- Email: [email protected]
MerTK Blockade Fuels Anti-tumor Immunity
Immunity
Previews MerTK Blockade Fuels Anti-tumor Immunity J. Magarian Blander1,*
1The Jill Roberts Institute for Research in Inflammatory Bowel Disease, Joan and Sanford I. Weill Department of Medicine, Department of Microbiology and Immunology, Sandra and Edward Meyer Cancer Center, Immunology and Microbial Pathogenesis Program, Weill Cornell Graduate School of Medical Sciences, Weill Cornell Medicine, Cornell University, New York, NY 10021, USA *Correspondence: [email protected] https://doi.org/10.1016/j.immuni.2020.01.015
Phagocytosis of apoptotic cells via the receptor MerTK is important for immune tolerance. In this issue of Immunity, Zhou et al. report that blockade of MerTK-mediated phagocytosis mobilizes anti-tumor immunity through a mechanism that involves the transport of tumor-derived cGAMP into macrophages via the ATPactivated channel P2X7R. The landscape of cancer immunotherapy is dominated by strategies to enhance T cell-mediated anti-tumor immunity. Chimeric antigen receptor (CAR) T cell immunotherapies comprised of genetically engineered T cells that attack cancer cells and immune-checkpoint inhibitors that reverse T cell exhaustion have achieved encouraging statistics of cancer remission and patient survival (O’Donnell et al., 2019). Harnessing the innate immune system against cancer is of much interest, in part when considering synergizing with adaptive immunity as well as intrinsic anti-tumor effects of innate immune cells. In this issue of Immunity, Zhou et al. (2020) report that systemic administration of an antibody to the receptor tyrosine kinase MerTK blocked efferocytosis by tumor-associated macrophages (TAMs) and stimulated anti-tumor immunity via a mechanism involving the cyclic GMP-AMP synthase (cGAS) and stimulator of inhibitor genes (STING) pathway. These findings open new avenues for exploiting innate immune processes in the fight against cancer (Figure 1). Resistance to cell death is a hallmark of cancer, ensuring survival of transformed cells as well as escape from phagocytosis. During early tumorigenesis, however, cell death shapes the emergence of more aggressive clones, inhibits immune surveillance, and drives tumor evolution toward malignant disease (Labi and Erlacher, 2015; Pitt et al., 2017). Zhou et al. hypothesized that blockade of efferocytosis in the tumor microenvironment would mediate a switch from apoptosis, an immunologically silent form of cell death, to immunogenic cell death of tumor cells. Uncleared apoptotic cells un-
dergo secondary necrosis and engage the innate immune system by releasing damage-associated molecular patterns (DAMPs) such as DNA, ATP, and highmobility-group box 1 protein (HMGB-1). Zhou et al. (2020) generated an antibody to the phagocytic receptor MerTK and showed that it blocked MerTK-mediated efferocytosis. MerTK presented an attractive therapeutic target because it was selectively expressed by TAMs, but not by intratumoral dendritic cells (DCs), and its expression across different human cancers correlated with an abundance of TAMs over other immune cells. The authors showed that systemic administration of anti-MerTK antibody to mice bearing MC38 colon carcinoma cells increased intratumoral apoptotic cell numbers soon after injection and was associated with circulating cell-free tumor-derived DNA indicative of lytic tumor cell death. When administered at early stages of tumor progression, anti-MerTK antibody significantly hindered tumor growth and conferred complete protection against tumor challenge, but its efficacy rested on the ability of tumor cells to undergo apoptosis. When tested on fully established tumors, however, antiMerTK antibody was only effective at preventing tumor growth when combined with immune checkpoint inhibitors antiPD-L1 or anti-PD-1. Established tumors may have developed resistance to cell death precluding apoptotic cell accumulation upon MerTK blockade. The efficacy of combining anti-PD-L1 or anti-PD-1 treatment with MerTK blockade could reflect improvement of the functionality of tumor infiltrating myeloid cells (Strauss et al., 2020), relief of the ‘‘don’t eat me’’ signal that tumor PD-L1 sends to TAMs
