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 Table of Contents  
REVIEW
Year : 2017  |  Volume : 3  |  Issue : 6  |  Page : 200-208

“Eating” Cancer cells by blocking CD47 signaling: Cancer therapy by targeting the innate immune checkpoint


Department of Microbiology, Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences and School of Basic Medicine, Peking Union Medical College, Beijing, China

Date of Web Publication29-Dec-2017

Correspondence Address:
Dr. Li Liu
Department of Microbiology, Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences and School of Basic Medicine, Peking Union Medical College, Beijing 100005
China
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ctm.ctm_26_17

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  Abstract 


Differing from the adaptive immune checkpoint mediated by programmed cell death-1 (PD-1) PD-1-ligand or CTLA-4, the CD47 and signal regulatory protein α (SIRPα) axis is emerging as a novel innate immune checkpoint of the immune cells of myeloid lineage. A balance should be established between the dual signals, the “Don't eat me signal” of CD47-SIRPα and the “Eat me signal” of calreticulin/low-density lipoprotein receptor-related protein. The enhanced expression of CD47 molecule has been found in many cancer tissues, including malignant blood tumors (acute myeloid leukemia) and solid tumors. A therapeutic value could be achieved by counteracting the expression of CD47 in cancer cells. In the recent years, great progress has been made to develop anticancer therapies by targeting CD47 (e.g., anti-CD47 antibody), in various types of cancer. However, there are a few challenges, like “antigen sink” in the clinical translation of CD47-mediated anticancer therapies, the attention to which is crucial.

Keywords: Antibody, CD47, immune checkpoint, phagocytosis, signal-regulatory protein a, tumor


How to cite this article:
Xiang YR, Liu L. “Eating” Cancer cells by blocking CD47 signaling: Cancer therapy by targeting the innate immune checkpoint. Cancer Transl Med 2017;3:200-8

How to cite this URL:
Xiang YR, Liu L. “Eating” Cancer cells by blocking CD47 signaling: Cancer therapy by targeting the innate immune checkpoint. Cancer Transl Med [serial online] 2017 [cited 2018 Apr 24];3:200-8. Available from: http://www.cancertm.com/text.asp?2017/3/6/200/221911




  Introduction Top


The most important function of the human immune system is its immune surveillance for invading pathogens and abnormally growing cells like cancer cells, in which both innate and adaptive immune responses play a role.[1],[2],[3],[4] T-cell-mediated immunity is activated by two pairs of receptor-ligand interactions: the interactions between T-cell receptor and major histocompatibility complex (MHC)-antigen complex on antigen-presenting cells (APCs) and a co-stimulatory signal between CD28 (Cluster of differentiation 28) on T-cell surface and B7 on APC.[5],[6],[7] T-cell activation also activates a CTLA-4-mediated negative signaling on T-cells, which also recognizes the B7 on APC to inhibit T-cell activation.[8] Different from CTLA-4, programmed cell death-1 (PD-1), another adaptive immune checkpoint, has more potent restriction on T-cell activation by interfering with the function of T-cell antigen receptor, through its interaction with its PD-1 ligand (PD-L1).[9],[10] Nonspecific immunity, however, also has indispensable function, for example, release of cytokines and macrophage-mediated phagocytosis.[11] Blocking these adaptive immune checkpoints such as CTLA-4 and PD-1 by monoclonal antibodies is of great therapeutic potential against many types of human cancers.[12],[13],[14]

CD47, a member of immunoglobulin superfamily, is a 50 kDa transmembrane protein. It binds to different counterparts such as thrombospondin and signal regulatory protein α (SIRPα).[11],[15] SIRPα expressed on the engulfing cells plays a negative role when it is activated by “self” cells that express the molecule ligand - CD47.[16] This interaction generates “Don't eat me” signal to prevent phagocytosis via inhibiting the phosphorylation of myosin II and the reshaping of the cytoskeleton. However, losing or reducing CD47 in the membrane may activate “eat me signal”; macrophages can then phagocytose the damaged or apoptotic cells.

Cancer cells may escape the immune surveillance of macrophages by upregulation of CD47 expression.[17] It is suggested that, in some malignant blood cancers such as acute myeloid leukemia (AML) and chronic myeloid leukemia, the abnormal increase in CD47 expression could guard the cancer cells from macrophage-mediated killing.[18] More recently, laryngeal squamous cell carcinoma and other solid tumors have also been shown to overexpress CD47, which is suggested as a potential therapeutic target in the treatment of the condition.[19] Further, CD47 has been identified as the regulator of melanoma tumor metastasis and immune evasion, targeting which, along with CD271, effectively suppress the metastatic melanoma condition.[20] These findings link CD47 molecule to the tumorigenesis, metastasis, and cancer progression.


