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Departments of
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Preclinical Development and
Clinical Science, Medarex, Bloomsbury, NJ 08804; and
Department of Research and Development, Medarex, Milpitas, CA 95035
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The mechanism by which antitumor responses are augmented as a result of treatment with anti-CTLA-4 mAbs is thought to be dependent on the inhibition of CTLA-4 engagement of the B7 costimulatory molecules expressed on APCs (1, 11). The B7 family of costimulatory molecules is critical in the induction of Ag-specific T cell activation through interactions with the constitutively expressed T cell molecule CD28 (12, 13). After a T cell becomes activated, CTLA-4 expression is induced on the surface of the cell. Due to a higher affinity, CTLA-4 efficiently competes with CD28 for binding to B7 molecules on the APC, leading to T cell inactivation. This mechanism of T cell inactivation by CTLA-4 is important in regulation of peripheral T cell homeostasis, an activity that is most dramatically demonstrated in CTLA-4 knockout mice, which exhibit massive lymphoproliferation leading to death at 3–4 wk of age (14, 15). Abs that block CTLA-4/B7 interactions appear to reversibly prolong T cell activation by preventing negative signals provided by the B7/CTLA-4 interactions and in so doing lower the threshold for T cell activation to Ag.
Thus far, the majority of studies on CTLA-4 blockade have been performed in mice. Evidence for a similar function in humans comes from limited in vitro studies demonstrating augmentation of T cell responses by CTLA-4 blockade (16). Furthermore, epidemiological data on polymorphisms in the CTLA-4 gene suggest a correlation with certain autoimmune disorders (17, 18). Primate models and human clinical trials will be required to elucidate the full potential of CTLA-4 blockade in humans. To this end, human mAbs directed against human CTLA-4 were generated using transgenic mice carrying human rather than mouse Ig genes. These mice possess introduced human H and L chain transgenes that undergo class switching and somatic mutations to produce human IgG1
and IgG3 Abs (19). This study describes the in vitro and in vivo activity of a human mAb specific for CTLA-4 referred to as 10D1. We have demonstrated that administration of mAb 10D1 to cynomolgus monkeys is well-tolerated and enhances immune responses to vaccines.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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BW-huCTLA-4/CD3
is a murine T cell hybridoma that expresses human CTLA-4 constitutively. This line was constructed by expressing the extracellular and transmembrane domains of human CTLA-4 (aa 1–190) fused to murine CD3 (aa 52–164). The CTLA-4/CD3 chimeric gene was then subcloned into the expression vector pBABE, which also contains a gene encoding for puromycin resistance, to create pBABE-huCTLA-4/CD3 (20). pBABE huCTLA-4/CD3 was transfected into the retroviral packaging line, 
-2, and a pool of puromycin-resistant cells was selected. BW5147 cells (ATCC no. TIB-47; American Type Culture Collection, Manassas, VA) were cocultured with the retroviral producer cells and selected for resistance to puromycin. A clone expressing high levels of human CTLA-4 at the cell surface was selected (BW-huCTLA-4CD3-3#7).
L cells which constitutively express mouse CTLA-4 were generated by transfection using a gene composed of the extracellular and transmembrane domains of murine CTLA-4 (aa 1–190) fused to the intracellular domain of murine CD28 (aa 179–219). L cells were also generated that express rhesus CTLA-4. A genomic clone from the rhesus genomic library (BD Biosciences/Clontech Laboratories, Palo Alto, CA) was isolated by hybridization with a human cDNA clone for CTLA-4. An exon encoding the extracellular domain of rhesus CTLA-4 was sequenced and a fusion between the signal peptide of human CTLA-4 (aa 1–36), the extracellular domain of rhesus CTLA-4 (aa 37–162), the transmembrane domain of human CTLA-4 (aa 162–190), followed by the intracellular domain of CD28 (aa 179–219) was created. The sequence of rhesus macaque CTLA-4 has two amino acid differences from the human sequence (21).
The human melanoma cell line SK-MEL-3 (American Type Culture Collection) was transfected to produce human GM-CSF (Invivogen, San Diego, CA). A subclone stably expressing GM-CSF was used for the vaccine studies and termed SK-Mel-GM. GM-CSF production was monitored by ELISA (R&D Systems, Minneapolis, MN).
