EMPTY L(D) MOLECULES CAPTURE PEPTIDES FROM ENDOCYTOSED HEPATITIS-B SURFACE-ANTIGEN PARTICLES FOR MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-I RESTRICTED PRESENTATION

Peptides recognized by CD8+ cytotoxic T lymphocytes in the context of major histocompatibility complex (MHC) class I molecules are usually d erived from endogenous proteins synthesized within the cell. Exogenous 22-nm hepatitis B surface antigen (HBsAg) particles are taken up by m any cells, and are processed in a novel peptide-transporter-independen t, endosomal or lysosomal pathway for class I (L(d))-restricted epitop e presentation. Here, we present evidence that 'empty' L(d) molecules derived from the cell surface are involved in presenting antigenic pep tides from endocytosed HBsAg particles. Intracellular assembly of pres entation-competent, trimeric L(d) molecules required endocytosis of th e exogenous antigen and 'empty' L(d) molecules. These data assign a fu nctional role to surface-associated, 'empty' MHC class I molecules.