mature dendritic cells, is a key event in the induction of a T cell response. After internalization by dendritic cells, proteins are enzymatically cleaved within endolysosomal compartments. Some of the resulting peptides, which are of considerably variable length, bind to HLA-DR molecules in a sequence dependent and HLA-DM-edited manner. It has been established that PTMs can increase the peptide binding affinity to MHC class II molecules, or interfere with the proteolysis of proteins. This may, in addition to the alterations introduced by the modified amino acid residue itself, result in the generation of new, naturally processed HLA-DR associated peptides, potentially giving rise to T cell epitopes. For some PTMs, such as maleylation and nitration, there is evidence that protein uptake by antigen presenting cells can be altered. We have studied whether there is a difference between the peptides derived from the allergen Bet v 1 presented via HLA-DR and those derived from post-translationally chemically modified Bet v 1 nitro. For this purpose, immature DCs were loaded with unmodified Bet v 1 or Bet v 1 nitro. After affinity purification of the HLA-DR peptide complexes, the HLA-DR associated peptides were isolated by acidic elution and identified by liquid chromatography-mass spectrometry and the identified Bet v 1 or Bet v 1 nitro derived peptides were compared with respect to peptide clusters, peptide length variants and copy number of peptides. Since changes in the pattern of presented HLA-DR associated peptides on DCs can also change the recognition by T lymphocytes, and since the conversion of tyrosine to nitrotyrosine has already been shown to affect the reactivity of T cells for other proteins, we also addressed the question whether peripheral blood mononuclear cells loaded with Bet v 1 nitro can activate T lymphocytes more efficiently than PBMCs loaded with unmodified Bet v 1. For this purpose Bet v 1-specific T cell lines were generated from birch pollen allergic patients and T cell proliferation towards unmodified Bet v 1 or Bet v 1 nitro was analyzed. Our study demonstrates that presentation of HLA-DR associated peptides was altered upon nitration of Bet v 1. Nitration resulted in a 2.9-fold increased number of identified peptide clusters, a 7.2-fold increase in the overall number of peptide length variants and a 12.2-fold increase in the copy number of identified peptides derived from major birch pollen allergen. An increase in allergen-derived peptide presentation was observed not only for sequence stretches containing tyrosine residues but also in regions devoid of tyrosine.