Immunization of transgenic mice and development of hybridomas
Transgenic HuMAb mice, strain HC2/KCo7, having four distinct genetic modifications were used for immunizations (22). To generate Abs, mice were immunized with BW-CTLA4/CD3 (5–10 x 106) injected i.p. Immune mice were given an i.v. boost with the extracellular domain of CTLA-4 4 days prior to harvesting spleens. The extracellular CTLA-4 fragment was prepared by proteolytic cleavage of the CTLA-4/Fc fusion protein (cat. no. 325-CT-200; R&D Systems) at a factor Xa protease cleavage site located after the C terminus of the CTLA-4 extracellular domain. Single cell suspensions of splenic lymphocytes from immunized animals were fused with the murine myeloma cell line P3X63Ag8.653 (American Type Culture Collection) in the presence of polyethylene glycol (23). Hybridomas were selected by the addition of hypoxanthine/aminopterin/thymidine 24 h after fusion. Hybridomas were first screened by a sandwich ELISA for human IgG producers. Human IgG-producing hybridomas were then selected based on reactivity with CTLA-4 by flow cytometry and ELISA.
Inhibition of CTLA-4/B7 binding
To demonstrate the ability of 10D1 to block the interaction of CTLA-4 to the ligands B7.1 and B7.2, competition assays were performed by flow cytometry and ELISA. Briefly, CTLA-4-expressing cells (BW-huCTLA-4/CD3) were incubated with varying concentrations of human anti-CTLA-4 mAb 10D1 (F(ab')2) or murine anti-CTLA-4 (BNI-3; BD Biosciences/BD PharMingen, San Diego, CA) together with either recombinant human B7.1-Ig (0.1 µg/ml) or B7.2-Ig (0.2 µg/ml; R&D Systems). The cells were washed, fixed in 1% paraformaldehyde, and their fluorescence analyzed with a FACSCalibur instrument. For ELISA experiments the B7-Ig was labeled with biotin, and detected with a streptavidin-alkaline phosphatase probe. Percent inhibition was calculated from the formula: ((mean fluorescence intensity (MFI) no inhibitor) - (MFI with inhibitor))/MFI no inhibitor x 100%.
Immunohistochemistry
Immunohistochemistry evaluations were performed by Pathology Associates International (Frederick, MD). Human tissues were obtained from the National Disease Research Interchange (Philadelphia, PA), embedded in Tissue-Tek OCT medium, frozen on dry ice, and stored below -70°C. Tissues were sectioned at 5 µm, fixed for 10 min in acetone before immunoperoxidase staining. Endogenous peroxidase activity was quenched through incubation with peroxidase solution for 5 min. FITC-labeled 10D1 was applied for 60 min at concentrations of 2.5 and 10 µg/ml. Rabbit anti-fluorescein Ab was applied for 30 min at a dilution of 1/500 (PBS with 1% BSA and 1 mg/ml heat-aggregated IgG). Slides were reacted for 30 min with DAKO Envision kit peroxidase-labeled polymer (DAKO, Carpinteria, CA). Diaminobenzidene was applied for 8 min using the substrate-chromogen solution supplied in the DAKO Envision kit. Slides were counterstained with hematoxylin, dehydrated, and coverslipped for light microscopic evaluation.
Binding to mouse/human/macaque CTLA-4
Cells lines that constitutively express human, murine, and rhesus CTLA-4 were used to determine the species cross-reactivity of 10D1. Cells were incubated with 10 µg/ml 10D1 or isotype control for 90 min at 4°C. After washing, the cells were further incubated with goat anti-human IgG conjugated to FITC. The cells were fixed in 1% paraformaldehyde and their fluorescence was analyzed on a FACSCalibur instrument.
Experimental design for hepatitis B surface Ag (HBsAg) 4 vaccine study in cynomolgus macaques
Eight male and eight female cynomolgus monkeys (Macaca fascicularis), 3–4 years of age and 3.5–5.6 kg, were obtained from Scientific Research International (Sparks, NV). The animal husbandry, test article administrations, and sample collections were performed by Sierra Biomedical (Sparks, NV). mAb 10D1 or an isotype-matched control human IgG (anti-RSV humanized IgG1; Synagis, MedImmune, Gaithersburg, MD) was administered i.v. (10 mg/kg) on days 1 and 29. All of the animals were vaccinated (i.m.) with 10 µg of HBsAg mixed with alum (Engerix-B; GlaxoSmithKline, Philadelphia, PA) on study days 2 and 30. Animals were monitored for general health twice daily and body weights were recorded weekly. At day 64, a macroscopic pathology examination was conducted and selected tissues were analyzed microscopically.
