Induction and detection of antibodies to squalene

An enzyme-linked immunosorbent assay (ELISA) utilizing antigen coated on hydrophobic polyvinyldiene fluoride (PVDF) membranes is described for detecting antibodies that bind to squalene (SQE). Because of the prior lack of availability of validated antibodies to SQE, positive controls for the assay were made by immunization with formulations containing SQE to create monoclonal antibodies (mAbs) that reacted with SQE. Among eight immunogens tested, only two induced detectable murine antibodies to SQE: liposomes containing dimyristoyl phosphatidylcholine, dimyristoyl phosphatidylglycerol, 71% SQE, and lipid A [L(71% SQE+LA)], and, to a much lesser extent, an oil-in-water emulsion containing SQE, Tween 80, Span 85, and lipid A. In each case, lipid A served as an adjuvant, but neither SQE alone, SQE mixed with lipid A, liposomes containing 43% SQE and lipid A, nor several other emulsions containing both SQE and lipid A, induced antibodies that reacted with SQE. Monoclonal antibodies produced after immunizing mice with [L(71% SQE+LA)] served as positive controls for developing the ELISA. Monoclonal antibodies were produced that either recognized SQE alone but did not recognize squalane (SQA, the hydrogenated form of SQE), or that recognized both SQE and SQA. As found previously with other liposomal lipid antigens, liposomes containing lipid A also induced antibodies that reacted with the liposomal phospholipids. However, mAbs were also identified that reacted with SQE on PVDF membranes, but did not recognize either SQA or liposomal phospholipid. The polyclonal antiserum produced by immunizing mice with [L(71% SQE+LA)] therefore contained a mixed population of antibody specificities and, as expected, the ELISA of polyclonal antiserum with PVDF membranes detected antibodies both to SQE and SQA. We conclude that SQE is a weak antigen, but that antibodies that specifically bind to SQE can be readily induced by immunization with [L(71% SQE+LA)] and detected by ELISA with PVDF membranes coated with SQE.

Proteasome inhibitors block the entry of liposome-encapsulated antigens into the classical MHC class I pathway

Liposome-encapsulated conalbumin (L(conalbumin)) is an antigen that is efficiently phagocytosed by bone marrow-derived macrophages and presented to effector cells as part of the major histocompatibility complex (MHC) class I complex. In this report, we show that the conalbumin component of L(conalbumin) is degraded to small peptide fragments and translocated to the area of the Golgi. Golgi localization is confirmed by co-localization of L(Texas red-conalbumin) (L(TR-conalbumin))with both NBD-ceramide, a lipid Golgi marker, and green fluorescent protein (GFP)-galactosyl transferase, a Golgi resident enzyme. Incubation of the cells with brefeldin A disrupts the Golgi and disperses the TR-conalbumin. Furthermore, when macrophages were incubated with another liposome-encapsulated antigen, L(ovalbumin), ovalbumin peptides were observed in the Golgi area and MHC class I-peptide complexes could be detected on the cell surface by both immunofluorescence microscopy and flow cytometry. The Golgi localization observed in vitro in cultured macrophages is mirrored by the in vivo uptake and Golgi localization of fluorescent L(conalbumin) in macrophages isolated from the spleen of a mouse injected with L(TR-conalbumin). The accumulation of peptide fragments in the Golgi is inhibited by the addition of the proteasome inhibitors, lactacystin and MG-132, demonstrating the role of the proteasome in this activity. In addition, when macrophages or a macrophage-derived cell line, are incubated with liposome-enccapsulated antigens and used as target cells in a cytotoxic T-cell (CTL) assay, the CTLs recognize the processed peptide-MHC complexes and kill the cells. In contrast, specific lysis of target cells by CTLs is inhibited when the target cells are first incubated with lactacystin. These results suggest that uptake and processing of L(antigen) follows the classical MHC class I pathway.

Naturally occurring antibodies to cholesterol: a new theory of LDL cholesterol metabolism

Although low-density lipoprotein (LDL) receptors regulate the disposition of two-thirds of circulating serum LDL cholesterol, non-LDL receptor mechanisms account for removal of one-third. Here, Carl Alving and Nabila Wassef propose that naturally occurring antibodies to cholesterol in normal human plasma also contribute to LDL cholesterol turnover by opsonizing LDL and other lipoproteins containing 'bad' cholesterol for removal by complement receptors.

Hemodynamic changes induced by liposomes and liposome-encapsulated hemoglobin in pigs: a model for pseudoallergic cardiopulmonary reactions to liposomes. Role of complement and inhibition by soluble CR1 and anti-C5a antibody

BACKGROUND

Intravenous administration of some liposomal drugs can trigger immediate hypersensitivity reactions that include symptoms of cardiopulmonary distress. The mechanism underlying the cardiovascular changes has not been clarified.

METHODS AND RESULTS

Anesthetized pigs (n=18) were injected intravenously with 5-mg boluses of large multilamellar liposomes, and the ensuing hemodynamic, hematologic, and laboratory changes were recorded. The significant (P<0.01) alterations included 79+/-9% (mean+/-SEM) rise in pulmonary arterial pressure, 30+/-7% decline in cardiac output, 11+/-2% increase in heart rate, 236+/-54% increase in pulmonary vascular resistance, 71+/-27% increase in systemic vascular resistance, and up to a 100-fold increase in plasma thromboxane B2. These changes peaked between 1 and 5 minutes after injection, subsided within 10 to 20 minutes, were lipid dose-dependent (ED50=4. 5+/-1.4 mg), and were quantitatively reproducible in the same animal several times over 7 hours. The liposome-induced rises of pulmonary arterial pressure showed close quantitative and temporal correlation with elevations of plasma thromboxane B2 and were inhibited by an anti-C5a monoclonal antibody (GS1), by sCR1, or by indomethacin. Liposomes caused C5a production in pig serum in vitro through classic pathway activation and bound IgG and IgM natural antibodies. Zymosan- and hemoglobin-containing liposomes and empty liposomes caused essentially identical pulmonary changes.