212 Immunity 52, February 18, 2020 ª 2020 Elsevier Inc.
preventing live tumor cell phagocytosis (Feng et al., 2019), or the overcoming of CD8+ T cell exhaustion (O’Donnell et al., 2019). By combining anti-MerTK and anti-PD-1 antibodies with Gemcitabine, a cytotoxic chemotherapeutic, the authors achieved an impressive tumor regression in all of the treated mice. The authors then examined the mechanistic underpinnings of enhanced antitumor immunity upon MerTK blockade. They found that TAMs exhibited a type-I interferon (IFN) signature soon after antiMerTK treatment and were the major source of type-I IFN. Despite systemic administration of anti-MerTK, the type-I IFN response was restricted to the tumor, and it was critical to the anti-tumor response as MerTK blockade was ineffective in tumor-bearing mice deficient for IFNAR, the receptor for type-I IFN. The type-I IFN signature made it likely that cross-priming of tumor-specific CD8+ T cells would be increased, and indeed, the authors found an increased frequency of proliferating intratumoral CD8+ T cells upon MerTK blockade. They also noted a significant increase in MHC-I presentation of tumor-derived peptide by TAMs, and to a lesser extent CD11b+ DCs, but not intratumoral CD103+ DCs. This was unexpected and left open the possibility of MerTK-independent TAM acquisition of tumor antigen from secondary necrotic cells. In a headto-head comparison in vitro, TAMs outperformed total DCs isolated from tumors in mediating antigen-specific CD8+ T cell activation. However, DCs and not macrophages specialize in cross-presentation and drive anti-tumor CD8+ T cell priming largely due to their migration and antigen trafficking to lymph nodes through which
Immunity
Previews
Figure 1. A Blocking Antibody that Targets MerTK on Tumor-Associated Macrophages Triggers a Cascade of Events that Control Early-Stage Tumors Antibody-mediated blockade of MerTK-dependent phagocytosis of apoptotic tumor cells results in the intratumoral accumulation of apoptotic tumor cells, which then undergo secondary necrosis. Tumor-expressed cGAS and host-expressed STING drive a type-I IFN response confined to the tumor site whereby cGAS in tumor cells produces cGAMP that activates STING in immune cells in a paracrine fashion. Extracellular ATP released by secondary necrotic tumor cells opens the purinergic P2X7 receptor channel on tumor-associated macrophages (TAMs), allowing cGAS entry and STING activation. TAMs acquire a type-I IFN signature and begin to produce IFN-b, while the tumor microenvironment exhibits signs of an inflammatory response to type-I IFN. This sequence of events culminates in the control of tumor progression. Effective anti-MerTK treatment requires responsiveness to type-I IFN through IFNAR, Batf3dependent CD103+ DC, and tumor-specific CD8+ T cells. Its effectiveness in late-stage tumors was contingent on co-administration of a cytotoxic chemotherapeutic agent and an immune checkpoint inhibitor.