Ab responses to HBsAg
For the mouse studies, Ab responses were determined by ELISA using recombinant HBsAg protein (Seradyne, Indianapolis, IN). Briefly, wells were coated with HBsAg (0.5 µg/ml), blocked, and then incubated with dilutions of plasma samples obtained before and after immunizations. Ab responses were measured with goat anti-murine IgG probe. The titer was determined as the highest dilution giving a positive value compared with preimmunization samples. For the macaque study, a commercial kit (Sanofi-Pasteur Diagnostics, Montreal, Quebec, Canada) was used for the quantitation of Abs to HBsAg. The HBsAg Ab concentration (mIU per milliliter) was determined from the standard curve according to the kit.
Surface marker expression and intracellular cytokine analysis
For determining surface marker expression, whole blood was divided into tubes (100–200 µl/tube) and stained for surface markers for 15 min at room temperature (anti-CD3, CD4, CD8, CD69, HLA-DR, and CD25; BD Biosciences). The RBCs were lysed and the remaining leukocytes were fixed in 1% paraformaldehyde. For the analysis of intracellular cytokines, whole blood was incubated overnight (37°C, 5% CO2) in the presence of HBsAg (Aldevron, Fargo, ND) or medium only along with costimulatory Abs (anti-CD28, anti-CD49d; BD Biosciences) and brefeldin A. RBCs were lysed and the remaining leukocytes were permeabilized (Perm 2 Solution; BD Biosciences) before staining for intracellular cytokines and activation marker. A mixture of Abs with established reactivity to macaques was used (anti-CD3, anti-CD8, anti-CD69 with either anti-TNF-
, anti-IL-2, or anti-IFN-
; BD Biosciences). The samples were analyzed on a (FACSCalibur) flow cytometer within 24 h of staining.
Experimental design for melanoma vaccine study
Six male and six female cynomolgus monkeys (M. fascicularis), 7–8 years of age and 2.4–6.4 kg, were obtained from Primate Products (Miami, FL). The animal husbandry, test article administrations, and sample collections were performed by Huntingdon Life Sciences (East Millstone, NJ). Ten milligrams per kilogram of mAb 10D1 were administered i.v. on days 0, 28, 56, 84, and 140 to six animals. The SKmel-GM vaccine was administered to all animals on days 0, 28, 56, 84, and 140. To prepare the vaccine, SKmel-GM cells were grown to confluency and harvested. The cells were treated with mitomycin C, washed several times, and resuspended to 1 x 107/ml in saline. The cell preparation (0.5 ml) was injected s.c. into animals. Each vaccine preparation was tested for endotoxin (<2 EU/ml) and GM-CSF production after 48 h (2–8 ng/ml per 106 cells). Animals were monitored for general health twice daily and body weights were recorded weekly. Hematology, pharmacokinetic analysis, and functional assays were performed prior to study initiation and periodically throughout the study. A complete macroscopic and microscopic pathology examination was performed at day 167.
Ab responses to melanoma vaccine
Flow cytometry. SKmel-3 or other cell lines were washed and adjusted to 3 x 105 cells/50 µl in PBS containing 0.1% BSA, before incubation with the plasma samples (final dilution = 1/1000) at 4°C. After washing, the cells were incubated with a FITC- conjugated goat anti-human IgG F(ab')2-specific reagent. The cell associated fluorescence was analyzed using a FACSCalibur, and the MFI of the samples was reported.
ELISA. Whole cell lysate was prepared from SKmel-3 cells or fresh frozen cynomolgus iris tissue by incubation of cells in Triton X-100 containing buffer. Solubilized proteins were separated from insoluble material by centrifugation. Plates were coated with lysates, and blocked with PBS containing 5% BSA. Plasma samples (1/200 dilution) were incubated on the plates. The bound monkey IgG was detected with alkaline phosphatase-labeled goat anti-human IgG Fc-specific probe. The assay was developed with a p-NPP substrate and the absorbance at 405 nm was determined.
Ab-dependent cellular cytotoxicity (ADCC)
Melanoma cell lines were used as targets for lysis by fresh human mononuclear cells purified from heparinized whole blood. Target cells were labeled with 100 µCi 51Cr for 1 h prior to combining with mononuclear cells and monkey plasma specimens. After 4-h incubation at 37°C, supernatants were collected and analyzed for radioactivity. Cytotoxicity was calculated by the formula: % lysis = (experimental cpm - target leak cpm)/(detergent lysis cpm - target leak cpm) x 100%. Specific lysis = % lysis with sample - % lysis without sample. Assays were performed in triplicates.
