Major histocompatibility complex class I loss due to the abnormality of β2-microglobulin gene is one of the mechanisms underlying delayed relapses in melanoma patients long after the initial positive responses to anti–PD-1 therapy. However, the tumor-specific reactivity of tumor-infiltrating lymphocytes in tumor lesions that lost major histocompatibility complex class I expression has not been well evaluated. We report the case of a 55-year-old woman with two metastatic melanoma lesions. After a 12-month period of successful tumor suppression by anti–PD-1 antibody therapy, one lesion started to grow again. We resected both lesions and examined the tumor cells and tumor-infiltrating lymphocytes. The shrinking lesion consisted of necrotic tissue and macrophages, and the enlarged lesion consisted of both necrotic tissue and viable tumor cells. The tumor cells completely lost major histocompatibility complex class I expression, but it was restored upon retroviral transduction of the normal β2-microglobulin gene. When we checked the tumor-specific reactivity of tumor-infiltrating lymphocytes derived from the relapsing lesion, we found that these tumor-infiltrating lymphocytes failed to recognize the native tumor cells derived from the lesion, but strongly recognized the major histocompatibility complex class-I–recovered cells by β2-microglobulin transduction. Our report emphasizes the limitations of T-cell–based immunotherapy and highlights the importance of developing alternative strategies for such cases.
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