naive CD8+ T cells recirculate (Roberts et al., 2016). Consistent with this, the authors found that the anti-tumor effects of MerTK blockade were lost in Batf3–/– mice, which lack the cross-presenting CD103+ DCs, and despite retention of the anti-MerTK-mediated type-I IFN response. CD8+ T cells were as critical as the Batf3-dependent CD103+ DC, because their depletion abrogated the anti-tumor effects of MerTK blockade. It remains to be tested whether intratumoral TAM cross-presentation upon MerTK blockade might enable expansion of tumor-specific CD8+ T cells infiltrating
the type-I IFN-remodeled tumor microenvironment. The cGAS-STING pathway, which responds to cytoplasmic DNA in settings such as viral infection or genomic DNA damage (Ablasser and Chen, 2019), is central to type-I IFN production, and thus, the authors examined its requirement in the context of MerTK blockade. Interestingly, the anti-tumor effects of MerTK blockade were lost in Sting / but not cGas / mice. Through an elegant set of experiments, the authors pinpointed a necessity for the enzymatic activity of cGAS, which produces cyclic
GMP-AMP (cGAMP), in tumor cells. Notably, when tumors were incapable of generating cGAMP, anti-PD-L1 treatment had no effect on early-stage tumors. It is unclear whether basal levels of tumorderived cGAMP play a role even when TAM efferocytosis is intact, perhaps by facilitating cross-priming of CD8+ T cells. cGAS deficiency is not expected to affect the release of DAMPs such as ATP and HMGB-1, which underlie the immunogenicity of cancer chemotherapy or radiotherapy driven secondary necrosis of tumor cells (Pitt et al., 2017). Yet these DAMPs appeared to take a back seat to cGAMP in the setting of anti-MerTK blockade. How does tumor-derived cGAMP gain access to intracellular STING in responding TAMs? This manner of STING activation in trans is reminiscent of the gapjunction mediated transfer of cGAS from producing to neighboring cells, where it promotes STING activation (Ablasser and Chen, 2019). The authors turned their attention to extracellular ATP, a DAMP present also in the tumor microenviroment; high ATP concentrations open the ligand-gated cation channel P2X7 receptor (P2X7R). They found that cultured macrophages produced type-I IFN only in response to the combination of cGAMP and ATP, and this was blocked by a specific pharmacological inhibitor of P2X7R. ATP enhanced the entry of fluorescent cGAMP into macrophages in a P2X7Rdependent manner, elicited phosphorylation of key mediators of the type-I IFN response, and led to IFN-b production. Consistent with these in vitro studies, anti-MerTK treatment was ineffective in tumor-bearing mice that had been treated with the P2X7R inhibitor or were deficient in P2X7R expression. Interestingly, expression of P2X7R was high in TAMs, consistent with their being the major type-I IFN producers in this setting. Finally, the authors showed that MC38 tumors engineered to overexpress CD39, an ectonucleotidase that hydrolyzes extracellular ATP, no longer displayed a type-I IFN signature and were refractory to anti-MerTK treatment. The authors did not test whether immunosuppressive adenosine, the product of ATP degradation, may also contribute to this loss of responsiveness (Sachet et al., 2017). The findings by Zhou et al. (2020) give pause for thought. They point to cGAMP Immunity 52, February 18, 2020 213
Immunity
Previews as a DAMP in its own right, and one that is released specifically by dying cancer cells and not normal cells, as a proxy of abnormal cGAS engagement by cytosolic chromatin fragments in cancer cells. In the setting of what we know about exploiting immunogenic cell death against cancer, the findings here also suggest that secondary necrotic cells elicit anti-tumor immunity differently than cells undergoing necroptosis, a programmed form of necrosis. Undefined nuclear factor kB (NF-kB)-dependent signals downstream of the RIPK1-RIPK3 necrosome complex that orchestrates necroptosis, but not the release of DAMPs, were important for eliciting anti-tumor immunity through intratumoral injection of necroptotic tumor cells (Snyder et al., 2019). As