Liposomes in immunology: impairment of the adjuvant effect of liposomes by incorporation of the adjuvant lysolecithin and the role of macrophages

Immunopotentiating conjugates

Current interest in the development of sub-cellular vaccines and in cancer immunotherapy has rekindled interest in immunopotentiating agents. In addition, peptide sequences of protective antigens are being synthesized or genetically engineered and it is likely that these peptides will have a small relative molecular mass and be non-immunogenic by themselves. A variety of experimental studies have shown that such protective antigens could be included in conjugated vaccines with carrier protein and/or immunopotentiator. The insertion of peptides with hydrophobic sequences into artificial lipid bilayers (liposomes) offers another mode of presentation of antigen to the immune system.

Lymphocyte transformation to membrane-conjugated, liposome-conjugated, or unconjugated pentadecylcatechol in the guinea pig

This study examined the in vitro immunogenicity of haptenated liposomes and compared them with haptenated biological membranes and unconjugated hapten. Peripheral-blood lymphocytes were obtained from guinea pigs topically sensitized with pentadecylcatechol (PDC) and immunologically naive guinea pigs. Lymphocyte transformations were studied by [3H]thymidine uptake. PDC failed to stimulate the lymphocytes from the immunologically naive group. There was significant blastogenesis in the cells of the sensitized group, but the degree of stimulation was dependent upon the manner in which the antigen was presented to them. Unconjugated hapten caused a low-level, dose-dependent mitogenesis in the sensitized T cells, and hapten-conjugated liposomes enhanced this response (P less than 0.05). By far the most effective immunogen was a haptenated biologic membrane. In all cases, the mitogenic response was macrophage dependent. It is possible that the haptenated biologic membranes were more effective than synthetic membranes (liposomes) because of the presence of membrane proteins that can conjugate with hapten and from a more effective immunogen.

The effect of Freund's adjuvants on blood-cerebrospinal fluid barrier permeability

In guinea pigs with experimental allergic encephalomyelitis induced by spinal cord homogenate--complete Freund's adjuvant (CFA) emulsions an increase in the albumin permeability of the blood--cerebrospinal fluid barrier occurred from day 10 post-inoculation (p.i.) onward. In animals inoculated with CFA alone an increased albumin permeability was also demonstrated but only between days 5 and 10 after inoculation; by day 14-16 p.i. the barrier permeability had returned to control values. A similar change was seen in animals inoculated with incomplete Freund's adjuvant (IFA) only. However, both CFA and CFA-cord induced a strong humoral immune response which was not seen in animals inoculated with IFA alone. These results may have important consequences for the understanding of the development of inflammatory diseases of the central nervous system.

The effect of surface sugars on liposomes in immunopotentiation

The effect of surface sugars of liposomes on the immunological responses to entrapped antigen has been investigated. alpha-Mannose and beta-galactose were grafted on the surface of liposomes containing lysozyme by covalent coupling of p-aminophenyl-D-glycosides to phosphatidyl ethanolamine liposomes using glutaraldehyde. Subcutaneous administration of antigen entrapped in beta-galactose liposomes stimulated an antibody response comparable to that elicited by sugar-free neutral liposomes. However, alpha-mannose bearing liposomes with entrapped lysozyme elicited an immune response similar to that induced by lysozyme in saline. Based on these observations it is suggested that alpha-mannose liposomes, that are specifically recognized by macrophages, are taken up rapidly by receptor mediated endocytosis and that the entrapped antigen is then rapidly degraded, resulting in low antibody production.

The role of macrophages in promoting the antibody response mediated by liposome-associated protein antigens

The effect of the in vivo depletion of macrophage function on the plaque-forming cell (PFC) response to liposome-associated bovine serum albumin (LSM-BSA) in mice was investigated. Suppression of macrophage activities was accomplished by the intraperitoneal administration of carrageenan (CGN), a sulphated polygalactose which selectively inhibits macrophage function without interfering with the reactivity of T- and B-lymphocytes. Thus, control animals injected with CGN were found to give a significantly suppressed PFC response to sheep red blood cells, a macrophage-dependent antigen, but not to E. coli lipopolysaccharide, a macrophage-independent antigen. The pretreatment of experimental animals with CGN virtually abolished their antibody response to LSM-BSA in terms of the circulating anti-BSA antibody titers and the BSA-specific PFC response. These results provide evidence to substantiate an obligatory role of macrophages in inducing the humoral response to liposome-associated protein antigens.

