News 21. June 2019
| by Laura Elizabeth Lansdowne, Science Writer, Technology Networks
By direct injection of engineered dying (necroptotic) cells into tumors, scientists have successfully triggered the immune system to attack cancer cells at multiple sites in the body and reduce tumor growth in mice. The study was published in Science Immunology 21. June 2019. This new strategy presents as a potential treatment method that could be used in combination with existing immunotherapies to "increase" the body's ability to fight cancer in the future.
We had the pleasure of talking to Andrew Oberst, the corresponding author of the study, to learn more about the type of cells used in the study, the design of the study and the limitations on how necroptotic cell death within the tumor leads to antitumor immunity and the team's plans for future studies.
Laura Lansdowne (LL): Can you tell us more about the type of dying cells used in the study?
Andrew Colonel (AO): We started using dying tumor cells; that is, we created tumors in mice using either melanoma or lung carcinoma tumor cells, and then introduced dying cells of the same type to those tumors. We found that when we introduced necroptotic (but not apoptotic) cells to these tumors, it activated the immune system to attack the tumor, reduce tumor growth and prolong survival. What was surprising to us, however, was that we also experienced a reduction in tumor growth when we introduced necroptotic cells of various type-murine fibroblasts to the tumor. This was surprising because these fibroblasts do not have any of the antigens present in the tumor. This indicated to us that the necroptotic cells acted on the tumor microenvironment (TME) instead of directly priming antigen-specific T cells.
LL: Can you give us an overview of the study design and any limitations? We used murine flank tumor models to study the immune response to necropototic cells within TME in vivo . We did it first by injecting necropototic tumor cells or fibroblasts; In these cases, we activated the key necroptosis-inducing kinase RIPK3 in these cells in vitro and then injected them into the tumors to undergo cell death in vivo . Next, we wanted to find a way to directly trigger necroptosis in tumor cells even in unmodified tumors. To do this, we constructed a form of the RIPK3 kinase, which was constitutively active so that only expression of this kinase was sufficient to rapidly induce necroptosis. We packaged a gene encoding this suicide enzyme in adeno-associated viruses (AAVs) and delivered them to tumors. We found that this approach also led to the activation of antitumor immune responses.
A limitation of our study design is that it depends solely on transplanted flank tumors (tumor cells injected under the skin of the mouse, which then grow into tumors). These models are widely used and useful because they are easy to work with and deliver fast results; However, they are also much less complex and generally easier to "cure" than autochthonic tumor models. So even though our study provides good evidence of the concept, a significant additional work is needed to verify these ideas in models that mimic human disease.
LL: How does necroptotic cell death in the tumor microenvironment (TME) lead to antitumor immunity?
AO: Our data show that necroptotic cells in TME produce cytokines and chemokines that recruit and activate phagocytes (macrophages and dendritic cells) within TME. Activation of these cells leads them to take up more material from the surrounding tumor and more easily display tumor-derived antigens to local CD8 + T cells. These activated CD8 + T cells can then control the tumor and can also function systemically and control tumors at distant sites. What is interesting and unexpected is that the necroptotic cells act on TME in general; they need not carry any tumor-associated antigen, but rather "turn on" tumor-resident phagocytes, which then promote anti-tumor immune response.
LL: Are there any plans you can share on future studies to validate your results?
AO: Our development of AAVs, which directly induce necroptosis, allows delivery of these genes to unmodified, autochthonous tumors; This is something we would like to explore by using more physiologically relevant tumor models. In addition, our data shows that it is the production of cytokines and chemokines by RIPK3 activation, which is immunogenic, non-necroptotic cell death itself . Considering how cells balance inflammatory transcription vs. cell death by RIPK3 activation, what governs these decisions and how different primary or tumor cell types involve these pathways will be important if we want to translate this work into further tumor settings.  Reference: AG Snyder, et al. Intratumoral activation of the necroptotic pathway components RIPK1 and RIPK3 enhances anti-tumor immunity. Science Immunology (2019) DOI: http://immunology.sciencemag.org/lookup/doi/10.1126/sciimmunol.aaw2004
Andrew Colonel spoke to Laura Elizabeth Lansdowne, Science Writer for Technology Networks.