Our studies reveal the involvement of IL-17 in the host innate immune response to the opportunistic bacterium MAC.
However, unanticipated was the finding that macrophages rather than T cells were the cellular source of IL-17. We provide data supporting the notion that IL-17 plays a role during early host immune responses against mycobacteria, evident shortly after in vitro exposure of macrophages to MAC. Moreover, detection of IL-17 in macrophages from coinfected lymph nodes of patients with AIDS suggests that IL-17 contributes to the immune response and persistence of MAC infection in the immunocompromised host, where IL-17 can lead to recruitment of new hosts and support IFN nonresponsiveness. IL-17F, one of the members of the IL-17 family with the most homology to IL-17A induced in activated monocytes [33
], was also elevated in MAC-infected macrophages.
IL-17A/F are potent inducers of inflammatory mediators, including chemokines such as CXCL10 [22
]. CXCL10 participates in recruitment of T lymphocytes, monocytes, and macrophages [23
], and elevated levels of CXCL10 correlate with poor response to treatment in patients with MAC infections [16
]. IL-17 may further sustain inflammation by enhancing the stability of chemokine mRNA transcripts [34
]. Therefore, enhanced CXCL10 induction by MAC and IL-17 suggests that this mechanism may be operational and contribute to continuous recruitment of new targets for bacterial and viral infections.
Our study shows that MAC induces IL-17 production through a mechanism involving the MAPK and NF-κB pathways. Recent reports have shown that chitin, a ubiquitous polysaccharide in fungi, insects, and parasites, regulates IL-17 and acute tissue inflammation in macrophages [35
] and that p38MAPK influences IL-17 [36
]. In addition, TLR2 stimulation in combination with T-cell receptor activation can promote Th17 differentiation [37
Prolonged IL-17 expression in mycobacteria-exposed macrophages correlated with high IRF4 and STAT3 levels, which are known to support IL-17 [14
]. These findings correlated with enhanced expression of IL-21, a regulator of IL-17 that is important for Th17 polarization and differentiation, representing an alternative pathway for generation of the Th17 subset [38
], which we considered to be a potential regulator of macrophage IL-17. The synergistic effect of MAC and IL-21 on IL-17 transcription can be explained, at least in part, by the fact that macrophages cultured with IL-21 or MAC show enhanced gene expression of IRF4, which is known to participate in the transcriptional regulation of IL-17 and IL-21 [27
]. IL-21, in turn, activates the MAPK pathway [39
] and influences phosphorylation of IRF4 [27
]. While we cannot rule out the participation of additional factors in the regulation of IL-17 in macrophages [14
], RORγt gene expression linked to regulation of IL-17 downstream of IRF4 [14
] was not significantly increased in macrophages by MAC (data not shown).
Despite the IL-17–propagated inflammatory response, macrophages in the immunocompromised host do not clear MAC,
in part because of enhanced SOCS1/3 interference with IFN-γ signaling [13
]. Here, we show that abrogation of IFN-γ–protective innate immune responses may be further exacerbated by IL-21–driven SOCS, consistent with its effects in dendritic cells [29
]. Higher SOCS expression is found in patients with tuberculosis and recurrent tuberculosis, and reduction of SOCS expression has been considered a plausible approach to improve host protective responses [40
]. SOCS may also interfere with protective antiviral activity of type I and type II interferons [13
]. Further complicating this scenario is the fact that infection of macrophages with MAC in the presence or absence of IL-21 enhanced levels of immunosuppressive CD274/PD-L1, as did coexposure to IL-17 and mycobacteria, likely via MAPK [30
]. This can be further aggravated by the ability of HIV-1 to enhance CD274/PD-L1 on macrophages, mediated via TLR [43
]. CD274/PD-L1, considered a marker for disease progression, and its receptor, PD1, are augmented on monocytes/macrophages and T cells during HIV-1 infection, suggesting dampening of the HIV-specific effector T-cell function [32
]. Although IL-21–producing CD4+
T cells have been associated with retroviral control by affecting CD8+
T lymphocytes in patients with a low viral load [45
], in immunocompromised hosts, IL-21 may deflect innate and adaptive protective immune responses by inducing SOCS and CD274 expression. In addition, the PD-1/PD-L1/PD-L2 pathway has been connected to suppression of effector T-cell function against Mycobacterium tuberculosis
Although appropriate levels of IL-17 are thought to have a protective role in response to infection, especially during the early stages of infection with HIV-1 and M. tuberculosis
, persistent IL-17 is also associated with tissue-damaging inflammation and negative outcomes [22
]. For instance, IL-17 has been reported to have immunopathological roles during infection with multidrug-resistant M. tuberculosis
and persistently elevated levels of antigen [48
]. IL-17 contributes to the pathogenesis of autoimmune/inflammatory conditions such as rheumatoid arthritis, systemic lupus erythematosis, multiple sclerosis, Sjögren syndrome, asthma, and Crohn disease [15
]. IL-17 in CD68+
monocytes/macrophages found in inflamed mucosa of patients with inflammatory bowel disease has been linked to induction and persistence of mucosal inflammation [49
]. Recently, tissue macrophages expressing IL-17 have been described in breast tumors, where they promote invasiveness [50
MAC regulates multiple host molecules in macrophages in vitro, corresponding to evidence of their dysregulation in vivo [13
]. Here, we exposed a new strategy used by this opportunistic pathogen to promote persistence within the macrophage. It is possible that, initially, IL-17 production by MAC-infected macrophages may aid in recruiting cells and thereby mediate resistance/protection activities that are important during the early innate immune response. In immunocompromised individuals and during advanced disease, dysregulated production of IL-17 in the absence of Th1 lymphocytes, IFN-γ, or appropriate counterregulatory mechanisms to disengage IL-17 responses could drive pathogenesis.