Radiation-Induced Immunogenic Modulation Enhances T-Cell Killing
Schematic model of tumor cell response to endoplasmic reticulum (ER) stress. ER stressors such as radiation increase the expression of multiple proteins implicated in antigen processing by triggering the Unfolded Protein Response, leading to translocation of calreticulin (CRT) to the cell surface, where it promotes better killing by killer T cells. Calreticulin also chaperones the loading of antigen-presenting molecule MHC I with peptide epitopes in the ER prior to its translocation to the cell surface.
For many types of cancer, including breast, lung, and prostate carcinomas, radiation therapy is the standard of care. However, limits placed on the tolerable levels of radiation exposure coupled with heterogeneity of biological tissue result in cases where not all tumor cells receive a lethal dose of radiation. Preclinical studies have shown that exposing tumor cells to lethal doses of radiation can elicit cell death while inducing some antitumor immunity, described as immunogenic cell death (ICD). However, in a clinical setting, immune responses elicited by radiation alone rarely result in protective immunity, as tumor relapse often occurs.
More recent studies have reported that sublethal radiation may play an active role due to its immunomodulatory effects on surviving tumor cells and/or immune system cells. James Hodge, Ph.D., Head of the Recombinant Vaccine Group in CCR’s Laboratory of Tumor Immunology and Biology, and his colleagues previously reported that radiation alters surviving tumor cells to make them more susceptible to killing by T cells. In 2013, the same group reported that chemotherapy-induced immunogenic modulation of tumor cells also enhances T-cell killing. Now, Hodge and his team have expanded on those findings by examining whether sublethal radiation could increase the susceptibility of breast, lung, and prostate cancer cells to lysis, or cellular breakdown, mediated by CD8+ cytolytic T cells (CTL), also known as killer T cells. The team also sought to discover the molecular mechanisms underlying the immunomodulation of these cell types. Toward that end, they focused on molecules known to enhance CTL-mediated tumor lysis: calreticulin—a molecular chaperone that assists in the folding of newly synthesized glycoproteins as well as antigen presentation—and components of the antigen-processing machinery (APM).
The researchers exposed human breast, lung, and prostate carcinoma cell lines to different levels of radiation. They observed dose-dependent cellular changes, with effects ranging from immunogenic modulation to the primary indicators of ICD, including the secretion of high-mobility group box 1 (HMGB1) protein, the cell-wide release of ATP, and, at high doses, cell death.
Breast, lung, and prostate carcinoma cells exposed to sublethal doses of radiation were significantly more sensitive to killer T cells. In addition, sublethal irradiation of these cell types induced widespread upregulation of six of seven APM components tested as determined by flow cytometry, increased the cell-surface expression of antigen-presenting molecules such as MHC I, and facilitated the translocation of calreticulin to the cell surface. Similar phenomena were observed following the sublethal irradiationof nude mice implanted with LNCaP (prostate adenocarcinoma) xenograft tumors.
The upregulation of calreticulin and its translocation to the cell surface following sublethal irradiation led Hodge and colleagues to hypothesize that the molecule plays a direct role in facilitating CTL-mediated lysis. Confirming their hypothesis, the team found that significantly reducing PERK expression—a serine/threonine kinase that mediates translocation of calreticulin to the cell surface—via knock down led to a decrease in the CTL-mediated lysis of tumor cells recovering from sublethal radiation. A calreticulin-blocking peptide also abrogated the increased T-cell killing of tumors recovering sublethal radiation, whereas exogenously applied calreticulin further increased lysis by killer T cells.
In summary, radiation therapy was found to induce a dose-dependent continuum of alterations in tumor cell biology, ranging from immunogenic modulation to immunogenic cell death. These results suggest that radiation therapy, even in sublethal doses, in combination with therapeutic cancer vaccines can induce a robust antitumor immune response in multiple types of carcinoma.Summary Posted: Wed, 01/01/2014
Gameiro SR, Jammeh ML, Wattenberg MM, Tsang KY, Ferrone S and Hodge JW. Radiation-induced immunogenic modulation of tumor enhances antigen processing and calreticulin exposure, resulting in enhanced T-cell killing. Oncotarget, December 31, 2013. PubMed Link