Dual blockade of PD-1 and PD-L1 is effective but causes severe pneumonitis in lung metastasis: Syngeneic mice study for validation of clinical observation

e14115 Background: In clinical practice, the shifting use of PD-1 inhibitors and its ligand PD-L1 inhibitors due to ineffectiveness or toxicity is becoming more common and has distinct responses. To validate clinical observation, we examined the anti-cancer effect, tumor microenvironment (TME) modulation and toxicity of dual blockade of PD-1 and PD-L1 in mice. Case Presentation: Three patients with metastatic lung adenocarcinoma received nivolumab (5, 5 and 11 doses); after the initial response and subsequent progression, they were shifted to receive atezolimumab (1, 1 and 4 doses) at intervals of 70, 38 and 43 days, respectively. All 3 patients obtained partial response according to iRECIST. Two of the 3 patients developed pneumonitis at days 3 and 15 after the first dose of atezolimumab and pneumonitis subsided by methylprednisolone injection for 7 and 4 days. Methods: The Balb/c mice bearing lung metastasis of CT26 colon cancer cells were treated. Assessments included optical imaging for tumor burden, multi-parameter flow cytometry for TME, pathology and immuohistochemistry. Results: The pathology of lung showed that all the mice (100%, n = 3 for each group) treated with a combination of anti-PD-1 and anti-PD-L1, either sequential or simultaneous, developed severe clustered inflammatory infiltrates, mucus filling in bronchus and thrombi in small vessels. Among infiltrating leukocytes, combinatorial blockade increased LyC6+ inflammatory monocytes and NKG2D+ T cells and reduced tumor-associated macrophages in lung. Peripheral leukocytes in spleen had no such alteration. In lung metastatic foci, the expression of PD-1 was noted mainly within tumors, and this expression was reduced by combinatorial blockade. PD-L1 expression had no change. For control of lung metastasis, the combinatorial treatments exhibited better control, and anti-PD-L1 followed by anti-PD-1 was the most effective. No significant pneumonitis was noted in single blockade groups. Conclusions: The combination of PD-1 and PD-L1 blockade, either sequential or concurrent, may have better tumor control but cause severe pneumonitis. Dual blockade of PD-1 and PD-L1 should be used with caution.