Immune-Mediated Inhibition of Metastases after Treatment with Local Radiation and CTLA-4 Blockade in a Mouse Model of Breast Cancer (1)
Breast cancer is often a systemic malignancy even at clinical presentation, when the tumor is resectable. Therefore, local treatment modalities need to be combined with an effective systemic treatment to significantly impact breast cancer survival. An excellent currently available model of metastatic breast cancer is the BALB/c-derived 4T1 tumor. After s.c. inoculation in the abdominal mammary fatpad, the primary tumor grows into a nodule with the histology of a high-grade breast cancer and sheds spontaneous systemic metastases to the lungs, liver, lymph nodes, bone marrow, and central nervous system. Therefore, this model is suitable for testing the effects of experimental therapies on metastatic disease.
Ionizing radiation therapy (RT) is a powerful inducer of apoptosis of tumor cells and therefore a widely used therapeutic tool in oncology. CTLA-4–mediated inhibition of T-cell activation can prevent the development of antitumor T-cell responses. However, the efficacy of CTLA-4 blockade as a single treatment is limited to intrinsically immunogenic tumors.
When injected s.c. in the mammary fatpad, 4T1 cells metastasized primarily by a hematogenous route and could be found in the lungs within 7 days, and nonmammary fatpad s.c. injection did not alter the metastatic ability of 4T1 cells. CTLA-4 blockade by the 9H10 mAb did not affect the tumor growth or survival of the mice. RT at a single dose of 12 Gy significantly delayed the growth of the primary irradiated tumor in the presence or absence of 9H10. The difference from RT + hamster IgG was not statistically significant. In contrast, the presence of 9H10 in combination with RT had a significant impact on the survival of the mice.
On day 35 post-tumor implantation, the median number of metastases was lower in all of the treated animals compared with controls. However, the decrease in lung metastases was significant only when RT and 9H10 were used in combination, consistent with our observation that only this treatment resulted in increased survival. In mice depleted of CD4+ or CD8+ T cells starting 3 days before RT treatment, the median number of metastases in mice treated with RT + 9H10 and depleted of CD8+ T cells was similar to control untreated animals.
In the presence of CTLA-4 blockade, complete regression of the irradiated tumor was seen in the majority of the mice and was associated with longer survival. One of seven mice was cured (no tumor recurrence for >100 days), rejected a challenge of 4T1 cells on day 111, and showed 4T1-specific cytolytic activity in the spleen 2 months after the rechallenge
The survival advantage obtained with the combination of RT and CTLA-4 blockade is encouraging and comparable to that obtained in the same 4T1 model by surgical removal of the primary tumor and vaccination with MHC class II- and B7.1-modified tumor cells combined with SEB superantigen.