IL-35 Regulates the Function of Immune Cells in Tumor Microenvironment

Abstract

Interleukin-35 (IL-35) is a heterodimeric cytokine composed of Epstein-Barr virus-induced gene 3 (EBI3) and IL-12p35 that has recently been shown to play diverse and important roles in the tumor microenvironment (TME). Owing to its immunosuppressive activity and ability to promote tumor growth and progression, IL-35 is widely recognized as a key mediator of TME status. Immune cells are key mediators of diverse tumor-related phenotypes, and immunosuppressive cytokines such as IL-35 can promote tumor growth and metastasis in TME. These influences should be considered together. Since tumor immunotherapy based on immune checkpoint blockade remains ineffective in many patients due to tumoral resistance, a new target or efficacy enhancing factor is urgently needed. Suppressing IL-35 production and activity has been demonstrated as an effective factor that inhibits tumor cells viability, and further investigation of this cytokine is warranted. However, the mechanistic basis for IL-35-mediated regulation of immune cells in the TME remains to be fully clarified. In the present review, we explore the roles of IL-35 in regulating immune cells within the TME. In addition, we highlight IL-35 as a specific immunological target and discuss its possible relevance in the context of immunotherapy. Lastly, we sought to summarize potential future research directions that may guide the advancement of current understanding regarding the role of this important cytokine as a regulator of oncogenesis.

Keywords: IL-35; anti-tumor immunity; regulatory immune cells; tumor immunotherapy; tumor microenvironment.

Figure 1

The anti- and pro-tumor effects of T-cell subsets in response to IL-35. Recent insight into the biology of IL-35 suggests that it can both suppress naive T cells (Tconv cells) and convert these naive Tconv cells into strongly suppressive induced Treg cells (iTr35 cells). Through a positive feedback loop associated with the expression of IL-35, Treg cells can inhibit the differentiation of CD4⁺ T cells into Th17 and Th1 cells. In addition, IL-35 is dispensable for the control of CD8⁺ T cells, and IL-3- dependent transcription signal depletion as well as the inhibitory receptor activation of CD8⁺ T cells play key roles in reducing CTL effector functions, which are critical for cellular anti-tumor responses. Given its broad immunoregulatory properties and its pro-tumor functions, IL-35 represents an important functional cytokine for Tregs that is secreted by multiple immunosuppressive cell subsets.

Figure 2

Interactions between IL-35 and other immune cells in TME. IL-35 regulates the activity of immune cells in the tumor microenvironment, and is mainly produced by Bregs, DCs, NK cells, TAMs, MDSC, and N2 neutrophils. Much like the differentiation of Treg cells, IL-35 can convert Bregs to an active subset (I35-Bregs) which secrete IL-35 to promote tumor growth. With increased inhibitory receptor expression and T cell depletion, IL-35⁺ DCs can slow primary tumor growth rates, unlike other IL-35⁺ immune cells. Furthermore, IL-35 mediates diverse functions in NK cells and is obligatory for promoting the early NK cell-mediated responses to enhance primary anti-tumor immunity. TAMs, MDSCs, and N2 cells recruited by IL-35 can similarly potentiate primary tumor growth and metastatic colonization in the tumor microenvironment.

Figure 3

IL-35 and PD-L1 in NSCLC immunotherapy. PD-L1 expression on tumor cells is the most studied biomarker in the context of predicting NSCLC patient immunotherapy outcomes. Further studies are needed to explore the relationship between IL-35 and PD-L1. Under conditions of EGF deprivation, IL-35 can reduce the expression of PD-L1 without impacting EGFR, which is also a target of NSCLC immunotherapy. A positive correlation between IL-35⁺Tregs and TTF-1⁺PD-L1⁺ cells has been observed in the TU region, and TTF-1 is a good prognostic factor associated with survival in NSCLC. In addition, CIP generation is often observed in patients undergoing PD-1 or PD-L1 antibody therapy. CIP patients have higher mean amounts of Th1 and Th17 cells and lower levels of Tregs in serum as compared to normal subjects, thus inducing IL-35 secretion in the plasma and BALF. The levels of IL-35 in the plasma and BALF can be measured for the diagnostic evaluation of CIP patients. Overall, the complex relationship between IL-35 and PD-L1 must be considered when discussing the potential role of IL-35 in tumor immunotherapy.