Liposome facilitated xenogeneic approach for studying human colon cancer immunity: carrier and adjuvant effect of liposomes.

Liposomes prepared with human LS174T colon tumour cell membranes induce specific primary xenogeneic immune responses in BALB/c splenocytes in vitro. Characterization of the adjuvant role of these liposomes was accomplished by determining the effect on immune induction of several modifications on the liposomal carrier. The results showed that the carrier effect of liposomes was mediated primarily by tumour antigens exposed on the outer surface. Trypsin treatment of the liposomes eliminated 95% of the surface protein and significantly (P less than 0.05) reduced the ability of liposomes to induce cytotoxic splenocytes. The generation of cytolytic activity with liposomes was dose-dependent, with a 10 micrograms protein threshold and a maximal response at 100 micrograms. 'Rigid' liposomes were shown to be significantly (P less than 0.05) more efficacious than fluid liposomes in inducing cytotoxicity. In addition, the data indicate that the xenogeneic cell-mediated immunity exhibits identical classes of effector cells as found in murine-murine reactions. Lymphocytes bearing the THY-1, Lyt-1 and Lyt-2 surface markers were necessary for immune induction. The role of Lyt-123 subpopulation was suggested by the inability to achieve normal cytolytic levels by reconstitution with Lyt-1 plus Lyt-2 cells. Adherent cells were, as expected, necessary for the generation of primary immunity. Indeed, the interaction of I-A+ adherent cells with liposomes for at least 8 h was required to generate subsequent maximal T cell cytotoxic activity. The phenotype of the cytotoxic effector cell was Thy-1+, Lyt-2+, and I-Ad-. If this were an allo-or syngeneic, and not a xenogeneic system, this study would be of less interest. However, when coupled with the known molecular homologies between murine and human lymphocyte antigens, these results suggest that the concept of cross species major histocompatibility complex (MHC) restriction is tenable. Thus the liposome is not only an effective antigen carrier, but also a functional adjuvant for in vitro induced cell-mediated immunity.