Original ArticlesMultiple myeloma cell-derived IL-32γ increases the immunosuppressive function of macrophages by promoting indoleamine 2,3-dioxygenase (IDO) expression
Introduction
Multiple myeloma (MM) is an incurable B cell tumor characterized by the accumulation of malignant plasma cells in the bone marrow (BM) [1]. The interaction of malignant plasma cells with the BM microenvironment contributes to their growth, proliferation, invasion, metastasis, and chemoresistance [2]. The MM–BM microenvironment is composed of cells, extracellular matrix, and soluble factors, among which macrophages (MΦs) are an abundant and important component [3,4]. Several studies have shown that the degree of MΦ infiltration can serve as a prognostic marker in newly diagnosed patients with MM [5,6]. Along with promoting angiogenesis through vasculogenic mimicry, MM-associated MΦs (mMΦs) protect MM cells from spontaneous and chemotherapy-induced apoptosis through both contact-mediated and non-contact-mediated mechanisms [7,8]. mMΦs therefore represent a potential target for myeloma treatment [9,10], and it is essential to explore the mechanisms underlying MΦ infiltration and the polarization of normal MΦs to mMΦs. Research has shown that MM cells recruit tumor-supportive MΦs and promote M2-like MΦ polarization through the CXCR4/CXCL12 axis [11]. Our previous study also showed that the chemokines CCL2, CCL3, and CCL14 promoted MΦ infiltration in the MM–BM microenvironment and increased MΦ proliferation [12]. While these studies indicated that cytokines from MM cells affect MΦs, few studies have focused on the MM cell-mediated immunosuppressive function of MΦs involving T cells.
Interleukin 32 (IL-32), a proinflammatory cytokine, was identified as natural killer cell transcript 4 (NK4) in activated natural killer cells and T cells [13]. There are more than nine alternatively spliced isoforms of IL-32, each with distinct functions. Among the variants, IL-32β is the most abundant isoform, and IL-32γ appears to be the most potent isoform [[14], [15], [16]]. The definitive receptor of IL-32 has not been identified, although proteinase 3 (PR3) has been revealed to be a specific IL-32-binding protein [17]. Along with being involved in numerous inflammatory and infectious diseases [18,19], IL-32 plays a crucial role in the development of both hematologic malignancies and solid tumors [20,21]. Some reports have shown that IL-32 promotes the progression of various malignancies, such as gastric cancer, lung cancer, and cutaneous T-cell lymphoma [[22], [23], [24]]. However, conflicting reports have shown a tumor suppressive role of IL-32 in colon cancer and melanoma [25,26]. We previously reported that IL-32 is overexpressed in the BM and peripheral blood of MM patients. Moreover, IL-32 induces the production of IL-6 in bone marrow stromal cells (BMSCs) [27]. Recently, another report showed that IL-32 promotes osteoclast differentiation in MM [28]. In this study, we further investigated the association between IL-32 expression and clinical features of newly diagnosed MM patients and explored the effects and underlying mechanisms of IL-32 and MΦs in the MM–BM microenvironment.
Section snippets
Cell culture
Human MM cell lines RPMI 8226, OPM2, CAG, ARK, MM.1S, ARP-1, H929, and LP-1 were generously provided by Dr. Qing Yi (Department of Cancer Biology, Lerner Research Institute, Cleveland Clinic, Cleveland, OH, USA). All MM cell lines were cultured in RPMI-1640 medium containing 1% l-glutamine and 10% fetal bovine serum (FBS; Thermo Fisher Scientific, Waltham, MA, USA). To prepare conditioned media (CM), MM cells were seeded at 5 × 105 cells/ml for 24 h. Culture supernatants were collected and
High IL-32 expression in MM correlated with clinical stage
We studied IL-32 expression in BM biopsies from newly diagnosed MM patients by immunohistochemical staining using anti-IgG as an isotype control (Sup.Fig. 1A). IL-32 was detectable in all patients, and approximately 73.9% (17/23) of the samples showed high expression (Fig. 1A). The expression levels of IL-32 and clinicopathological details of the patients are shown in Table 1. IL-32 expression was significantly and positively associated with the International Staging System (ISS) stage
Discussion
We report herein that high IL-32 expression in MM patients was associated with more advanced clinical stage. IL-32γ in MM cells induced IDO production in MΦs through PR3, which was dependent on the activation of the STAT3 and NF-κB pathways. IDO-positive IL-32γ-educated MΦs suppressed the proliferation of CD4+ T cells and their IL-2, IFN-γ, and TNF-α production in response to polyclonal stimulation.
IL-32 is a proinflammatory cytokine involved in various inflammatory diseases and cancers [18].
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
We thank all study participants for their contributions to this project and appreciate professor Qing Yi for guiding the experiments. We thank Elsevier (https://wileyeditingservices.com/en/) for English language editing.
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These authors have contributed equally to this work.