The selective targeting of tumor-associated carbohydrate Ags by the induction of serum Abs that trigger apoptosis of tumor cells as a means to reduce circulating tumor cells and micrometastases would be an advantage in cancer vaccine development. Some plant lectins like Griffonia simplicifolia lectin I and wheat germ agglutinin mediate the apoptosis of tumor cells. We investigated the possibility of using these lectins as templates to select peptide mimotopes of tumor-associated carbohydrate Ags as immunogens to generate cross-reactive Abs capable of mediating apoptosis of tumor cells. In this study, we show that immunization with a mimotope selected based on its reactivity with Griffonia simplicifolia lectin I and wheat germ agglutinin induced serum IgM Abs in mice that mediated the apoptosis of murine 4T1 and human MCF7 cell lines in vitro, paralleling the apoptotic activity of the lectins. Vaccine-induced anti-carbohydrate Abs reduced the outgrowth of micrometastases in the 4T1 spontaneous tumor model, significantly increasing survival time of tumor-bearing animals. This finding parallels suggestions that carbohydrate-reactive IgM with apoptotic activity may have merit in the adjuvant setting if the right carbohydrate-associated targets are identified.

Natural carbohydrate-reactive IgM Abs are implicated in mediating the apoptosis of tumor cells, and these circulating natural Abs are suggested as a mechanism of innate immune surveillance against cancer cells (1). mAbs directed against carbohydrate Ags expressed on tumor cells that trigger apoptosis have been described (2, 3) and provide a possibility for their application in the immunotherapy of disseminated cancer cells. The selective targeting of tumor-associated carbohydrate Ags by the induction of serum Abs that trigger apoptosis as a means to eradicate metastases could therefore be an advantage in vaccine development (4).

In our approach to induce sustained immunity against cancer cells, we are developing peptide mimotopes of tumor-associated carbohydrate Ags. Toward this end, we have shown that peptide mimotopes can induce humoral responses that mediate tumor-specific complement-dependent cytotoxicity (CDC)3 (5, 6) and tumor-specific cellular responses (7). We have also shown that priming with peptide mimotopes followed by boosting with carbohydrate Ag can prolong the IgM response in mice, a benefit that is perceived to be of value for cancer vaccines in humans (8). Because it is expected that complement inhibitors expressed on a tumor cell surface can impair CDC, we have turned our attention to a strategy of developing mimotopes that will induce Abs that trigger apoptotic mechanisms.

The lectins Griffonia simplicifolia I (GS-I), reactive with the α-galactose (αGal) and α-N-acetylgalactosamine (αGalNAc) moieties, and wheat germ agglutinin (WGA), reactive with N-acetylglucosamine (GlcNAc) and sialic acid moieties, mediate apoptosis of various murine and human tumor cell lines (9, 10, 11, 12, 13, 14). Screening combinatorial peptide libraries expressing a large collection of peptide sequences with lectins or anti-carbohydrate Abs has indicated a feasible strategy to produce immunogens for inducing carbohydrate cross-reactive immune responses (15). We hypothesize that using GS-I and WGA as templates to define and select peptide mimotopes will enable us to induce serum Abs capable of mediating apoptosis of tumor cells upon mimotope immunization.

The feasibility of this immunological strategy is presented in the current study, in which we find that carbohydrate-reactive IgM induced by a peptide mimotope in a DNA vaccine format or synthesized as a multiple Ag peptide (MAP) suppresses the outgrowth of metastases in the spontaneous murine mammary tumor 4T1 model. These observations further support the premise that Ab-inducing vaccines against carbohydrates can be used in the adjuvant setting, where circulating tumor cells and micrometastases are the primary targets (16, 17).

The 4T1 and MCF7 cell lines were purchased from American Type Culture Collection. The 4T1 cells were maintained in complete DMEM medium (Cellgro; Mediatech), supplemented with 100 μg/ml penicillin-streptomycin (Cellgro), 2 mM l-glutamine (Cellgro), and 10% (v/v) FBS (ATLAS Biologicals). The MCF7 cells were maintained in American Type Culture Collection complete growth medium, which contains MEM (Eagle) supplemented with 0.1 mM nonessential amino acids, 1 mM sodium pyruvate, 0.01 mg/ml bovine insulin, 100 μg/ml penicillin-streptomycin, and 10% FBS. The ARK cell line was established at the Arkansas Cancer Research Center from bone marrow aspirate of a patient with multiple myeloma (18). This cell line was kept in a complete RPMI 1640 medium supplemented with 100 μg/ml penicillin-streptomycin, 1 mM sodium pyruvate, 10% FBS, and 2500 mg/ml glucose. Biotinylated GS-I and WGA lectins were purchased from Vector Laboratories. FITC-conjugated streptavidin was purchased from Sigma-Aldrich. Peptides were synthesized as MAP (Bio-Synthesis). Synthetic carbohydrate probes incorporated into a polyacrylamide (PAA) matrix were purchased from Glycotech. Vybrant apoptosis kit no. 3 was purchased from Molecular Probes. Fluorometric caspase activity detection kit was purchased from BD Biosciences. Apoptag, In Situ Apoptosis Detection kit S7100, was purchased from Chemicon International.

Oligonucleotides were synthesized and inserted into a secretory plasmid vector. First, the oligonucleotides were cloned between the restriction site NotI and HindIII in an intermediate shuttle vector pSL1180 (Amersham Biosciences) and then transferred to pSecTag2/HygroB (Invitrogen Life Technologies). Cloning was confirmed in each step by DNA sequencing. Constructs were designated as 104-, 105-, and 107-pSec based on the cloned peptide sequence.

