Elsevier

European Journal of Cancer

Volume 76, May 2017, Pages 100-109
European Journal of Cancer

Review
Cancer immunotherapy targeting the CD47/SIRPα axis

https://doi.org/10.1016/j.ejca.2017.02.013Get rights and content

Abstract

The success of cancer immunotherapy has generated tremendous interest in identifying new immunotherapeutic targets. To date, the majority of therapies have focussed on stimulating the adaptive immune system to attack cancer, including agents targeting CTLA-4 and the PD-1/PD-L1 axis. However, macrophages and other myeloid immune cells offer much promise as effectors of cancer immunotherapy. The CD47/signal regulatory protein alpha (SIRPα) axis is a critical regulator of myeloid cell activation and serves a broader role as a myeloid-specific immune checkpoint. CD47 is highly expressed on many different types of cancer, and it transduces inhibitory signals through SIRPα on macrophages and other myeloid cells. In a diverse range of preclinical models, therapies that block the CD47/SIRPα axis stimulate phagocytosis of cancer cells in vitro and anti-tumour immune responses in vivo.

A number of therapeutics that target the CD47/SIRPα axis are under preclinical and clinical investigation. These include anti-CD47 antibodies, engineered receptor decoys, anti-SIRPα antibodies and bispecific agents. These therapeutics differ in their pharmacodynamic, pharmacokinetic and toxicological properties. Clinical trials are underway for both solid and haematologic malignancies using anti-CD47 antibodies and recombinant SIRPα proteins. Since the CD47/SIRPα axis also limits the efficacy of tumour-opsonising antibodies, additional trials will examine their potential synergy with agents such as rituximab, cetuximab and trastuzumab. Phagocytosis in response to CD47/SIRPα-blocking agents results in antigen uptake and presentation, thereby linking the innate and adaptive immune systems. CD47/SIRPα blocking therapies may therefore synergise with immune checkpoint inhibitors that target the adaptive immune system. As a critical regulator of macrophage phagocytosis and activation, the potential applications of CD47/SIRPα blocking therapies extend beyond human cancer. They may be useful for the treatment of infectious disease, conditioning for stem cell transplant, and many other clinical indications.

Introduction

The field of immuno-oncology has rapidly translated hypothetical concepts into clinical strategies, yielding a new era of cancer investigation and treatment. A large emphasis has been placed on therapies that stimulate the adaptive immune system to attack cancer, in particular T cells. This is rightfully due to the success of immune checkpoint inhibitors targeting CTLA-4 and the PD-1/PD-L1 axis, which disable inhibitory signals to T cells and generate anti-tumour responses. However, both adaptive and innate immune cells are endowed with specialised functions to eliminate pathogens, and many of these functions can be redirected against tumours. Cells of the myeloid lineage are the most abundant immune cells in the body, yet few immunotherapeutic approaches have been aimed at stimulating them to attack cancer. Macrophages in particular have remarkable potential as mediators of anti-cancer therapies based on their robust ability to perform phagocytosis. However, macrophages have a complex relationship with tumours, and in many cases they may promote tumour growth or metastasis. The CD47/signal regulatory protein alpha (SIRPα) axis is a critical molecular interaction that inhibits the activation of macrophages and other myeloid cells against tumours and thereby acts as a myeloid-specific immune checkpoint (Fig. 1). Therapies targeting the CD47/SIRPα axis have demonstrated success in a wide range of preclinical models and are now under investigation in clinical trials for both solid and haematologic malignancies. The CD47/SIRPα axis has emerged as one of the most promising new targets for immuno-oncology.

Section snippets

Discovery of CD47 as a ‘marker of self’

CD47 is a 50 kDa multipass transmembrane protein with an extracellular region composed of a single immunoglobulin superfamily (IgSF) domain. Early studies described CD47 as a molecule expressed by a variety of ovarian cancers [1], a cell-surface protein that interacted with integrins on haematopoietic cells [2] and a glycoprotein on the surface of red blood cells [3]. A unifying theme to these studies and subsequent investigations was widespread expression of CD47 on both normal and malignant

Molecular characterisation of the CD47/SIRPα interaction

Extensive biophysical characterisation of the CD47/SIRPα interaction has been performed, including crystallographic analysis of the extracellular domain of SIRPα alone an in complex with CD47 [10], [11]. These studies have demonstrated that the distal, N-terminal domain of SIRPα is responsible for contacting CD47 [11], [12]. SIRPα was first characterised as a receptor tyrosine kinase that associates with the inhibitory phosphatases SHP-1 and SHP-2 [13]. SIRPα has three extracellular IgSF

