Effect of combination treatment of 15 cGy total body irradiation and OK-432 on spontaneous lung metastasis and mitogenic response of splenocytes in mice

Optimizing Radiotherapy with Immunotherapeutic Approaches

Several factors must be considered to successfully integrate immunotherapy with radiation into clinical practice. One such factor is that concepts arising from preclinical work must be tested in combination with radiation in preclinical models to better understand how combination therapy will work in patients; examples include checkpoint inhibitors, tumor growth factor-beta (TGF-β) inhibitors, and natural killer (NK) cell therapy. Also, many radiation fields and fractionation schedules typically used in radiation therapy had been standardized before the introduction of advanced techniques for radiation planning and delivery that account for changes in tumor size, location, and motion during treatment, as well as uncertainties introduced by variations in patient setup between treatment fractions. As a result, radiation therapy may involve the use of large treatment volumes, often encompassing nodal regions that may not be irradiated with more conformal techniques. Traditional forms of radiation in particular pose challenges for combination trials with immunotherapy. This chapter explores these issues in more detail and provides insights as to how radiation therapy can be optimized to combine with immunotherapy.

Antitumor Effects by Low Dose Total Body Irradiation

Total-body irradiation (TBI) with 0.02-0.25 Gy has been reported to have antitumor effects. In mice, low-dose TBI induces tumor growth delay, antimetastatic effects, suppressive effects on the incidence of spontaneous thymiclymphoma, sensitization of tumor to ionizing radiation, and decrease in TD50 value. In artificial metastasis, 0.20 Gy TBI suppressed lung metastasis when it was conducted between 3 h before and 3 h after tumor cell injection into a tail vein. In spontaneous metastasis, 0.15-0.20 Gy TBI suppressed lung metastasis. Irradiation with 0.15 Gy twice a week from 11 weeks of age for 40 weeks significantly suppressed the incidence of spontaneous thymic lymphoma in AKR/J mice, which caused prolonged life span. Low-dose TBI has been used in the clinical treatment of lymphomatous malignancies including chronic lymphocytic leukaemia (CLL) and non-Hodgkin's lymphoma (NHL). The usual practice was to give 0.1 Gy TBI three times a week or 0.15 Gy TBI two times a week to a total dose of 1.5 Gy. Despite this low total dose, low-dose fractionated TBI could induce long-term remissions and was as effective as the chemotherapy to which it was compared. Experimental data suggest that the antitumor effects of low-dose TBI could be explained by immune enhancement, induction of apoptosis, and intrinsic hypersensitivity to low-dose irradiation. Possible mechanisms of immune enhancement are elimination of the T-suppressor subset of lymphocytes and augmentation of the immune response including alteration of cytokine release and enhanced proliferative activity of lymphocytes to mitogenic stimuli.

Applications of hormesis in toxicology, risk assessment and chemotherapeutics

There is much debate over the fundamental shape of the dose-response curve in the low-dose zone, particularly in the fields of toxicology and risk assessment. The defaults, principally accepted dose-response models in the major texts in these areas and in government regulatory activities, are a threshold model for non-carcinogens and a linear model for most carcinogens. We have argued that in properly designed studies the U-shaped hormetic response predominates and is more fundamental. In this article, a broad range of basic issues associated with the acceptance of U-shaped dose responses as central to toxicology, pharmacology and their applications to risk assessment and medicine will be discussed.

Induction of interleukin-1beta and interleukin-6 mRNA by low doses of ionizing radiation in macrophages

We have previously reported the antimetastatic effects and augmentation of immune responses, which would be a mechanism of the antimetastatic effects, of 0.1 to 0.2 Gy total body irradiation. To elucidate the cellular mechanisms of the augmentation of immune response, we investigated the effects of low-dose irradiation on gene expression of interleukin-1beta (IL-1beta) and IL-6 using mouse peritoneal macrophages in vitro. Absolute mRNA quantification was carried out using competitive polymerase chain reaction. Gene expression of IL-1beta and IL-6 was increased 1 to 2 hr after 2.0 Gy irradiation and then decreased to below the basal expression level 4 hr after irradiation. Irradiation with 0.1 Gy increased IL-6 expression 2 hr after irradiation, but it did not affect IL-1beta expression. Downregulation of IL-1beta and IL-6 observed 4 hr after 2.0 Gy irradiation was not observed with 0.1 Gy irradiation. The protein kinase C (PKC) inhibitor H7 and the phosphatidylinositol 3-kinase (PI3-kinase) inhibitor wortmannin inhibited induction of IL-1beta and IL-6 expression, which suggests that radiation-induced IL-1beta and IL-6 expression is achieved by PKC- and PI3-kinase-mediated signaling.

Apoptosis: biphasic dose responses

The features of apoptotic dose responses typically have been evaluated over a broad dose-response continuum to differentiate apoptotic and necrotic responses. The present article specifically addresses the dose-response relationship for apoptosis in detail. It was determined that a number of well-documented cases exist in which the dose-response relationship for apoptosis is biphasic. The underlying mechanisms and biomedical implications of this biphasic dose-response phenomenon are discussed.

Hormesis: U-shaped dose responses and their centrality in toxicology

The fundamental nature of the dose response is neither linear or threshold, but rather U-shaped. When studies are properly designed to evaluate biological activity below the traditional toxicological threshold, low-dose stimulatory responses are observed with high frequency and display specific quantitative features. With a few exceptions, the low-dose stimulatory response is usually not more than twofold greater than the control response, with a stimulatory zone that is more variable, ranging from less than tenfold to more than several orders of magnitude of the dose. Considerable mechanistic evidence indicates that hormetic effects represent overcompensation in response to disruptions in homeostasis that are mediated by agonist concentration gradients with different affinities for stimulatory and inhibitory regulatory pathways.

The immunobiology of low-dose total-body irradiation: more questions than answers

Low-dose total-body irradiation (TBI) is used in the treatment of chronic lymphocytic leukemia and low-grade non-Hodgkin's lymphoma. The usual practice is to give very low individual fractions (0.1 to 0.25 Gy) several times a week, to a total dose of 1.5 to 2 Gy. Despite this low dose, low-dose TBI can induce long-term remissions in the majority of patients. Immune enhancement, rather than direct radiation cell killing, is one of the suggested mechanisms by which low-dose TBI can exert its effect. Data from animal experiments have shown that low-dose TBI could enhance the immune response through (1) augmenting the proliferative reactive response of the T cells to mitogenic stimulation; (2) altering cytokine release, particularly the activation of interferon gamma and Il2 production; (3) increasing the expression of Il2 receptors on the T-cell surface; (4) facilitating signal transduction in T lymphocytes; (5) increasing splenic catecholamine content and lowering the serum corticosterone level; and (6) eliminating a particularly radiosensitive subset of the suppressor T cells. Data for humans, though scarce, suggest that at least some of these mechanisms occur in patients treated with low-dose TBI. Whether these immunomodulatory effects are responsible for the clinical outcome is not yet clear. Much is still unknown about the immunobiology of low-dose TBI, its clinical potential, and the possible synergism with chemotherapy, biological response modifiers, or immunotherapy. This lack of comprehensive knowledge hampers the optimal and widespread use of this intriguing and potentially useful treatment modality in clinical practice.