Background: Hemophagocytic lymphohistiocytosis (HLH) is a rare and life-threatening state of immune hyperactivation. It has the highest mortality rate among all hematological immune-related adverse events (irAEs) of immune checkpoint inhibitors (ICIs) when treating various cancers. However, the predisposing factors of HLH have rarely been mentioned in previous research.Case Description: Herein, we report 2 cases of HLH following treatment with pembrolizumab. A patient was diagnosed with thymic carcinoma (TC) and possible Sjögren's syndrome (SS), while another was diagnosed with non-small cell lung cancer (NSCLC) and Epstein-Barr virus (EBV) infection, and both were positive for antinuclear antibodies. Both cases experienced transient immune-related fever on day 7 after pembrolizumab administration and splenomegaly on day 10. Then recurrent high-grade fever appeared, and liver function impairment, highly elevated ferritin, and hypertriglyceridemia were tested. After the diagnosis of HLH, both patients were treated with dexamethasone and etoposide without relapse in our follow-up.Conclusions: Considering the widespread use of ICIs and the high mortality rate of HLH, the immunerelated fever, splenomegaly, and other signs of hyperinflammation after the infusion of ICIs, are worthy of attention to the presence of HLH. Preexisting autoimmune diseases (ADs) or positive antibodies, concomitant infection, and the setting of thymic epithelial tumors (TET) may be predisposing factors for HLH. And increased caution is needed before the initiation of ICIs for patients with 2 or more predisposing factors.

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