Six- to 8-wk-old BALB/c female mice were purchased from The Jackson Laboratory. To establish tumor, each mouse was inoculated s.c. in the abdominal mammary gland with 1 × 105 4T1 cells. Tumor growth was measured using a caliper and was recorded as the mean of two orthogonal diameters ((a + b)/2) (7). DNA immunization was performed as described earlier with minor modifications (6). Each mouse received three i.m. injections (weekly intervals) of 50-μg DNA construct resuspended in 100 μl of PBS. Immunization started 4 days posttumor transplant. Peptide immunization was performed as described earlier (5). Each mouse received 50 μg of MAP and 10 μg of QS-21 (Antigenics, Inc.) i.p., both resuspended in 100 μl of PBS.

Spontaneous metastases were measured by methods described by Pulaski and Ostrand-Rosenberg (19, 20) as clonogenic assays. Briefly, 4 wk after tumor inoculation, mice were sacrificed, and the lungs and the livers were harvested. Following this, the number of clonogenic cells was determined by growing harvested cells in medium containing 6-thioguanine (21). The animal studies have been reviewed and approved by Institutional Animal Care and Use Committee of University of Arkansas for Medical Sciences.

Staining, acquisition, and analysis were performed as described earlier (22). Briefly, cells were incubated with biotinylated GS-I and WGA lectins (10 μg/ml) for 30 min and then stained with FITC-conjugated streptavidin at 2 μg/ml for another 30 min on ice. For inhibition assay, biotinylated GS-I lectin (10 μg/ml) was combined with serial concentrations of the peptide and incubated overnight at +4°C. Lectin/peptide mix was then added to the cells, and lectin binding was visualized as above. Mean fluorescence intensity was calculated from duplicates for each carbohydrate concentration, and percentage of inhibition was calculated as follows: (1 − (mean of test tubes/mean of control tubes (only GS-I))) × 100.

ELISA was performed as described before (5). Inhibition ELISA was performed as described earlier (7). Mean absorbance was calculated from duplicates for each carbohydrate concentration, and percentage of inhibition was calculated as follows: (1 − (mean of test wells/mean of control wells)) × 100.

4T1 cells were plated into wells of a 96-well plate in medium containing 5% FBS. Lectins were added to some wells at various concentrations, and after overnight incubation, wells were washed; cells were fixed, stained with trypan blue, and counted; and the percentage of cytotoxicity was calculated using the following formula: (1 − (alive cells in experimental wells/alive cells in control wells)) × 100. Cytotoxicity of MCF7 cells was assayed by using Celltiter 96 Aqueous One Solution (Promega) according to the manufacturer’s instructions. Briefly, 3–5 × 104 cells were seeded in wells of 96-well plates, and lectins were added and incubated overnight. Then, the provided solution was added, absorbance was read, and the percentage of cytotoxicity was calculated using the following formula: (1 − (absorbance of test well/absorbance of control well)) × 100. To assess apoptosis, we used Vybrant apoptosis kit no. 3 (Molecular Probes) based on manufacturer’s instructions. Briefly, cells were incubated with or without the lectins and then harvested and stained with red-fluorescent propidium iodide (PI) and FITC-conjugated annexin V. After staining, dead cells show red and green fluorescence, apoptotic cells show green fluorescence, and live cells show no fluorescence. For detection of serum-mediated cytotoxicity or apoptosis, cell culture was established in complete medium overnight, and then the medium was replaced with the medium supplemented with 5% heat-inactivated mouse serum, and incubation was continued. Caspase activity was assayed by a fluorometric kit (BD Biosciences) using the manufacturer’s instructions. Briefly, cells were coincubated with medium supplemented with the lectins or the mice sera. Then, cells were trypsinized and counted, and cell lysate was prepared. Total protein was quantitated using a BCA Protein Assay kit (Pierce). Activity of each caspase was detected by using the specific fluorogenic substrates peptides VDVAD (caspase 2), DEVD (caspase 3), IETD (caspase 8), and LEHD (caspase 9) conjugated to 7-amino-4-methyl-coumarin. Fluorescence intensity was measured using FLx 800 Microplate Fluorescence Reader (Bio-Tek Instruments) and corrected in regard to protein concentration. Lysates of nontreated cells (in case of lectins) or cells treated with normal mouse serum prepared at the same time points were used as negative controls.

Cells were grown in 24-well plates on coverslips overnight. Culture medium was replaced with fresh medium containing 1% BSA and premixed 5% immunized mouse serum and anti-mouse IgM-FITC and incubated for 4 h. Slides were then washed in PBS and scanned with an inverted confocal microscope (Zeiss LSM 410) to a maximum depth of 25 μm, and representative images were captured.

Sections of lung and primary tumor from mice at 7, 14, and 21 days postinoculation were fixed in 10% neutral buffered formalin, processed, and embedded in paraffin, sectioned at 6 μm, stained with H&E, and examined under a light microscope. Serial sections were stained by the TUNEL method using Apoptag Peroxidase In Situ Apoptosis Detection kit S7100 (Chemicon International) based on the manufacturer’s instructions. Briefly, 6-μm sections were deparaffinized, rehydrated, and treated with 20 μg/ml Proteinase K for 15 min at room temperature. Sections were washed with two changes of dH2O for 2 min each. Endogenous peroxidases were blocked with 3% H2O2 in PBS for 5 min and washed with three changes of PBS. Equilibration buffer containing digoxigenin-conjugated nucleotides was placed directly onto the section for 10 s. Sections were incubated with TdT enzyme in a humidified chamber at room temperature for 1 h. Sections were then incubated for 10 min at room temperature in stop-wash buffer, rinsed in three changes of PBS for 1 min each, and incubated with anti-digoxigenin conjugate for 30 min at room temperature. Sections were washed in four changes of PBS, stained with 0.5% (w/v) methyl green counterstain, and evaluated with a light microscope.