a reflection of having undergone modifications associated with the process of apoptosis, secondary necrotic cells release inflammatory HMGB-1 irreversibly bound to nucleosomes, while cells undergoing primary necrosis—caused by trauma such as injury or burns—release free HMGB-1 (Sachet et al., 2017). Secondary necrotic cells also release less ATP than cells undergoing primary necrosis. Chemotherapy-induced tumor necrosis secondary to apoptosis elicited anti-tumor immunity through NLRP3-dependent interleukin (IL)-1b and Toll-like receptor 4 (TLR4)-dependent cross-presentation, by apoptotic tumor cell release of ATP and HMGB-1, respectively (Pitt et al., 2017). These differences illustrate how a distinct innate immune response could be engaged depending on the mode of death and the transformed status of the cell—a type-I IFN response, an inflamma-
214 Immunity 52, February 18, 2020
some-dependent IL-1b response, or a necroptosis-dependent NF-kB response. Whether these responses have equivalent anti-tumor efficacy requires future investigations. Side-by-side comparisons of an inflammasome versus type-I IFN response, for example, would have important implications to the therapeutic implementation of NLRP3 versus STING targeted strategies. Lastly, it is worth noting that cancer cells increase the expression of ‘‘don’t eat me’’ signals such as CD47, which interacts with the SIRPa receptor on macrophages to inhibit phagocytosis. In fact, blocking CD47-SIRPa interaction enables the phagocytosis of live cancer cells and is currently being evaluated in phase I and/or II clinical trials in patients with advanced solid and hematological cancers (Feng et al., 2019). These trials aim to accomplish the reverse of anti-MerTK blockade, which is to enhance rather than block tumor cell phagocytosis. AntiCD47 blockade enables the clearance of live tumor cells, while the success of anti-MerTK treatment is contingent on tumor cell apoptosis as a prelude to the secondary necrosis that releases the critical combination of cGAMP with ATP. Nevertheless, therapeutic exploitation of innate immune clearance of dying cancer cells promises to be an exciting new area of cancer immunotherapy.
REFERENCES Ablasser, A., and Chen, Z.J. (2019). cGAS in action: Expanding roles in immunity and inflammation. Science 363.
Feng, M., Jiang, W., Kim, B.Y.S., Zhang, C.C., Fu, Y.X., and Weissman, I.L. (2019). Phagocytosis checkpoints as new targets for cancer immunotherapy. Nat. Rev. Cancer 19, 568–586. Labi, V., and Erlacher, M. (2015). How cell death shapes cancer. Cell Death Dis. 6, e1675. O’Donnell, J.S., Teng, M.W.L., and Smyth, M.J. (2019). Cancer immunoediting and resistance to T cell-based immunotherapy. Nat. Rev. Clin. Oncol. 16, 151–167. Pitt, J.M., Kroemer, G., and Zitvogel, L. (2017). Immunogenic and Non-immunogenic Cell Death in the Tumor Microenvironment. Adv. Exp. Med. Biol. 1036, 65–79. Roberts, E.W., Broz, M.L., Binnewies, M., Headley, M.B., Nelson, A.E., Wolf, D.M., Kaisho, T., Bogunovic, D., Bhardwaj, N., and Krummel, M.F. (2016). Critical Role for CD103(+)/CD141(+) Dendritic Cells Bearing CCR7 for Tumor Antigen Trafficking and Priming of T Cell Immunity in Melanoma. Cancer Cell 30, 324–336. Sachet, M., Liang, Y.Y., and Oehler, R. (2017). The immune response to secondary necrotic cells. Apoptosis 22, 1189–1204. Snyder, A.G., Hubbard, N.W., Messmer, M.N., Kofman, S.B., Hagan, C.E., Orozco, S.L., Chiang, K., Daniels, B.P., Baker, D., and Oberst, A. (2019). Intratumoral activation of the necroptotic pathway components RIPK1 and RIPK3 potentiates antitumor immunity. Sci. Immunol. 4. Strauss, L., Mahmoud, M.A.A., Weaver, J.D., Tijaro-Ovalle, N.M., Christofides, A., Wang, Q., Pal, R., Yuan, M., Asara, J., Patsoukis, N., and Boussiotis, V.A. (2020). Targeted deletion of PD-1 in myeloid cells induces antitumor immunity. Sci. Immunol. 5. Zhou, Y., Fei, M., Zhang, G., Liang, W.-C., Lin, W., Wu, Y., Piskol, R., Ridgway, J., McNamara, E., Huang, H., et al. (2020). Blockade of the Phagocytic Receptor MerTK on Tumor-Associated Macrophages Enhances P2X7R-Dependent STING Activation by Tumor-Derived cGAMP. Immunity 52, this issue, 357–373.
