(P 378) HIGH DOSE RADIATION THERAPY PROMOTES IMMUNE SUPPRESSIVE MACROPHAGE PHENOTYPES, WHEREAS LOW DOSE RADIATION THERAPY PROMOTES IMMUNOGENIC MACROPHAGE PHENOTYPES IN A MURINE MODEL OF SOFT TISSUE SARCOMA

Objective: Soft tissue sarcomas (STS) are aggressive tumours of mesenchymal origin, which are treated with neoadjuvant radiation therapy (RT) of 2Gy in 25 fractions and surgical resection. While RT is typically used to improve local control in combination with surgery, there is a growing body of evidence that RT modulates the immune compartment of tumours. Interestingly, there is evidence that macrophages are highly abundant in STS after RT and appear to express immune suppressive markers. These macrophage phenotypes have also been shown to inhibit anti-tumour T cells. Thus, macrophages may be dampening the potential of these immune cells. Herein, we assessed how RT influences both myeloid and T cell populations after RT.

Methods: Syngeneic, murine STS were engrafted in C57Bl/6 mice, then used for RT experiments when tumours were ~500-800mm3 in size. Tumours were treated with three RT regimens; 1) 6Gy x5 clinical RT, 2) 8Gy x3 high dose hypofractionated RT, and 3) 8Gy x1 low dose RT. A SmART+ (Precision X-ray) irradiator was used to deliver RT. A CT image was acquired prior to treatment, then image planning was completed to deliver RT to the entire tumour. Tumours were assessed 72hrs after RT by flow cytometry to evaluate macrophages and lymphocytes.

Results: We found that 6Gy x5 and 8Gy x3 increased the proportion of TGFB1+ immune suppressive macrophages, whereas the 8Gy x1 reduced CD206+ and TGFB1+ macrophages and increased CD80+ immunogenic macrophages. Interestingly, the 6Gy x5 and 8Gy x3 led to increased CD3e+ and CD8+ T cells, as well as increased activated CD8 T cells. In contrast, the 8Gy x1 did not increase T cells, but increased activated CD8 T cells.

Conclusion: High doses of RT induce an immune suppressive macrophage phenotype 72hrs after treatment, whereas low dose induces immunogenic macrophage phenotypes. However, all doses exhibited increased activated T cells. We will assess various RT regimens in combination with anti-PD1 or CAR T immunotherapy.