  The “Don't Eat Me” and the “Eat Me” Signal Top


The “Don't eat me” signal, CD47, also previously known as ovarian tumor-associated antigen (OA3), 1D8 antigen, or integrin-associated protein (IAP), was initially identified as the ovarian tumor antigen, recognized by monoclonal antibody (mAb)-OVTL3, which was suggested to be of potential in the diagnosis of ovarian cancer.[21],[22] A year later, in 1987, Miller et al.[23] reported that a novel cell surface antigen, recognized by mAb 1D8, was uncovered from Chinese hamster-human somatic cells that was absent in Rhnull erythrocytes. The full length of CD47/IAP was first detected using the mAb-B6H12, which was able to specifically block the Arg-Gly-Asp (RGD)-mediated enhancement of neutrophil-mediated phagocytosis,[24] and was subsequently cloned from U937 cDNA library.[25] Using mAb-OVTL3, Campbell et al.[26] also isolated a 323 amino acid ovarian tumor-associated antigen named OA3, which was later shown to be identical to CD47/IAP.[22],[27] Structurally, CD47 contains an extracellular N-terminal hydrophilic Ig superfamily domain and an intracellular carboxyl-terminal hydrophobic domain with five transmembrane helices plus a short intracellular tail.[25],[26] SIRPα which is another member of Ig superfamily has been identified as the receptor to CD47.[28],[29],[30] SIRPα, a single transmembrane molecule, encompasses three Ig superfamily domains extracellularly plus a cytoplasmic domain containing four immunoreceptor tyrosine-based inhibitory motifs (ITIMs). The activation of SIRPα upon CD47 binding induces tyrosine phosphorylations within the selected two ITIMs of SIRPα, likely by Src family kinases. This then leads to the recruitment of SH2-containing phosphatase-1 (SHP-1) and 2 (SHP-2) to the cytoplasmic tail of SIRPα,[31] which in turn releases the autoinhibition of the intramolecular interaction between SH2 domain and PTP domain to trigger the catalytic activity of PTP domain of SHP1 and SHP2, which will lead to the dephosphorylation of various downstream signaling molecules of SIRPα, thus inhibiting phagocytosis.[15],[32] CD47 may also influence leukocyte migration by affecting the binding of integrin αvβ3 with its ligand. Further, CD47-SIRPα interaction is also known to facilitate the transendothelial migration of neutrophils from the blood vessel into the inflammatory tissues,[33],[34] and monocyte trafficking in the brain.[35]

The “Eat me” signal was initially identified as phosphatidylserine (PtdSer) on apoptotic cells, which could be recognized by PtdSer receptor on the phagocytes.[36] The apoptosis is a programmed cell death with no inflammatory outcome, while necrosis is a special cell death that can be categorized into either caspase-1 associated pro-inflammatory (pyroptosis) or inflammatory cell death (necroptosis).[37],[38],[39],[40] Morphologically, the apoptotic cells commonly demonstrate the blebbing with no membrane disruption,[38] while both pyroptosis and necroptosis are undergoing membrane rupture.[39],[40] In normal cells, PtdSer usually resides at the inner leaflet of the lipid bilayer membrane. As cells undergo apoptosis, PtdSer is flipped on to the outside of the membrane, which then activates its cognate receptor on the surrounding phagocytes, promoting the engulfment of the apoptotic cell.[41],[42] In addition, the transmembrane molecule such as calreticulin (CRT) also functions as the “Eat me” signal, which is often found upregulated in the apoptotic cells [Figure 1]a, and is recognized by low-density lipoprotein receptor-related protein (LRP) on the phagocytes, subsequently facilitating engulfment of the apoptotic cells.[43] The therapeutic value of CRT in cancer has been first elucidated by Obeid et al.,[44] who discovered that drug-mediated exposure of CRT on tumor cell surface could elicit tumor cell clearance by the dendritic cell-mediated phagocytosis.[45]
Figure 1: The role of the innate immune checkpoint during cellular homeostasis, pathogenesis, or therapeutic condition. (a) The “Eat me” signal is usually enhanced by the overexpression of CRT on the apoptotic cells which could be recognized by LRP on macrophages. (b) CD47 is upregulated on the surface of macrophages to generate a stronger “Don't me signal” carried out by CD47-SIRPα axis. (c) Blocking the ligation of CD47 and SIRPα by anti-CD47 antibody promotes the clearance of the targeted cancer cells by unblocked “Eat me signal.” (d) A homeostasis between the “Eat me” signal and the “Don't eat me” signal can be reached in normal microenvironment. LDL: Low-density lipoprotein; LRP: LDL receptor-related protein; CRT: Calreticulin; SIRPα: Signal regulatory protein α; CD47: Cluster of differentiation 47