CONCLUSIONS

The intense, nontachyphylactic, highly reproducible, complement-mediated pulmonary hypertensive effect of minute amounts of intravenous liposomes in pigs represents a unique, unexplored phenomenon in circulation physiology. The model provides highly sensitive detection and study of cardiopulmonary side effects of liposomal drugs and many other pharmaceutical products due to "complement activation-related pseudoallergy" (CARPA).

Trafficking of liposomal antigen to the trans-Golgi of murine macrophages requires both liposomal lipid and liposomal protein

Major histocompatibility complex (MHC) class I molecules found on antigen-presenting cells present peptides derived from cytoplasmic proteins to T cells. In contrast, peptides from exogenous proteins are mostly presented by class II molecules. It has been well established that liposomes can serve as an efficient delivery system for entry of exogenous protein antigens into the MHC class I pathway. Our previous studies utilizing fluorophore-labeled proteins encapsulated in liposomes demonstrated that after phagocytosis of the liposomes by bone marrow-derived macrophages (BMs), the processed peptides were subsequently visualized in the trans-Golgi, while free conalbumin was excluded from the trans-Golgi area. In the present study, we investigated whether liposomal lipids follow the same intracellular route as the liposomal proteins after phagocytosis by BMs. Multilamellar liposomes with different lipid compositions that also contained fluorescent phospholipids (empty liposomes) were incubated with murine BMs. Our results indicate that although empty liposomes were avidly phagocytosed by macrophages, the fluorescent liposomal lipids did not localize to any particular area of the cell but were distributed throughout the cell. In contrast, when a protein was encapsulated in the liposomes, the liposomal lipids were no longer dispersed throughout the cell, but were concentrated and localized in the trans-Golgi area. Furthermore, when the liposomes contained a fluorescent-labeled protein, the fluorescent peptides also localized to the trans-Golgi. These results demonstrate that the combination of both liposomal lipids and liposomal protein is required for Golgi-specific targeting of liposomal antigens. Transport of both liposomal lipids and liposomal proteins to the Golgi complex, a major subcellular organelle in the passage of MHC class I molecules, might explain why antigens encapsulated in liposomes readily induce cytotoxic T lymphocytes.

Liposomes containing lipid A serve as an adjuvant for induction of antibody and cytotoxic T-cell responses against RTS,S malaria antigen

Encapsulation of soluble protein antigens in liposomes was previously shown to result in processing of antigen via the major histocompatibility complex class I pathway, as evidenced by costaining of the trans-Golgi region of murine bone marrow-derived macrophages (BMs) by fluorophore-labeled liposomal antigen and by a trans-Golgi-specific fluorescent lipid. Evidence is presented here that free or liposome-encapsulated RTS,S, a particulate malaria antigen consisting of hepatitis B particles coexpressed with epitopes from the Plasmodium falciparum circumsporozoite protein, also was localized in the trans-Golgi after incubation with BMs, suggesting processing by the class I pathway. An in vivo cytotoxic T-lymphocyte (CTL) response was detected, however, only after immunization with RTS,S encapsulated in liposomes containing lipid A and not after immunization with free RTS,S or with RTS,S encapsulated in liposomes lacking lipid A. Therefore, intracellular delivery of antigen containing CTL epitopes to the Golgi of BMs does not necessarily result in a CTL response in vivo unless an additional adjuvant, such as liposomes containing lipid A, is utilized. Encapsulation of RTS,S in liposomes containing monophosphoryl lipid A (MPL) resulted in a dose-dependent enhancement of the NANP-specific immunoglobulin G (IgG) antibody response compared to that of free RTS,S. The IgG1 and IgG2a subclasses predominated after immunization with RTS,S encapsulated in liposomes containing MPL. These results demonstrate that encapsulation of a lipid-containing particulate antigen, such as RTS, S, in liposomes containing lipid A can enhance both humoral and cellular immune responses.

Liposomes containing lipid A serve as an adjuvant for induction of antibody and cytotoxic T-cell responses against RTS,S malaria antigen

Encapsulation of soluble protein antigens in liposomes was previously shown to result in processing of antigen via the major histocompatibility complex class I pathway, as evidenced by costaining of the trans-Golgi region of murine bone marrow-derived macrophages (BMs) by fluorophore-labeled liposomal antigen and by a trans-Golgi-specific fluorescent lipid. Evidence is presented here that free or liposome-encapsulated RTS,S, a particulate malaria antigen consisting of hepatitis B particles coexpressed with epitopes from the Plasmodium falciparum circumsporozoite protein, also was localized in the trans-Golgi after incubation with BMs, suggesting processing by the class I pathway. An in vivo cytotoxic T-lymphocyte (CTL) response was detected, however, only after immunization with RTS,S encapsulated in liposomes containing lipid A and not after immunization with free RTS,S or with RTS,S encapsulated in liposomes lacking lipid A. Therefore, intracellular delivery of antigen containing CTL epitopes to the Golgi of BMs does not necessarily result in a CTL response in vivo unless an additional adjuvant, such as liposomes containing lipid A, is utilized. Encapsulation of RTS,S in liposomes containing monophosphoryl lipid A (MPL) resulted in a dose-dependent enhancement of the NANP-specific immunoglobulin G (IgG) antibody response compared to that of free RTS,S. The IgG1 and IgG2a subclasses predominated after immunization with RTS,S encapsulated in liposomes containing MPL. These results demonstrate that encapsulation of a lipid-containing particulate antigen, such as RTS, S, in liposomes containing lipid A can enhance both humoral and cellular immune responses.

