PD-1: Its Discovery, Involvement in Cancer Immunotherapy, and Beyond.

On December 10, 2018, I was sitting among the big crowd of audience, as one of the invited guests to the ceremony, in the Stockholm Concert Hall. When King of Sweden Carl XVI Gustaf bestowed the diploma and medal of Nobel Prize of Physiology or Medicine 2018 on Dr. Tasuku Honjo and shook his hand for a while, surrounded by the thunderous applause and energetically blessing orchestral music, I thought that it had been a long journey for the molecule that we had first isolated in the early 1990s. Although it was truly a commemorable moment in the history of the programmed death-1 (PD-1) research, I believe we still have a long way to go. In this review article, I will explain why I think so, particularly by focusing on the potential role(s) that PD-1 appears to play in self-nonself discrimination by the immune system.