As a proinflammatory cytokine, IL-36 is involved in many inflammatory reactions and plays an active role in immunity. Therefore, excessive IL-36 level often means that the body has a certain degree of damage [18]. Previous reports on IL-36 have mostly assessed its involvement in various chronic inflammatory and immuno-pathological processes mediated by Th2 cells, including psoriasis, rheumatoid arthritis, inflammatory colorectal diseases, etc. [19–21]. Recent evidence suggests IL-36 also promotes CD4+ T cell-dependent type 1 immune reactions. Binding to the mouse bone marrow-derived dendritic cell (BMDC) surface receptor IL-36R, IL-36β upregulates CD80, CD86 and MHC-II molecules, and stimulates the activation of CD4+ T cells and splenocytes to synthesize IFN-γ, IL-4 and IL-17 [9]. IL-36 directly acts on CD4+ T cells to increase cell division and IL-2 secretion; in addition, IL-36 synergizes with IL-12 in promoting Th1 polarization of naive CD4+ T cells [7].
This analysis revealed that IL-36β was markedly downregulated in pancreatic cancer tissue samples compared with adjacent tissue specimens, which indicates IL-36β has a potential function in antitumor immunity and may be related to tumor progression. This might be because IL-36β is mainly secreted by immune cells. However, tumor cells are predominant in the microenvironment of tumor tissues, and immune effector cells are in an immunosuppressive state, resulting in fewer immune cells infiltrating; therefore, tumor IL-36β amounts are reduced compared with the normal tissue. The above mouse experiments illustrated that tumor IL-36β overexpression inhibited tumor growth, and IL-36β achieved antitumor effects by inducing the proliferation of CD8 + T and NK cells in the TME. To further validate the antitumor effect of IL-36β, we transfected tumor cells in vivo with IL-36β adenovirus to increase IL-36β amounts in tumors, and the IL-36β overexpression adenovirus could act as an immunopotentiator to induce CD8 + T, NK, and γδT cell aggregation in the TME, thereby exerting antitumor effects.
As shown above, IL-36β stimulated CD8 + T cells to synthesize high IFN-γ and IL-2 amounts. Microarray analysis showed that after IL-36β stimulation of human and mouse CD8 + T cells, let-7c-5p amounts were decreased consistently. Subsequently, the expression of let-7c-5p in mouse CD8 + T cells was increased and decreased by the lentiviral technique, respectively. It was found that let-7c-5p silencing increased IFN-γ and IL-2 synthesis by CD8 + T cells. Meanwhile, when IL-36β was used to stimulate CD8 + T cells, these effects were further enhanced. High expression of let-7c-5p yielded opposite results. These findings indicate IL-36β promotes the production of IFN-γ and IL-2 by downregulating let-7c-5p in CD8 + T cells. Therefore, we hypothesized that IL-36β affects the function of let-7c-5p through a certain signaling pathway.
The let-7 family was one of the first miRNA groups to be found in Caenorhabditis elegans in 2000 [22]. The human let-7 family consists of 13 miRNAs, including let-7c. Several reports have indicated let-7c is a tumor suppressor that is downregulated or absent in multiple human tumors, including lung, ovarian, prostate and colon cancers [23, 24]. Let-7c-5p, belonging to the 1et-7 family, also has anticancer function. New research shows that CDKN2B-AS1, an oncogenic lncRNA of HCC, promotes NAP1L1-dependent PI3K/AKT/mTOR pathway by sponging let-7c-5p [25]. Xin Zhao et al. [26] also found IL-36β promotes CD8 + T cell activation by inducing mTORC1 via PI3K/Akt, IKK and MyD88 signaling, enhancing antitumor immune responses. Fu X et al. [27] found that let-7c-5p decreases cell proliferation and enhances apoptosis via ERCC6 in breast cancer. Wells AC et al. [28] unveiled a new let-7-dependent mechanism acting as a molecular brake to control the degree of CD8+T cell responses. As shown above, IL-36β increased IFN-γ and IL-2 amounts by downregulating let-7c-5p in CD8+ T cells, which has been reported by no previous studies. To summarize, direct and indirect experiments have confirmed IL-36β enhances the anti-tumor effects of CD8+ T cells by downregulating the micro-RNA let-7c-5p.
Current cancer treatments include surgery, chemotherapy and radiotherapy. With in-depth assessment of tumor etiology and immune responses, tumor immunotherapy has become the fourth treatment modality [29]. PD-1 monoclonal antibody has had great success in clinical practice for the treatment of melanoma [30]. However, the response to this treatment approach for other solid tumors may be limited since it relies on the response of spontaneous T cells to the malignancy. It is known that some cytokines can enhance tumor immunogenicity, thereby helping active lymphocytes or reversing their incompetent state, to ultimately exert anti-tumor effects. This study suggests that IL-36β has an anti-tumor activity, and let-7c-5p plays an important role in IL-36β-induced CD8 + T cell-mediated immune response, but the specific mechanism remains to be further investigated experimentally.