Soluble major histocompatibility complex molecules in immune regulation: highlighting class II antigens

Summary The involvement of major histocompatibility complex (MHC) antigens in the development and regulation of immune response has been well defined over the years, starting from maturation, antigenic peptide loading, migration to the cell membrane for recognition by the T‐cell receptor and recycling for immune response cessation. During this intracellular trafficking, MHC antigens find a way to be excreted by the cells, because they can be found as soluble MHC class I (sMHC‐I) and class II (sMHC‐II) molecules in all body fluids. Although secretion mechanisms have not been sufficiently studied, sMHC molecules have been shown to display important immunoregulatory properties. Their levels in the serum have been shown to be altered in a variety of diseases, including viral infections, inflammation, autoimmunities and cancer, etc. while they seem to be involved in a number of physiological reactions, including maintenance of tolerance, reproduction, as well as mate choice vis‐à‐vis species evolution. The present review aims to present the thus far existing literature on sMHC molecules and point out the importance of these molecules in the maintenance of immune homeostasis. Representative T‐cell receptor (TCR)–MHC interactions in the presence or not of an antigen‐presenting cell (APC). All combinations may take place in the context of immune recognition, with higher or lower affinities. Each combination results in a different type of activation, dictating the fate of the immune response.