The CD47/SIRPα axis in cancer

Expanding on knowledge of CD47 in bone marrow engraftment, the functional role of CD47 in protecting haematopoietic stem cells (HSCs) was further examined. HSCs intermittently enter into a migratory phase during which they exit the bone marrow and circulate systemically through the blood stream [23]. These migratory HSCs upregulate CD47 on their surface to avoid removal by macrophages of the reticuloendothelial system [23]. A similar mechanism is employed by acute myelogenous leukaemia (AML)

Mechanisms of action

Throughout these studies, the predominant mechanism of action investigated has been macrophage activation. In vitro, CD47 blockade stimulates macrophage phagocytosis of cancer cells, a finding that has been extensively validated by microscopy and flow cytometry [24], [25], [28], [31], [33]. In vivo, treatment of mice with liposomal clodronate—which depletes macrophages—abrogates the tumouricidal effects of CD47-blocking therapies, indicating macrophage are required for robust anti-tumour

Approaches to targeting the CD47/SIRPα axis

A variety of therapeutics targeting the CD47/SIRPα axis are under preclinical and clinical investigation, including conventional antibodies, recombinant polypeptides, and bispecific molecules (Table 1). For each of these agents, there exist benefits and tradeoffs for successful translation to the clinic. Special considerations pertain to each agent's efficacy, toxicity, and pharmacokinetic and pharmacodynamic properties.

To date, anti-CD47 antibodies are the best characterised therapies

Combining CD47/SIRPα blockade with tumour-opsonizing antibodies

Although CD47-blocking antibodies are sufficient to induce phagocytosis as single agents, macrophage phagocytosis is dependent on the integration of pro-phagocytic (‘eat me’) and anti-phagocytic (‘don't eat me’) signals. CD47 acts as a predominant inhibitory signal that prevents macrophage phagocytosis of cancer cells. A number of pro-phagocytic signals have also been described, including intrinsic molecules that contribute to immunological cell death such as phosphatidylserine and calreticulin

Considerations for clinical development of CD47/SIRPα-blocking therapies

As described above, one of the greatest distinctions between agents targeting the CD47/SIRPα interaction is whether they act as a ‘monotherapy’ against cancer or an ‘adjuvant’ to tumour-binding antibodies. Any agent that contains an Fc region that engages activating Fc receptors on macrophages has the potential to act as a monotherapy for cancer. These therapies include anti-CD47 antibodies and SIRPα-Fc fusion proteins. As a class, they pose a greater risk for on-target toxicity to normal cells

The CD47/SIRPα axis and the adaptive immune system

Beyond activation of innate immune cells, mounting evidence suggests CD47 blockade can also initiate or augment adaptive immune responses. In a mouse ovalbumin (OVA) tumour model, treatment with anti-CD47 antibodies resulted in greater phagocytosis and antigen presentation [34]. Transfer of macrophages that had engulfed tumour cells into immunocompetent mice was able to protect the mice from a subsequent challenge with B16 cells expressing OVA [34]. These results suggested CD47 blockade could

Additional applications of CD47/SIRPα-blocking therapies

The principles of targeting the CD47/SIRPα axis can also be applied to other non-malignant states of disease. In theory, diseased cells may be more susceptible to macrophage phagocytosis, and any therapeutic antibody that depletes cells by engaging immune effector functions could be enhanced by combination with CD47-blocking agents. Possible combinations include those with antibodies targeting persistently infected cells, such as broadly neutralising HIV antibodies [69], [70]. Therapeutic

Conclusions

Myeloid cells hold much promise as effectors of cancer immunotherapy, and therapeutic targeting of the CD47/SIRPα axis may unlock their potential. Preclinical studies indicate CD47/SIRPα-blocking therapies are effective against a broad range of cancers, and ongoing clinical trials will determine their efficacy as single agents and as combination therapies. The application of these therapeutics extends beyond human cancer, and additional preclinical and clinical studies will reveal their benefit

Conflict of interest statement

K.W. is an inventor of US patent applications pertaining to CD47-blocking assigned to Stanford University. K.W. declares consulting and/or equity ownership in Alexo Therapeutics, Inc. and Forty Seven Inc.

Acknowledgements

The author thanks I.L. Weissman, members of the Weissman laboratory, E. Weiskopf and C. Weiskopf for support and helpful discussions. The author declares no funding sources or writing assistance for this manuscript.

References (76)

  • S. Jaiswal et al.

    CD47 is upregulated on circulating hematopoietic stem cells and leukemia cells to avoid phagocytosis

    Cell

    (2009)
  • R. Majeti et al.

