The Journal of Immunology, 1998, 160: 3637-3641.
Copyright © 1998 by The American Association of Immunologists
Cutting Edge: Liposomal Formulation of a Self Lymphoma Antigen Induces Potent Protective Antitumor Immunity
Larry W. Kwak1,*,
Robin Pennington*,
Larry Boni
,
Augusto C. Ochoa2,
,
Richard J. Robb
and
Mircea C. Popescu
*
Department of Experimental Transplantation and Immunology, Medicine Branch, Division of Clinical Sciences, National Cancer Institute, Bethesda, MD 20892;
Biomira USA, Cranbury, NJ 08512; and
SAIC-Frederick, Frederick, MD 21702
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Abstract
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We developed a liposome carrier for a model nonimmunogenic, self
Ag. This carrier reproducibly converted lymphoma Ig into a potent tumor
rejection Ag in mice. A single immunization induced protection against
challenges representing 20 to 100 times the minimum lethal dose of
parental tumor. This protective effect required minimal amounts of
incorporated Ag and IL-2 and elicited specific Abs (compared with free
Ag or liposomal control Ig which did not elicit any specific Abs);
depletion experiments demonstrated a requirement for effector
CD4+ and CD8+ T cells. Head-to-head
comparisons, indicating superior potency and induction of specific T
cell activation, distinguished liposomal from prototype,
carrier-conjugated Ag. These results provide a strategy for formulating
weak tumor or other clinically important Ags into vaccines.
 |
Introduction
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The ability
to induce a potent, specific, and systemic immune response against
inherently poor immunogenic Ags is critical to vaccine development.
Various experimental strategies have been explored with candidate tumor
Ags to overcome this limitation, including formulation of Ag with
immunologic adjuvants, genetic fusion with immunogenic carriers, and
paracrine delivery of cytokines or costimulatory molecules. However,
these methods generally function to enhance the immunogenicity of Ags
or tumors that are already somewhat immunogenic; only rarely do they
convert a truly nonimmunogenic Ag to an immunogenic one (1, 2, 3).
Our laboratory has studied syngeneic, lymphoma-derived Ig as a model
tumor Ag. The recognition sites of the B cell Ag receptor contain
determinants, which are defined by their unique variable region
sequences, that can themselves be recognized as Ags. Since B cell
malignancies are clonally restricted to the synthesis of a single Ig,
these determinants, Ids, can serve as tumor-specific Ags (4). Lynch et
al. initially demonstrated that tumor Id-specific Abs and resistance to
myeloma growth could be elicited in syngeneic mice, and this
observation was later supported by others (5, 6, 7, 8, 9, 10). Subsequently, our
studies of carrier-conjugated lymphoma-derived Id immunization in human
patients showed that humoral responses could be induced against the
specific Id on each patients tumor (11).
Unmodified Ig (free Id protein) isolated from murine 38C13 lymphoma is
completely nonimmunogenic in syngeneic mice. Physical modification of
this lymphoma-derived Id by either chemical or genetic fusion to an
immunogenic carrier protein, such as keyhole limpet hemocyanin
(KLH)3, elicits
Id-specific Abs; however, there is little evidence for an effector T
cell response (10, 12). Liposomes can be effective vaccine carriers,
inducing, in some cases, specific T cell immunity (13). Therefore, we
explored a novel liposome formulation, consisting of free 38C13 Id,
lipid, and human rIL-2 (liposomal lymphoma-derived Id (lipo-Id)).
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Materials and Methods
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Mice and tumor
We obtained 6 to 12-wk-old C3H/HeN Mtv-negative female mice from
the Animal Production Area of the National Cancer Institute, Frederick
Cancer Research and Development Center (NCI-FCRDC), Frederick, MD. The
carcinogen-induced, C3H 38C13 B cell lymphoma has been previously
described (14).
Liposomal vaccine preparation
Free IgM (38C13 Id) was isolated by somatic cell hybridization
(15), and 38C13 Id and a control IgMkappa (4C5) (both
from R. Levy, Stanford University, Stanford, CA) were purified from
ascites. Where indicated, a second control IgMkappa
(TEPC-183, Sigma, St. Louis, MO) was used. Id-KLH was prepared by
glutaraldehyde conjugation at a 1:1 ratio of Id and KLH (10). The
liposomal entrapment procedure of Anderson (16) was modified as
follows: Aqueous Id, IL-2, and mouse serum albumin were added to dry,
powdered dimyristoylphosphatidylcholine, warmed to 37°C, and
subjected to three cycles of vortexing, sonication, freezing at
-70°C, and thawing at 37°C. Following dilution with 1 mM PBS (pH
7), samples were washed three times by centrifugation (12,000
revolutions per min). Final concentrations were typically 40 mM
dimyristoylphosphatidylcholine, >5 x 105
international units/ml IL-2, and various amounts of Id Ag as indicated.
