Programmed cell death 1 (PD-1) and its ligand 1 (PD-L1) inhibitors have quickly become standard of care for patients with advanced non-small cell lung cancer and increasing numbers of other cancer types. In this report, we discuss the clinical history, pathological evaluation, and genomic findings in a patient with metastatic lung squamous cell cancer (SCC) who developed severe nivolumab-induced pneumonitis preceding durable clinical remission after three doses of nivolumab.A patient with chemotherapy-refractory, metastatic lung SCC developed symptomatic pneumonitis by week 4 after nivolumab treatment, concurrently with onset of a potent antitumor response. Despite discontinuation of nivolumab after three doses and the use of high dose oral corticosteroids for grade 3 pneumonitis, continued tumor response to a complete remission by 3 months was evident by radiographic assessment. At the time of this submission, the patient has remained in clinical remission for 14 months. High PD-L1 expression by immunohistochemistry staining was seen in intra-alveolar macrophages and viable tumor cells in the pneumonitis and recurrent tumor specimens, respectively. Tumor genomic profiling by FoundationOne targeted exome sequencing revealed a very high tumor mutation burden (TMB) corresponding to 95-96 percentile in lung SCC, i.e., 87.4-91.0 and 82.9 mut/Mb, respectively, in pre- and post-nivolumab tumor specimens. Except for one, the 13 functional genomic alterations remained the same in the diagnostic, recurrent, and post-treatment, relapsed tumor specimens, suggesting that nivolumab reset the patient's immune system against one or more preexisting tumor-associated antigens (TAAs). One potential TAA candidate is telomerase reverse transcriptase (TERT) in which an oncogenic promoter -146C>T mutation was detected. Human leukocyte antigen (HLA) typing revealed HLA-A*0201 homozygosity, which is the prevalent HLA class I allele that has been used to develop universal cancer vaccine targeting TERT-derived peptides.Nivolumab could quickly reset and sustain host immunity against preexisting TAA(s) in this chemotherapy-refractory lung SCC patient. Further mechanistic studies are needed to characterize the effective immune cells and define the HLA-restricted TAA(s) and the specific T cell receptor clones responsible for the potent antitumor effect, with the aim of developing precision immunotherapy with improved effectiveness and safety.