Introduction- A wide variety of normal cellular events including cell signaling, DNA replication and protein degradation are regulated by protein post-translational modification (PTM). Oligopeptides are liberated from proteins during their intracellular degradation via the proteasome and a few of them retain their PTMs and along with native peptides are transported into the endoplasmic reticulum, where they are loaded onto Human Leukocyte Antigen (HLA) class 1 molecules and presented on the surface of antigen presenting cells. The composition of these HLA-peptide complexes dictates T cell recognition and any subsequent immune response. Various studies have shown that in cancer, a dysregulation in different signaling cascades occurs which increases PTMs. The aim of this study is to identify modified proteins and peptides in different hematological cancers including different subtypes of leukemias and myelomas.
Methods- 1 X 109 cells of human acute monocytic leukemia cell line, THP-1 (HLA-A*02:01, B*15:11, C*03:03, DRB1*01:01, 15:01, DQB1*05:01, 06:02, DPB1*02:01, 04:02) were lysed. Immunoaffinity purification was used to extract peptides from HLA class I and II alleles. Direct proteome analysis of the peptide ligands was performed using liquid chromatography and tandem mass chromatography (LC-MS/MS).
Results- A total of 5788 peptides were identified from HLA-A*02:01, 2259 peptides from pan HLA class I and 2027 peptides for pan HLA class II. Several different modifications were found; prominently oxidation and deamidation along with more rarely detected phosphorylation (n=37) and methylation (n=100). These rarer modified peptides will be tested in cellular assays, using peripheral blood mononuclear cells hematological cancer patients and healthy donors (as a control group) to determine their ability to evoke a T cell mediated immune response.
Conclusion- This study implicates PTM proteins and peptides as an immunological signature of ‘transformed’ cancer cells that can lead to development of newer diagnostic tools and better immunotherapeutic approaches.