Retroelements in the human MHC class II region.

Molecular genetic studies of the human major histocompatibility complex (MHC) have led to the identification of more than 200 genes. Besides the large number of genes in the MHC, densely clustered areas of retroelements have been identified. These include short and long interspersed elements (SINEs and LINEs), and human endogenous retroviruses (HERVs). The presence of retroelements in the MHC provides a clear example of how these elements affect the genome plasticity of the host. Comparative analyses of these retroelements have proven highly useful in evolutionary studies of the MHC. Recently, HERV-encoded superantigens have been implicated as candidate autoimmune genes in type I diabetes and multiple sclerosis. In addition, genetic analyses have revealed that autoimmune diseases show strong associations with MHC class II genes. The intriguing correlations between retroviral encoded antigens, MHC class II genes and the development of autoimmune disease merit intense future investigations of retroelements, in particular those endogenous retroviruses located in the MHC class II region proper.