, AG028878.Conflict of interest: There is no conflict of interest.

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