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The immunomodulatory role of CD47 has been revealed by an elegant experiment in CD47 knock-out mice.[46] Since the expression of CD47, but not MHC, could be detected in red blood cells (RBCs), it was assumed that CD47 might function as a “self-”marker being recognized by the host immune system. Indeed, wild-type recipient mice, but not CD47-/- mice, eliminated the donor CD47-/- RBCs. The authors further showed that the clearance of opsonized RBCs in autoimmune hemolytic anemia was mainly due to the phagocytosis by macrophage that was regulated by the interaction between CD47 and SIRPα.[47] In this case, CD47 on RBC functioned as a “Don't eat me” signal through the interaction with its cognate receptor SIRPα on macrophage, which may generate a negative signal to prevent phagocytosis. The expression of CD47 is prevalent and most often upregulated in cancer tissues [Figure 1]b,[26],[48] while the expression of its cognate receptor SIRPα is relatively limited and particularly abundant in myeloid cells (macrophages, monocytes, granulocytes, and dendritic cells) and neurons.[49] Disrupting the ligation of CD47 and SIRPα or reduced CD47 expression could promote the clearance of viable or apoptotic cells by phagocytes similar to CRT/LRP mediated phagocytosis [Figure 1]c.[43] A homeostasis should therefore be reached between the “Eat me” signal and the “Don't eat me” signal [Figure 1]d. Thus, blockade of CD47 by various means, such as anti-human CD47 antibodies, has acquired great therapeutic benefits in treating cancers.


  Anti-CD47 Antibody and Antitumor Effect Top


Many researches pay attention to the antitumor effects of the anti-CD47 antibody; however, the underlying mechanism is not yet clear. Possible mechanisms are as follows: (1) The inhibition of the CD47-SIRPα signaling pathway by anti-CD47 antibody could increase the phagocytosis of tumor cells by macrophages and eventually clearing them.[16] This pathway is acknowledged as the major contributor to the antitumor effect of anti-CD47 antibody. Supporting the mechanism, it was shown that, in the absence of macrophages, anti-CD47 antibody had no effect on lymphoblast viability in three malignant B-cell lines.[50] (2) The Fc domain of antibody may activate the antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity (CDC), which then complements the T-cell function in tumor cell clearing.[51] The blockage of CD47-SIRPα signaling axis by anti-CD47 antibody promotes T-cell-mediated antitumor effect, through the activation of innate immune response in dendritic cells responding to the stimulation of tumor antigen.[52] Several studies have demonstrated the therapeutic effect of the anti-CD47 antibody in several cancer conditions.

Acute myeloid leukemia

AML is a life-threatening disease in children and teenagers all over the world. The disease often relapses soon after discontinuing the treatment, mainly due to the remnant leukemic stem cells' (LSCs) resistance to therapy. The status of LSCs is the important factor in determining the treatment and prognosis of AML. MOLM-13 cell, an established leukemia cell line from an AML (acute monocytic leukemia variant) patient,[53] had a low level of CD47 expression and failed to be engrafted into immunodeficient mice. However, mice transplanted with CD47-overexpressed MOLM-13 cells succumbed to the disease accompanying with high infiltration of leukemia cells in both bone marrow and spleen.[18] Consistently, increased CD47 expressions in three independent cohorts of adult AML were associated with worse overall survival.[54] Therefore, CD47 might be an ideal therapeutic target for AML. In contrast, the results of the study from Galli et al.,[55] who employed a tissue microarray-based analysis, revealed that the upregulation of CD47 in bone marrow was indeed directly correlated with the tumor loads in patients, but could not serve as a prognostic marker.