HIV-1 neutralizing antibodies in the genital and respiratory tracts of mice intranasally immunized with oligomeric gp160

Because mucosal surfaces are a primary route of HIV-1 infection, we evaluated the mucosal immunogenicity of a candidate HIV-1 vaccine, oligomeric gp160 (o-gp160). In prior studies, parenteral immunization of rabbits with o-gp160 elicited broad neutralizing serum Ab responses against both T cell line-adapted HIV-1 and some primary HIV-1 isolates. In this study, nasal immunization of mice with o-gp160, formulated with liposomes containing monophosphoryl lipid A (MPL), MPL-AF, proteosomes, emulsomes, or proteosomes with emulsomes elicited strong gp160-specific IgG and IgA responses in serum as well as vaginal, lung, and intestinal washes and fecal pellets. The genital, respiratory, and intestinal Abs were determined to be locally produced. No mucosal immune responses were measurable when the immunogen was given s.c. Abs from sera and from vaginal and lung washes preferentially recognized native forms of monomeric gp120, suggesting no substantial loss in protein tertiary conformation after vaccine formulation and mucosal administration. Inhibition of HIV-1MN infection of H9 cells was found in sera from mice immunized intranasally with o-gp160 formulated with liposomes plus MPL, proteosomes, and proteosomes plus emulsomes. Formulations of o-gp160 with MPL-AF, proteosomes, emulsomes, or proteosomes plus emulsomes elicited HIV-1MN-neutralizing Ab in lung wash, and formulations with proteosomes, emulsomes, or proteosomes plus emulsomes elicited HIV-1MN-neutralizing Ab in vaginal wash. These data demonstrate the feasibility of inducing both systemic and mucosal HIV-1-neutralizing Ab by intranasal immunization with an oligomeric gp160 protein.

Visualization of peptides derived from liposome-encapsulated proteins in the trans-Golgi area of macrophages

Exogenous proteins are generally not presented through the major histocompatibility complex (MHC) class I pathway, yet several recent studies show that particle-associated antigens induce a CD8+ T-cell response. Therefore, a pathway must exist in vivo for the presentation of exogenous antigens on class I molecules. In the present study, we investigated the intracellular fate of liposome-encapsulated Texas Red (TR)-conjugated protein in cultured bone marrow-derived macrophages (BMs). After phagocytosis of liposomes, the fluorescent liposomal protein, initially associated with the liposomal lipids in phagosomes, later entered the cytoplasm, and the processed protein was subsequently visualized in the trans-Golgi as a fluorescent peptide. Experiments performed with BMs from transporter associated with antigen processing (TAP1) knock-out mice demonstrated that the translocation of peptides into the trans-Golgi area was dependent upon TAP1 protein. We conclude that delivery of liposomal proteins or peptides to the cytoplasm of phagocytes and subsequent transport of peptides to the Golgi via the classical MHC class I pathway involving TAP proteins might explain the known propensity of liposomal antigens to induce cytotoxic T-lymphocytes (CTLs).

Complement activation and thromboxane secretion by liposome-encapsulated hemoglobin in rats in vivo: inhibition by soluble complement receptor type 1

Intravenous administration of liposome-encapsulated hemoglobin (LEH) in rats led to an early (within 15 min) decline of hemolytic complement (C) activity in the plasma along with a significant, parallel rise in thromboxane B2 (TXB2) levels. The TXB2 response was inhibited by co-administration of soluble C receptor type 1 (sCR1) with LEH, as well as by C depletion with cobra venom factor. These observations provide evidence for a causal relationship between LEH-induced C activation and TXB2 release, and suggest that sCR1 could be useful in attenuating the acute respiratory, hematological and hemodynamic side effects of LEH described earlier in the rat.

Complement activation in vitro by the red cell substitute, liposome-encapsulated hemoglobin: mechanism of activation and inhibition by soluble complement receptor type 1

BACKGROUND

Liposome-encapsulated hemoglobin (LEH) has been developed as an emergency blood substitute, yet its effect on human complement has never been explored. Considering that complement activation is a major pathogenic factor in the respiratory distress syndrome that often develops in trauma and shock, LEH-induced complement activation may be a critical safety issue.

STUDY DESIGN AND METHODS

Various LEH and corresponding empty liposomes were incubated with normal human sera, and various markers of complement activation (serum levels of C4d, Bb, SC5b-9, and CH50; C5a-induced granulocyte aggregation; membrane deposition of C3b) were measured. Incubations were also performed in the presence of (ethylene-bis[oxyethylenenitrilo]tetraacetic acid) (EGTA) and Mg++ (EGTA/Mg++) and soluble complement receptor type 1.

RESULTS

LEH and liposomes activated human complement, as indicated by significant changes in one or more markers. The effect was primarily due to the presence of the phospholipid vehicle; small, unilamellar, highly homodispersed vesicles induced the greatest degree of complement activation. Complement activation was partially inhibited by EGTA/Mg++. The latter finding, together with the parallel increases in serum C4d and Bb, suggests activation of both the classical and alternative pathways. Soluble complement receptor type 1 (0.05-20 micrograms/mL) efficiently inhibited all vesicle-induced complement activation.

CONCLUSION

Because of complement activation, the use of LEH for transfusion may require careful evaluation of safety. Soluble complement receptor type 1 may be useful as a prophylactic agent for complement activation-related complications of liposome infusions.