Liposomes had a bimodal distribution consisting of both small (mean
diameter of 177 nm) and medium/large (mean diameter of 2 µm)
vesicles. Based on analysis with an anti-38C13 Id mAb (S1C5, M.
Kaminski, University of Michigan, Ann Arbor, MI) and electron
microscopy, Id and IL-2 were distributed on both the outer surface of
the vesicles and between the inner lipid bilayers.
The concentration of incorporated Id for each individual preparation
was determined by sandwich ELISA after dissolution of lipid vesicles
with detergent. The unknown was captured by rabbit anti-mouse IgM
and detected by biotinylated rabbit anti-mouse IgM (17).
Streptavidin-europium was added as a final step, and europium
fluorescence was recorded by a 1232 delayed fluorescence immunoassay
time-resolved fluorometer (Wallac, Gaithersburg, MD). Experiments with
a control IgM demonstrated that similar exposure to detergent did not
influence detection by ELISA. Furthermore, the sum total of
incorporated and unincorporated Id was equivalent to the amount of
input Id, based on protein content, within a few percentage points
(data not shown).
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Results and Discussion
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Mice (10 per group) that had been previously immunized with
lipo-Id and challenged with 20 times the minimum lethal dose of
lymphoma 2 wk later demonstrated significantly prolonged survival as
well as protective immunity (30%, Fig. 1
A). This was in
distinct contrast to controls that had been immunized either with
liposomes containing comparable amounts of a control IgM (lipo-control
Id, p = 0.001) or with free Id
(p = 0.002). The potency of lipo-Id was also
apparent against a substantially larger tumor inoculum (100 times the
minimum lethal dose; p = 0.001 vs control liposomes,
data not shown). The requirement for liposomal incorporation of Id or
IL-2 was evaluated by the experiment shown in Figure 1
B.
Formulations consisting of both Id and IL-2 entrapped in liposomes
(lipo-Id), Id only in liposomes that was then mixed with an equivalent
amount of free IL-2 (lipo-Id (free IL-2)), or IL-2 only in liposomes
subsequently mixed with free Id (lipo-IL-2 (free Id)) were used to
immunize mice (10 per group), followed by tumor challenge. Optimal
protective immunity was observed in mice immunized with liposomes
containing both Id and IL-2 (p < 0.01 compared
with controls immunized with either free Id or IL-2 alone), suggesting
that physical entrapment of Ag and IL-2 was critical.

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FIGURE 1. Immunization with liposomal, but not free, tumor-derived Id elicits
protective antitumor immunity. A, Id or a control IgM
(lipo-control Id) was incorporated into liposomes; the dose of Id
administered per mouse is indicated in parentheses. Mice (10 per group)
were immunized i.p. and challenged i.p. with 2 x 103
lymphoma cells 2 wk later. B indicates the requirement for
both Ag and IL-2 incorporation. Formulations consisted of both Id and
IL-2 entrapped in liposomes (lipo-Id), Id only in liposomes that was
then mixed with an equivalent amount of free IL-2 (lipo-Id (free
IL-2)), or IL-2 only in liposomes subsequently mixed with free Id
(lipo-IL-2 (free Id)). C shows the effect of Ag dose and a
comparison of potency with KLH-conjugated Id. Serial dilutions of input
Id were used to prepare liposomes containing equal concentrations of
other components. D shows IL-2 dose response. Serial
dilutions of input IL-2 were used to prepare liposomes containing equal
concentrations of other components (40 µg Id per mouse).
BD, Mice (1020 per group) received a single i.p.
immunization and were then challenged with tumor as in A.
Statistical comparisons of survival (as compared with free Id) were
made on the basis of nonparametric Mantel-Cox log rank p
values (BMDP Statistical Software, Los Angeles, CA).
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Given the above results, we sought to optimize Ag incorporation. Four
lipo-Id vaccines, delivering Id doses of 0.4 to 40 µg/mouse, were
prepared by serially diluting the amount of input Id while holding the
concentrations of other components constant. Mice given a single
immunization with lipo-Id and challenged with tumor demonstrated a
clear dose-dependent effect on protective immunity, with those vaccines
delivering 40, 10, and 2 µg Id producing superior survival compared
with free Id (Fig. 1
C). In this experiment, we also
compared the potency of lipo-Id with a previously determined optimal
dose of KLH-conjugated Id (50 µg Id). Matched for Ag dose, the
protective effect of lipo-Id (40 µg Id) was superior to that of
Id-KLH (p = 0.036, Fig. 1
C).
