Processing of proteins in autophagy vesicles of antigen-presenting cells generates citrullinated peptides recognized by the immune system.

Our laboratory has been investigating for some time the nature of the response of T lymphocytes in autoimmunity in the reactions against self-proteins that result in a number of diseases, such as type 1 diabetes, multiple sclerosis, rheumatoid arthritis (RA) and others. T cells recognize peptides generated from proteins that are processed by antigen-presenting cells (APC). The peptides may derive from exogenous proteins or from the normal catabolism of self-proteins. The peptides complexed to major histocompatibility complex (MHC) molecules constitute the chemical entity that is engaged by the antigen-receptor of T cells. An important hypothesis postulates that self-peptides that suffer post-translational modifications in the APC may form neo-antigens that are recognized by the immune system and form the target of autoimmunity. Our interest in citrullination in the context of antigen processing and presentation stemmed from studies suggesting that an immune response to citrullinated self-peptides may be involved in autoimmunity. In a first publication, we found T cells that specifically recognized citrullinated peptides after immunization of inbred mice with standard foreign proteins. We used the small protein hen-egg white lysozyme. These T cells only recognized the citrullinated peptide and not the unmodified one, thus proving that a neo-epitope had been created by this modification. But how this modification took place was not known. Our recent report describes a central role for autophagy in citrullination of peptides by APC.