The Kaplan-Meier method was used to estimate survival rates, and the log-rank test was performed to compare two groups. Fisher exact test was used for comparisons made between animals regarding liver metastasis. Other statistical comparisons between means were performed using Student’s t test. Differences between groups were considered significant if p < 0.05. Statistica software was used for analyses.

Both WGA and GS-I bound to murine 4T1 and human MCF-7 breast cell lines with WGA showing stronger reactivity for both cell lines as assessed by flow cytometry (Fig. 1,A). Both lectins mediated cell death upon coincubation overnight with either cell line, validating the induction of cytotoxicity by the lectins (9, 10, 11, 14) (Fig. 1,B). Supplementary experiments demonstrated that the cytotoxicity effect of the GS-I and WGA lectins was mediated through an apoptotic pathway as measured by an annexin V assay (Fig. 1 C), with WGA displaying a more dramatic apoptotic effect on MCF7 cells than GS-I.

FIGURE 1.

GS-I and WGA bound to the breast cell lines and mediated cytotoxicity and apoptosis. A, Lectins were coincubated with the indicated cell lines at 10 μg/ml for 30 min on ice. Similar amount of biotin was used as negative control. Binding was visualized with FITC-conjugated streptavidin using flow cytometry. Mean fluorescence intensity for each histogram is shown. B, Cells were seeded and coincubated with indicated concentration of the lectins overnight. Percentage of cytotoxicity was calculated based on the average of three replications as explained in Materials and Methods. C, Cells were incubated with or without lectins at 20 μg/ml for 4 h. Cells were then harvested, washed, and stained with PI (FL-2) and annexin V (FL-1) as explained in Materials and Methods. Percentage of cells in each quadrant is shown. Both lectin-treated and nontreated cells were stained. Control stands for cells that were not treated with the lectin but were stained with PI and annexin V. In 4T1 cells, the experiments with WGA and GSI were performed separately, so for each experiment we run a separate nontreated stained control. D and E, Cells were incubated with or without WGA and GS-I lectins at 10 μg/ml overnight and then were harvested, and caspase activity was measured. To compare the magnitude of activation over a period of time, fold increase of the activation, induced by GS-I lectin, as measured by fluorescence intensity of treated cells over nontreated cells at the same time point is presented (E). ∗, p < 0.05; ∗∗, p < 0.01; ns, not significant compared with control. All experiments above were repeated at least three times.

FIGURE 1.

GS-I and WGA bound to the breast cell lines and mediated cytotoxicity and apoptosis. A, Lectins were coincubated with the indicated cell lines at 10 μg/ml for 30 min on ice. Similar amount of biotin was used as negative control. Binding was visualized with FITC-conjugated streptavidin using flow cytometry. Mean fluorescence intensity for each histogram is shown. B, Cells were seeded and coincubated with indicated concentration of the lectins overnight. Percentage of cytotoxicity was calculated based on the average of three replications as explained in Materials and Methods. C, Cells were incubated with or without lectins at 20 μg/ml for 4 h. Cells were then harvested, washed, and stained with PI (FL-2) and annexin V (FL-1) as explained in Materials and Methods. Percentage of cells in each quadrant is shown. Both lectin-treated and nontreated cells were stained. Control stands for cells that were not treated with the lectin but were stained with PI and annexin V. In 4T1 cells, the experiments with WGA and GSI were performed separately, so for each experiment we run a separate nontreated stained control. D and E, Cells were incubated with or without WGA and GS-I lectins at 10 μg/ml overnight and then were harvested, and caspase activity was measured. To compare the magnitude of activation over a period of time, fold increase of the activation, induced by GS-I lectin, as measured by fluorescence intensity of treated cells over nontreated cells at the same time point is presented (E). ∗, p < 0.05; ∗∗, p < 0.01; ns, not significant compared with control. All experiments above were repeated at least three times.

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Using 4T1 cells, we further confirmed the induction of apoptosis, measuring activation of caspases 2, 3, 8, and 9. Overnight incubation of 4T1 cells with GS-I and WGA activated all indicated caspases (Fig. 1,D). Activation was observed to start as early as 4 h postincubation for all caspases. Activation of caspases 2 and 3 were consistently increased in treated cells compared with nontreated cells as depicted by fold increase of fluorescent intensity over a period of time (Fig. 1 E), suggesting their critical role in cytotoxicity.

We have defined peptides that mimic multiple carbohydrate Ags (6) that include peptides 105, 106, and 107 (Table I) or selected against specific carbohydrate-reactive Abs like peptides 104 and 109 (Table I) (23). Peptide 107 was chosen for our studies based on its consistent reactivity with both GS-I and WGA (Fig. 2,A), further indicating that this peptide can functionally mimic multiple types of tumor-associated carbohydrate structures. In inhibition assays, we observed that peptide 107 binding to WGA was inhibited by GlcNAc (Fig. 2 B) and that the peptide significantly inhibited GS-I lectin binding to 4T1 cells (C), suggesting that the peptide binds at or near the carbohydrate binding site of the two lectins.

Table I.

