Recycling class I MHC antigens: dynamics of internalization, acidification, and ligand-degradation in murine T lymphoblasts.

We have previously shown that activated T lymphocytes spontaneously internalize their own surface class I MHC antigens and that this phenomenon is specific for these cells since it does not occur in B lymphocytes even after activation. The present work was aimed at defining the quantitative aspects of this phenomenon and, in particular, at the elucidation of the route of the internalized class I MHC antigens. We intended to determine if the internalized molecules are delivered to the lysosomal compartment and digested there or if instead they are brought back to the plasma membrane in a recycling pathway similar to that described for various cell surface proteins known to be engaged in the process of receptor-mediated endocytosis. We have devised a flow cytometric assay based on the use of fluorochrome-labeled monoclonal anti-H-2K antibodies to measure the kinetics of H-2K internalization. Comparison of the time necessary for the internalization of one-half of the surface H-2K molecules in activated T lymphocytes, which is approximately 1 hour, with the half-life of these molecules on the same cells, which is 14 hours, clearly indicates that the internalized molecules are not degraded but are instead recycled. The recycling takes place in an endosomal compartment with an average pH of about 5.6. The monoclonal anti-H-2K antibody used in these studies was not eluted from the H-2K molecules at this pH and is recycled along with them. On the other hand, protein A bound to the Fc of the anti-H-2K antibody was eluted at the low pH of the endosomes, delivered to lysosomes, and digested. We have therefore defined a novel phenomenon, namely the recycling of class I MHC antigens, which occurs selectively in T lymphocytes. The features of this phenomenon are similar to the recycling of surface receptors which mediate the endocytosis of a variety of extracellular ligands in different cells. However, no physiological extracellular ligand is known for class I MHC antigens. It is a reasonable speculation that upon activation T lymphocytes recycle their own surface class I MHC antigens as part of the complex machinery whereby these lymphocytes recognize and respond to non-self moieties on the plasma membranes of presentor or target cells.