The CD1 system

CD1 was originally defined as a cluster of mAb directed against leukocyte surface determinants. The antigens recognised appeared to share three main features: a restricted tissue expression (cortical thymocytes and some dendritic cells), a structure consisting of a heterodimer of an alpha chain of- 45 kDa associated with beta-2 microglobulin (b2m) and a lack of significant polymorphism. This phenotype is also characteristic of mouse TL antigens. By analogy, it was assumed that CD1 would be encoded in the human MHC and would belong to the category of non-classical MHC class I antigens. Although the function of these molecules is unknown, they are considered to be functionally distinct from classical MHC class I, which are ubiquitously expressed, highly polymorphic and known to function in the binding and presentation of foreign peptides to T cells. The study of the molecular genetics of CD1 has partly overturned these expectations, revealing fundamental differences between CD 1 and non-classical MHC class I antigens, as well as supporting the view that the former must have an important function. As a comprehensive review on CD 1 has just recently been published, this paper will only focus on the developments which are both most recent and, in our opinion, most relevant to the biological function of CD 1.