Previews MerTK Blockade Fuels Anti-tumor Immunity J. Magarian Blander1,*
1The Jill Roberts Institute for Research in Inflammatory Bowel Disease, Joan and Sanford I. Weill Department of Medicine, Department of Microbiology and Immunology, Sandra and Edward Meyer Cancer Center, Immunology and Microbial Pathogenesis Program, Weill Cornell Graduate School of Medical Sciences, Weill Cornell Medicine, Cornell University, New York, NY 10021, USA *Correspondence: [email protected] https://doi.org/10.1016/j.immuni.2020.01.015
Phagocytosis of apoptotic cells via the receptor MerTK is important for immune tolerance. In this issue of Immunity, Zhou et al. report that blockade of MerTK-mediated phagocytosis mobilizes anti-tumor immunity through a mechanism that involves the transport of tumor-derived cGAMP into macrophages via the ATPactivated channel P2X7R. The landscape of cancer immunotherapy is dominated by strategies to enhance T cell-mediated anti-tumor immunity. Chimeric antigen receptor (CAR) T cell immunotherapies comprised of genetically engineered T cells that attack cancer cells and immune-checkpoint inhibitors that reverse T cell exhaustion have achieved encouraging statistics of cancer remission and patient survival (O’Donnell et al., 2019). Harnessing the innate immune system against cancer is of much interest, in part when considering synergizing with adaptive immunity as well as intrinsic anti-tumor effects of innate immune cells. In this issue of Immunity, Zhou et al. (2020) report that systemic administration of an antibody to the receptor tyrosine kinase MerTK blocked efferocytosis by tumor-associated macrophages (TAMs) and stimulated anti-tumor immunity via a mechanism involving the cyclic GMP-AMP synthase (cGAS) and stimulator of inhibitor genes (STING) pathway. These findings open new avenues for exploiting innate immune processes in the fight against cancer (Figure 1). Resistance to cell death is a hallmark of cancer, ensuring survival of transformed cells as well as escape from phagocytosis. During early tumorigenesis, however, cell death shapes the emergence of more aggressive clones, inhibits immune surveillance, and drives tumor evolution toward malignant disease (Labi and Erlacher, 2015; Pitt et al., 2017). Zhou et al. hypothesized that blockade of efferocytosis in the tumor microenvironment would mediate a switch from apoptosis, an immunologically silent form of cell death, to immunogenic cell death of tumor cells. Uncleared apoptotic cells un-
dergo secondary necrosis and engage the innate immune system by releasing damage-associated molecular patterns (DAMPs) such as DNA, ATP, and highmobility-group box 1 protein (HMGB-1). Zhou et al. (2020) generated an antibody to the phagocytic receptor MerTK and showed that it blocked MerTK-mediated efferocytosis. MerTK presented an attractive therapeutic target because it was selectively expressed by TAMs, but not by intratumoral dendritic cells (DCs), and its expression across different human cancers correlated with an abundance of TAMs over other immune cells. The authors showed that systemic administration of anti-MerTK antibody to mice bearing MC38 colon carcinoma cells increased intratumoral apoptotic cell numbers soon after injection and was associated with circulating cell-free tumor-derived DNA indicative of lytic tumor cell death. When administered at early stages of tumor progression, anti-MerTK antibody significantly hindered tumor growth and conferred complete protection against tumor challenge, but its efficacy rested on the ability of tumor cells to undergo apoptosis. When tested on fully established tumors, however, antiMerTK antibody was only effective at preventing tumor growth when combined with immune checkpoint inhibitors antiPD-L1 or anti-PD-1. Established tumors may have developed resistance to cell death precluding apoptotic cell accumulation upon MerTK blockade. The efficacy of combining anti-PD-L1 or anti-PD-1 treatment with MerTK blockade could reflect improvement of the functionality of tumor infiltrating myeloid cells (Strauss et al., 2020), relief of the ‘‘don’t eat me’’ signal that tumor PD-L1 sends to TAMs
