Cell-sized, supported artificial membranes (pseudocytes): Response of precursor cytotoxic T lymphocytes to class I MHC proteins

Novel cell-sized, supported artificial membranes bearing class I antigens have been prepared by a simple dialysis procedure and then used to study the requirements for antigen recognition by precursor cytotoxic T lymphocytes (CTL). The membranes were made by mixing lipid, H-2 antigen, and C₁₈ alkylated 5 μm silica beads in deoxycholate, and dialyzing to remove the detergent. The H-2 antigen-bearing, cell-sized beads, termed pseudocytes (artificial cells), were able to simulate generation of secondary CTL responses with the same specificity as alloantigen-bearing spleen cells. Comparative analyses demonstrated that the size of an antigen-bearing structure, and thus its potential for multivalent interaction, was a critical determinant of effectiveness of antigen recognition, and showed that H-2 antigen was recognized as effectively on cell-sized beads as on allogeneic spleen cells. Generation of a response to antigen on the cell-sized beads was completely dependent on addition of lymphokines to the cultures. Thus, unlike liposomes, H-2 antigen on beads was not available to accessory cells for stimulation of Ia-dependent production of lymphokines by T helper cells. These results, as well as direct observations by microscopy, strongly indicate that antigen is recognized on the surface of the beads. Despite effective stimulation of secondary CTL responses, antigen on beads was completely inactive in stimulating a primary CTL response by naive spleen cells. The results of mixing experiments by using beads and alloantigen-bearing cells or plasma membrane vesicles indicate that the lack of a primary response may result from a requirement for a soluble factor(s) that is not needed for generation of secondary responses. The unique advantages of cell-size supported membranes of studying antigen recognition by T cells are discussed. The beads can be handled and used like antigen-bearing in functional assays, while possesssing well-defined, readily varied, and easily quantitated composition.