By: Nicholas Garaffo, Biochemistry and Molecular Biology, 20’
Authors’ Note: I originally wrote this piece for my UWP 104E class Writing in the Science’s, but I have since expanded my topic and complicated my original analysis. Ultimately, I submitted this piece to the Norman J. Lang Prize, was awarded second place, and presented my research to the UC Davis college deans. I chose to focus my literary review on cell signaling pathways because I hope to study such topics in my PhD. This topic has impacted my life personally because my grandmother was diagnosed with non-hodgkin’s lymphoma my freshman year of college. In fact, during this review the drugs she was treated with were mentioned, and the CD47-SIRPa pathway may actually be used to treat such a disease.
According to the American Cancer Institute, in 2018, cancer had an estimated 1,735,350 new cases and 609,640 people died in the United States alone1. Like many deadly diseases, cancer has found ways to evade the immune system. Many cancers overexpress CD47—a widely expressed “don’t eat me” signal—which interacts with the immune cells’ signal receptor protein alpha (SIRPα), to prevent programmed cell removal (PrCR) 2. ‘Don’t eat me’ signals are a class of cell surface proteins that tell the immune system the corresponding cell is healthy and performing properly. Recent advances have been made to target the CD47-SIRPα pathway to prevent the antiphagocytic activity seen in many cancers. The scope of this review is limited to two new methods used to inhibit the CD47-SIRPα pathway: anti-CD47 and SIRPα antibodies, and small peptide inhibitors. The antibodies for CD47 have shown effectiveness in clinical trials. Antibody inhibition for CD47 and SIRPα were compared, and SIRPα produced better cell type specific inhibition, but similar on-target healthy cell phagocytosis caused anemia in both trials. Several factors, including degradation and inability to penetrate dense tumors, hinder antibody treatment in all cancer patients; therefore, small peptide inhibitors offer an alternate route for inhibition to occur.
PrCR is an efficient and accurate process that clears dead, dying, or infectious cells. Phagocytic macrophages—neutrophils, dendritic cells and monocyte derivatives—perform PrCR, and acts independently of apoptosis—programmed cell rupture. Without such processes apoptosis would release cellular contents, such as proinflammatory signals, into the extracellular space3, 4. Such signals can activate inflammatory responses leading to organ and tissue damage. Cells that are under oxidative stress release chemotactic factors that attract immune cells4. Once the macrophage locates the infected cell, it recognizes the cell through “don’t eat me” or “eat me” ligands to prevent or induce cell engulfment, respectively. The scope of this review is limited to a single signal—receptor interaction between CD47—a widely expressed transmembrane protein5—and signal receptor protein alpha (SIRPα)—a receptor expressed on phagocytic immune cells. CD47 links to SIRPα and acts as a “don’t eat me” signal to prevent cellular phagocytosis.
Macrophages activate specific transcription factors in response to environmental cues. Notch signaling describes the macrophages’ internal protein cascade upon receptor-ligand interactions. The macrophage responds by adjusting its polarization into either phagocytic, categorized as the M1 polarization, or non-phagocytic (M2)6. This is important because a macrophages’ phenotype is environmentally dependent on surrounding cell signals, and plays a critical role in PrCR. Upon binding CD47, SIRPα initiates a signal transduction via src homology-2 domain recruitment, a large protein complex. This complex importantly contains two tyrosine phosphatases: SHP-1 and SHP-2, which both interact with various proteins for signaling. Once activated, SHP-1 propagates a downstream antiphagocytic signal (M2) through an unknown mechanism2, 7, 8. Naturally, this ensures macrophages do not engulf healthy cells. In fact, a single CD47-SIRPα interaction is capable of preventing phagocytosis9.
One mechanism cancer uses to evade the immune system is through the CD47-SIRPα pathway. For cancer to propagate it must: prevent apoptosis, divide rapidly, and evade the immune system11. Many cancers overexpress CD47 and it is hypothesized that CD47 accumulation acts as a camouflage. Since CD47 is sufficient to prevent PrCR of healthy cells, when cancers overexpress this signal they can effectively prevent phagocytic clearance. Therefore, inhibiting the CD47-SIRPα pathway is a favorable route for therapeutics2. Efforts have been made to target CD47 and SIRPα individually through monoclonal antibodies (mAb) and high-affinity small peptides. These methods, coupled with known cancer therapeutics like Rituximab, have been shown to decrease tumor cell density in vitro, in vivo, and in clinical trials14. The main goal here is to assess the potential adverse effects presented in each therapeutic. Major hurdles include the potential for other phagocytic inhibitors, off-target effects, and the lack of long-term effects.