Wang et al.[56] studied the effect of anti-CD47 antibody on LSCs, the results of which revealed a significant drop in the number of leukocytes in peripheral blood in response to anti-CD47 antibody, as compared with that of the anti-IgG1 antibody. Moreover, anti-CD47 antibody worked well with cytarabine toward targeting LSCs and depleting leukemia cells, meanwhile significantly increasing the overall survival rate. Ponce et al.[57] created a local inhibitory checkpoint mAb (licMAB) by fusing the endogenous SIRPα domain with the N-terminus of the light chain of an antibody targeting CD33, which is an AML-specific antigen. This licMABs could disrupt the CD47-SIRPα axis, enhancing the macrophage-mediated phagocytosis of AML cells and eliminating AML cell lines and patient-derived AML cells.

Myeloma

The expression level of CD47 is positively associated with disease progression in multiple myeloma (MM). A 10-fold increase in CD47 expression was observed as the disease progression from monoclonal gammopathy of undetermined significance to MM.[58] Jarauta et al.[59] demonstrated that, carfilzomib, a potential next-generation drug for MM treatment, could induce both apoptosis and autophagy of the targeted cancer cells. Blocking autophagy with chloroquine further promoted carfilzomib-mediated immunogenic cell death, which was in direct correlation with the enhanced CRT exposure on MM cells. In an early study, Kikuchi et al.[60] constructed bivalent single-chain antibody fragments (scFvs) against CD47 and proved that these scFvs exhibited great therapeutic potential against human myeloma cancer. Later, Kim et al.[61] demonstrated the potential of anti-CD47 antibody in killing myeloma cells. They found that the myeloma cells in 73% of MM patients overexpressed CD47. Then, they demonstrated that, at cellular level, the anti-CD47 antibody could enhance the macrophage-mediated phagocytosis of myeloma cells. Using the xenotransplantation model, they revealed that the anti-CD47 antibody could not only decrease the tumor burden by inhibiting the proliferation of myeloma cells, but also significantly increase the tumor response rate from 40.7% to 73.7%. Furthermore, they discovered that the anti-CD47 antibody inhibited the growth of myeloma cells even within the human bone marrow microenvironment, which is otherwise known to protect myeloma cells from chemotherapy, while promoting their proliferation. This research thus suggests of the potential value of anti-CD47 antibody in the treatment of hematologic tumors.

Ovarian cancer

Ovarian cancer is a common solid cancer in women. Brightwell et al.[62] found that 79.2% of the ovarian cancer patients increasingly expressed CD47. After chemotherapy, although not significant, a linear relationship between patients' overall survival rate and CD47 expression could be observed. Median survival period of CD47high patients was about 38 months, while that of CD47low patients could reach up to 45 months. Only 50% of CD47high patients showed a complete response to anti-CD47 therapy, which was significantly lower than that of CD47low patients (65%). High-grade serous ovarian carcinoma (HGSOC) is a subtype of ovarian cancer with relatively poor prognosis. Li et al.[63] showed that the expression level of CD47 was directly correlated with the poor survival in HGSOC patients. More recently, Liu et al.[64] investigated the antitumor effect of anti-CD47 antibody in ovarian cancer, under both in vitro and in vivo conditions. Using SK-OV-3 cells (an ovarian cancer cell line) co-cultured with THP-1-derived macrophages, they found that phagocytic index increased significantly after CD47 blockade. The in vivo experiment demonstrated a significant decrease in tumor growth in response to anti-CD47 antibody, as compared to anti-HLA control group. These studies indicate that CD47 could be a potential therapeutic target in treating ovarian cancer.

Osteosarcoma

Osteosarcoma is an aggressive malignant bone cancer that is most often observed in children. As mentioned above, the phagocytic process depends on the coordination between the enhanced “Eat me” signal such as CRT and the reduced “Don't eat me” signal such as CD47 [Figure 1]c. Zhang et al.[65] found that, in osteosarcoma, CRT may serve as a prognostic marker since a high level vs. a low level of CRT expression could be found in normal vs. osteosarcoma tissues, nonmetastatic vs. metastatic tissues, and chemotherapy vs. nonchemotherapy group. The study by Xu et al.[66] revealed that CD47 was overexpressed in osteosarcoma tissue as compared with that of the adjacent normal tissues, as assessed through mRNA, protein, and immunochemistry analysis. Also, patients with high CD47 expression had shorter overall survival period. Using a mouse model, they also showed that after treating KRIB (osteosarcoma cell line)-injected nude mice with anti-CD47 antibody (B6H12) for 45 days, the tumor bearing was significantly decreased, as demonstrated by a lower tibial weight, which was nearly half of the IgG control group. Furthermore, the pulmonary metastasis rate decreased from 75% to 11% in the anti-CD47-treated group. Using osteosarcoma mice as a model, Kawano et al.[67] showed that the combined treatment of dendritic cells and doxorubicin further enhanced doxorubicin-mediated anti-osteosarcoma effect. Overall, these results indicate that targeting either the “Eat me” signal or “Don't eat me” signal may offer a new therapeutic option for the treatment of osteosarcoma.