212 Immunity 52, February 18, 2020 ª 2020 Elsevier Inc.
preventing live tumor cell phagocytosis (Feng et al., 2019), or the overcoming of CD8+ T cell exhaustion (O’Donnell et al., 2019). By combining anti-MerTK and anti-PD-1 antibodies with Gemcitabine, a cytotoxic chemotherapeutic, the authors achieved an impressive tumor regression in all of the treated mice. The authors then examined the mechanistic underpinnings of enhanced antitumor immunity upon MerTK blockade. They found that TAMs exhibited a type-I interferon (IFN) signature soon after antiMerTK treatment and were the major source of type-I IFN. Despite systemic administration of anti-MerTK, the type-I IFN response was restricted to the tumor, and it was critical to the anti-tumor response as MerTK blockade was ineffective in tumor-bearing mice deficient for IFNAR, the receptor for type-I IFN. The type-I IFN signature made it likely that cross-priming of tumor-specific CD8+ T cells would be increased, and indeed, the authors found an increased frequency of proliferating intratumoral CD8+ T cells upon MerTK blockade. They also noted a significant increase in MHC-I presentation of tumor-derived peptide by TAMs, and to a lesser extent CD11b+ DCs, but not intratumoral CD103+ DCs. This was unexpected and left open the possibility of MerTK-independent TAM acquisition of tumor antigen from secondary necrotic cells. In a headto-head comparison in vitro, TAMs outperformed total DCs isolated from tumors in mediating antigen-specific CD8+ T cell activation. However, DCs and not macrophages specialize in cross-presentation and drive anti-tumor CD8+ T cell priming largely due to their migration and antigen trafficking to lymph nodes through which
Immunity
Previews
Figure 1. A Blocking Antibody that Targets MerTK on Tumor-Associated Macrophages Triggers a Cascade of Events that Control Early-Stage Tumors Antibody-mediated blockade of MerTK-dependent phagocytosis of apoptotic tumor cells results in the intratumoral accumulation of apoptotic tumor cells, which then undergo secondary necrosis. Tumor-expressed cGAS and host-expressed STING drive a type-I IFN response confined to the tumor site whereby cGAS in tumor cells produces cGAMP that activates STING in immune cells in a paracrine fashion. Extracellular ATP released by secondary necrotic tumor cells opens the purinergic P2X7 receptor channel on tumor-associated macrophages (TAMs), allowing cGAS entry and STING activation. TAMs acquire a type-I IFN signature and begin to produce IFN-b, while the tumor microenvironment exhibits signs of an inflammatory response to type-I IFN. This sequence of events culminates in the control of tumor progression. Effective anti-MerTK treatment requires responsiveness to type-I IFN through IFNAR, Batf3dependent CD103+ DC, and tumor-specific CD8+ T cells. Its effectiveness in late-stage tumors was contingent on co-administration of a cytotoxic chemotherapeutic agent and an immune checkpoint inhibitor.
