Cd1

CD1, Cluster of differentiation 1, consists of a family of several human glycoproteins that are expressed on the surface of various antigen-present cells. They belong to the so-called MHC-like glycoproteins and occur in humans in 5 different isoforms: CD1a-CD1e. The coding gene is located on chromosome 1 q22-q23.

CD1-glycoproteins are divided into 2 groups:

• Group 1: CD1a, CD1b and CD1c; these glycoproteins are expressed by antigen presenting cells.
• Group 2: CD1d; this glycoprotein is expressed by a large number of different cells.
• CD1e: intermediate, intracellular form of CD1 (this protein is not expressed on the cell surface). The function of CD1e is still unclear.

CD1 is expressed in the endoplasmic reticulum (ER) by most B cells, but also by epithelial cells. CD1a-c is also found on dendritic cells. In the endoplasmic reticulum, the CD1 proteins are assembled and loaded with lipids.

The quarternary structure of the CD1 molecule is very similar to that of the MHC complex. It is composed of several segments that form an antigen-binding groove similar to the MHC-1 complex, which is made up of hydrophobic amino acids. The CD1 molecule binds and presents exogenous lipid antigens of different lipid classes such as glycolipids, phosphlipids, sphingolipids, etc. so that they can be recognised by T-cell receptors. Mycobacterial lipids are preferably processed and presented by CD1a-CD1c.

The CD1 presentation pathway is important for antimicrobial defence.