SURFACE EXPRESSION OF THE INVARIANT CHAIN (CD74) IS INDEPENDENT OF CONCOMITANT EXPRESSION OF MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II ANTIGENS

Whether or not intracellular transport and surface expression of the i nvariant chain (Ii; CD74) occurs independent of the presence of major histocompatibility complex (MHC) class II molecules was examined by co mparing the class II-negative mutant lymphoblastoid cell line 174 x CE M.T2 (T2) and its class II-positive parental cell line 174 x CEM.T1 (T 1). We found a similar proportion of Ii being transported to the Golgi complex in T1 and T2, as monitored by the degree of sialic acid addit ion to glycan side chains of Ii. In agreement with this result, T1 and T2 expressed comparable amounts of Ii at the cell surface, as measure d by flow cytometry. This indicates that, although not associated with class II molecules, a proportion of Ii is transported to the plasma m embrane. Both in TI and T2, surface Ii (sIi) was rapidly internalized with a half-life of 3-4 min, suggesting that some Ii enters the endocy tic route via the cell surface after being internalized. Our data demo nstrate transport of Ii on a route alternative to the endocytic pathwa y. This alternative route could also account for delivery of newly syn thesized class II-Ii complexes to processing compartments in antigen-p resenting cells.