In a separate experiment, the superior potency of lipo-Id was also
apparent at suboptimal, lower Ag doses. Nine mice immunized with
lipo-Id (2 µg Id) demonstrated 33% protection, while 10 mice per
group immunized with Id-KLH containing 50, 10, or 2 µg Id per mouse
demonstrated 40, 30, and 0% protection, respectively
(p = 0.007 for lipo-Id vs Id-KLH (2 µg Id),
data not shown). Neither free nor liposomal IL-2 (without Ag) enhances
protective immunity when mixed with either free Id or Id-KLH in this
model (18); thus, the superiority of lipo-Id over Id-KLH cannot be
attributed to the simple addition of IL-2.
In parallel, input IL-2 was serially diluted while the dose of Id (40
µg) was held constant. A clear IL-2 dose-dependent effect on
protective immunity was also observed (Fig. 1
D). It
was striking that vaccines containing as little as 1/10 the prototype
amount of IL-2 were capable of inducing significant protection
(p = 0.004 vs free Id).
To investigate the mechanism by which lipo-Id induces protective
antitumor immunity, we initially determined antiidiotypic Ab levels
elicited by the vaccines in Figure 1
C (Fig. 2
). A clear dose-dependent effect of
encapsulated Id was apparent on the Ab response (15, 7, 1, and 0.1
µg/ml, respectively). The induction of a humoral response by lipo-Id
was distinct from free Id, which failed to induce any Ab. However,
compared with Id-KLH immunization (55 µg/ml), these levels of Ab were
uniformly low, even though two lipo-Id preparations had produced
superior or equivalent tumor protection, respectively (Fig. 1
C). Both lipo-Id and Id-KLH predominantly induced
IgG1 isotype Abs; some IgG2a isotype Abs were also induced (data not
shown). The apparent lack of correlation between Ab response and tumor
protection suggested that Ab could not fully account for the protection
induced by lipo-Id.

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FIGURE 2. Humoral responses. Mean antiidiotypic Ab levels of five mice/group from
the experiment in Figure 1 C were determined at the time of
tumor challenge. Serum was serially diluted over microtiter plates
coated with lymphoma-derived Id which had been affinity purified using
an anti-Id mAb. Binding of serum Abs to Id was detected by goat
anti-mouse IgG horseradish peroxidase-conjugated Abs. Serum
antiidiotypic Ab levels were quantitated by comparing sera titration
curves with a standard curve obtained with a known concentration of a
mixture of purified anti-Id mAbs. Specificity for Id was demonstrated
by the lack of binding to a control IgM (TEPC-183, data not
shown).
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Next, we investigated splenic T cell proliferation in vitro. The
representative experiment shown in Figure 3
A demonstrates a
dose-dependent response to free Id after priming with lipo-Id, but not
after priming with liposomes without Ag, Id-KLH, or free Id. The
specificity of the response for Id was formally demonstrated in a
separate experiment by both the lack of parallel response to a control
IgM (4C5) and the lack of response to 38C13 Id after priming with
liposomes containing another control IgM (lipo-control Id, TEPC-183,
Fig. 3
B). Thus, at a lipo-Id concentration (10 µg)
that produced tumor protection equivalent to that of Id-KLH, there was
a profound difference in the ability of the former, but not the latter,
to induce T cell activation.

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FIGURE 3. In vitro Id-specific T cell responses. Results from two separate
experiments are shown. Splenocytes obtained from two to three
mice/group that had been immunized i.p. 2 wk earlier as indicated were
pooled and enriched for T cells by passing over nylon wool and then
placed in 96-well flat-bottom microtiter plates in quadruplicate (200
µl, 2 x 105 cells/well) with free Id at
various concentrations (A), or with a single
concentration of Id or one of two control IgMs
(B). Normal syngeneic splenocytes (2000 rad,
5 x 105) were added as a source of APCs. Cultures
were maintained at 37°C, 5% CO2 for 5 to 7 days, and 18
to 24 h before harvesting 1 µCi [3H]thymidine (2
Ci/mmol, New England Nuclear Research Products, Boston, MA) in 50 µl
medium was added to each well. Incorporated radioactivity was measured
in an LKB 1205 Betaplate liquid scintillation counter (Gaithersburg,
MD).
