Original ArticlesLipid mediator lipoxin A4 inhibits tumor growth by targeting IL-10-producing regulatory B (Breg) cells
Introduction
B lymphocytes are well known to function as antibody-synthesizing machines to protect the host against invasive pathogens. However, mounting evidence indicates that B cells also play an essential role in various immune pathologies including malignant diseases [1], [2]. Previous studies showed that B cells inhibited the induction of T cell-dependent tumor immunity and B cells can induce CD8 T cells to become anergic [3]. On the other hand, depletion of B cells enhances anti-tumor cytotoxic T cell response, leading to effective clearance of tumor cells [4]. In clinic, there are reports that the presence of B cells is correlated with worse outcome [5], [6], regardless of other contrary findings. It has now become clear that a subset of IL-10-producing regulatory B (Breg) cells play a pivotal role in mediating immunosuppression. It has been reported that Breg cells can suppress Th1 and Th17 responses [7]. In murine models, adoptive transfer of Breg cells can ameliorate the symptoms of experimental autoimmune encephalomyelitis (EAE) [8]. In human systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA), disease activities are closely related to the number and function of Breg cells [9], [10]. Initially, IL-10 production is considered as a hallmark of Breg cells; however, other cytokines like lymphotoxin and IL-35 produced by B cells also display suppressive properties [11], [12]. In addition to producing immunosuppressive cytokines, Breg cells also act directly on natural killer (NK) and dendritic cells to restrain their activities [13], [14]. Furthermore, Breg cells indirectly induce and maintain Treg cells as immune suppressors [15]. Currently, though the mechanisms by which Breg cells regulate immune responses have been widely investigated, how Breg cell numbers and their function are controlled remains poorly understood.
Lipoxin A4, an endogenous eicosanoid, is highlighted in the regulation of inflammation [16]. Lipoxin A4 is synthesized locally from arachidonic acid at inflammation sites via a transcellular biosynthesis mechanism [17]. By specifically binding to two distinct receptors: formyl peptide receptor 2 (FPR2), a surface membrane G-protein-coupled receptor, and the aryl hydrocarbon receptor (AhR), a ligand-activated nuclear transcription factor, LXA4 strongly inhibits the trafficking of leukocytes to the inflammatory site and stimulates the phagocytosis of apoptotic cells by tissue macrophages [18], [19]. Roles of LXA4 in tumorigenesis have been explored using different tumor models [20], [21], [22]. Although those results have shown anti-tumor effects of LXA4, the underlying mechanism remains unclear. In this study, we provide evidence that LXA4 exerts the anti-tumor effect through inhibiting the generation of IL-10 producing Breg cells. Our findings suggest that LXA4 functions as an endogenous immune modulator against tumors.
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Cell lines and mice
Murine hepatocarcinoma H22 (BALB/c), melanoma B16 (C57BL/6) and colorectal carcinoma CT26 (BALB/c) cell lines were purchased from the China Center for Type Culture Collection (Wuhan, China) and cultured according to the guidelines given. Female BALB/c and C57BL/6 mice, 8- to 10-week-old, were purchased from the Center of Medical Experimental Animals of Hubei Province (Wuhan, China). Female BALB/c nude and CB-17 SCID mice, 6- to 8-week-old, were purchased from Vital River Laboratory Animal
Anti-tumor effects of LXA4 in different murine tumor models
To investigate the anti-tumor effect of LXA4, a murine H22 hepatocarcinoma tumor model was first tested. After subcutaneous injection of H22 tumor cells (2 × 105), the tumor-bearing mice were intraperitoneally administered with BML-111, an agonist of LXA4 receptor, once per two days. The result showed that the administration of BML-111 significantly inhibited H22 tumor growth (Fig. 1A). Besides the H22 tumor model, murine B16 melanoma and CT26 colon cancer tumor models were also conducted.
Discussion
Lipid metabolites are critical in the immunoregulation of both innate and adaptive immune responses. Many lipid mediators show immunosuppressive action, and potentially favor tumor immune evasion. However little is known about the process of restraining tumor immune evasion by lipid mediators. In this study, we demonstrate that arachidonic acid-derived lipid mediator LXA4 can upregulate anti-tumor immunity by targeting an important immunosuppressive B cell subset, Breg cells.
The role of B cells
Conflict of interest
The authors declare no conflict of interest.
Acknowledgments
This work was supported by National Basic Research Program of China (2014CB542100), National Science Fund for Distinguished Young Scholars of China (81225021), National Natural Science Foundation of China (81472653), and Special Fund of Health Public Welfare Profession of China (201302018).
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Z. Wang and Q. Cheng contributed equally to this work.