Synthetic human MUC1 peptides are important candidates for therapeutic cancer vaccines. To explore whether a human MUC1 peptide BP25 (STAPPAHGVTSAPDTRPAPGSTAPP) can be rendered immunogenic by incorporation in liposomes, the effects of physical association of the peptide with liposomes on immune responses were investigated. Lipid conjugated and nonconjugated MUC1 peptides were incorporated in liposomes with a composition of distearoylphosphatidylcholine/cholesterol/dimyristoylphosphatidylglyc erol (3:1:0.25, molar ratio) containing monophosphoryl lipid A (1% w/w of the total lipids). Liposomes were characterized for peptide retention by HPLC and for surface peptide display of MUC1 epitopes by flow cytometry. C57BL/6 mice were immunized with lipopeptide alone, peptide mixed with peptide-free liposomes, and peptide associated with liposomes in entrapped or surface-exposed forms. T cell proliferative responses, cytokine patterns, and antibody isotypes were studied. Results showed that immune responses were profoundly influenced by the liposome formulations. Physically associated, either encapsulated or surface-exposed, peptide liposomes elicited strong antigen-specific T cell responses, but not lipopeptide alone or peptide mixed with peptide-free liposomes. Analysis of the cytokines secreted by the proliferating T cells showed a high level of IFN-gamma and undetectable levels of IL-4, indicating a T helper type 1 response. Thus, physical association of the peptide with liposomes was required for T cell proliferative responses, but the mode of association was not critical. On the other hand, the nature of the association significantly affected humoral immune responses. Only the surface-exposed peptide liposomes induced MUC1-specific antibodies. A domination of anti-MUC1 IgG2b over IgG1 (94 versus 6%) was observed. Our results support the hypothesis that different immune pathways are stimulated by different liposome formulations. This study demonstrated that a liposome delivery system could be tailored to induce either a preferential cellular or humoral immune response.

A studies of cytotoxic cells in athymic nude mice demonstrate higher cytotoxic activity of NK cells and macrophages than in their euthymic counterparts. The higher level of endogenous cytotoxic activity can be considered as complementary to the deficiency or lack of thymus dependent T lymphocytes and their functions. However, with increased age of mice some T lymphocytes and their functions can be demonstrated. By stimulation of splenocytes and lymph node cells in vitro with IL-2 or anti CD3 antibody cytotoxic activity towards P-815 (NK resistant, LAK sensitive) target cells can be generated. There exist data, which indicate that the cytotoxic activity is exerted by extrathymic pre-T lymphocytes with TcR gamma delta antigenic phenotype. The differences in transplantability of human tumors in athymic nude mice cannot be explained by defect in antigen recognition and in immune response of athymic nude mice, recipients of the xenografted material. The biological relevance in vivo of high endogenous cytotoxicity of NK cells observed in many strains of athymic nude mice remains obscure. The availability of new immunodeficient mouse models, e.g. scid mice deficient in B and T lymphocytes and with low level of NK cells, in which not only xenografted human tumor grow but human lymphoid cell can be transplanted as well, opens new and broader experimental possibilities, in which new preclinical immunotherapeutical approaches can be applied.

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