Peptides used in the study

PeptidePeptide Sequence
104 GGIMILLIFSLLWFGGA 
105 GGIYYPYDIYYPYDIYYPYD 
106 GGIYWRYDIYWRYDIYWRYD 
107 GGIYYRYDIYYRYDIYYRYD 
109 GGARVSFWRYSSFAPTY 
PeptidePeptide Sequence
104 GGIMILLIFSLLWFGGA 
105 GGIYYPYDIYYPYDIYYPYD 
106 GGIYWRYDIYWRYDIYWRYD 
107 GGIYYRYDIYYRYDIYYRYD 
109 GGARVSFWRYSSFAPTY 
FIGURE 2.

Peptide 107 reacted with the lectins in a specific manner, and immunization induced serum IgM Abs reactive with the carbohydrates and the cells. A, Plates were coated with the 107 peptide, and dose-dependent reactivity of indicated lectins was assessed. B, WGA binding to peptide 107 was inhibited by competitive carbohydrate GlcNAc. GalNAc was used as negative control. Results present the mean value ± SD. ∗∗, p < 0.01; ∗∗∗, p < 0.0005 compared with inhibition of GalNAc at the same concentration. C, GS-I binding to the 4T1 cells was inhibited by peptide 107 as revealed in a FACS assay. Peptide 109 was used as the negative control. Bars show SD based on three replications. ∗, p < 0.025; ∗∗, p < 0.01; ∗∗∗, p < 0.0005 compared with inhibition of peptide 109 at the same concentration. D, Mice (10/group) were immunized three times with 107-pSec and the respective vector plasmid pSec. Animals were bled 10 days after the last boost. For each group, sera were pooled for 10 mice. ELISA plates were coated with indicated carbohydrates and reactivity of IgM Abs was detected. End-point Ab titer against Galα1,3Galβ-PAA was 1:2560, whereas it was 1:1280 for GlcNAcβ1,4GlcNAC-β-PAA. The titers were determined as the highest sera dilution with significantly higher OD compared with preimmune sera using paired Student’s t test at p < 0.05. E–G, Binding of the serum to 4T1 (E), MCF7 (F), and ARK (G) cells was detected by flow cytometry and presented by mean fluorescence intensity (MFI).

FIGURE 2.

Peptide 107 reacted with the lectins in a specific manner, and immunization induced serum IgM Abs reactive with the carbohydrates and the cells. A, Plates were coated with the 107 peptide, and dose-dependent reactivity of indicated lectins was assessed. B, WGA binding to peptide 107 was inhibited by competitive carbohydrate GlcNAc. GalNAc was used as negative control. Results present the mean value ± SD. ∗∗, p < 0.01; ∗∗∗, p < 0.0005 compared with inhibition of GalNAc at the same concentration. C, GS-I binding to the 4T1 cells was inhibited by peptide 107 as revealed in a FACS assay. Peptide 109 was used as the negative control. Bars show SD based on three replications. ∗, p < 0.025; ∗∗, p < 0.01; ∗∗∗, p < 0.0005 compared with inhibition of peptide 109 at the same concentration. D, Mice (10/group) were immunized three times with 107-pSec and the respective vector plasmid pSec. Animals were bled 10 days after the last boost. For each group, sera were pooled for 10 mice. ELISA plates were coated with indicated carbohydrates and reactivity of IgM Abs was detected. End-point Ab titer against Galα1,3Galβ-PAA was 1:2560, whereas it was 1:1280 for GlcNAcβ1,4GlcNAC-β-PAA. The titers were determined as the highest sera dilution with significantly higher OD compared with preimmune sera using paired Student’s t test at p < 0.05. E–G, Binding of the serum to 4T1 (E), MCF7 (F), and ARK (G) cells was detected by flow cytometry and presented by mean fluorescence intensity (MFI).

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We have previously shown that immunization with peptide mimotopes of carbohydrate Ags formulated as MAPs or as DNA vaccines can elicit carbohydrate-reactive IgM serum Abs (5, 24). To expand this latter strategy, the sequence of peptide 107 was translated into oligonucleotides and DNA constructs (Table II), and groups of mice were immunized with the 107-pSec DNA constructs and serum reactivity with carbohydrate probes was assayed (Fig. 2,D). Serum IgM Abs from 107-pSec-immunized mice bound to Gal and GlcNAc oligosaccharides. We detected Ag-specific serum Abs of IgM isotype reactive with 4T1 and MCF7 cells by flow cytometry (Fig. 2, E and F), with no Ag-specific IgG isotype detected. Ag-specific serum IgM Abs were not detected against the myeloma cell line ARK cells (Fig. 2 G) because reactivity of the preimmune serum to ARK cells was similar to the reactivity of serum from 107-pSec-immunized mice. The ARK cell line was not reactive with the GS-I lectin and only marginally with WGA (data not shown).

Table II.

