Open access Commentary J Immunother Cancer: first published as 10.1136/jitc-2020-000802 on 5 April 2020. Downloaded from Immunogenicity of cell death driven by immune effectors Lorenzo Galluzzi,1,2,3,4,5 Giulia Petroni,1 Guido Kroemer5,6,7,8,9,10 To cite: Galluzzi L, Petroni G, ABSTRACT immunogenic cell death (ICD)4 was missing. Kroemer G. Immunogenicity Whether cell death caused by T lymphocytes and natural Novel findings from the laboratories of Dr of cell death driven by killer (NK) cells would be immunogenic per se has been a Melero and Dr Pardo elegantly resolve this immune effectors. Journal for matter of intense debate. Two back- to-back papers from ImmunoTherapy of Cancer conundrum as they demonstrate that cancer the Melero’s and Pardo’s groups have now resolved this 2020;8:e000802. doi:10.1136/ cells succumbing to immune effectors display conundrum, demonstrating that T and NK cell–mediated jitc-2020-000802 sufficient antigenicity and adjuvanticity to cytotoxicity represents indeed a bona fide variant of initiate adaptive immune responses in immu- LG and GK are joint senior immunogenic cell death. 5 6 authors. nocompetent hosts. By harnessing both parental and genet- Accepted 21 March 2020 ically modified mouse cancer cell lines Preclinical and clinical data accumulating including MC38 colorectal carcinoma cells over the past two decades demonstrate and EL4 lymphoma cells, these authors that—irrespective of treatment modality— elegantly demonstrated that cancer cells the success of cancer therapy most often succumbing to antigen- specific CTLs or NK relies on the activation of a robust immune cells in vitro display surrogate markers of ICD response with systemic outreach.1 A key role such as the exposure of calreticulin (CALR) in this context is played by so- called “epitope on the surface of the plasma membrane and spreading,” that is, the diversification of the release of high mobility group Box 1 an adaptive immune response that initially (HGMB1) into the extracellular space. When targets only one or a few antigenic determi- inoculated in immunocompetent syngeneic nants but ultimately involves multiple T- cell mice as a prophylactic vaccine, such dying clones specific for a variety of epitopes. cells conferred robust protection against a Indeed, the vast majority of solid neoplasms subsequent challenge with living cells of the display an elevated antigenic heterogeneity same parental type, as well as with different http://jitc.bmj.com/ that generally compromises the efficacy of cells genetically engineered to express a immune responses targeting a single antigen, shared antigen. In particular, ovalbumin as demonstrated by a plethora of therapeutic (OVA)- expressing MC38 exogenously pulsed vaccines ultimately failing clinical testing.2 with an immunogenic epitope from premel- From an immunological perspective, epitope anosome protein (PMEL, best known as spreading obligatorily relies on several steps: gp100) and then exposed to gp100- specific on September 23, 2021 by guest. Protected copyright. (1) the release of novel antigenic material CTLs could successfully vaccinate C57BL/6 by cancer cells succumbing to the initial mice against a challenge with OVA- expressing immune response; (2) the uptake of such EL4 cells (commonly known as EG7 cells). material by antigen-presenting cells (APCs) This elegant experimental design demon- and their migration to secondary lymphoid strated epitope spreading in vivo.5 6 organs or tertiary lymphoid structures; and In line with an expanding literature on © Author(s) (or their 7 employer(s)) 2020. Re- use (3) the cross-priming of naive T-cell clones in the molecular determinants of ICD, mouse permitted under CC BY-NC. No the context of sufficient co- stimulatory signals cancer cells killed by CTLs or NK cells failed commercial re- use. See rights for them to acquire effector functions.3 Thus, to protect syngeneic immunocompetent and permissions. Published by the death of cancer cells attacked by CD8+ hosts from a challenge with antigenically BMJ. cytotoxic T lymphocytes (CTLs) must be able, compatible cells on the antibody-mediated For numbered affiliations see + end of article. at least in some instances, to re- initiate the depletion of CD8 CTLs as well as on the cancer- immunity cycle and ultimately lead whole- body knockout of Prf1, which encode Correspondence to to the generation of additional CTLs with the central cytotoxic molecule perforin Dr Lorenzo Galluzzi; broadened specificity.3 Until now, however, 1. The prophylactic effect of cancer cells deadoc80@ gmail. com formal demonstration that cellular cytotox- succumbing to CTLs or NK cells was also lost Guido Kroemer; icity as mediated by CTLs and natural killer in mice lacking the HMGB1 receptor Toll- kroemer@ orange. fr (NK) cells constitutes a bona fide variant of like receptor 4 (TLR4), as well as in mice Galluzzi L, et al. J Immunother Cancer 2020;8:e000802. doi:10.1136/jitc-2020-000802 1 Open access J Immunother Cancer: first published as 10.1136/jitc-2020-000802 on 5 April 2020. Downloaded from this notion, BCL-X L overexpression had little effects on sensitivity to death by immune effectors, which correlated with a minimal influence on vaccination efficacy.5 Thus, the impact of CASP3 on the efficacy of vaccination with cancer cells succumbing to CTLs and NK cells appears to be largely limited to its ability to precipitate cell death in this setting, which does not apply to ICD driven by other stimuli including radiation.8–10 Early work from the late Jurg Tschopp and colleagues demonstrated that the lytic granules of CTLs contain high levels of CALR, which was interpreted as a safeguard mechanism to prevent PRF1 activation by Ca2+ ions prior to granule exocytosis, largely relying on the ability of CALR to chelate Ca2+.11 Thus, it is possible that (at least part of) the CALR molecules detected on the surface of cancer cells attacked by CTLs (or NK cells) may be provided in trans by the latter (rather than in cis by the former) along with PRF1 and the other cytotoxic mole- Figure 1 Cellular cytotoxicity as a potential source of cules contained in lytic granules. Vaccination experiments adjuvanticity in trans. In the context of chemotherapy- with Calr−/− cancer cells are urgently awaited to clarify induced and radiation therapy–induced immunogenic the relative contribution of endogenous vs exogenous cell death (ICD), the phagocytosis-stimulatory molecule calreticulin (CALR) is exposed on the surface of dying CALR to the immunogenic potential of cellular cytotox- cells as a consequence of anterograde transport from icity. Such a mechanistic exploration may be particularly the endoplasmic reticulum (ER) to the plasma membrane important for the development of next-generation CAR T via the Golgi apparatus (GA) (A). When ICD is initiated by cells endowed with superior cytotoxic functions and also cytotoxic T lymphocytes (CTLs) or natural killer (NK) cells, with the capacity to transfer adjuvanticity to their target, however, surface-exposed CALR may be provided by de facto favoring epitope spreading (figure 1). Irrespective immune effectors (at least in part). This mechanism could be of this and other unresolved conundrums, the current harnessed to improve the therapeutic efficacy of CAR T cells findings from Dr Melero’s and Dr Pardo’s groups shed by maximizing their ability to deliver adjuvant- like signals to their targets, ultimately resulting in superior epitope spreading new light onto the contribution of ICD to therapeutically (B). PRF1, perforin 1. relevant immune responses in the context of the cancer- immunity cycle. that are deficient in cross-priming as a consequence of Author affiliations http://jitc.bmj.com/ stimulator of interferon response cGAMP interactor 1Department of Radiation Oncology, Weill Cornell Medical College, New York, NY, 1 (Sting1) or basic leucine zipper transcription factor, USA 5 6 2Sandra and Edward Meyer Cancer Center, New York, NY, United States ATF- like 3 (Batf3) deletion. Conversely, the results 3 from Pardo and colleagues appear to challenge the Caryl and Israel Englander Institute for Precision Medicine, New York, NY, United States current literature in proposing that caspase 3 (CASP3), 4Department of Dermatology, Yale School of Medicine, New Haven, CT, United States a key regulator of apoptosis with multipronged immu- 5Université de Paris, Paris, France on September 23, 2021 by guest. Protected copyright. nosuppressive effects including the ability to shut down 6Equipe labellisée par la Ligue contre le cancer, INSERM U1138, Centre de type I interferon production by STING1,8–10 is required Recherche des Cordeliers, Paris, France 7 for the immunogenicity of cancer cells succumbing to Metabolomics and Cell Biology Platforms, Gustave Roussy Comprehensive Cancer 5 Institute, Villejuif, France immune effectors. This interpretation was based on the 8Pôle de Biologie, Hôpital Européen Georges Pompidou, AP- HP, Paris, France ability of chemical pan-caspase inhibitor (Q- VD- OPh) 9Suzhou Institute for Systems Medicine, Chinese Academy of Sciences, Suzhou, and a dominant-negative variant of CASP3, but not the China overexpression of BCL2- like 1 (BCL2L1, a potent anti- 10Department of Women's and Children's Health, Karolinska University Hospital, Stockholm, Sweden apoptotic molecule best known as BCL- XL), to reduce (to some degree) immunological protection conferred Contributors LG conceived the article and wrote the first version of the manuscript by cancer cells dying on attack by antigen-specific