The invention provides a compound represented by the formula (I)
wherein R
1
is a hydrogen atom or a hydroxyl group; two R
2
are the same or different and each is a hydrogen atom or a hydroxyl group; R
3
is a hydrocarbon group having 8-14 carbon atoms, which optionally has substituent(s); and R
4
is a hydrocarbon group having 18-24 carbon atoms, which optionally has substituent(s), or a salt thereof. The invention also provides a method of treating an autoimmune disease or suppressing immunity by administering the aforementioned compound or a salt thereof to a patient in need thereof.