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Background: The combination of PD-1 inhibitors and cytotoxic drugs is reported to enhance anti-tumor activity in non-small cell lung cancer; however, the underlying synergistic mechanisms remain uncertain. This retrospective case series was designed to investigate objective response and survival rates of salvage chemotherapy following nivolumab and explore the immunohistochemical profiles of tumor-infiltrating immune cells., Methods: The medical records of 37 patients administered nivolumab were retrospectively reviewed. Overall response rate and progression-free survival were compared among three groups: salvage chemotherapy following nivolumab, nivolumab therapy alone, and chemotherapy preceding nivolumab., Results: Eight cases met the study criteria. Salvage chemotherapy following nivolumab improved the overall response rate to 62.5% (95% confidence interval [CI] 34.4-90.6%; P = 0.004) and median progression-free survival to six months (95% CI 4.6-7.4; P = 0.016), compared to nivolumab alone and preceding chemotherapy. The response to salvage chemotherapy was not associated with tumor PD-L1 expression. A partial response was achieved in four cases with ≤ 5% and ≤ 2.9 cells/mm 2 of PD-1 + immune cells, whereas stable disease and progressive disease were observed in three cases with ≥ 30% and ≥ 12.7 cells/mm 2 . Responders had fewer PD-1 + immune cells than non-responders (percentage P = 0.028; density P = 0.034)., Conclusion: Salvage chemotherapy following nivolumab improved anti-tumor activity regardless of tumor PD-L1 status, but nivolumab following chemotherapy did not. The presence of few PD-1 + tumor-infiltrating immune cells may serve as a potential predictor of response to salvage chemotherapy. Further studies involving a large cohort are needed to clarify how nivolumab re-sensitizes the tumor immune microenvironment to chemotherapy., (© 2018 The Authors. Thoracic Cancer published by China Lung Oncology Group and John Wiley & Sons Australia, Ltd.)