Complement activation in human serum by liposome-encapsulated hemoglobin: the role of natural anti-phospholipid antibodies

In exploring the occurrence and mechanism of liposome-encapsulated hemoglobin (LEH)-induced complement (C) activation, we found that normal human serum contained low titers of IgG and IgM class natural antibodies with reactivity against LEH, and that the amount of vesicle-bound IgM significantly correlated with LEH-induced C consumption. IgM binding to LEH was inhibited by phosphocholine and ATP, but not by choline chloride. These data suggest that naturally occurring antibodies play a key role in LEH-induced C activation, and that a major portion of these antibodies are directed against the phosphate moiety on the phospholipid headgroups of liposome bilayers.

Liposomal subunit vaccines: effects of lipid A and aluminum hydroxide on immunogenicity

Protein and peptide antigens frequently are only slightly immunogenic when utilized alone in vaccines. Formulation of these antigens in a carrier vehicle, particularly when an adjuvant is included, often results in markedly enhanced immune responses. Encapsulation of peptide and protein antigens in liposomes generally results in a relatively slight enhancement of antibody production compared with that observed with the antigen alone. However, when lipid A is included in the liposomes, immunogenicity is markedly increased compared both with antigen alone and with antigen encapsulated in liposomes lacking lipid A. The enhancement of the immune response caused by lipid A is dependent on the liposomal lipid A dose. Aluminum salts, such as aluminum hydroxide and aluminum phosphate, act as adjuvants for some antigens and are used in a variety of human vaccines. When liposomes containing encapsulated protein or peptide antigens were adsorbed with aluminum hydroxide, an enhancement of the antibody response was observed with some antigens, whereas with other antigens the presence of aluminum hydroxide either had no effect or resulted in a diminished antibody response. Immunogenicity of protein and peptide antigens can be enhanced by formulation in liposomes containing lipid A and, depending on the antigen, can be enhanced further by adsorption of the liposomal antigen formulation with aluminum salts.

Antibodies to cholesterol: biological implications of antibodies to lipids

Injection of silicone gel or silicone oil intraperitoneally into BALB/c mice induced the formation of antibodies that reacted by ELISA with highly purified crystalline cholesterol and, to a much lesser extent, antibodies that reacted with a phospholipid (dimyristoyl phosphatidylglycerol). Although IgM and IgG antibodies to cholesterol were detected, the titers of IgG antibodies were low when compared with IgM. The titers of IgM antibodies to cholesterol in certain sera exhibited activities that reached baseline values at dilutions as high as 1:5000, thus making them equivalent to titers that have been previously published for ascites fluid containing murine monoclonal antibodies to cholesterol. The antibodies to cholesterol induced by silicone compounds are indistinguishable in their binding to crystalline cholesterol from naturally-occurring antibodies to cholesterol in normal human serum. They are also indistinguishable from antibodies induced by a proposed vaccine to cholesterol that is currently in late preclinical development for prevention of hypercholesterolemia in humans. The anti-cholesterol vaccine, which consists of liposomes heavily laden with cholesterol as an antigen and lipid A as an adjuvant, induces antibodies that react with low density lipoproteins (LDL) and opsonize them for removal by liver macrophages. It appears that silicone gel or silicone oil causes recruitment and adsorption of cholesterol at the injection site, and also serves as an adjuvant that may have immunostimulant properties similar to lipid A for inducing antibodies to lipids. Antibodies to lipids such as cholesterol or phospholipids are not harmful to intact cell membranes because of steric hindrance from surrounding lipids and larger macromolecules that block binding of the antibodies.

Immunization with cholesterol-rich liposomes induces anti-cholesterol antibodies and reduces diet-induced hypercholesterolemia and plaque formation

Immunization of rabbits with a protein-free formulation consisting of liposomes containing 71% cholesterol and lipid A as an adjuvant induced anticholesterol antibodies that caused complement-dependent lysis of liposomes lacking lipid A. The antibodies, immunoglobulin G (IgG) and immunoglobulin M (IgM), also recognized nonoxidized crystalline cholesterol as an antigen by enzyme-linked immunosorbent assay (ELISA). The effects of immunization against cholesterol on elevations in serum cholesterol and development of atherosclerosis were examined in rabbits fed a diet containing 0.5% to 1.0% cholesterol. Although the mean serum cholesterol level, mainly in the form of very-low-density lipoprotein cholesterol, rose as much as 60-fold in the nonimmunized rabbits, the elevation was significantly less--as much as 35% lower--in the immunized rabbits. Elevation of serum cholesterol was accompanied by an apparent drop in the level of antibodies on initiating the diet, followed by a rebound on stopping the diet, thus suggesting that the antibodies were adsorbed to cholesterol that was present in circulating lipoproteins. When lipoprotein fractions--composed of either very-low-density and intermediate-density lipoproteins derived from cholesterol-fed nonimmunized rabbits or human low-density lipoproteins--were tested as capture antigens by solid-phase ELISA, reactivity was observed with IgG and IgM antibodies present in the serum of immunized rabbits. Immunization also resulted in a marked decrease in the risk of developing atherosclerosis. Analysis of aortic atherosclerosis by quantitative histologic examination and fatty streaks by automated morphometric probability-of-occurrence mapping showed diminished atherosclerosis in most areas of the aorta in vaccine recipients. It is proposed that immunization with liposomes containing 71% cholesterol and lipid A can reduce diet-induced hypercholesterolemia and atherosclerosis.