naive CD8+ T cells recirculate (Roberts et al., 2016). Consistent with this, the authors found that the anti-tumor effects of MerTK blockade were lost in Batf3–/– mice, which lack the cross-presenting CD103+ DCs, and despite retention of the anti-MerTK-mediated type-I IFN response. CD8+ T cells were as critical as the Batf3-dependent CD103+ DC, because their depletion abrogated the anti-tumor effects of MerTK blockade. It remains to be tested whether intratumoral TAM cross-presentation upon MerTK blockade might enable expansion of tumor-specific CD8+ T cells infiltrating
the type-I IFN-remodeled tumor microenvironment. The cGAS-STING pathway, which responds to cytoplasmic DNA in settings such as viral infection or genomic DNA damage (Ablasser and Chen, 2019), is central to type-I IFN production, and thus, the authors examined its requirement in the context of MerTK blockade. Interestingly, the anti-tumor effects of MerTK blockade were lost in Sting / but not cGas / mice. Through an elegant set of experiments, the authors pinpointed a necessity for the enzymatic activity of cGAS, which produces cyclic
GMP-AMP (cGAMP), in tumor cells. Notably, when tumors were incapable of generating cGAMP, anti-PD-L1 treatment had no effect on early-stage tumors. It is unclear whether basal levels of tumorderived cGAMP play a role even when TAM efferocytosis is intact, perhaps by facilitating cross-priming of CD8+ T cells. cGAS deficiency is not expected to affect the release of DAMPs such as ATP and HMGB-1, which underlie the immunogenicity of cancer chemotherapy or radiotherapy driven secondary necrosis of tumor cells (Pitt et al., 2017). Yet these DAMPs appeared to take a back seat to cGAMP in the setting of anti-MerTK blockade. How does tumor-derived cGAMP gain access to intracellular STING in responding TAMs? This manner of STING activation in trans is reminiscent of the gapjunction mediated transfer of cGAS from producing to neighboring cells, where it promotes STING activation (Ablasser and Chen, 2019). The authors turned their attention to extracellular ATP, a DAMP present also in the tumor microenviroment; high ATP concentrations open the ligand-gated cation channel P2X7 receptor (P2X7R). They found that cultured macrophages produced type-I IFN only in response to the combination of cGAMP and ATP, and this was blocked by a specific pharmacological inhibitor of P2X7R. ATP enhanced the entry of fluorescent cGAMP into macrophages in a P2X7Rdependent manner, elicited phosphorylation of key mediators of the type-I IFN response, and led to IFN-b production. Consistent with these in vitro studies, anti-MerTK treatment was ineffective in tumor-bearing mice that had been treated with the P2X7R inhibitor or were deficient in P2X7R expression. Interestingly, expression of P2X7R was high in TAMs, consistent with their being the major type-I IFN producers in this setting. Finally, the authors showed that MC38 tumors engineered to overexpress CD39, an ectonucleotidase that hydrolyzes extracellular ATP, no longer displayed a type-I IFN signature and were refractory to anti-MerTK treatment. The authors did not test whether immunosuppressive adenosine, the product of ATP degradation, may also contribute to this loss of responsiveness (Sachet et al., 2017). The findings by Zhou et al. (2020) give pause for thought. They point to cGAMP Immunity 52, February 18, 2020 213
Immunity
Previews as a DAMP in its own right, and one that is released specifically by dying cancer cells and not normal cells, as a proxy of abnormal cGAS engagement by cytosolic chromatin fragments in cancer cells. In the setting of what we know about exploiting immunogenic cell death against cancer, the findings here also suggest that secondary necrotic cells elicit anti-tumor immunity differently than cells undergoing necroptosis, a programmed form of necrosis. Undefined nuclear factor kB (NF-kB)-dependent signals downstream of the RIPK1-RIPK3 necrosome complex that orchestrates necroptosis, but not the release of DAMPs, were important for eliciting anti-tumor immunity through intratumoral injection of necroptotic tumor cells (Snyder et al., 2019). As