Antitumor immune responses induced by ionizing irradiation and further immune stimulation

Abstract

The therapy of cancer emerged as multimodal treatment strategy. The major mode of action of locally applied radiotherapy (RT) is the induction of DNA damage that triggers a network of events that finally leads to tumor cell cycle arrest and cell death. Along with this, RT modifies the phenotype of the tumor cells and their microenvironment. Either may contribute to the induction of specific and systemic antitumor immune responses. The latter are boosted when additional immune therapy (IT) is applied at distinct time points during RT. We will focus on therapy-induced necrotic tumor cell death that is immunogenic due to the release of damage-associated molecular patterns. Immune-mediated distant bystander (abscopal) effects of RT when combined with dendritic cell-based IT and the role of fractionation of radiation in the induction of immunogenic tumor cell death will be discussed. Autologous whole-tumor-cell-based vaccines generated by high hydrostatic pressure technology will be introduced and the influence of cytokines and the immune modulator AnnexinA5 on the ex vivo generated or in situ therapy-induced vaccine efficacy will be outlined. RT should be regarded as immune adjuvant for metastatic disease and as a tool for the generation of an in situ vaccine when applied at distinct fractionation doses or especially in combination with IT to generate immune memory against the tumor. To identify the most beneficial combination and chronology of RT with IT is presumably one of the biggest challenges of innovative tumor research and therapies.

Fig. 1

Immunogenicity of therapy-induced dying and dead tumor cells. Cytotoxic agents such as chemotherapeutics (CT), ionizing radiation administered by radiotherapy (RT), hyperthermia (HT) when applied along with RT and/or CT as well as cell death modifiers (CDM) such as the pan caspase inhibitor zVAD-fmk already alter the tumor cell phenotype very early after their application. Stress proteins such as HSP70 (white circle) and recognition molecules for phagocytes such as phosphatidylserine (PS, black circle) get exposed. Later on, the cells undergo cell death via apoptosis or necrosis. The latter exists in an accidental and programmed form. While necrotic cells lose their membrane integrity resulting in the release of immune-activation-damage-associated molecular patterns (DAMPs) such as HMGB1 (triangle), ATP (rectangle), or HSP70 (white circle), apoptotic ones maintain their integrity and DAMPS stay hidden. Apoptotic cells get swiftly cleared and recognized via PS, and an immune-suppressive microenvironment is created by the release of anti-inflammatory cytokines by macrophages. In contrast, DAMPs mature and activate DCs and foster the cross-presentation of tumor-cell-derived antigens to T cells. A specific cellular antitumor immune response is started. Additionally, DAMPs may also directly activate cells of the innate immune systems such as natural killer (NK) cells. The necrotic immunogenic form of tumor cell death can be fostered by impairing the clearance of apoptotic cells by macrophages with AnnexinA5 (AnxA5) or by inducing massive amounts of apoptotic cells in multimodal therapy settings. The latter can then be regarded as inducers of an in situ vaccine. Immunogenic tumor cell death forms can also be induced ex vivo, by killing biopsy-derived fresh tumor cells with techniques that result in complete cell death of the tumor cells by concomitantly increasing their immunogenicity. In such a way prepared whole tumor cells by high hydrostatic pressure (HHP) technology are currently tested as tumor vaccines in preclinical mouse models

Fig. 2

Immune-activating properties of radiotherapy. The medical use of ionizing radiation in radiotherapy (RT) primarily aims to induce DNA damage in tumor cells (local tumor control), but additionally alters the tumor cell surface and its microenvironment. Via the generation of reactive oxygen species (ROS) and provoking endoplasmic reticulum (ER) stress, immune-activating cytokines and chemokines, exosomes as well as damage-associated molecular patterns (DAMPs) get released. This is strongly fostered by additional immune therapy (IT). On the tumor cell surface, molecules, ligands, and receptors that promote lysis of the tumor cells by cytotoxic T lymphocytes and contribute to immune stimulation get exposed after RT. Therefore, RT acts both, targeted on the tumor cell and non-targeted on the microenvironment perceived as the immune-mediated distant bystander (abscopal) effects of this classical local tumor treatment

Fig. 3

Crucial aims when combining distinct fractionation schemes of radiotherapy (RT) with immune therapy (IT). The presumably biggest challenge of innovative multimodal tumor therapies is to identify the most beneficial combination and chronology of distinct chemotherapeutic and radiation protocols with immune therapies (IT). Besides local tumor control, systemic antitumor immunity should be induced. To provide the immune cells time to act and react after radiotherapy (RT), longer breaks between the single radiations might be beneficial and could be further utilized for application of IT that converts the immune-suppressive tumor microenvironment into a more activating one. Urgent need exists of further identifying the immunogenic properties of RT alone given in norm-, hypo-, or oligo-fractionated doses. To achieve long-lasting antitumor immunity, characterized by immunological memory against the individual tumor including its metastases, the immune system should be boosted again at the end of the classical therapy after a yet-to-be defined time window