Pharmacokinetics
Analysis of plasma concentration for mAb 10D1 was performed by ELISA. Briefly, microtiter wells were coated with CTLA-4-Ig (R&D Systems) and blocked with PBS containing 5% BSA. Dilutions of plasma samples were incubated on the plates, and the bound 10D1 was detected with alkaline phosphatase-labeled goat anti-human IgG (Fab') probe. The assay was developed with a p-NPP substrate and the absorbance at 405 nm was determined on a plate reader. Sample dilutions falling into the linear portion of a standard curve were used for determining the plasma concentrations.
Statistics
Statistical analysis of data was obtained using the Student t test and SIGMAPLOT software (SPSS Science, Chicago, IL).
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Results |
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Fully human mAbs specific for human CTLA-4 were generated using transgenic mice expressing human Ig genes. Spleen cells from mice that developed human Ab titers against CTLA-4 were used to generate hybridomas using standard myeloma fusion technology originally developed by Kohler and Millstein (23). Approximately 70 hybridomas secreting human IgG mAbs were generated with specificity for CTLA-4. One hybridoma (10D1) was selected for further development based on binding specificity, affinity, and capacity to block ligand binding.
The predominant mechanism whereby Abs to CTLA-4 are thought to mediate their immune stimulatory activities is through blockade of the receptor-ligand interactions. Therefore, we characterized the ability of mAb 10D1 to inhibit binding between CTLA-4 and its costimulatory ligands, B7.1 and B7.2. Fig. 1 demonstrates blocking of ligand binding to CTLA-4-expressing cells by F(ab')2 of 10D1 using a flow cytometry method. The murine anti-human CTLA-4 mAb (BNI-3) and 10D1 effectively blocked ligand binding with similar kinetics (IC50 
1–3 µM). Similarly, mAb 10D1 was found to efficiently block B7 binding to CTLA-4 in ELISA-based competition assays with purified recombinant molecules (data not shown). In addition, 10D1 Ab inhibited the binding of soluble CTLA-4Ig to L cell transfectants expressing murine B7.2 (data not shown).
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CTLA-4 blockade enhances humoral responses to HBsAg vaccine
Studies regarding CTLA-4 blockade in mice have focused on cellular responses and have not reported on humoral immune responses. However, as Ab titers are an important and readily measured immune correlate for vaccines, we performed a preliminary study in mice to examine the effect of CTLA-4 blockade on Ab responses to HBsAg. Administration of the hamster anti-murine CTLA-4 (75 µg/dose, 
3 mg/kg) at both the prime and the boost immunizations enhanced the Ab titers to HBsAg compared to the control group (Fig. 4A). After the boost, 50% of the animals treated with anti-CTLA-4 mAb reached high anti-HBsAg titers (>105), compared to only one of eight in the control group, although this difference did not quite reach statistical significance. We did not observe a difference in the IgG isotype responses to HBsAg, which generated a predominantly Th2 profile with or without anti-CTLA-4 mAb.
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T cell activation was also investigated in vaccinated animals. Analysis of Ag-specific T cell responses was attempted using recombinant HBsAg protein to stimulate intracellular cytokine responses with fresh samples of whole blood. One animal (of four) in the 10D1 group demonstrated a consistent, Ag-specific stimulation of TNF- and IL-2 in both CD8- and CD8 T cells (at weeks 7 and 9). Fig. 5 illustrates the Ag-specific TNF- response at week 7. IFN- was not detected, however, this may have been due to the relatively weak staining with anti-IFN- Ab used in these studies. Using whole protein to prime in vitro T cell responses limits the sensitivity of these assays, therefore, we may be underestimating the Ag-specific T cell responses.
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Potentiation of immune responses to a melanoma vaccine in cynomolgus monkeys with mAb 10D1
Based on the encouraging results from the initial study with HBsAg vaccine, a second experiment was initiated to examine the efficacy and safety of mAb 10D1 in combination with a cancer vaccine. The vaccine used in this study was a human melanoma cell line (SKmel-3) transfected to express GM-CSF (SKmel-3-GM). Previous studies in murine models have demonstrated synergistic effects of CTLA-4 blockade combined with a similar vaccine (7, 8, 24). In addition, a whole cell vaccine would allow for investigation of autoimmune reactions to a variety of normal tissues, despite the fact that the cellular vaccine was of human origin. A protocol was designed for support of potential clinical applications, and to investigate chronic dosing of mAb 10D1.
