The drug is based on a fungus known as "winter-insect-summer-plants". An excerpt describing the fungus:
Dr. Fujita says he reasoned that an even more powerful immunosuppressant chemical ought to be present in a group of Asian fungi known in Chinese and Japanese as "winter-insect-summer-plants." These fungi attack insects in the winter with their chemical arsenal. By summertime, the insect is dead and its corpse has been transformed into a vessel for the blooming fungus. Ironically, the same properties that make the chemical deadly in the insect world may also have a helpful side for people suffering from certain autoimmune diseases, in which an overactive immune-system response causes the body to attack its own cells.
There are many natural remedies used in Chinese and Japanese medicine, including herbs and fungus. Perhaps many of these work because they are immunosuppressive.