Antibody and cytotoxic T-lymphocyte responses to a single liposome-associated peptide antigen

The development of peptide-based vaccines that elicit antibody (Ab) and cellular immune responses has been hampered by the lack of highly immunogenic formulations. In this study, we compared the induction of Ab and cytotoxic T-lymphocyte (CTL) responses to a peptide derived from the V3 loop of HIV-1 gp120 (P18 and its cysteine-glycine derivative (CG-P18)) when incorporated into liposomes with lipid A (LA) or mixed with aluminum hydroxide. P18-specific CTL were only observed with liposomes with LA. P18-specific Ab responses were found with liposomes containing CG-P18 but not P18. Increased surface expression of the former, resulted in enhancement of the Ab response without loss of CTL induction. Thus, the manner in which a peptide is localized can influence the outcome of the response induced by highly immunogenic liposome formulations.

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.

A vaccine-elicited, single viral epitope-specific cytotoxic T lymphocyte response does not protect against intravenous, cell-free simian immunodeficiency virus challenge

Protection against simian immunodeficiency virus (SIV) challenge was assessed in rhesus monkeys with a vaccine-elicited, single SIV epitope-specific cytotoxic T-lymphocyte (CTL) response in the absence of SIV-specific antibody. Strategies were first explored for eliciting an optimal SIV Gag epitope-specific CTL response. These studies were performed in rhesus monkeys expressing the major histocompatibility complex (MHC) class I gene Mamu-A*01, a haplotype associated with a predominant SIV CTL epitope mapped to residues 182 to 190 of the Gag protein (p11C). We demonstrated that a combined modality immunization strategy using a recombinant Mycobacterium bovis BCG-SIV Gag construct for priming, and peptide formulated in liposome for boosting, elicited a greater p11C-specific CTL response than did a single immunization with peptide-liposome alone. Vaccinated and control monkeys were then challenged with cell-free SIVmne by an intravenous route of inoculation. Despite a vigorous p11C-specific CTL response at the time of virus inoculation, all monkeys became infected with SIV. gag gene sequencing of the virus isolated from these monkeys demonstrated that the established viruses had no mutations in the p11C-coding region. Thus, the preexisting CTL response did not select for a viral variant that might escape T-cell immune recognition. These studies demonstrate that a potent SIV-specific CTL response can be elicited by combining live vector and peptide vaccine modalities. However, a single SIV Gag epitope-specific CTL response in the absence of SIV-specific antibody did not provide protection against a cell-free, intravenous SIV challenge.

Complement activation by liposome-encapsulated hemoglobin in vitro: the role of endotoxin contamination

Incubation of liposome-encapsulated Hb (LEH) with rat serum at 37 degrees C led to accelerated decay of serum hemolytic complement (C) activity (CH50/ml). Empty liposomes (L) caused less decrease of CH50/ml, whereas free Hb had no effect on C activity. The LEH- and L-induced increases in C consumption were unlikely a consequence of endotoxin (LPS) contamination, as spiking of rat serum with LPS caused reduction in C only at levels significantly higher than those detectable in LEH or L. LPS-induced C consumption was not potentiated by free hemoglobin.

Complement activation in rats by liposomes and liposome-encapsulated hemoglobin: evidence for anti-lipid antibodies and alternative pathway activation

Intravenous injection of hemoglobin-containing liposomes (LEH) caused a significant reduction in plasma hemolytic complement activity in rats on a time scale of minutes. Liposomes without hemoglobin also caused complement consumption, but less than LEH, while free hemoglobin was without effect. Consistent with complement activation, the LEH-induced drop in plasma hemolytic complement activity was closely paralleled by an increase in plasma thromboxane B2 level. Studies to determine the mechanism of complement activation demonstrated the presence of natural antibodies in rat serum against all lipid components of LEH, thus, the potential for classical pathway activation. Yet, in vitro incubation of LEH with rat serum showed that: 1) EGTA/Mg++, which inhibits complement activation through the classical pathway, did not inhibit complement consumption by LEH, and 2) the use of serum preheated at 50 degrees C, which inhibits C activation through the alternative pathway by selectively depleting factor B, effectively reversed the complement-consuming effect of LEH. Consequently, LEH-induced complement activation in rat serum seems to involve primarily the alternative pathway.

Liposomes as carriers for vaccines

A liposome vaccine formulation that has been successfully used in both animal immunization studies and clinical trials is described. Issues concerning the choice of components for the liposomal vaccine formulation are discussed, especially with respect to the lipid components and the adjuvant. A procedure is described for manufacturing liposomal vaccines using Good Manufacturing Practices as promulgated by the U.S. Food and Drug Administration. Quality control testing for clinical use is described, with particular emphasis on aspects relevant to liposomes. Utilization issues are discussed, including injection volumes, antigen and adjuvant doses, and routes of administration.

Complement-dependent phagocytosis of liposomes

In this article we describe an in vitro model for complement-dependent phagocytosis of liposomes. We have previously reported that complement-opsonized liposomes are avidly ingested by murine peritoneal or bone marrow-derived cultured macrophages. However, when the liposomes contained certain lipids, including phosphatidylinositol, ganglioside GM1, and sulfogalactosyl ceramide, that have been identified as causing prolonged circulation time in vivo, complement-dependent phagocytosis of the liposomes was greatly suppressed. We identify certain additional factors associated with suppressed complement-dependent phagocytosis, including, liposomal negative charge and liposomal prostaglandin E2 or thromboxane B2. Possible mechanisms responsible for suppression of complement dependent phagocytosis are suggested. We propose that suppression of complement-dependent phagocytosis could be a contributing factor in the promotion of increased circulation time of 'stealth' liposomes and that complement opsonization probably plays a role in vivo in removing liposomes from the circulation.

