Purpose: Lung cancer is the leading cause of cancer-related mortality in the United States. Radiation, a common component of treatment, can cause acute damage to critical organs including the lungs and the heart, but the serious toxicities from radiotherapy alone is relatively rare. A recent addition to the treatment regimen is immunotherapy, such as anti-PD-1 antibody, which blocks the inhibition of activated T cells. Combining anti-PD-1 treatment and thoracic radiation has potential for improving the outcomes of locally advanced lung cancer over traditional chemoradiation regimens, but the effect of combining these therapies on non-malignant lung tissue has not yet been investigated in preclinical models.
Materials and Methods: 6–8 week old C57Bl/6 mice were treated with either anti-PD-1 antibody or control IgG with or without thoracic radiation (20Gy), and survival was monitored as an end point to determine any potential increase of toxicity from the combination therapy. Immune cell infiltration into the irradiated cardiac and lung tissues was analyzed via flow cytometry and histologically.
Results: At 21 days post-radiation, 70% of animals in the IgG + radiation group survived, significantly more than the anti-PD-1 + radiation group (36%; p=0.0169). T cell counts were significantly elevated in both cardiac and pulmonary tissues after combination therapy as compared to anti-PD-1 antibody alone (heart: 6.1 vs 22.4, p<0.001; lung 3.4 vs 20.8, p<0.001) or control IgG plus radiation (heart 11.3 vs 22.4, p<0.05; lung 12.2 vs 20.8, p<0.05) in flow cytometric studies. Histologic analysis confirmed this increase in the comparison of anti-PD-1 antibody alone versus antibody plus irradiation (heart: 464 vs 679 cells per field, p<0.001; lung: 780 vs 1109, p<0.001) and control IgG plus radiation or combination therapy (heart: 526 vs 679 cells per field, p<0.001; lung: 848 vs 1109, p<0.05).
Conclusions: Combining anti-PD-1 antibody and thoracic irradiation results in T cell infiltration into lung and heart tissue and increases mortality in a preclinical model. We conclude that healthy tissue damaged by irradiation is more susceptible to further damage by activated T cells.