Lung cancer

The most common form of lung cancer is non-small cell lung cancer (NSCLC). Zhang et al.[68] developed a therapeutic strategy to target both CD47 and autophagy in NSCLC, using a novel anti-CD47 antibody, SIRPαD1-Fc. They found that this fusion antibody could exert its anti-NSCLC effect by promoting the phagocytosis by macrophage and enhancing autophagy in cancer cells. Another study showed that targeting the cancer stem cells (CSCs) of NSCLC using the anti-CD47 antibody markedly increased the survival time of the transplanted mice.[69] Zhao et al.[70] identified CD47 to play an important role in NSCLC cell invasion, migration, and metastasis, all of which were inhibited, in a CDC-42-dependent manner, by downregulating CD47. Small cell lung cancer (SCLC) is an aggressive subtype of lung carcinoma, and no effective therapy is available as of today. Weiskopf et al.[71] demonstrated that CD47 is significantly overexpressed in SCLC cells, blocking of which by the anti-CD47 antibody could enhance the macrophage-mediated phagocytosis of the tumor cells. In addition, the animal experiment showed a marked decrease in the tumor volume in the anti-CD47 antibody-treated group, compared to control animals (160.2 mm 3vs. 837.8 mm 3, respectively). Further, combining anti-CD47 antibody therapy with another antibody-targeting CD56 promoted the phagocytosis of SCLC by macrophages and inhibited the tumor growth as compared to single antibody treatment.

Laryngeal squamous cell carcinoma

Kim et al.[72] reported that the inhibition of CD47 may enhance the natural killer cell-mediated cytotoxicity in head-and-neck squamous cell carcinoma cell lines, while its increased expression had the opposite effect. Sakakura et al.[73] compared the effect of alteration in the CD47 antigen on M1 and M2, two subtypes of macrophage, in head-and-neck squamous cell carcinoma condition,in vivo and in vitro. They found that the phagocytic ability of M1 subtype was significantly inhibited by the overexpression of CD47 in tumor cells. Yang et al.[19] showed that CD47 expression was significantly increased in laryngeal squamous cell carcinoma tissue, in comparison with adjacent normal tissues. Further, the antitumor effect of three different monoclonal antibodies, two anti-CD47 antibodies (BRIC126 and B6H12.2), and one anti-SIRPα antibody, on laryngeal squamous cell carcinoma, was evaluated. Results showed that all the three antibodies exerted potent antitumor effect and could inhibit tumor growth in a rat model injected with Hep2 cells. Moreover, combined therapy with CD47-siRNA and the anti-SIRPα antibody had a more potent effect than either alone.

Leiomyosarcoma

Leiomyosarcoma can produce colony-stimulating factor and recruit tumor-associated macrophages. As an important part of the tumor microenvironment, tumor-associated macrophages promote tumor growth and proliferation, which means that the majority of tumor-associated macrophages stay in the M2 subtype.[74],[75] Xu et al.[66] found that the anti-CD47 antibody inhibits the M2 polarization to effectively control the proliferation of osteosarcoma. Edris et al.[74] evaluated the effect of anti-CD47 antibody on the function of tumor-associated macrophage in leiomyosarcoma condition. The results showed that the anti-CD47 antibody significantly enhanced macrophage-mediated phagocytosis of leiomyosarcoma cells in vitro. Further, the anti-CD47 antibodies were used as therapeutic drugs to treat the xenotransplantation nude mice model, which was subcutaneously injected with human leiomyosarcoma cells. Treatment with anti-CD47 antibody significantly decreased the average tumor mass by four times in LMS05 (leiomyosarcoma cell line)-injected nude mice and reduced tumor volume by 30 times in LMS04 (leiomyosarcoma cell line)-injected nude mice, in comparison with an IgG1 injection control group. Moreover, the anti-CD47 antibody significantly decreased lymph node metastasis number, metastasis lymph node weight, and tumor cell proportion in lung tissues, indicating that the anti-CD47 antibody can also prevent cancer metastasis. However, a Phase II study using the PD-1 antibody on advanced uterine leiomyosarcoma (ULMS) patients failed to reach a significant therapeutic benefit.[76] This suggests that the single immune checkpoint antibody may not be sufficient for ULMS treatment and a combination therapy might need to be considered in the future.

