Breaking the Immune Tolerance to Apoptotic Cancer Cells Ingested by Phagocytes
- 1. School of Life Science and Biotechnology, Shanghai Jiao Tong University, Shanghai, China
- 2. School of Life Science and Biotechnology, Shanghai Jiao Tong University, Shanghai, China
Cell loss in malignant disease is a very significant component of tumor dynamics and apoptosis is a common process in highgrade malignancy, with high apoptosis indices generally reflecting poor prognosis and a likely indication of the rapidity of apoptotic cell (AC) clearance in situ . Activation of apoptosis is a wellestablished approach to cancer therapy. However, constitutive or therapy-induced apoptosis of tumor cell populations generates an immunosuppressive environment that protects malignant tissue from potential host anti-tumor immune mechanisms. It has been postulated that apoptosis contributes to oncogenesis through recruitment and appropriate activation of tumor-associated macrophages that support tumor growth and evolution; direct and indirect trophic effects resulting in net increases in tumor cell numbers; and anti-inflammatory and tolerogenic properties that suppress innate and adaptive anti-tumor immune responses .
Rapid removal of ACs is considered central to the resolution of inflammation and in preventing autoimmune disease . In health, more than 1 × 109 ACs are cleared from the human body each day in the immune system alone , underscoring the importance of tightly regulated mechanisms to prevent the activation of proinflammatory responses to self. ACs express various autoantigens, and their persistence is thought to generate harmful autoimmunity through the activation of self-reactive lymphocytes [5,6]. Conversely, the efficient removal of ACs can generate an immunoregulatory milieu and promote the resolution of inflammation, a process that relies on their recognition and engulfment by professional phagocytes macrophages and dendritic cells [3,7,8]. Phagocytosis of ACs usually results in an anti-inflammatory state with the induction of immunosuppressive cytokines such as TGF-β, IL-10 and prostaglandin E2, and inhibition of proinflammatory cytokines such as IL-12 and TNFα. We first demonstrated how AC-derived signals inhibited IL-12 gene expression . Cell-cell contact with ACs via phosphatidylserine (PS) was sufficient to induce profound inhibition of IL-12 production by activated macrophages. The inhibition did not involve autocrine or paracrine actions of IL-10 and TGF-β. We identified a novel zinc finger nuclear factor, named GC binding protein (GC-BP), which was induced following phagocytosis of ACs by macrophages. GC-BP, activated via tyrosine phosphorylation upon AC-induction, selectively inhibited IL-12 p35 gene transcription by binding to its promoter in vitro and in vivo, thus decreasing IL-12 production. Blocking GC-BP by RNA interference restored IL-12p35 gene transcription and IL-12 synthesis. GC-BP activity was regulated via tyrosine phosphorylation in response to ACs .
Subsequently, we reported that IL-10 production stimulated by ACs was regulated at the point of transcription in a manner dependent on p38 mitogen-activated protein kinase, partially on the scavenger receptor CD36, and required cell-cell contact but not phagocytosis . Furthermore, we showed that ACinduced transcriptional activation of IL10 was mediated by pre-B cell leukemia transcription factor-1b (Pbx-1) and another Hox cofactor Pbx-regulating protein 1 (Prep-1). This study also revealed a novel role of the two developmentally critical factors in the regulation of homeostasis in the immune system.
Recent studies indicate that IL-23 is over-expressed by macrophages and DCs in human and mouse tumors and antagonistically regulates local inflammatory responses in the tumor microenvironment and infiltration of intraepithelial lymphocytes. IL-23 upregulates proinflammatory cytokines IL17 and IL-22, matrix metalloproteinase MMP9 and increases angiogenesis. In addition, IL-23 reduces tumor infiltration of cytotoxic T lymphocytes (CTLs), in contrast to IL-12’s promotion of CTL infiltration. Conversely, blocking IL-23 via genetic deletion or antibody-mediated neutralization causes increased CTL infiltration into the transformed tissues and protects against chemically induced carcinogenesis and transplanted tumors .
We recently observed that in the absence of microbial stimuli, human and mouse myeloid DCs and macrophages engulfing ACs produced substantial amounts of IL-23, IL-6, and TGF-b, the essential cytokines for the development of Th17 cells that have been strongly implicated in human tumor immunity . Furthermore, we identified two novel transcription factors, LRRC16B and FLJ44967, as critically important and direct for the induction of IL-23 in phagocytes exposed to ACs.
Taken together, these data supports our hypothesis that phagocytosis of apoptotic cancer cells (ACCs) induces immunosuppressive cytokines and inhibits immunostimulatory cytokines in part through the induction of GC-BP, Pbx-1/ Prep-1, LRRC16B and FLJ44967, (collectively abbreviated as GPLF), generating an immunosuppressive microenvironment favoring tumor progression. Targeting these molecules, instead of interfering with the phagocytosis itself, will have anti-tumor benefits without causing overt autoimmune damage (Figure 1)
Our research in this area is focused on three aspects: (1) investigating how the cancer-promoting cytokine IL-23 is induced during phagocytosis of ACCs through the two novel transcription factors we have identified, LRRC16B and FLJ44967; (2) assessing the effects of tageting the three major transcription regulators (GC-BP, Pbx-1/Prep-1, and LRRC16B/FLJ44967) induced in the phagocyte/ACC interaction that regulate the expression of IL-12, IL-10, and IL-23, respectively, in a mouse tumor model via immunization using ACC-pulsed dendritic cells; (3) deciphering the role of TGF-β expression and signaling in phagocytes in regulating immune tolerance to ACCs via genetic targeting of the TGF-β gene and its signaling pathway in the meyloid compartment.
This research carries the potential of uncovering novel inner workings that underlie some of the most profound networks of nature and evolution. It will conceptually move the field forward, and inspire the development of innovative strategies to overcome immune tolerance to self tumor antigens in cancer-therapeutic modality and vaccination.
Kim HJ, Zheng H, Liu X, Du Q, Zhang L, et al. (2013) Breaking the Immune Tolerance to Apoptotic Cancer Cells Ingested by Phagocytes. J Immunol Clin Res 1: 1001.
This work has been supported by a grant from the NIH to X.M. (R01 AI045899).
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