ATP specifically bound as a hapten to a monoclonal anti-phospholipid antibody retains phosphate donor activity

We have previously reported that each of four monoclonal antibodies to a phospholipid, phosphatidylinositol phosphate (PIP), has a phosphate binding subsite in the antigen binding site that can bind ATP (Molec. Immunol. 21, 863-868, 1984). We have now observed that antibody-bound ATP has the ability to donate a phosphate group in the phosphorylation reaction of glucose to glucose-6-phosphate catalyzed by hexokinase. The phosphorylation reaction proceeds equally efficiently when ATP is provided as free (nonbound) ATP or as antibody-bound ATP. We conclude that an anti-phospholipid antibody can serve as a carrier of a functionally active nucleotide.

Adjuvant effects of liposomes containing lipid A: enhancement of liposomal antigen presentation and recruitment of macrophages

Liposomes containing lipid A induced potent humoral immune responses in mice against an encapsulated malaria antigen (R32NS1) containing NANP epitopes. The immune response was not enhanced by lipid A alone or by empty liposomes containing lipid A. Experiments to investigate the adjuvant mechanisms of liposomes and lipid A revealed that liposome-encapsulated R32NS1 was actively presented by bone marrow-derived macrophages to NANP-specific cloned T cells. The degree of presentation was related to the amount of liposomal antigen added per macrophage in the culture medium. At high cell densities, poor presentation occurred when liposomes lacked lipid A but excellent presentation occurred when the liposomes contained lipid A. Liposomes containing lipid A and encapsulated antigen also activated gamma interferon-treated macrophages to produce nitric oxide. Macrophage activation and antigen presentation occurred with liposomes that could not be detected by the Limulus amebocyte lysis assay. Intraperitoneal injection of liposomal lipid A caused a marked increase in the recruitment of immature (peroxidase-positive) macrophages to the peritoneum. On the basis of these experiments, we propose that the mechanism of the adjuvant action of liposomal lipid A is partly due to increased antigen presentation by macrophages and partly due to recruitment of an increased number of macrophages serving as antigen-presenting cells.

Phagocytosis of liposomes by macrophages: intracellular fate of liposomal malaria antigen

Liposomes containing a synthetic recombinant protein were phagocytosed by macrophages, and the internalized protein was recycled to the cell surfaces where it was detected by enzyme-linked immunosorbent assay. The transit time of the liposome-encapsulated protein from initial phagocytosis of liposomes to appearance of protein on the surfaces of macrophages was determined by pulse-chase experiments. The macrophages were pulsed with liposomes containing protein and chased with empty liposomes, and vice versa. The amount and rate of protein antigen expression at the cell surfaces depended on the quantity of encapsulated protein ingested by the macrophages. Although liposomes were rapidly taken up by macrophages, the liposome-encapsulated protein was antigenically expressed for a prolonged period (at least 24 h) on the cell surface. Liposomes were visualized inside vacuoles in the macrophages by immunogold electron microscopy. The liposomes accumulated along the peripheries of the vacuoles and many of them apparently remained intact for a long time (greater than 6 h). However, nonliposomal free protein was also detected in the cytoplasm surrounding these vacuoles, and it was concluded that the free protein in the cytoplasm was probably en route to the macrophage surface. Exposure of the cells to ammonium chloride did not inhibit the appearance of liposomal antigenic epitopes on the cell surface, and this suggests that expression of the liposomal antigenic epitopes at the surface was not a pH-sensitive phenomenon. There was no significant effect of a liposomal adjuvant, lipid A, on the rate or extent of surface expression of the liposomal protein.

Complement-dependent phagocytosis of liposomes by macrophages: suppressive effects of "stealth" lipids

We have previously reported that complement-opsonized liposomes composed of dimyristoyl phosphatidylcholine and cholesterol are actively phagocytozed by murine peritoneal macrophages and that such complement-induced phagocytosis can be suppressed by the presence of liposomal phosphatidylinositol (Proc. Natl. Acad. Sci. USA 81, 1984). We now report suppressive effects of other liposomal lipids, including monosialoganglioside (GM1) and sulfogalactosylceramide. Complement-dependent phagocytosis was almost completely suppressed by liposomes containing GM1 or phosphatidylinositol and partially suppressed when liposomes contained sulfogalactosylceramide. Although the mechanism of suppression of complement-induced phagocytosis by these liposomal lipids is not yet completely understood, it does not seem to involve the early stages of complement activation resulting in opsonization of liposomes with complement. We conclude that suppression of complement-induced phagocytosis by phosphatidylinositol, GM1, or sulfogalactosylceramide occurs at a step after liposome opsonization.

Antibodies to liposomal phosphatidylcholine and phosphatidylsulfocholine

Antibodies against dimyristoyl phosphatidylsulfocholine or dimyristoyl phosphatidylcholine were raised in rabbits after injection of liposomes containing phosphatidylsulfocholine or phosphatidylcholine, cholesterol, and lipid A. The antibody activities were assayed by complement-dependent immune damage to liposomes and by a solid-phase, enzyme-linked immunosorbent assay using purified dimyristoyl phosphatidylcholine or dimyristoyl phosphatidylsulfocholine as antigen. Each antiserum raised against phosphatidylsulfocholine reacted with liposomes containing phosphatidylcholine, and each antiserum raised against phosphatidylcholine reacted with liposomes containing phosphatidylsulfocholine. However, adsorption of dimyristoyl phosphatidylsulfocholine antiserum with liposomes containing dimyristoyl phosphatidylcholine removed all activity against dimyristoyl phosphatidylcholine, but did not eliminate antibody activity against dimyristoyl phosphatidylsulfocholine. These results indicate that the antiserum against phosphatidylsulfocholine contained mixed populations of antibodies. Polyclonal antisera that have been appropriately adsorbed can therefore be obtained with a high degree of specificity for phosphatidylsulfocholine and such antisera can distinguish between phosphatidylsulfocholine and phosphatidylcholine.

