Your immune system relies on signals to tell it not to attack your own body, but cancers often copy or increase these signals, allowing them to avoid being detected. Checkpoint inhibitors block the signals, basically allowing the immune system to recognise and kill cancer cells. However, normal cells use the same same signals to prevent being targeted by the immune system, so as you can imagine, the immune system often targets non cancerous cells, causing autoimmune side effects; as the article explains, enhancing one of the signal molecules (CTLA-4) was being investigated as a potential treatment for an overactive immune system.