    CD47 is an adverse prognostic factor and therapeutic antibody target on human acute myeloid leukemia stem cells

    Cell

    (2009)
  • M.P. Chao et al.

    Anti-CD47 antibody synergizes with rituximab to promote phagocytosis and eradicate non-Hodgkin lymphoma

    Cell

    (2010)
  • M. Ngo et al.

    Antibody therapy targeting CD47 and CD271 effectively suppresses melanoma metastasis in patient-derived xenografts

    Cell Rep

    (2016 Aug 9)
  • T. Yi et al.

    Splenic dendritic cells survey red blood cells for missing self-CD47 to trigger adaptive immune responses

    Immunity

    (2015)
  • E.J. Brown et al.

    Integrin-associated protein (CD47) and its ligands

    Trends Cell Biol

    (2001)
  • J.S. Isenberg et al.

    CD47 is necessary for inhibition of nitric oxide-stimulated vascular cell responses by thrombospondin-1

    J Biol Chem

    (2006)
  • V. Mateo et al.

    Mechanisms of CD47-induced caspase-independent cell death in normal and leukemic cells: link between phosphatidylserine exposure and cytoskeleton organization

    Blood

    (2002)
  • Y. Kikuchi et al.

    Apoptosis inducing bivalent single-chain antibody fragments against CD47 showed antitumor potency for multiple myeloma

    Leuk Res

    (2005)
  • Y. Wu et al.

    Phosphatidylserine recognition by phagocytes: a view to a kill

    Trends Cell Biol

    (2006)
  • A.M. Anniss et al.

    Expression of CD47 (integrin-associated protein) decreases on red blood cells during storage

    Transfus Apher Sci

    (2002)
  • D. Ito et al.

    Canine lymphoma as a comparative model for human non-Hodgkin lymphoma: recent progress and applications

    Vet Immunol Immunopathol

    (2014)
  • S.M. Rue et al.

    Identification of a candidate therapeutic antibody for treatment of canine B-cell lymphoma

    Vet Immunol Immunopathol

    (2015)
  • L.G. Poels et al.

    Monoclonal antibody against human ovarian tumor-associated antigens

    J Natl Cancer Inst

    (1986)
  • E. Brown et al.

    Integrin-associated protein: a 50-kD plasma membrane antigen physically and functionally associated with integrins

    J Cell Biol

    (1990)
  • W.J. Mawby et al.

    Isolation and characterization of CD47 glycoprotein: a multispanning membrane protein which is the same as integrin-associated protein (IAP) and the ovarian tumour marker OA3

    Biochem J

    (1994)
  • P.A. Oldenborg et al.

    Role of CD47 as a marker of self on red blood cells

    Science

    (2000)
  • B.R. Blazar et al.

    CD47 (integrin-associated protein) engagement of dendritic cell and macrophage counterreceptors is required to prevent the clearance of donor lymphohematopoietic cells

    J Exp Med

    (2001)
  • K. Takenaka et al.

    Polymorphism in Sirpa modulates engraftment of human hematopoietic stem cells

    Nat Immunol

    (2007)
  • A. Kharitonenkov et al.

    A family of proteins that inhibit signalling through tyrosine kinase receptors

    Nature

    (1997)
  • A.N. Barclay et al.

    The SIRP family of receptors and immune regulation

    Nat Rev Immunol

    (2006)
  • S. Adams et al.

    Signal-regulatory protein is selectively expressed by myeloid and neuronal cells

    J Immunol

    (1998)
  • K. Weiskopf et al.

    Myeloid cell origins, differentiation, and clinical implications

    Microbiol Spectr

    (2016)
  • A.N. Barclay et al.

    The interaction between signal regulatory protein alpha (SIRPalpha) and CD47: structure, function, and therapeutic target

    Annu Rev Immunol

    (2014)
  • R.K. Tsai et al.

    Inhibition of “self” engulfment through deactivation of myosin-II at the phagocytic synapse between human cells

    J Cell Biol

    (2008)
  • M.P. Chao et al.

    Therapeutic antibody targeting of CD47 eliminates human acute lymphoblastic leukemia

    Cancer Res

    (2011)
  • D. Kim et al.

    Anti-CD47 antibodies promote phagocytosis and inhibit the growth of human myeloma cells

    Leukemia

    (2012)
  • S.B. Willingham et al.

    The CD47-signal regulatory protein alpha (SIRPa) interaction is a therapeutic target for human solid tumors

    Proc Natl Acad Sci U S A

    (2012)
  • Cited by (0)

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