Macrophages Promote Circulating Tumor Cell-Mediated Local Recurrence following Radiotherapy in Immunosuppressed Patients

Although radiotherapy (RT) decreases the incidence of locoregional recurrence in breast cancer, patients with triple-negative breast cancer (TNBC) have increased risk of local recurrence following breast-conserving therapy. The relationship between RT and local recurrence is unknown. Here, we tested the hypothesis that recurrence in some instances is due to the attraction of circulating tumor cells to irradiated tissues. To evaluate the effect of absolute lymphocyte count on local recurrence after RT in patients with TNBC, we analyzed radiation effects on tumor and immune cell recruitment to tissues in an orthotopic breast cancer model. Recurrent patients exhibited a prolonged low absolute lymphocyte count when compared with nonrecurrent patients following RT. Recruitment of tumor cells to irradiated normal tissues was enhanced in the absence of CD8+ T cells. Macrophages (CD11b+F480+) preceded tumor cell infiltration and were recruited to tissues following RT. Tumor cell recruitment was mitigated by inhibiting macrophage infiltration using maraviroc, an FDA-approved CCR5 receptor antagonist. Our work poses the intriguing possibility that excessive macrophage infiltration in the absence of lymphocytes promotes local recurrence after RT. This combination thus defines a high-risk group of patients with TNBC.Significance: This study establishes the importance of macrophages in driving tumor cell recruitment to sites of local radiation therapy and suggests that this mechanism contributes to local recurrence in women with TNBC that are also immunosuppressed.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/78/15/4241/F1.large.jpg Cancer Res; 78(15); 4241-52. ©2018 AACR.

Orthovoltage X-Rays Exhibit Increased Efficacy Compared with γ-Rays in Preclinical Irradiation

Radionuclide irradiators (137Cs and 60Co) are commonly used in preclinical studies ranging from cancer therapy to stem cell biology. Amidst concerns of radiological terrorism, there are institutional initiatives to replace radionuclide sources with lower energy X-ray sources. As researchers transition, questions remain regarding whether the biological effects of γ-rays may be recapitulated with orthovoltage X-rays because different energies may induce divergent biological effects. We therefore sought to compare the effects of orthovoltage X-rays with 1-mm Cu or Thoraeus filtration and 137Cs γ-rays using mouse models of acute radiation syndrome. Following whole-body irradiation, 30-day overall survival was assessed, and the lethal dose to provoke 50% mortality within 30-days (LD50) was calculated by logistic regression. LD50 doses were 6.7 Gy, 7.4 Gy, and 8.1 Gy with 1-mm Cu-filtered X-rays, Thoraeus-filtered X-rays, and 137Cs γ-rays, respectively. Comparison of bone marrow, spleen, and intestinal tissue from mice irradiated with equivalent doses indicated that injury was most severe with 1-mm Cu-filtered X-rays, which resulted in the greatest reduction in bone marrow cellularity, hematopoietic stem and progenitor populations, intestinal crypts, and OLFM4+ intestinal stem cells. Thoraeus-filtered X-rays provoked an intermediate phenotype, with 137Cs showing the least damage. This study reveals a dichotomy between physical dose and biological effect as researchers transition to orthovoltage X-rays. With decreasing energy, there is increasing hematopoietic and intestinal injury, necessitating dose reduction to achieve comparable biological effects.

SIGNIFICANCE

Understanding the significance of physical dose delivered using energetically different methods of radiation treatment will aid the transition from radionuclide γ-irradiators to orthovoltage X-irradiators.