Anaphylactoid reactions mediated by autoantibodies to cholesterol in miniature pigs

Antoantibodies to cholesterol were detected and purified from normal (nonimmunized) pig serum. The antibodies were assayed by ELISA with crystalline cholesterol as an Ag and by C-dependent damage to cholesterol-laden liposomes. Intravenous injection of liposomes containing cholesterol into anesthetized animals caused decreased hemolytic complement titers, and induced a reaction consisting of transient neutropenia, thrombocytopenia, respiratory distress, cyanosis, pulmonary and systemic hypertension, and decreased cardiac output. Plasma levels of thromboxane B2 and 6-keto-prostaglandin F1 alpha increased 1300 and 200%, respectively, and leukocyte and platelet counts decreased by 36 and 38%, respectively. Injection of cholesterol-free liposomes did not induce the reaction. These results show that naturally occurring autoantibodies to cholesterol can initiate C activation and can be associated with anaphylactoid reaction to exogenously administered cholesterol in pigs.

Anaphylactoid reactions mediated by autoantibodies to cholesterol in miniature pigs

Antoantibodies to cholesterol were detected and purified from normal (nonimmunized) pig serum. The antibodies were assayed by ELISA with crystalline cholesterol as an Ag and by C-dependent damage to cholesterol-laden liposomes. Intravenous injection of liposomes containing cholesterol into anesthetized animals caused decreased hemolytic complement titers, and induced a reaction consisting of transient neutropenia, thrombocytopenia, respiratory distress, cyanosis, pulmonary and systemic hypertension, and decreased cardiac output. Plasma levels of thromboxane B2 and 6-keto-prostaglandin F1 alpha increased 1300 and 200%, respectively, and leukocyte and platelet counts decreased by 36 and 38%, respectively. Injection of cholesterol-free liposomes did not induce the reaction. These results show that naturally occurring autoantibodies to cholesterol can initiate C activation and can be associated with anaphylactoid reaction to exogenously administered cholesterol in pigs.

Prostaglandin and thromboxane in liposomes: suppression of the primary immune response to liposomal antigens

Liposomes containing lipid A as adjuvant and also containing prostaglandin E2 or thromboxane B2 were examined for the ability to influence induction of humoral immunity against liposomal protein or lipid antigens in rabbits. The protein antigen consisted of cholera toxin that was bound to ganglioside GM1 on the surface of the liposomes. High titers of anti-cholera toxin antibodies were produced and IgM and IgG responses were detected. When the immunizing liposomes contained either prostaglandin E2 or thromboxane B2 as part of the lipid bilayer, the primary immune response, involving both IgM and IgG antibodies, was greatly reduced. The secondary immune response observed after a boosting immunization was not suppressed by liposomal eicosanoids. A similar inhibitory effect on the primary response was observed when liposomal lipid antigens were examined instead of cholera toxin. An inhibitory effect of liposomal prostaglandin E2 on the phagocytic uptake of opsonized liposomes by cultured macrophages was also observed, suggesting that liposomal eicosanoids can have direct local effects on macrophages that might influence the immune response to liposomal antigens.

Cross-reactions of nucleic acids with monoclonal antibodies to phosphatidylinositol phosphate and cholesterol

Four monoclonal IgM antibodies to phosphatidylinositol phosphate (PIP), four antibodies to cholesterol and one antibody to liposomes containing phosphatidylcholine, cholesterol and dicetyl phosphate were tested for reactivity with denatured DNA. Three of four antibodies to PIP cross-reacted strongly with denatured DNA. The other antibodies did react with denatured DNA but only very weakly. The binding to DNA was competed by synthetic polynucleotides. In competitive assays, one of the anti-PIP antibodies was particularly reactive with poly(dT) and another with poly(I) and poly(dG). Binding of an anti-cholesterol antibody to ssDNA was also inhibited by poly(I) and poly(dG). Two of the anti-PIP antibodies were also reactive with mononucleotides, and all four bound inositol hexaphosphate. High concns of nucleosides did not compete for binding, indicating that phosphate is involved in the binding site. Phospholipids, particularly those containing inositol phosphate, also competed for binding to DNA, but to varying extents, indicating a variable overlap in the antibody binding site for DNA and phospholipid determinants. These antibodies, induced by immunization with liposomes, showed cross-reactivity characteristics often found with certain types of autoantibodies, but they did not bear the H130 idiotype, which was identified on IgM anti-DNA autoantibodies from MRL-lpr/lpr mice.

Phosphatidylinositol liposomes opsonized by concanavalin A stimulate phosphatidylinositol turnover in macrophages

We have previously found that concanavalin A binds specifically to inositol and phosphatidylinositol. In the present study we demonstrate that binding of concanavalin A to liposomes containing phosphatidylinositol influences the uptake of such liposomes by macrophages. Although resident mouse peritoneal macrophages normally ingest liposomes only to a slight extent, attachment of concanavalin A to the liposomes caused a marked enhancement of phagocytosis. Furthermore induction of phagocytosis of concanavalin A-opsonized liposomes was associated with increased phosphatidylinositol turnover. We conclude that (a) concanavalin A can serve as an opsonizing agent for liposomes; (b) opsonization results from binding of concanavalin A to phosphatidylinositol on the liposomal surface; and (c) concanavalin A-induced phagocytosis is associated with increased phosphatidylinositol turnover in the macrophages.

