Biosynthesis and glycosylation of the invariant chain associated with HLA-DR antigens

The biosynthesis and glycosylation of the invariant (I) chain associated with HLA-DR α-β chain complexes has been examined in human B lymphoblastoid cell lines. Two-dimensional polyacrylamide gel electrophoresis of HLA-DR immune precipitates from extracts of metabolically labeled cells demonstrated a processed form of the I chain, called Ip, which consists of a series of spots beginning near the basic I chain spot and ending near the acidic α spots. Pulse/chase labeling yielded information on the kinetics of processing of the invariant chain, as well as the finding that the association of Ip with α-β chain complexes appears to be transient. Neuraminidase treatment of immune precipitates confirmed that Ip was a form of the I chain containing up to eight sialic acid residues. Labeling in the presence of tunicamycin suggested the presence of both N-linked and O-linked oligosaccharide units on the invariant chain. This processing pattern does not appear to be an artifact of B cell lines; it was seen in peripheral blood lymphocytes, freshly purified monocytes, and in an interleukin 2-dependent continuous T cell line that expresses HLA-DR. In addition, there appears to be a free pool of the invariant chain and its processed form that is not found associated with DR α and β chains.