Oligonucleotide sequences used for cloning and making DNA constructs

PeptideOligonucleotide Sequencesa
104 agcttGGCGGCATCATGATCCTGCTGATCTTCTCCCTGCTGTGGTTCGGCGGCGCCTAAgc 
 ggccgcTTAGGCGCCGCCGAACCACAGCAGGGAGAAGATCAGCAGGATCATGATGCCGCCa 
105 agcttGGCGGCATCTACTACCCCTACGACATCTACTACCCCTACGACATCTACTACCCCTACGACTAAgc 
 ggccgcTTAGTCGTAGGGGTAGTAGATGTCGTAGGGGTAGTAGATGTCGTAGGGGTAGTAGATGCCGCCa 
107 agcttGGCGGCATCTACTACCGCTACGACATCTACTACCGCTACGACATCTACTACCGCTACGACTAAgc 
ggccgcTTAGTCGTAGCGGTAGTAGATGTCGTAGCGGTAGTAGATGTCGTAGCGGTAGTAGATGCCGCCa  
PeptideOligonucleotide Sequencesa
104 agcttGGCGGCATCATGATCCTGCTGATCTTCTCCCTGCTGTGGTTCGGCGGCGCCTAAgc 
 ggccgcTTAGGCGCCGCCGAACCACAGCAGGGAGAAGATCAGCAGGATCATGATGCCGCCa 
105 agcttGGCGGCATCTACTACCCCTACGACATCTACTACCCCTACGACATCTACTACCCCTACGACTAAgc 
 ggccgcTTAGTCGTAGGGGTAGTAGATGTCGTAGGGGTAGTAGATGTCGTAGGGGTAGTAGATGCCGCCa 
107 agcttGGCGGCATCTACTACCGCTACGACATCTACTACCGCTACGACATCTACTACCGCTACGACTAAgc 
ggccgcTTAGTCGTAGCGGTAGTAGATGTCGTAGCGGTAGTAGATGTCGTAGCGGTAGTAGATGCCGCCa  
a

Lowercase letters show sequence of the restriction sites used for cloning. Uppercase letters show the peptide-encoding sequences.

To assess serum-mediated cell cytotoxicity, cells were coincubated with serum from peptide encoded DNA- or vector-alone-immunized mice (control), and the percentage of dead cells was detected after overnight coincubation. We observed ∼20% dead cells in wells containing 107-pSec serum when wells supplemented with pSec (vector)-immunized serum were used as control. In concert with the GS-I and WGA effects, 107-pSec-immunized serum mediated apoptosis of both 4T1 and MCF7 cell lines with more pronounced effects on 4T1 cells (Fig. 3,A). Paralleling the lectin cytotoxicity, we observed a significant increase in activation of caspases 2 and 3 after overnight incubation with serum from 107-pSec-immunized mice (Fig. 3,B). We estimated the fold increase in activation of caspases induced by 107-pSec-immunized mouse serum over normal mouse serum (Fig. 3,C). Activation of caspases 2 and 3 was detected as early as 4 h postincubation with serum Abs from 107-pSec-immunized mice. After overnight incubation, we observed an increase in activation for all caspases studied; however, caspases 2 and 3 showed the highest fold increase in activation over normal-mouse serum-treated cells (Fig. 3 C).

FIGURE 3.

Serum-mediated apoptosis in 4T1 and MCF7 cells. A, Medium containing 5% FBS was supplemented with 5% of heat-inactivated serum from 107-pSec or pSec (control)-immunized mice. Cells were incubated for 16 h with mouse-serum-supplemented medium, and then cells were harvested, washed, and stained with PI (FL-2) and annexin V (FL-1) as explained in Materials and Methods and analyzed by flow cytometry. B, Cells were incubated with medium supplemented with 107-immunized or normal mouse sera as above. Cells were then harvested after overnight incubation, and caspase activity was measured. ∗, p < 0.05; ns, not significant compared with cells incubated with normal mouse serum (control). C, To compare the magnitude of activation over a period of time, fold increase of the activation, as estimated by fluorescence intensity of cells treated with 107-immunized serum over cells treated with normal mouse serum at the same time point is graphed. D, IgM Abs were visualized as internalized aggregates. The immunized serum was premixed with FITC-conjugated anti-mouse IgM, and then the mixture was added to 4T1 cell, and the distribution of staining was visualized using a Zeiss LSM 410 inverted confocal microscope. FITC-conjugated anti-mouse IgM was added alone in similar conditions as negative control (E).

FIGURE 3.

Serum-mediated apoptosis in 4T1 and MCF7 cells. A, Medium containing 5% FBS was supplemented with 5% of heat-inactivated serum from 107-pSec or pSec (control)-immunized mice. Cells were incubated for 16 h with mouse-serum-supplemented medium, and then cells were harvested, washed, and stained with PI (FL-2) and annexin V (FL-1) as explained in Materials and Methods and analyzed by flow cytometry. B, Cells were incubated with medium supplemented with 107-immunized or normal mouse sera as above. Cells were then harvested after overnight incubation, and caspase activity was measured. ∗, p < 0.05; ns, not significant compared with cells incubated with normal mouse serum (control). C, To compare the magnitude of activation over a period of time, fold increase of the activation, as estimated by fluorescence intensity of cells treated with 107-immunized serum over cells treated with normal mouse serum at the same time point is graphed. D, IgM Abs were visualized as internalized aggregates. The immunized serum was premixed with FITC-conjugated anti-mouse IgM, and then the mixture was added to 4T1 cell, and the distribution of staining was visualized using a Zeiss LSM 410 inverted confocal microscope. FITC-conjugated anti-mouse IgM was added alone in similar conditions as negative control (E).

Close modal

It is argued that the above-mentioned lectins trigger apoptosis via intracellular signaling following internalization (9, 14). To better understand the mechanism of apoptosis by serum, we investigated the possibility of internalization of serum Abs. Murine serum was mixed with FITC-conjugated anti-mouse IgM, and then the mixture was added to cells, and fluorescence microscopy was used to visualize the binding pattern of murine IgM to cells. We observed that IgM Abs bound to the cell surface and that coincubation resulted in the appearance of IgM in small aggregates that were present in the interior of the cells due to internalization of the IgM Abs in endocytic vesicles (Fig. 3,D). We did not detect internalization of anti-mouse IgM alone (Fig. 3 E).

