The invention provides an effective amount of a diketopiperazine having the following formula for use in the treatment of a T-cell mediated autoimmune disease:wherein:R1 and R2, which may be the same or different, each is:(a) a side chain of an amino acid, wherein the amino acid is glycine, alanine, valine, norvaline, α-aminoisobutyric acid, 2,4-diaminobutyric acid, 2,3-diaminobutyric acid, leucine, isoleucine, norleucine, serine, homoserine, threonine, aspartic acid, asparagine, glutamic acid, glutamine, lysine, hydroxylysine, histidine, arginine, homoarginine, citrulline, phenylalanine, p-aminophenylalanine, tyrosine, thyroxine, cysteine, homocysteine, methionine, penicillamine or ornithine； provided, however, that when R1 is the side chain of asparagine or glutamine, then R2 cannot be the side chain of lysine or ornithine, and when R1 is the side chain of lysine or ornithine, then R2 cannot be the side chain of asparagine or glutamine；(b) a derivative of a side chain of an amino acid, wherein the amino acid is one of those recited in (a), and the derivatized side chain has:(i) an -NH2 group replaced by an -NHR3 or -N(R3)2 group, wherein each R3 may independently be a substituted or unsubstituted alkyl, cycloalkyl, heterocycloalkyl, aryl, alkylaryl, arylalkyl or heteroaryl；(ii) an -OH group replaced by an -O-PO3H2 or -OR3 group, wherein each R3 may independently be a substituted or unsubstituted alkyl, cycloalkyl, heterocycloalkyl, aryl, alkylaryl, arylalkyl or heteroaryl；(iii) a -COOH group replaced by a -COOR3 group, wherein each R3 may independently be a substituted or unsubstituted alkyl, cycloalkyl, heterocycloalkyl, aryl, alkylaryl, arylalkyl or heteroaryl；(iv) a -COOH group replaced by a -CON(R4)2 group, wherein each R4 may independently be H or a substituted or unsubstituted alkyl, cycloalkyl, heterocycloalkyl, aryl, alkylaryl, arylalkyl or heteroaryl；(v) an -SH group replaced by -S-S-CH2-CH(NH2)-COOH or -S-S-CH2-CH2-CH(NH2)-COOH；(vi)
