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The mechanism of pseudoprogression after immunotherapy. (A) T cells were inactivated by the PD-L1 and CTLA-4 presented by tumor cells or antigen-presenting cells (APCs). (B) T cells were reactivated after the administration of immune checkpoint inhibitors such as anti-PD-1/PD-L1/CTLA-4. (C) Activated T cells infiltrate tumor lesions and kill the tumor cell. (D) Antigens released by the death of tumor cells attract more infiltrating inflammatory cells. (E) Shrinking tumor tissues can cause vascular tears and hemorrhage in locoregional lesions. (F) The inflammatory response and hemorrhage cause the edema of lesions. (G) The necrotic byproducts of dead tumor cells cannot be absorbed immediately and accumulate in locoregional lesions. Inflammatory cell infiltration, hemorrhage, edema and necrosis enlarged the lesions in imageologic assessments and indicate pseudoprogression.