To assess whether vaccination with peptide 107 could arrest metastases, we established the 4T1 tumor in mammary fat pads and then started immunization with the plasmids containing the DNA sequences of 104, 105, and 107 mimotopes. Peptides 104 and 105 are carbohydrate mimics and induction of carbohydrate and cell-reactive functional serum Abs by immunization with these peptides has been shown in our previous works (23, 25, 26). Peptide 107 binds to both GS-I and WGA lectins, and the data suggest that this peptide may mimic multiple oligosaccharides involved in tumor cell apoptosis. To better evaluate peptide 107 as an antitumor apoptosis-mediating immunogen, we also included peptides 104 and 105 in the immunization regimen.

Immunization with 107-pSec plasmid induced slight tumor shrinkage in comparison to all other groups that was temporary (data not shown). However, 107-pSec immunization significantly (p = 0.021) increased survival time of the tumor-bearing animals compared with immunization with vector (pSec) only (Fig. 4). At day 39, 80% of the 107-pSec-immunized mice were alive, whereas 80% of vector-immunized animals died, with the remaining 20% sacrificed at day 42 because of appearance of signs of morbidity based on the animal use protocol. At this same day point, 80% of 107-pSec-immunized mice were still alive. Although at day 56, no mice were alive in the 104-pSec- and 105-psec-immunized groups, 40% of the mice in the 107-pSec group were still alive in fair condition. These mice were later sacrificed at day 68 postinoculation due to a large tumor burden as required by the animal use protocol.

FIGURE 4.

Immunization of tumor-bearing animals with DNA construct of the 107 peptide induced an increase in survival rate. Mice were inoculated with 105 cells s.c. in mammary ducts, and immunization was started 4 days later with 7-day intervals. The percentage of survival in immunized groups was estimated by the Kaplan-Meier method. Increase in the survival rate observed in 107-pSec-immunized mice is statistically significant at p = 0.021 compared with pSec (vector)-immunized mice by log-rank test.

FIGURE 4.

Immunization of tumor-bearing animals with DNA construct of the 107 peptide induced an increase in survival rate. Mice were inoculated with 105 cells s.c. in mammary ducts, and immunization was started 4 days later with 7-day intervals. The percentage of survival in immunized groups was estimated by the Kaplan-Meier method. Increase in the survival rate observed in 107-pSec-immunized mice is statistically significant at p = 0.021 compared with pSec (vector)-immunized mice by log-rank test.

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To further characterize the effects of immunization on metastases to distant organs, we repeated the challenge experiment as above. Lung and liver samples were harvested, and the presence of metastatic cells was detected and quantified by clonogenic assay (Table III). The immunization regimen had no effect on lung metastases but had significant positive effects on reducing liver metastasis (Fisher exact test, p = 0.018). Of 12 mice used for 107-pSec therapeutic DNA immunization, only 2 were found positive for tumor in the liver compared with 8 of 12 positive livers in pSec (vector)-immunized animals.

Table III.

Number of mice detected positive for distant organ metastasis of 12 total mice

ImmunizationOrgans
LungLiver
pSec (vector) 12 
107-pSec 12 2a 
ImmunizationOrgans
LungLiver
pSec (vector) 12 
107-pSec 12 2a 
a

, p = 0.018 compared with vector immunized, by Fisher exact test.

Therapeutic immunization had no significant effect either on the size of primary tumor or on lung metastasis. To test whether earlier immunization and accumulation of Abs at the time of tumor challenge modified the outcome of immunization, in a follow-up study, mice were immunized before the challenge and after the challenge with either the peptide or DNA constructs. Mice were bled 7 days after the third prophylactic immunization and then challenged with 4T1 cells 3 days after blood collection. Serum was collected individually, and all sera were pooled for 10 mice in each group. Binding of pooled serum to 4T1 cells and its apoptotic activity was assayed (Table IV). As shown in Table IV, immunization with both peptide 107 and 107-pSec DNA vaccines generated serum Abs that bound to 4T1 cells and mediated their apoptosis. Peptide immunization generated Abs of higher end-point titer, which mediated a more pronounced apoptotic effect.

Table IV.

Characterization of serum after DNA and peptide immunization

Test DescriptionPeptide ImmunizationDNA ImmunizationGS-I
Naive106107pSec (vector)106-pSec107-pSec
End-point titera  1:1280 1:2560  1:160 1:160  
Annexin bindingb 184 (±39) 228 (±53)f 436 (±64)d 171.5 (±43) 244.5 (±32)f 324 (±36)c 902 (±89)e 
PI bindingb 32 (±3.8) 29.8 (±1.2)f 32 (±2.9)f 40.33 (±3.2) 38.58 (±4.1)f 52.29 (±4.5)c 111 (±21)d 
Test DescriptionPeptide ImmunizationDNA ImmunizationGS-I
Naive106107pSec (vector)106-pSec107-pSec
End-point titera  1:1280 1:2560  1:160 1:160  
Annexin bindingb 184 (±39) 228 (±53)f 436 (±64)d 171.5 (±43) 244.5 (±32)f 324 (±36)c 902 (±89)e 
PI bindingb 32 (±3.8) 29.8 (±1.2)f 32 (±2.9)f 40.33 (±3.2) 38.58 (±4.1)f 52.29 (±4.5)c 111 (±21)d 
a

Binding of IgM Abs to 4T1 cells was titered. Titer was determined based on mean fluorescence intensity of flow cytometry assay. The highest serum dilution with higher mean fluorescence intensity than the preimmune serum is shown as the end-point titer.

b

Mean fluorescence intensity for annexin V and PI (±SD) is shown.

c

, p < 0.05;

d

, p < 0.025;

e

, p < 0.005;

f

, not significant compared with naive serum or pSec (vector) serum as negative controls. GS-I was compared with either negative control.