Encapsulation of immunoadjuvant [14C]peptidoglycan monomer into liposomes. Effect on metabolism and immune response in mice

14C-labeled peptidoglycan monomer was encapsulated into negatively charged, multilamellar liposomes composed of egg phosphatidylcholine, cholesterol and dicetylphosphate. Excretion and tissue distribution of the label in mice were studied after intravenous injections. Encapsulation of peptidoglycan monomer into liposomes as compared to free peptidoglycan monomer, resulted in increased retention of the label, particularly in the liver and to a lesser extent in spleen. The excretion was drastically reduced and delayed even after 4 days when cholesterol-rich (phosphatidylcholine/cholesterol, 7:5 molar ratio) liposomes were used for encapsulation of peptidoglycan monomer. Peptidoglycan monomer and liposomes, when tested separately, stimulate the immune response to sheep erythrocytes in mice. However, there was no significant additive or synergistic effect when peptidoglycan monomer was encapsulated into liposomes.

Liposomes as immunological carriers for the preparation of antimannosyl antibodies

Antiserum was raised against an aminophenyl derivative of D-mannose grafted on to a liposomal surface. As characterized by immunodiffusion, quantitative precipitation and hapten inhibition, the antiserum was found to contain mannose specific antibodies in addition to antibodies against the aromatic phenyl group.

The adjuvant effect of liposomes in eliciting anti-galactosyl antibodies

Negatively-charged, multilamellar liposomes, covalently coupled with p-aminophenyl-beta-D-galactopyranoside, elicited the rabbits a nearly equivalent anti-galactosyl immune response in both the presence and the absence of complete Freund's-adjuvant (CFA). The antibody response obtained using liposomes both as the carrier and adjuvant was found to be better than that obtained through the conventional way of using protein carrier in CFA. The results suggest an effective role of liposomes as adjuvant in the production of sugar-specific antibodies.

Liposomes as immunological adjuvants in eliciting antibodies specific to the synthetic polypeptide poly(LTyr, LGlu)-poly(DLAla)--(LLys) with high frequency of site-associated idiotypic determinants

The antibody response to the synthetic polypeptide, poly(LTyr, LGlu)-poly(DLAla)--poly(LLys), [(T,G)-A--L], injected entrapped in liposomes which served as adjuvant has been analyzed. The liposomes used were composed of phosphatidylcholine, cholesterol, dicetylphosphate and DL alpha-tocopherol (molar ratios as 4:3:0.1:0.5) and therefore, were negatively charged. Since the (T,G)-A--L is also negatively charged, no free complexes were formed. The (T,G)-A--L was found to be entrapped inside the enclosed volume of the liposomes, and no (T,G)-A--L antigenic determinants could be detected on the liposomal membranes. Injection of high-responder C3H.SW (H-2b) mice with (T,G)-A--L-bearing liposomes demonstrated that the i.p. and the i.v. routes of immunization were efficient in eliciting (T, G)-A--L specific antibodies, whereas the i.d. injection led to poor antibody responses. The latter route of immunization is the most effective when (T,G)-A--L is injected in complete Freund's adjuvant (CFA). When low doses (0.1 and 1 microgram) of (T, G)-A--L were used for immunization, the liposomes were better adjuvants than CFA. The effectiveness of the liposomes as immunological adjuvants was also shown in their ability to induce high-potential, primed memory cells. The pattern of low (H-2k,a) and high (H-2b) responsiveness to (T,G)-A--L was retained following immunization with (T,G)-A--L entrapped in liposomes, as tested in two pairs of congenic strains. (T,G)-A--L-specific antibodies induced by injection with 1 microgram antigen entrapped in liposomes bear the (T,G)-A--L site-related idiotypic markers of C3H.SW (Igh-1a) mice in a significantly higher frequency than the homologous idiotypes, namely the antibodies elicited in this strain against (T,G)-A--L in CFA. Thus, liposomes may serve as adjuvants for the production of relatively restricted (T,G)-A--L-specific antibodies of high quality.

Liposomes in immunology: multilamellar phosphatidylcholine liposomes as a simple, biodegradable and harmless adjuvant without any immunogenic activity of its own

Multilamellar phosphatidylcholine liposomes were coated with the antigens human serum albumin (HSA) or bovine gamma globulin (BGG) simply by suspending the liposomes in a solution of the antigens. Antigen coated phosphatidylcholine liposomes showed nearly the same adjuvant activity after intravenous injection as liposomes of a more complex phospholipid composition. Since phosphatidylcholine liposomes are biodegradable, harmless, easily obtainable, have no immunogenic activity of their own and may be administered intravenously, they seem to be a promising immunoadjuvant.

Endotoxin enhanced adjuvant effect of liposomes, particularly when antigen and endotoxin are incorporated within the same liposome

Endotoxin has been shown to enhance the adjuvant effect of liposomes to protein antigens. When a weak antigen and endotoxin were associated with the same liposomes, antibody production could be detected 3 days (72 hours) after injection of the antigen-liposome-endotoxin preparation. When antigen and endotoxin were associated with different liposomes and these antigen-liposome and endotoxin-liposome preparations were injected simultaneously, the adjuvant effect of the liposomes was enhanced also, but not enough to detect antibody production on day 3.