Drug transfer into lymphoblasts mediated by liposomes bound to distinct sites on H-2 encoded I-A, I-E, and K molecules.

Protein A from S. aureus was covalently coupled to small sonicated liposomes containing methotrexate (MTX) and carboxyfluorescein (CF). These liposomes were incubated with mitogen-stimulated CBA T or B cells preincubated with one of a panel of murine IgG2 monoclonal antibodies directed at distinct sites on H-2Kk, I-Ak, or I-Ek molecules. Liposomes became bound to the cells as a consequence of the interaction of protein A on the liposome and antibody on the cell. The number of bound liposomes was measured by fluorescence, and the endocytosis of liposomes was evaluated by the MTX-mediated reduction in radiolabeled deoxyuridine incorporation. T cells were sensitive to MTX in liposomes bound to all anti-H-2Kk antibodies tested, but B cells were essentially insensitive, even though they expressed more H-2Kk, bound more liposomes directed at this molecule, and were equally sensitive to free MTX. By contrast, B cells were sensitive to liposome-encapsulated MTX when liposomes were bound to any antibody directed at I-Ak. The situation was more complex with respect to I-Ek. Antibodies directed at one part of the molecule were quite effective at drug delivery, while antibodies directed at other, spatially separate domains were much less effective, even though the number of liposomes bound to these various determinants was the same. The results suggest differential regulation of the internalization of H-2K by T and B cells, and are compatible with the existence of functionally distinct I-E molecules.