In both peptide and DNA-immunized animals, tumor grew significantly slower. All preimmunized mice developed tumor upon challenge; however, tumor growth was significantly slower in peptide 107-immunized animals (Fig. 5, A and B). In contrast to our therapeutic immunization regimen, in prophylactic immunization, we observed inhibition of primary tumor growth during the whole experiment. Examination of TUNEL-stained slides revealed significantly more apoptotic bodies per square millimeter in the primary tumors of immunized mice at 7 days post-tumor inoculation compared with control mice (p = 0.0072; Fig. 5 C).

FIGURE 5.

Immunization with the peptide 107 inhibited tumor growth and reduced lung metastases. Mice (10 per group) were immunized with peptide (A) or DNA constructs (B) weekly for 3 wk and then inoculated with tumor at day 10 after the third immunization. Immunization was resumed at day 7 posttransplant and continued every week for 3 more weeks. Tumor size was measured during this period of time and are illustrated as the mean values ± SD. ∗, p < 0.025; ∗∗, p < 0.01, compared with 106 (A) or vector (B) immunization. C, Mice from peptide-immunized groups were sacrificed weekly, lungs and primary tumors were harvested, sections were prepared, and TUNEL staining was performed as described in Materials and Methods. Apoptotic bodies were counted in five randomly chosen microscopic fields in sections of primary tumors, and averages per square millimeter are shown. ∗, p = 0.0072; ns, not significant compared with naive tumor-bearing animals. D, Peptide-immunized mice were sacrificed at day 28, lungs were collected, and clonogenic assay was performed. The number of clonogenic cells was determined and illustrated as the mean values of 10 mice per group ± SD. ∗, p = 0.009; ns, not significant compared with the naive mice. E and F, Groups of mice were left unimmunized (E) or were immunized as explained above (F) and then inoculated with the 4T1 tumor. Mice from each group were sacrificed weekly, lungs were harvested, sections were prepared, and H&E staining was performed on prepared slides. Slides prepared from mice sacrificed 21 days postinoculation are shown. Bars, 50 um.

FIGURE 5.

Immunization with the peptide 107 inhibited tumor growth and reduced lung metastases. Mice (10 per group) were immunized with peptide (A) or DNA constructs (B) weekly for 3 wk and then inoculated with tumor at day 10 after the third immunization. Immunization was resumed at day 7 posttransplant and continued every week for 3 more weeks. Tumor size was measured during this period of time and are illustrated as the mean values ± SD. ∗, p < 0.025; ∗∗, p < 0.01, compared with 106 (A) or vector (B) immunization. C, Mice from peptide-immunized groups were sacrificed weekly, lungs and primary tumors were harvested, sections were prepared, and TUNEL staining was performed as described in Materials and Methods. Apoptotic bodies were counted in five randomly chosen microscopic fields in sections of primary tumors, and averages per square millimeter are shown. ∗, p = 0.0072; ns, not significant compared with naive tumor-bearing animals. D, Peptide-immunized mice were sacrificed at day 28, lungs were collected, and clonogenic assay was performed. The number of clonogenic cells was determined and illustrated as the mean values of 10 mice per group ± SD. ∗, p = 0.009; ns, not significant compared with the naive mice. E and F, Groups of mice were left unimmunized (E) or were immunized as explained above (F) and then inoculated with the 4T1 tumor. Mice from each group were sacrificed weekly, lungs were harvested, sections were prepared, and H&E staining was performed on prepared slides. Slides prepared from mice sacrificed 21 days postinoculation are shown. Bars, 50 um.

Close modal

To compare the effect of preimmunization on metastasis, we collected lungs at day 28 posttumor transplant from both DNA- and peptide-immunized mice and performed the clonogenic assay. In the peptide-immunized group (with 10 mice per group), 30% of 107-peptide-immunized animals were free of lung-associated tumor cells. In addition, comparison of the average of clonogenic lung metastases showed a significant reduction of lung metastatic cells in the 107-immunized group (Fig. 5,D). In the DNA-immunized group, all mice tested had established lung metastasis; however, pSec-107-immunized mice had a smaller average of colonies than pSec-106 and pSec vector control (data not shown). Consistent with results from the clonogenic assay, metastatic lesions in the lungs from peptide 107-immunized group were significantly smaller than those from the naive group (Fig. 5, E and F).

The presence of natural carbohydrate-reactive IgM Abs in humans capable of mediating apoptosis in tumor cells has been reported (1). In an effort to develop a strategy to induce sustained immunity targeting carbohydrate Ags on metastatic tumor cells, we have used lectins as a model template to further define and develop immunogens that elicit tumor-specific apoptosis triggering IgM Abs. GS-I consists of two isolectins, GS-I-B4 and GS-I-A4, with different carbohydrate specificity. GS-I-B4 is more specific for αGal, whereas the A4 isolectin has higher specificity toward αGalNAc-containing ligands (27, 28). GS-I also binds to the neolactoseries Ags Lewis Y and Lewis b (our unpublished observation). WGA binds to both GlcNAc and sialic acid (29). Therefore, to broaden the application, we decided to select for a peptide that could potentially mimic the various tumor-associated carbohydrate structures.

