Cell surface expression of class II histocompatibility antigens occurs in the absence of the invariant chain

The invariant chain is a glycoprotein transiently associated with the alpha and beta subunits of class II antigens of the major histocompatibility complex during their transport to the cell surface. An expression assay with cDNA clones transfected into simian COS cells was used to test whether the invariant chain is required for assembly and transport of human class II antigens. COS cells do not express detectable levels of RNA from the endogenous invariant chain gene. Cell surface expression of the DP, DQ, and DR antigens was observed in COS cells transfected with the respective alpha and beta chain cDNA clones. Analysis of RNA from the transfected cells showed that the human genes were transcribed in COS cells and that the endogenous simian class II and invariant chain genes were not induced. Cotransfections with an invariant chain cDNA clone did not alter the levels of class II antigens at the cell surface. Biosynthetic labeling and immunoprecipitation demonstrated that the invariant chain cDNA was expressed into a protein which associated with DR alpha and beta chains. Efficient expression of DR antigen in absence of invariant chain was also observed at the surface of a human fibroblast line stably transfected with DR alpha and beta cDNA. This study demonstrates that expression of all three human class II antigens can be achieved with cDNAs cloned in expression vectors. Furthermore, cell surface expression of class II major histocompatibility complex antigens can occur in absence of invariant chain. The postulated role of the invariant chain in class II antigen transport to the cell surface must be reevaluated. The invariant chain may rather be involved in functional properties of class II molecules such as antigen presentation.