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To definitively determine the role of effector T cells in protection,
we depleted specific subsets in vivo in immunized mice. Mice primed
with lipo-Id received depleting mAbs that were specific for either
CD4+ or CD8+ T cells (or both) or normal rat
IgG before tumor challenge (Fig. 4
).
Depletion of either subset, either alone or in combination, was
associated with markedly reduced survival (p =
0.012 and 0.004, respectively, vs rat IgG-treated controls), suggesting
a requirement for both CD4+ and CD8+ T cells.
However, these results do not formally exclude a minor role for
antiidiotypic Abs that are found intact in mAb-treated mice (data not
shown), especially given the incomplete abrogation of protection.

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FIGURE 4. Effect of in vivo T cell depletion. Mice received a single i.p.
immunization with lipo-Id and were randomly assigned to receive
treatment with a depleting mAb specific for CD4+ or
CD8+ T cells (GK1.5 and 53.672, respectively, ammonium
sulfate purified ascites, preclinical repository, NCI-FCRDC), both
mAbs, or with normal rat IgG (Sigma) 2 wk later on every other day for
three i.p. doses (0.4 mg/day, eight mice/group). Controls immunized
with free Id also received treatment with normal rat IgG. All mice were
then challenged i.p. with 2 x 103 lymphoma cells.
Flow cytometry (27) analysis of splenocytes from normal mice treated
with these mAbs in parallel 1 and 2 wk after treatment confirmed a
>95% depletion of the appropriate subset with normal levels of the
other subset. p values refer to comparisons with non-T
cell-depleted, lipo-Id-vaccinated mice.
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These experiments demonstrate the successful conversion of
nonimmunogenic, free lymphoma-derived Id into a vaccine, without
structural modification of the Ag by chemical or genetic fusion. As a
"self" Ag, Id provided a rigorous test for this carrier and,
consequently, a relevant model for other poorly immunogenic tumor or
infectious disease Ags. The potency of this formulation was apparent by
both the small amount of incorporated Ag actually required, and by
comparison with the optimal dose of prototype, carrier-conjugated Id.
It is possible, however, that the reduction in Ag dose required for
immunogenicity may vary for other tumor Ags.
The precise cellular mechanisms through which T cells induced by
lipo-Id mediate antitumor immunity remain to be elucidated. Liposomes
are known to target Ag to lymphoid organs for processing in either MHC
class I or II presentation pathways by APCs (19, 20). However, IL-2
incorporation in the same vesicle was also required. Preliminary
studies suggest that the sustained release of IL-2 occurs, and that
IL-2 on the vesicle surface may be capable of causing the association
and internalization of liposomes with cells expressing IL-2R (M.C.
Popescu, unpublished observations). Further studies of in situ
interactions between APCs, T cells, and other potential non-T cell
effectors are in progress.
The ability of lipo-Id to induce Id-specific T cells also distinguishes
this formulation from Id-KLH (Fig. 3
), which is currently being tested
in clinical trials (11, 21), as well as from Id-cytokine fusions (22)
and DNA vaccines (23, 24), none of which have been reported to prime
for a CD4+ or CD8+ T cell response to Id in
murine models. Dendritic cells pulsed with Id may represent an
alternative vaccine to induce T cell immunity (25).
Finally, evidence suggesting that T cells recognize idiotypic
determinants as processed peptides with MHC was provided by the
unexpected cross-reactivity of murine CD8+ influenza
hemagglutinin-specific T cell clones with a 10 amino acid peptide that
was derived from a myeloma Ig heavy chain variable region (26).
Furthermore, we recovered a human donor-derived myeloma Id-specific
CD4+ T cell line from a marrow transplant recipient that
was blocked by anti-MHC class II Abs (21).
Future studies with this potent carrier should be aimed at formulating
other cytokines of potential interest for vaccine development.
Ultimately, the simple and reproducible formulation of this carrier
should facilitate its translation to clinical trial.
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Acknowledgments
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We thank Ronald E. Gress for critically reading the manuscript and
Steve Weeks for technical assistance.
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Footnotes
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1 Address correspondence and reprint requests to Dr. Larry W. Kwak, National Cancer Institute, Frederick Cancer Research and Development Center, Bldg. 567, Rm. 205, Frederick, MD 21702. E-mail address: 
2 Current address: Louisiana State University Medical Center, New Orleans, LA 70112. 
3 Abbreviations used in this paper: KLH, keyhole limpet hemocyanin; lipo-Id, liposomal lymphoma-derived Id. 
Received for publication December 2, 1997.
Accepted for publication February 11, 1998.
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