Specific binding of concanavalin A to free inositol and liposomes containing phosphatidylinositol

We previously reported that concanavalin A could bind specifically to liposomes containing phospholipids and lacking glycoconjugates (Biochem. Biophys. Res. Comm. 74, 208, 1977). In the present study we show that the binding of concanavalin A to the liposomes was greatly increased (up to 5 fold) by the presence of phosphatidylinositol in the liposomes. Furthermore, the binding of concanavalin A to phosphatidylinositol-liposomes was specific and could be inhibited by either alpha-methyl mannoside or by myo-inositol. We also found that concanavalin A-induced lymphocyte mitogenesis could be inhibited either by alpha-methyl mannoside or by myo-inositol. Simultaneous addition of both inhibitors to concanavalin A and liposomes showed that inhibition was non-competitive: alpha-methyl mannoside was more inhibitory to liposomes lacking phosphatidylinositol, and myo-inositol was more inhibitory to liposomes containing phosphatidylinositol. This suggests that the binding site for inositol might be different than that for mannose. Equilibrium dialysis and Scatchard plots revealed 4 binding sites each for inositol and mannose at neutral pH. The binding constants of concanavalin A were 0.13 X 10(4) and 0.25 X 10(4) liters/mole respectively for inositol and mannose. We conclude that concanavalin A binds specifically to the inositol portion of phosphatidylinositol.

Phosphate-binding specificities of monoclonal antibodies against phosphoinositides in liposomes

Monoclonal antibodies against phosphatidylinositol phosphate were produced after injecting a mouse with liposomes containing dimyristoylphosphatidylcholine, cholesterol, phosphatidylinositol phosphate and lipid A. The antibodies raised were IgM (kappa) and their activities were assayed by complement-dependent damage to liposomes lacking lipid A but containing the rest of original immunizing mixture of lipids. Three of the four antibodies selected cross-reacted with liposomes containing phosphatidylinositol instead of phosphatidylinositol phosphate; and two of the antibodies cross-reacted with liposomes containing phosphatidylinositol diphosphate. Each of the antibodies had a phosphate-binding specificity. Each also cross-reacted with liposomes containing sulfogalactosyl ceramide, but not with liposomes containing galactosyl ceramide, or gangliosides or with liposomes containing lipid A but lacking phosphoinositides. Recognition of sulfogalactosyl ceramide probably occurred because the chemical characteristics of the sulfate group were sufficiently similar to those of phosphate to allow recognition by the antibody. The phosphate-binding specificity was further confirmed by inhibition by phosphocholine, inositol hexaphosphate, ATP, AMP and even sodium phosphate, but not by choline or inositol.

Suppression of phagocytic function and phospholipid metabolism in macrophages by phosphatidylinositol liposomes

Increased phagocytosis of complement-opsonized vesicles was accompanied by increased phosphatidylinositol (PtdIns) turnover in murine macrophages. However when PtdIns was also present as one of the lipids in the opsonized liposomes, it reduced both phagocytosis and stimulation of endogenous PtdIns turnover. These suppressive effects did not occur with liposomes containing PtdIns phosphate (PtdIns-P). When a monoclonal IgM "anti-PtdIns-P" antibody that bound to inositol phosphate was substituted for antigalactosyl ceramide antibodies for activating complement in the opsonizing process, enhanced phagocytosis occurred normally but increased cellular PtdIns turnover did not occur. Therefore the data show that, although PtdIns-P cannot replace PtdIns for suppressing PtdIns turnover, PtdIns-P can be induced to be suppressive after specific binding to an antibody that recognizes inositol phosphate. We conclude that ingestion of complement-opsonized liposomes by macrophages and complement-induced turnover of cellular PtdIns are separate but related phenomena that can be independently modulated by the polar group of liposomal PtdIns.

Effects of negatively charged lipids on phagocytosis of liposomes opsonized by complement

Ingestion of liposomes opsonized by specific antibody plus complement was investigated in vitro. Although the antibodies alone (IgM) did not have an opsonizing effect, in the presence of such antibodies uptake and ingestion of liposomes by mouse peritoneal macrophages was enhanced 5- to 10-fold by addition of complement. Phagocytosis of complement-opsonized liposomes was strongly dependent on the charge of the liposomal lipids. The presence of a negatively charged (i.e., acidic) lipid profoundly suppressed the uptake of the liposomes. Each of three acidic liposomal lipids, phosphatidylserine, phosphatidylinositol and dicetyl phosphate, suppressed liposome uptake. We conclude that opsonization of liposomes with complement greatly stimulates ingestion of liposomes by murine macrophages. However, most of the opsonic enhancement conferred by complement can be prevented by the presence of negatively charged membrane lipids.

Inhibition of growth in young mice treated with pentazocine: reversal by naltrexone

One week old mice were injected subcutaneously once daily with d,1-methadone (5 mg/kg), pentazocine, naltrexone, naloxone, nalorphine or nalbuphine, each at 10 mg/kg. The remaining half of each litter was used as control. Only methadone and pentazocine groups showed reduced weight gain after 3 weeks of treatment (P < 0.01). Injection of pentazocine in dosages of 5-20 mg/kg inhibited weight gain and protein synthesis in a dose-related manner. The incorporation of labeled leucine was followed in brain, liver and muscles. Methadone and pentazocine groups showed a significant decrease in protein synthesis in all tissues studied. The nalbuphine, nalorphine, naloxone, and naltrexone-treated groups incorporated leucine normally, correlating with normal weight gain. These data suggest that pentazocine, unlike the other mixed agonist-antagonists and antagonists, adversely affects the growth of very young animals when administered chronically. A specific opioid effect is suggested by the fact that naltrexone given concomitantly with the pentazocine prevents development of the biochemical lesion.