Intracellular and cell surface heterotypic associations of human leukocyteantigen-DR and human invariant chain

The intracellular and cell-surface heterotypic associations of HLA-DR in th e presence and absence of the invariant chain were investigated. Simultaneo us confocal microscopy imaging of the Golgi apparatus and HLA-DR molecules revealed that cells transfected only with HLA-DR and not the invariant chai n or HLA-DM, accumulate class II molecules mostly in the Golgi apparatus, p roximal to the cell nucleus. In cont rast, in cells transfected with both H LA-DR and the invariant: chain, or HLA-DR, the invariant chain and HLA-DM, the class II molecules are more evenly distributed in intracellular compart ments. Confocal microscopy and flow cytometry revealed that in the absence of the invariant chain, a greater number of HLA-DR molecules are transporte d to nr the cell surface. Biochemical experiments and nonequilibrium pH gra dient electrophoresis revealed that HLA-DR associates with surface invarian t chain in the presence of HLA-DM. In cells that lack HLA-DM, no cell-surfa ce association of HLA-DR and Ii was observed. Taken together, these results reveal two separate and distinct functions for surface and intracellular i nvariant chain subsets. The intracellular invariant chain "arrests" the cla ss II molecules in the endocytic pathway. In contrast, cell-surface invaria nt chain associates with class II molecules at the cell surface, possibly p laying a role in recycling empty class II molecules or as an accessory mole cule. (C) American Society for Histocompatibility and Immunogenetics, 1999. Published by Elsevier Science Inc.