Malignant melanoma

Malignant melanoma is one of the most aggressive types of cancer that patients are usually in their terminal stage upon diagnosis. Ngo et al.[20] reported that CD47 expression was directly correlated with the metastatic state of malignant melanoma. Fu et al.[77] investigated Chinese melanoma patients by immunohistochemistry approach and found that the expression of CD47 can serve as an independent prognostic marker for disease severity and patient survival. To improve the malignant melanoma treatment is the focus of current research worldwide. Wang et al.[78] created a novel nanomedicine called liposome-protamine-hyaluronic acid (LPH) (siCD47) that combined LPH with CD47-specific siRNA, the intravenous injection of which could significantly inhibit melanoma growth in experimental animals. CD271-positive melanoma cells are considered to be the cancer stem cells responsible for the growth of malignant melanoma with the tendency of metastasis.[79],[80],[81] Anti-CD47 antibody treatment markedly altered the tumor microenvironment of CD271-positive melanoma cells, in which the well-differentiated macrophage increased, while the inflammatory monocytes, neutrophils, and pro-metastasis macrophages significantly decreased. Moreover, the combination of the anti-CD47 antibody and the cytotoxic anti-CD271 antibody markedly enhanced the antitumor effect in metastatic melanoma.[20]


  Application Prospect for Anti-CD47 Antibody Top


The antitumor effect of anti-CD47 antibody, a blocker for CD47-SIRPα signaling, has been tested in different hematologic and solid tumors. Animal models have been used to verify its broad application prospect. However, this type of therapeutic approach is still in its earlier stage and needed to be further improved before being widely used in the clinic. The aspects that need consideration include targeting, sources, combined therapy, and clinical consideration as follows.

Targeting

As we stated earlier, normal cells also express CD47 molecule and thus could also get exposed to anti-CD47 antibody, when treated. This poses the challenge to target anti-CD47 antibody against tumor cells alone, sparing normal cells. In addition, the CD47-expressing normal cells, such as red blood cells, may cause “antigen sink” preventing the CD47 antibody from reaching the intended tumor cells.[82] To solve this problem, Dheilly et al.[83] developed a bispecific antibody by combining an anti-CD19 (tumor-specific membrane molecule) or mesothelin antibody with the anti-CD47 antibody, forming a bispecific κλ body, which showed a better therapeutic effect under both in vitro and in vivo assessments. Non-Hodgkin's lymphoma is the common adult hematological cancer in which the involved lymphocytes (B-cells) express CD20 as a specific surface antigen. Rituximab is an anti-CD20 mAb used as the first-line drug to treat this disease. Piccione et al.[82] studied the antitumor effect of bispecific antibody, by combining rituximab with the anti-CD47 mAb, in non-Hodgkin's lymphoma. The results showed that this combination could significantly decrease the antigen sink, by lowering the deposition of antibody on the surface of RBCs. Further,in vivo assessment of this bispecific antibody showed a higher inhibition of lymphoma burden along with longer overall survival time in human non-Hodgkin's lymphoma-grafted mice. Weiskopf et al.[84] conducted a similar study on canine diffuse large B-lymphoma, using bispecific antibody (anti-CD47 and anti-CD20 antibodies) approach, demonstrating a potent synergistic effect of the combination. The above results indicate that the bispecific antibody might be a potential solution to solve the targeting issues, and a promising approach for clinical translation.

