194 Internalization of Class I Mhc Molecules in T Cells

A vast port ion o f the immunological l i terature has been dedicated, in the last 15 years, to the role of the major histocompatibility complex (MHC) ~ in the immune response. T h e r e is now agreement (1, 2) that the molecules coded by the genes of the MHC are essential for cellular interactions and ultimately for self, not-self distinction, at least for what concerns the response o f the T system and for the product ion of antibodies to T -dependen t antigens. In spite o f the extensive efforts and o f the considerable advances in the immunogenet ics and biochemistry o f the MHC system, several important aspects of the immune function o f this system are still unknown. We have chosen to re-examine the cytology of histocompatibility antigens, with a view to some more recently investigated aspects o f cell receptor dynamics, as a contr ibut ion to the knowledge o f the cellular basis of MHC function. This paper is dedicated to Class I MHC molecules and T cells. It shows that Class I MHC molecules are spontaneously internalized at a rapid rate by activated T lymphoid cells in a manner that has many similarities with the recently elucidated internalization and recycling o f different receptors in nonimmunocytes (3).