PTM (post-translational modification) is a chemical modification that a protein undergoes during or after its translation. Most of these are reversibly regulated by enzymes, adding and removing modifications. Examples of PTM include：1) formation of tertiary structure of proteins by disulfide bonds; 2) addition of carbohydrate chains to secretory proteins and membrane-bound proteins; 3) regulation of enzyme activity and on/off of signal transduction by phosphorylation and dephosphorylation; 4) degradation of proteins by ubiquitination and SUMOylation.

These modifications play an important role in regulating protein folding, targeting to specific intracellular compartments, interaction with ligands or other proteins, and ultimately, immunogenic properties. For instance, citrullination is the best-characterized PTM in the field of rheumatology, and anti-cyclic citrullinated peptides are the gold standard for the diagnosis of rheumatoid arthritis (RA). In this special issue, we focus on PTM in the regulation of immune cells.