Antibody targeting of CD47 and SIRPα shows inhibition of anti-phagocytic signaling
Antibody targeting of CD47 is an effective therapeutic for specific cancers. Acute myelogenous leukemia (AML) is maintained by self-renewing leukemia stem cells (LSC) which evade phagocytosis through increased CD47 expression2, 4. By targeting CD47, researchers hope to activate a focused immune response against tumor cells. Both, in vitro and in vivo analysis of an anti-CD47 antibody (B6H12.2) in an AML LSC model reported a 3-5 fold increase in phagocytosis compared to macrophages and tumor cells alone12. In contrast, an anti-SIRPα antibody reported an increased phagocytosis only when coupled with trastuzumab—a known breast cancer therapeutic10. This contradiction is important, firstly, because it shows antibodies alone are insufficient to increase phagocytosis. Secondly, it hypothesizes other “don’t eat me” signals continue to inhibit phagocytosis after the CD47-SIRPα has been blocked. Lastly, it shows two alternate ways to inhibit the CD47-SIRPα pathway. The anti-SIRPα antibody is argued as a favored cancer therapeutic because CD47 is widely expressed across cell types. Targeting CD47 may cause unwanted on-target CD47 phagocytosis. Despite this possibility, an in vivo analysis of B6H12.2 reported no additional phagocytic activity even with equivalently coated cells4. However, therapeutic exposure only lasted 14 days and animal models were sacrificed afterwards; therefore, long term effects have not been assessed.
Anti-CD47 antibody development towards human variant
A limitation to antibody therapeutics is inter-species variation. B6H12.2s’ affinity decreased from mice to humans due to CD47 variation. Therefore, a human anti-CD47 antibody (5F9) was produced and grafted to immunoglobulin G4 scaffold (IgG4)13. The resulting antibody (Hu5F9-G4) was tested in vitro for its affinity towards human CD47 and
revealed strong attraction, illustrated by the incredibly small amount of dissociation betweenCD47-SIRPa (KD=1×10-12). Hu5F9-G4 was further tested in cynomolgus monkeys to assess potential toxicity in a human-like model. No serious adverse events were characterized except dose dependent anemia which was expected due to the high CD47 expression on red blood cells and reverted naturally after antibody treatment2. However, using healthy monkeys was a limitation to this study; tumor cell phagocytosis was not assessed in vivo. Furthermore, the toxic effects were only tested in a three week period and no long-term effects were characterized.
Clinical trials for the human CD47 antibody variant
Clinical trials of Hu5F9-G4 antibody coupled with rituximab are currently being conducted. Toxicity and effectiveness were assessed in 22 patients with aggressive and indolent lymphoma (this can be thought of as metastatic and benign cancer, respectively)14. From this sample, 50% had an objective response and 36% had a complete response. Furthermore, by day 28, white and red blood cells had approximately 100% of their CD47 receptors occupied. This is important because blocking all CD47-SIRPα interactions is needed for effective results and, since all cells are not degraded, other signals must be preventing phagocytosis in healthy cells9. As seen in other animal models, dose-dependent anemia was the most common side-effect but normal levels of red blood cells reverted at lower dosages or after the treatment period2. This coupled treatment showed promising results for patients with aggressive and indolent lymphoma.
High-affinity small peptides as an alternate CD47-SIRPα inhibitor
Another issue with antibody therapeutics is their poor permeability into dense tumors15. Given this hurdle, an alternate route is small peptide inhibitors against the CD47-SIRPα pathway. By antagonizing CD47 or SIRPα, the small peptides should block any anti-phagocytic signaling and allow PrCR to occur. Small peptides are highly specific antagonists modeled after invariable regions of their target. By analyzing the human SIRPαs’ binding domain, a competitive antagonist for human CD47 was produced16. The high-affinity SIRPα monomer (CV1) was tested in vitro to assess its affinity towards human and mouse CD47. CV1 presented the same inhibition between human and mouse CD47 variants (50,000-fold affinity increase and KD=34.0 pm). Since small peptides are modeled after invariable regions, their affinities are similar between species. This is important because affinity testing for humans can now be estimated through animal models; thereby, eliminating toxic and costly human trials. Furthermore, ex vivo co-treatment of CV1 with anti-Her2/neu—a well studied breast cancer antibody—increased phagocytosis of human breast cancer cells compared to anti-Her2/neu alone. This coupled treatment was tested in vivo and revealed increased anti-tumor responses in a mouse breast cancer model. Co-treatment illustrates the possibility for more “don’t-eat-me” signals present on cancer cells. Despite CV1s’ efficacy, its high affinity caused on-target CD47 binding across all cell types. Although this high-affinity is wanted in therapeutics, unwanted red blood cell phagocytosis occurred and resulted in anemia. This side-effect, however, is common between all CD47 inhibitors and naturally reverted after treatment16,17.
