SS1P Immunotoxin Induces Markers of Immunogenic Cell Death and Enhances the Effect of the CTLA-4 Blockade in AE17M Mouse Mesothelioma Tumors

Our plain-English summary of SS1P Immunotoxin Induces Markers of Immunogenic Cell Death and Enhances the Effect of the CTLA-4 Blockade in AE17M Mouse Mesothelioma Tumors, by Y. Leshem, E.M. King, R. Mazor, et al (Toxins, 2018).

The study

• Authors: Y. Leshem, E.M. King, R. Mazor, Y. Reiter, I. Pastan
• Journal: Toxins
• Published: 2018;10(11)
• Cited by: 30 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

SS1P is an anti-mesothelin immunotoxin composed of a targeting antibody fragment genetically fused to a truncated fragment of Pseudomonas exotoxin A. Delayed responses reported in mesothelioma patients receiving SS1P suggest that anti-tumor immunity is induced. The goal of this study is to evaluate if SS1P therapy renders mesothelioma tumors more sensitive to cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) immune checkpoint blockade. We evaluated the ability of SS1P to induce adenosine triphosphate (ATP) secretion and calreticulin expression on the surface of AE17M mouse mesothelioma cells. Both properties are associated with immunogenic cell death. Furthermore, we treated these tumors with intra-tumoral SS1P and systemic CTLA-4. We found that SS1P increased the release of ATP from AE17M cells in a dose and time-dependent manner. In addition, SS1P induced calreticulin expression on the surface of AE17M cells. These results suggest that SS1P promotes immunogenic cell death and could sensitize tumors to anti-CTLA-4 based therapy. In mouse studies, we found that the combination of anti-CTLA-4 with intra-tumoral SS1P induced complete regressions in most mice and provided a statistically significant survival benefit compared to monotherapy. The surviving mice were protected from tumor re-challenge, indicating the development of anti-tumor immunity. These findings support the use of intra-tumoral SS1P in combination with anti-CTLA-4.

In plain English

This was a cell and mouse study of a cancer drug, not of asbestos exposure. The authors used AE17M mouse mesothelioma cells, which came from mice treated with asbestos (the article does not name the fiber type) and were modified to carry human mesothelin. In culture, the immunotoxins SS1P and LMB-100 killed these cells. SS1P also raised two signs that the authors link to immunogenic cell death. ATP in the medium rose from 14 pM in untreated cells to 3177 pM at 100 ng/mL SS1P. Surface calreticulin rose from 1.2% of untreated live cells to 15% after 24 hours. In mice, SS1P given into a vein did not slow tumor growth. Injected into the tumor, it slowed growth only for a time. In one experiment, tumor-injected SS1P plus anti-CTLA-4 antibody eradicated tumors in seven of eight mice, versus two of eight given anti-CTLA-4 with a salt-solution (PBS) injection into the tumor. The combination gave a statistically significant survival benefit. All nine cured mice rejected a second tumor challenge. The authors concluded this supports testing the combination in patients.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory cell-culture experiments and animal (mouse) experiments testing anti-mesothelin immunotoxins alone and with anti-CTLA-4 antibody
• Who was studied: AE17M mouse mesothelioma cells (AE17 cells derived from the peritoneal cavity of asbestos-treated C57BL/6 mice, modified to express human mesothelin); female wild-type C57BL/6 mice, 6-9 weeks old, with tumor cells implanted under the skin of the flank; 8 mice per group in the main combination experiment
• Exposure: Treatments studied: the immunotoxins SS1P and LMB-100 (in culture; SS1P given intravenously at 4 µg or into the tumor at 8 or 10 µg in mice), and anti-CTLA-4 antibody (25 µg into the abdominal cavity every four days). Asbestos is mentioned only as the origin of the cell line; no fiber type or dose is given.
• Main results:
  • In culture, the concentration of SS1P that halved AE17M cell viability was 3 ng/mL; for LMB-100 it was 17 ng/mL
  • ATP in the medium of AE17M cells rose from 14 pM (untreated) to 67, 795 and 3177 pM at 6.25, 25 and 100 ng/mL SS1P (p less than 0.01)
  • Surface calreticulin was found on 1.2% of untreated live AE17M cells, and on 9.1% after 21 hours and 15% after 24 hours of SS1P (p less than 0.01)
  • In mice, SS1P given intravenously did not affect AE17M tumor growth
  • SS1P injected into the tumor reduced tumor size on days 12, 14 and 16, but the effect was temporary
  • In the Figure 5 experiment, all AE17M tumors grew with tumor-injected SS1P alone
  • In the Figure 5 experiment, in mice given tumor-injected PBS plus anti-CTLA-4, tumor growth slowed in 3 of 8 mice and tumors were eradicated in two of those 8 mice
  • In the Figure 5 experiment, in mice given both tumor-injected SS1P and anti-CTLA-4, tumors were completely eradicated in 7 of 8 mice
  • Median survival was 18 days with SS1P alone and 20 days with PBS plus anti-CTLA-4. The combination gave a significant survival benefit over the control groups (p less than 0.05).
  • When LMB-100 was combined with anti-CTLA-4, complete regression rates ranged from 25% to 50%
  • All 9 mice that had eradicated their tumors rejected a second injection of AE17M cells. Tumors grew in the 5 mice that had never had a tumor.
• The authors' conclusion: The authors concluded that injecting anti-mesothelin immunotoxins into tumors strengthened the effect of anti-CTLA-4 in a mouse mesothelioma model and induced markers of immunogenic cell death. In their view, this gives preclinical support for testing the combination in patients.
• Limitations the authors state:
  • The AE17M tumors did not respond to intravenous immunotoxin, so the authors could not test whether intravenous immunotoxin makes tumors more sensitive to anti-CTLA-4

Topics

The National Library of Medicine indexes this study under subjects our site covers: Mesothelioma.

Medical terms in this study, explained in our Lexicon: CTLA-4, Mesothelioma, Mesothelin.

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