Sources

One of the critical factors affecting antibody-mediated therapy is its source, the host incompatibility of which could induce immune response resulting in its rejection. The commonly studied anti-CD47 antibody is of mouse origin, such as B6H12. Since mouse is the common study model, seldom rejection of the antibody occurs, and is thus not addressed in such studies. However, if these antibodies were to be used in a human clinical trial, immune reaction to this mouse-origin antibody could be induced. Therefore, it is essential to develop a humanized antibody or a modified antibody to substitute the mouse-origin one. Liu et al.[85] first generated humanized anti-CD47 antibody (Hu5F9-G4) by transferring a complementarity-determining region of the mouse-origin antibody (5F9) with human IgG4. The affinity of this humanized antibody was compared to that of mouse-origin one, which showed no significant difference in antitumor effect between the two. The antitumor effect of the humanized antibody was confirmed in both cellular and animal models of human AML. In 2016, Zeng et al.[86] used a phage display technology to isolate a fully humanized anti-CD47 antibody, ZF1. This fully humanized antibody (ZF1) had a similar affinity to tumor cells as that of mouse-origin anti-CD47 antibody (B6H12). Tumor burden was effectively relieved by ZF1 in leukemic cells (CCRF and U937) engrafted in leukemia animal models. This study lays a preclinical foundation for the application of ZF1 in future leukemia therapy.

Combination therapy

Chemotherapy drugs are frequently used as the first line of cancer treatment in the clinics. However, a long-time repeated use of the chemotherapy drugs may result in drug resistance and thus therapeutic failure. Immunotherapy, by targeting the immune checkpoint, has become a novel and promising strategy to tackle cancer. Combination therapy with molecular targeting drugs and traditional chemotherapy drugs may give a better therapeutic effect. Cytarabine is the first-line drug for acute leukemia in the clinic. Wang et al.[56] showed that, when combined, anti-CD47 antibody significantly increase the antitumor effect of cytarabine on acute leukemia, as compared to single cytarabine or anti-CD47 antibody treatment. Furthermore, the “order of administration” of the chemotherapy drug and the anti-CD47 antibody needs a careful consideration. Liu et al.[52] investigated the relationship of drug order and antitumor effect. They found that when the lymphoma-transplanted mice were given cyclophosphamide or paclitaxel 1 day before anti-CD47 antibody treatment, it would make better tumor therapy than either single anti-CD47 antibody or chemotherapy treatment given after the anti-CD47 antibody administration. This indicates that the chemotherapy before anti-CD47 antibody treatment could enhance the antitumor effect of the combination. They also found that changing treatment order could not induce a significant inhibition of tumor cell growth, indicating that a different order of treatment with chemotherapy and anti-CD47 antibody could even generate an opposite effect. These results suggest future investigators to focus on the interaction of traditional chemotherapy drug and a polyclonal antibody to find the best treatment schedule suitable for different cancer conditions. On the other side, for patients with tumors that cannot be fully resected, cytoreductive surgery is of choice to relieve the tumor burden and to enhance the chemotherapeutic effect. Zhu et al.[87] found that the injection of the anti-CD47 antibody after cytoreductive surgery could significantly extend the overall survival time of the glioblastoma xenograft-engrafted nude rats, from 69 days to 81.5 days. At the same time, more CD68-expressing macrophages were recruited, with more anti-inflammatory factors being produced. This study indicated that the combination of surgery and anti-CD47 antibody needs to be taken into consideration in future tumor therapy.

Clinical trials on anti-CD47 antibody

Before entering into full clinical trial, the safety and efficacy of the new drug must be carefully evaluated using a preclinical animal model. Pietsch et al.[88] found that, in primate animal model, the treatment with a higher dose (1 mg/kg) of the anti-CD47 antibody could cause 40% decrease in hemoglobin within RBCs, suggesting that higher dose of the anti-CD47 antibody might be toxic. Finding the therapeutic window and the best dosage is essential for a new drug study. Currently, five new clinical trials assessing cancer therapy using an anti-CD47 mAb are in progress, as follows: NCT02216409, NCT02678338, NCT02367196, NCT02641002, and NCT02663518.[89] The clinical application of anti-CD47 antibody might be hopefully established in the next few years, after the completion of these clinical trials.


  Conclusion Top


CD47–SIRPα interaction acts as an innate immune checkpoint, offering a promising target for cancer immune therapy. The problems associated with using an anti-CD47 antibody as an anti-cancer drug include: the lack of successful clinical trials, the specificity of tumor targeting, and the uncompleted defined pathway of CD47-mediated tumor immune escaping. Therefore, there is a long way ahead before an ideal therapeutic strategy could be launched.

Financial support and sponsorship

This work was supported by grants from National Natural Science Foundation of China (No. 81272230, 81550030) and CAMS Initiative for Innovative Medicine (CAMS-I2M: 2017-I2M-3-007).

Conflicts of interest

There are no conflicts of interest.



 
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