a reflection of having undergone modifications associated with the process of apoptosis, secondary necrotic cells release inflammatory HMGB-1 irreversibly bound to nucleosomes, while cells undergoing primary necrosis—caused by trauma such as injury or burns—release free HMGB-1 (Sachet et al., 2017). Secondary necrotic cells also release less ATP than cells undergoing primary necrosis. Chemotherapy-induced tumor necrosis secondary to apoptosis elicited anti-tumor immunity through NLRP3-dependent interleukin (IL)-1b and Toll-like receptor 4 (TLR4)-dependent cross-presentation, by apoptotic tumor cell release of ATP and HMGB-1, respectively (Pitt et al., 2017). These differences illustrate how a distinct innate immune response could be engaged depending on the mode of death and the transformed status of the cell—a type-I IFN response, an inflamma-
214 Immunity 52, February 18, 2020
some-dependent IL-1b response, or a necroptosis-dependent NF-kB response. Whether these responses have equivalent anti-tumor efficacy requires future investigations. Side-by-side comparisons of an inflammasome versus type-I IFN response, for example, would have important implications to the therapeutic implementation of NLRP3 versus STING targeted strategies. Lastly, it is worth noting that cancer cells increase the expression of ‘‘don’t eat me’’ signals such as CD47, which interacts with the SIRPa receptor on macrophages to inhibit phagocytosis. In fact, blocking CD47-SIRPa interaction enables the phagocytosis of live cancer cells and is currently being evaluated in phase I and/or II clinical trials in patients with advanced solid and hematological cancers (Feng et al., 2019). These trials aim to accomplish the reverse of anti-MerTK blockade, which is to enhance rather than block tumor cell phagocytosis. AntiCD47 blockade enables the clearance of live tumor cells, while the success of anti-MerTK treatment is contingent on tumor cell apoptosis as a prelude to the secondary necrosis that releases the critical combination of cGAMP with ATP. Nevertheless, therapeutic exploitation of innate immune clearance of dying cancer cells promises to be an exciting new area of cancer immunotherapy.
REFERENCES Ablasser, A., and Chen, Z.J. (2019). cGAS in action: Expanding roles in immunity and inflammation. Science 363.
Feng, M., Jiang, W., Kim, B.Y.S., Zhang, C.C., Fu, Y.X., and Weissman, I.L. (2019). Phagocytosis checkpoints as new targets for cancer immunotherapy. Nat. Rev. Cancer 19, 568–586. Labi, V., and Erlacher, M. (2015). How cell death shapes cancer. Cell Death Dis. 6, e1675. O’Donnell, J.S., Teng, M.W.L., and Smyth, M.J. (2019). Cancer immunoediting and resistance to T cell-based immunotherapy. Nat. Rev. Clin. Oncol. 16, 151–167. Pitt, J.M., Kroemer, G., and Zitvogel, L. (2017). Immunogenic and Non-immunogenic Cell Death in the Tumor Microenvironment. Adv. Exp. Med. Biol. 1036, 65–79. Roberts, E.W., Broz, M.L., Binnewies, M., Headley, M.B., Nelson, A.E., Wolf, D.M., Kaisho, T., Bogunovic, D., Bhardwaj, N., and Krummel, M.F. (2016). Critical Role for CD103(+)/CD141(+) Dendritic Cells Bearing CCR7 for Tumor Antigen Trafficking and Priming of T Cell Immunity in Melanoma. Cancer Cell 30, 324–336. Sachet, M., Liang, Y.Y., and Oehler, R. (2017). The immune response to secondary necrotic cells. Apoptosis 22, 1189–1204. Snyder, A.G., Hubbard, N.W., Messmer, M.N., Kofman, S.B., Hagan, C.E., Orozco, S.L., Chiang, K., Daniels, B.P., Baker, D., and Oberst, A. (2019). Intratumoral activation of the necroptotic pathway components RIPK1 and RIPK3 potentiates antitumor immunity. Sci. Immunol. 4. Strauss, L., Mahmoud, M.A.A., Weaver, J.D., Tijaro-Ovalle, N.M., Christofides, A., Wang, Q., Pal, R., Yuan, M., Asara, J., Patsoukis, N., and Boussiotis, V.A. (2020). Targeted deletion of PD-1 in myeloid cells induces antitumor immunity. Sci. Immunol. 5. Zhou, Y., Fei, M., Zhang, G., Liang, W.-C., Lin, W., Wu, Y., Piskol, R., Ridgway, J., McNamara, E., Huang, H., et al. (2020). Blockade of the Phagocytic Receptor MerTK on Tumor-Associated Macrophages Enhances P2X7R-Dependent STING Activation by Tumor-Derived cGAMP. Immunity 52, this issue, 357–373.