PD-1 blockade is a cancer immunotherapy effective in various types of cancer. In a fraction of treated patients, however, it causes rapid cancer progression called hyperprogressive disease (HPD). With our observation of HPD in ∼10% of anti–PD-1 monoclonal antibody (mAb)-treated advanced gastric cancer (GC) patients, we explored how anti–PD-1 mAb caused HPD in these patients and how HPD could be treated and prevented. In the majority of GC patients, tumor-infiltrating FoxP3highCD45RA−CD4+ T cells [effector Treg (eTreg) cells], which were abundant and highly suppressive in tumors, expressed PD-1 at equivalent levels as tumor-infiltrating CD4+ or CD8+ effector/memory T cells and at much higher levels than circulating eTreg cells. Comparison of GC tissue samples before and after anti–PD-1 mAb therapy revealed that the treatment markedly increased tumor-infiltrating proliferative (Ki67+) eTreg cells in HPD patients, contrasting with their reduction in non-HPD patients. Functionally, circulating and tumor-infiltrating PD-1+ eTreg cells were highly activated, showing higher expression of CTLA-4 than PD-1− eTreg cells. PD-1 blockade significantly enhanced in vitro Treg cell suppressive activity. Similarly, in mice, genetic ablation or antibody-mediated blockade of PD-1 in Treg cells increased their proliferation and suppression of antitumor immune responses. Taken together, PD-1 blockade may facilitate the proliferation of highly suppressive PD-1+ eTreg cells in HPDs, resulting in inhibition of antitumor immunity. The presence of actively proliferating PD-1+ eTreg cells in tumors is therefore a reliable marker for HPD. Depletion of eTreg cells in tumor tissues would be effective in treating and preventing HPD in PD-1 blockade cancer immunotherapy.

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A 20‐year‐old man was admitted with acute respiratory failure. He had started smoking 20 heat‐not‐burn cigarettes (HC) per day 6 months previously, then purchased a second device for smoking HC to increase smoking to 40 cigarettes per day 2 weeks before hospitalization. Acute eosinophilic pneumonia (AEP) was diagnosed based on medical history, chest high‐resolution computed tomographic findings, and bronchoalveolar lavage fluid eosinophilia. On starting treatment with prednisolone, the patient exhibited complete recovery. A relationship between cigarette smoking and AEP has been suggested. HC were released in September 2015 in Japan, Italy, and Switzerland. HC attract attention as a cigarette generating less harmful substances than a conventional cigarette. We herein report the first case of AEP caused by smoking HC. HC are expected to spread around the world. In the same way as a conventional cigarette, HC should be recognized as a potential cause of AEP.

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