Major histocompatibility complex class I molecules internalized via coated pits in T lymphocytes

Abstract

Endocytosis of the major histocompatibility complex (MHC)-encoded class I and class II molecules has been the subject of recent investigations. Class I molecules, which are key elements in T cell-mediated cytotoxicity¹., are differentially endocytosed by different cell types. Fibroblasts internalize their class I molecules via uncoated cell surface vesicles and tubular invaginations when these molecules are cross-linked with multivalent ligands^2,3. T lymphocytes internalize their class I molecules spontaneously, but B lymphocytes do not internalize them at all^.4–6. Here we describe a morphological investigation of the mechanism by which class I molecules are endocytosed by T lymphocytes. We show that, unlike fibroblasts, T lymphocytes spontaneously internalize 20–40% of their class I molecules in a process involving coated pits and coated vesicles. Thus, the endocytic pathway of class I molecules in T lymphocytes is similar to those of other more classical cell-surface receptors involved in receptor-mediated endocytosis⁷. In contrast, the same class I molecules remained on the cell surface in B lymphocytes. These data show that class I molecules are differentially regulated in T and B lymphocytes and fibroblasts.