The invention provides an agent comprising: a cyclic peptide comprising a T cell antigen peptide； wherein in cyclised form the T cell antigen peptide is not capable of eliciting a T cell response； and wherein the T cell antigen peptide is rendered capable of eliciting a T cell response by selective cleavage of one or more cleavage sites in the cyclic peptide. The agent may be used to prevent or treat a condition characterised by the presence of unwanted cells.