Groups of six cynomolgus monkeys were dosed monthly (except for the fourth month) with s.c. injections of 5 x 106 SKmel-3-GM cells for a total of five doses. The SKmel-3-GM cells were pretreated with mitomycin C, and produced 2–8 ng of GM-CSF/106 cells/48 h. One group of monkeys was given an i.v. bolus of mAb 10D1 (10 mg/kg) on the same days as the vaccine. Samples were drawn for immune response analysis 2 wk following each immunization.
Treatment with 10D1 induced a dramatic enhancement of humoral responses to the vaccine as observed by both flow cytometry and ELISA (Fig. 6). Ab responses to cell surface Ags were noted after the first vaccine dose, peaked after the second dose, and remained elevated throughout the study. Interestingly, Ab responses to total cellular Ags (ELISA with whole cell lysate) peaked later in the course of the study (day 97). In the 10D1 group, five of six animals developed a strong Ab response to the vaccine (Fig. 6C). In contrast, only one of six animals in the vaccine alone group had a good response at the dilution used in these assays.
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Discussion |
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In this report, we demonstrate that Ab-mediated inhibition of CTLA-4 increased Ag-induced Abs in both mice and primates. Our findings are in contrast to the study reported by Horspool et al. (25), which showed CTLA-4 blockade suppressed humoral responses to a DNA vaccine. The reason for this discrepancy is unclear, but most likely is related to differences in the vaccine models used. In agreement with our findings, Zheng and Monestier (26) have recently reported that CTLA-4 blockade augments autoantibody production in a mouse model of mercury-induced autoimmunity. The mechanism by which CTLA-4 blockade may enhance Ab responses may involve enhancing T cell help for germinal center B cells. Walker et al. (27) have recently demonstrated that CTLA-4 expression down-regulates germinal center B cell proliferation. In their mouse model, CTLA-4 blocking Abs combined with immunization significantly enhanced the size of germinal centers.
In the absence of macaque melanoma cell lines, we used a human cell vaccine in this study. The xenogeneic cells clearly elicited Ab responses that were not specific to the melanoma vaccine. However, by comparison of several human tumor cell lines with varying HLA molecules we observed a consistently greater reactivity to melanoma-derived cell lines, indicating that at least some of the anti-vaccine Abs may have specificity for melanoma Ags. Furthermore, two of the six animals treated in combination with anti-CTLA-4 developed significant anti-self Abs that reacted strongly with lysate derived from melanoma-rich tissue. The biological consequences of the enhanced immune responses induced with anti-CTLA-4 treatment are unclear. Abs can have significant antitumor activity, and to support the biological significance of the elicited immune response, we demonstrated enhanced ADCC activity against melanoma cell lines in the animals treated with anti-CTLA-4.
In addition to Ab responses, we measured Ag-specific T cell activity in 2 of 10 monkeys that were treated with mAb 10D1, compared to 0 of 10 animals given only the vaccines. However, as these studies were unable to use MHC-matched peptides for inducing T cell activation, we have possibly underestimated the number of animals with Ag-specific T cells.
The potent effect of anti-CTLA-4 Abs on immune responses raises the concern of inducing unrestricted anti-self reactions that lead to autoimmune disease. In fact, transgenic mice deficient in CTLA-4 succumb to a lymphoproliferative disease at 3–4 weeks of age (14, 15). Moreover, combining CTLA-4 blockade with experimental autoimmune diabetes and encephalomyelitis exacerbates the disease in those models (28, 29, 30, 31, 32). However, studies in most mouse models indicate that Abs to CTLA-4 given to adult animals enhance responses specifically to the Ags used for vaccination, without generating nonspecific autoimmunity (1). More recently, the expression of CTLA-4 on regulatory T cells (T reg) that contribute to tolerance of self-Ags has raised additional concerns that CTLA-4 blockade can induce undesired autoimmunity (33, 34).
Our experiments in nonhuman primates have shown that chronic administration (6 mo) of mAb 10D1 did not result in treatment-related pathology upon complete macroscopic and microscopic evaluation, with the exception of slight irritation at the vaccine injection site of two animals. Furthermore, there were no changes in hematology, coagulation, or clinical chemistry at any time interval related to administration of the Ab. The absence of side effects is noteworthy despite the ability of CTLA-4 blockade to establish potential autoreactive anti-self responses in these animals. Importantly, the monkeys did not develop any detectable Ab response to the 10D1 mAb and high levels of active circulating Ab were maintained for duration of the study.
