Methods and compositions for noninvasive detection of organ transplant rejection

A method comprising:

• i) contacting a sample with a nanosensor, said nanosensor comprising:
• a) a peptide substrate which is capable of being cleaved by a target protease, wherein said target protease has increased activity when encountering a transplant rejection or an autoimmune disease, and
• b) a detectable reporter coupled to said peptide substrate;
• ii) detecting a level of said detectable reporter.