Antigen presentation by liposomes as model system for T-B cell interaction

Intracellular processing of liposome-encapsulated antigens by macrophages depends upon the antigen

Two proteins, a recombinant malaria protein (R32NS1) and conalbumin, were encapsulated in separate liposomes. The mechanisms of presentation of unencapsulated and liposome-encapsulated R32NS1 and conalbumin to antigen-specific T-cell clones were investigated in in vitro antigen presentation assays using murine bone marrow-derived macrophages (BMs) as antigen-presenting cells. A much lower concentration of liposomal antigen than of unencapsulated antigen was required for T-cell proliferation. Liposome-encapsulated conalbumin required intracellular processing by BMs for antigen-specific T-cell proliferation, as determined by inhibition with chloroquine, NH4Cl, leupeptin, brefeldin A, monensin, antimycin A, NaF, and cycloheximide and by treatment of BMs with glutaraldehyde. Liposome-encapsulated conalbumin therefore follows the classical intracellular antigen processing pathway described for protein antigens. Similarly, unencapsulated conalbumin also required intracellular processing for presentation to antigen-specific T cells. In contrast, both unencapsulated R32NS1 and liposome-encapsulated R32NS1 were presented to T cells by BMs without undergoing internalization and intracellular processing. These results suggest that the antigen itself is the major element that determines whether a requirement exists for intracellular processing of liposomal antigens by macrophages.

Liposomal adjuvanticity: effect of encapsulation and surface-linkage on antibody production and proliferative response

The immunoadjuvanticity of liposomal antigens, namely encapsulated and surface-linked conalbumin, was studied at different levels of an immune response including immunoglobulin production, blastogenic response and lymphokine production in sensitization conditions compatible with vaccine designs. The results demonstrated that both liposomal formulations stimulate all properties analyzed with respect to free antigen but significantly differ in some of their inductive capabilities, suggesting that they follow different routes in the immune network. Thus, although both liposomal antigens are capable of inducing potent humoral responses characterized by increased production of IgM and IgG2a, covalently linked antigen stimulates a quasi-polyclonal blastogenic response accompanied by the simultaneous secretion of IL-2 and IFN gamma, while encapsulated antigen which is less blastogenic mainly induces IL-2 secretion. We can conclude that: first, both antigenic formulations induce a Th1 type of activation and might therefore potentiate cell-mediated immunity, but surface-linkage favors a more rapid maturation of the response and a much more intense help induction. Second, although a strong adjuvanticity can be observed whatever the route of sensitization, namely intraperitoneally, intravenously or subcutaneously, the intravenous injections induce the better potentiation. The in vitro data were all compared to those obtained with naïve mice, allowing dissociation of the contribution of the in vitro rechallenge from that of the in vivo immunization. These differences observed between the liposomal antigens might be taken advantage of while formulating vaccines specifically suited to meet required needs and suggest that covalently linked antigen might be particularly useful in situations where induction of cell-mediated immunity is of prime importance.

Liposomes as carriers of antigens and adjuvants

Liposomes have been widely used as carriers of protein or peptide antigens. Antigenic materials can be attached to the outer surface, encapsulated within the internal aqueous spaces, or reconstituted within the lipid bilayers of the liposomes. The natural tendency of liposomes to interact with macrophages has served as the primary rationale for utilizing liposomes as carriers of antigens. Liposomes also serve as carriers of a variety of adjuvants and mediators, including lipid A, muramyl dipeptide and its derivatives, interleukin-1, and interleukin-2. Research utilizing in vitro cell culture models has demonstrated that liposomes containing both appropriate antigens and major histocompatibility gene complex molecules can induce antigen-specific genetically restricted cytotoxic T lymphocytes. Liposomes induce immune reactions through classical interactions with antigen presenting cells. However, modelling experiments have also demonstrated that liposomes can even substitute for antigen presenting cells, and cell-free genetically restricted and nonrestricted presentation of antigens by liposomes to helper T lymphocytes has been demonstrated. Liposomes are successful for inducing potent immunity in vivo and they are now being employed in numerous immunization procedures and as vehicles for candidate vaccines.

Parameters affecting the immunogenicity of a liposome-associated synthetic hexapeptide antigen

The effect of liposome association on the immunogenicity of the hexapeptide IRGERA was investigated. When administered in the absence of a carrier and adjuvant this peptide, which corresponds to a linear epitope located at the C-terminus of histone H3, was not immunogenic. When mice were immunized with the peptide covalently linked to the surface of small unilamellar vesicles containing monophosphoryl lipid A as adjuvant, a relatively long-lasting response with memory cell induction was observed. The anti-peptide antibodies raised in this way reacted with the cognate sequence in the native histone. In contrast, coupling of the peptide to the surface of large vesicles yielded both an IgM and IgG response of short duration whereas encapsulation of the free peptide in large vesicles was ineffective. These results indicate that with short synthetic peptides, liposomes provide a substitute for a carrier protein. However, an adjuvant has to be incorporated in the vesicles in order to obtain an efficient immune response. Such an approach may be useful for designing synthetic vaccines.