A solution to CD47 on-target side-effects is antagonizing SIRPα instead. CD47 is expressed widely across cell lines, while SIRPα is present on a subset of macrophages; therefore, SIRPα is arguably the favored target for cancer therapeutics10. One potential SIRPα antagonist, which showed similar potency as CV1, is Velcro-CD47– a high-affinity CD47 variant synthesized through a novel protein “velcro” technique9. Through in vitro analysis, Velcro-CD47 enhanced mAb-mediated phagocytosis by inhibiting anti-phagocytic signals. It is important to note that the small peptide inhibitors do not, by themselves, promote phagocytosis. While antibodies illicit a targeted immune response, small peptides rely on the immune systems’ natural clearance or other cancer therapeutics to clear cancer cells.
Other small peptide therapeutics for CD47-SIRPα inhibition include 4N1K and its derivative PKHB1. There has been substantial evidence that 4N1K increases PrCR in vivo18,19,20,21. Several papers highlight a difference between CD47 +/+ and CD47 -/- tumor cells removal upon 4N1K treatment22. Unlike B6H12/Hu5F9, 4N1K is able to potentiate PrCR of chronic lymphocytic leukemia (CLL) in soluble conditions; however, in human serum, 4N1K is degraded by proteases faster than antibodies-more than 90% was degraded in an 1-hour incubation18. This therapy, therefore, requires more injections for an accurate response. Furthermore, 4N1K has conflicting evidence for its CD47 specificity, and may cause off-target effects23. In order to combat these issues, two terminal residues were replaced on 4N1K with their D analogues. This new therapeutic, PKHB1, lasted longer in human serum, maintained its solubility, and continued to bind CD47. PKHB1 was then tested in vivo and showed higher rates of CLL PrCR 18. PKHB1 is currently in pre-clinical trials for CLL treatment.
Cancer therapeutics continue to progress towards more accurate and less toxic forms. In turn, this eliminates the need for deleterious options like chemotherapy. CD47-SIRPα presents a target for future immunological therapeutics. Although anemia and off-target effects must be further assessed, CD47-SIRPα inhibitors present a feasible and effective option.
Anti-CD47 antibodies increase tumor cell phagocytosis in a coupled therapy with Rituximab and show accurate responses in Phase I clinical trials14. In response to phagocytosis of healthy CD47-expressing cells, anti-SIRPα antibodies have been developed which illustrated similar phagocytic responses in vivo with higher cell type specificity10. Regardless of the target, blocking the CD47-SIRPα pathways still cause anemia in patients. This is an expected and treatable side-effect that naturally reverts after a short-term treatment. No long-term effects of antibody treatments have been assessed and remains a limitation to these studies. To combat the limitations seen in antibody treatments, small peptide inhibitors are being developed for the CD47-SIRPα pathway. Velcro-CD47 presented a novel protein manufacturing technique and provided a high-affinity peptide to prevent inhibitor signals16. 4N1K has been shown to increase tumor cell phagocytosis between CD47 +/+ and CD47-/- but small peptide inhibitors are hindered by their short half-life in blood serum due to protease activity.
By blocking the CD47-SIRPα pathway and other inhibitor signals, researchers can trigger a natural immune clearance of cancer cells. Although differences between CD47 and SIRPα therapeutics, long-term effects, and 4N1K off-target effects must be further assessed, preliminary research indicates this pathway as a potential target for future therapeutics.
- “Cancer Statistics”. National Cancer Institute, 2019, https://www.cancer.gov/about-cancer/understanding/statistics.
- Oldenborg PA, Zheleznyak A, Fang Y, Lagenaur CF, Gresham HD, Lindberg FP. “Role of CD47 as a Marker of Self on Red Blood Cells.” Science. 2001;288: 2051- 54
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