The 107-peptide was identified and selected based on its reactivity with both GS-I and WGA with an expectation that, upon immunization, serum Abs cross-reactive with multiple carbohydrate structures would be induced. The peptides 104, 106, 107, and 109 all bind to an anti-Lewis Y Ab (23, 25, 26) and induce serum Abs cross-reactive with Lewis Y-expressing breast cancer cells, which mediated tumor cell killing by a CDC mechanism (5). We have shown before that mimotopes of core structures of tumor-associated carbohydrate Ags are particularly advantageous to augment the cross-reactive carbohydrate immune response because they can function as priming agents to expand B cells to multiple carbohydrate Ags upon boosting with nominal Ag (30). Such peptides are considered as multiple Ag mimotopes and would simplify the vaccine strategy by obviating the need to develop multivalent immunogens based upon each individual immunogen.

Administration of the peptide and its genetic form (107-pSec construct) as vaccine induced serum IgM Abs that bound to cells and mediated apoptosis of both 4T1 and MCF7 cells, with a more pronounced effect on the murine cell line. This is not surprising in that MCF7 cells are not uniform in expressing full-length caspase 3 and display diminished susceptibility to apoptotic stimuli (31). Because the GS-I-reactive Gal epitope is not expressed on human cells, the observation that the serum Abs can trigger apoptosis of MCF7 cells suggests that αGalNAc, GlcNAc, sialic acid moieties, and perhaps the Lewis Y Ag may be expressed on glycoproteins associated with signaling functions, and reactivity with these glycoproteins may mediate the internalization of the serum Abs.

Internalized serum IgM suggests that the mode of action of the induced Abs might be similar to the lectins. Like the lectins, specific binding to the cell surface oligosaccharides should be the prerequisite for the Ab-mediated cell killing. Internalized Abs can trigger a wide spectrum of intracellular signals that might contribute to apoptosis. For example, the serum Abs can induce their effect via mitochondria activation as it has been proposed for the lectins (14). Internalized carbohydrate-reactive IgM via its interaction with rafts may lead to modulation of cellular ceramide, which results in the induction of apoptosis (32). Such mechanisms are currently under study. The caspase activation results further suggest a similarity between apoptosis mediated by the lectins and the serum Abs, because both predominantly mediated activation of caspases 2 and 3 in 4T1 cells.

We propose that immunization with peptide 107 generates serum Abs that bind to a subpopulation of cells that express the target lectin-defined carbohydrate epitopes on the 4T1 surface. Serum Ab binding is followed by internalization of the Abs, and apoptosis and lysis of cells, which appears as a temporary tumor regression because we observed only a marginal effect on the size of the tumor at the primary site of inoculation (data not shown). Ags released from lysed cells may still initiate an immune response, which inhibits further metastatic growth and a statistical increase in survival rate. Segregated analysis of the effects of therapy on metastasis to distant organs highlighted a definite influence on metastatic tumor in the liver. But why does the immunization affect only liver metastasis? Staining of tissues with lectins clearly showed that there are lectin-positive cells also in the lung and the solid tumor (data not shown). We postulate that by the time an effective level of the IgM titer is reached, tumor burden in the lung and at the site of primary inoculation exceeds a treatable limit. This is akin to studies in which treatment of cells with carbohydrate-reactive Ab are effective only a few days after tumor challenge (16). Likewise, vaccination efficacy has been implicated to be dependent on the size of tumor (7, 16, 19).

To account for timing, appearance, and the strength of the Ab response, and its influence on the outcome of the vaccination strategy, we immunized naive mice, and then challenged them with the tumor and continued the immunization. DNA immunization significantly slowed the growth of the solid tumor especially at the earlier stages. The in situ apoptosis assay revealed an increase in apoptosis of tumor cells from immunized mice at 7 days post-tumor transplant compared with controls. Peptide immunization produced a stronger Ab response, which strikingly blocked initiation of the lung metastasis in 30% of animals tested, with the number of lung metastasis also significantly reduced. This is a significant achievement because curing this tumor is difficult. Others showed that even prophylactic immunization in a cell-based vaccine targeting 4T1 cells with MHC and B7.1 transfectants, although effectively reducing lung metastasis, neither affected the growth of the primary tumor nor blocked lung metastasis (19, 33). IL-12 therapy proved to be an effective strategy in reducing the number of 4T1 lung metastases. However, it failed to affect primary tumor growth or afford protection against the initiation of tumor growth in the lung (34).

In patients with advanced stages of breast cancer, antitumor immune responses may still be present, and appropriate stimulation of immunity may have beneficial effects in immune augmentation leading to at least an increase of survival rate. Although the immunotherapy proposed here is not curative for aggressive tumors at advanced stages, it may postpone the progression of metastatic disease. It is generally recognized that treatment with mAbs or vaccines inducing Abs must be restricted to the adjuvant setting, where the targets are circulating tumor cells and micrometastases (16). Therefore, induction of apoptotic IgM Abs might be targeted as an endpoint in the development of vaccination strategies against cancer especially in the adjuvant setting for metastatic disease. Our findings point to the fact that cell apoptosis can be specifically targeted through a cross-reactive immune response to a surrogate Ag, which has implications for the design of novel approaches for cancer vaccines.

We thank Charlotte Read Kensil of Antigenics, Inc., for the QS-21.

The authors have no financial conflict of interest.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

1

This work was supported by grants from the U.S. Army Breast Cancer Program (DAMD17-01-1-0366) and the National Institutes of Health (CA-089480) to T.K.-E.

3

Abbreviations used in this paper: CDC, complement-dependent cytotoxicity; GS-I, Griffonia simplicifolia lectin I; Gal, galactose; GalNAc, N-acetylgalactosamine; GlcNAc, N-acetylglucosamine; WGA, wheat germ agglutinin; MAP, multiple Ag peptide; PAA, polyacrylamide; PI, propidium iodide.

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