Analysis of the requirement for β2‐microglobulin for expression and formation of human CD1 antigens

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Abstract
Human CD1 form a group of nonpolymorphic leukocyte surface molecules with homology to major histocompatibility complex (MHC) proteins. Recent findings in human and in mouse demonstrate the capacity of CD1 molecules to present nonpeptide components like lipids or lipoglycans as well as peptides. We studied the involvement of β2-microglobulin (β2m) in expression of the classic human CD1 proteins CD1a, CD1b, and CD1c. The β2m-deficient human melanoma cell line FO-1 was transiently transfected with either CD1a, CD1b, or CD1c DNA alone, or in combination with β2m using the adenovirus-enhanced receptor-mediated transfer infection system. Only co-transfection of FO-1 cells with CD1 + β2m resulted in the detection of CD1 Ag by monoclonal antibodies (mAb). This indicated that CD1 mAb recognized determinants are dependent on β2m and raised the question whether β2m-free forms of CD1 can be expressed. Therefore, to visualize CD1 molecule expression independently of β2m, we expressed tagged recombinant forms. A full-length CD1b construct tagged at the very C terminus with a small peptide was transported to the plasma membrane only when β2m was co-transfected. β2m involvement in the transport of CD1 was confirmed by expression of soluble forms of CD1a, CD1b, and CD1c in three different cell types. Analogous to tagged full-length CD1b, secretion of the soluble CD1 constructs was strictly dependent on β2m. The soluble CD1 chimeras were secreted as complexes with endogenous β2m. Thus, similar to its role for MHC class I expression, β2m is essential for processing and surface transport of the classic human CD1 molecules CD1a, CD1b, and CD1c.