During the course of the study, no significant changes were observed in the lymphocyte subsets that were evaluated. However, phenotyping of T cells using the standard activation markers CD25, CD69, and HLA-DR cannot address the effects of CTLA-4 blockade on small numbers of Ag-specific lymphocytes. Nevertheless, these results are noteworthy in that CTLA-4 blockade does not significantly impact large numbers of T cells in a model where "normal" complement of naive and memory T cells are present. This is distinguished from the murine system where the majority of in vivo models describe the effects of CTLA-4 blockade on naive T cells. Studies of the role of CTLA-4 in memory cell responses suggest that the effects of CTLA-4 blockade may be more potent in secondary responses than in naive cells (35).
CTLA-4 is constitutively expressed at the surface of CD4+CD25+ T reg cells in mouse and human (33, 34, 36). Yet, the role of CTLA-4 in the function of these cells is controversial. Anti-CTLA-4 Abs administered in high doses can moderate the protective function of CD25+CD4+ T reg cells, leading to gastritis and intestinal inflammation in murine models (33, 34). However, Sutmuller et al. (37) have demonstrated that removal of this subset is synergistic with CTLA-4 blockade in a murine model of tumor immunotherapy, suggesting that anti-CTLA-4 treatment does not impact the T reg population. In vitro studies in the human system have suggested that anti-CTLA-4 Abs have no effect on the suppressive function of these cells (38, 39). Although we have not directly assessed the CD4+CD25+CTLA-4+ subset in primates, we have demonstrated that there is no alteration in the overall numbers of CD4+CD25+ T cells after chronic exposure to anti-CTLA-4 Ab. The lack of pathology associated with anti-CTLA-4 treatment suggests that interfering with T reg cells by CTLA-4 blockade is not sufficient to induce clinically significant autoimmune reactions in healthy primates.
The safe toxicology profile of anti-CTLA-4 treatment in monkeys has permitted the initiation of clinical studies in cancer patients with mAb 10D1 (referred to as MDX-010). These clinical trials used a single dose of anti-CTLA-4 at 3 mg/kg in patients with hormone refractory prostate cancer and metastatic melanoma. Ab treatment with a single dose was tolerated well and demonstrated clear signs of immunological activity and antitumor effects, including significant clinical regressions. In addition, several of the patients in the melanoma trial had received prior immunotherapy including various vaccines. From immunohistochemical analysis of post-treatment tumor biopsies from these patients, it appears that CTLA-4 blockade enhanced tumor-directed immune responses in patients that received cellular vaccines (40). These promising initial results have prompted several studies using MDX-010 as a single agent and as a single agent in combination with chemotherapy as well as combinations with vaccines. In a preliminary report of 14 patients receiving anti-CTLA-4 together with melanoma peptides, several tumor responses (two complete responses and one partial response) have been observed (41). Interestingly, in these responder patients as well as some patients that did not meet the criteria for a frank clinical response, significant autoimmune reactions were manifested in specific organs, including skin, gut, liver, and adrenal gland. This suggests that CTLA-4 blockade activates anti-self responses; in contrast, while anti-self responses were also noted in the nonhuman primate studies, there was no evidence of these or any other autoimmune pathology. These initial studies in primates and in humans suggest that the negative regulatory function of CTLA-4 is an important target in the manipulation of the immune system for effective antitumor therapy.
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Acknowledgments |
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Footnotes |
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2 Current address: Diadexus, South San Francisco CA, 94080.
3 Current address: Cancer Therapy Evaluation Program, National Cancer Institute, Rockville, MD 20852.
4 Abbreviations used in this paper: HBsAg, hepatitis B surface Ag; MFI, mean fluorescence intensity; ADCC, Ab-dependent cellular cytotoxicity; T reg, T regulatory.
Received for publication May 9, 2003. Accepted for publication September 30, 2003.
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monoclonal antibodies from a novel strain of minilocus transgenic mice. Nat. Biotechnol. 14:845.[Medline]
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) or in combination with 10D1 (•) were analyzed by ELISA at a 1/200 dilution. To normalize for individual background variability, values are expressed as the fold-increase in OD405 relative to pretreatment values. Pretreatment (background) OD405 values ranged from 0.051 to 0.152.