Latest Advances in the Understanding of Active-Specific

C h a p t e r 22

Latest Advances in the Understanding of ActiveSpecific Immunotherapy for Brain Tumors 1

YangLiu1,2,Qingzhong Kong , Ka-yun Ng2, and Kevin Lillehei1,3 Downloaded by NATL UNIV OF SINGAPORE on May 5, 2018 | https://pubs.acs.org Publication Date: May 15, 2000 | doi: 10.1021/bk-2000-0752.ch022

1Division of Neurosurgery and department of Pharmaceutical Sciences, University of Colorado Health Sciences Center, Denver, CO 80262

Over the years, numerous attempts have been tried to establish the effective use of immunotherapy for treatment of brain tumors. However, generation of active immune responses in most of these studies was only possible in the absence of viable tumor cells, suggesting that immunotherapy can only be used as preventive therapy. Few studies today have demonstrated success in using immunotherapy to treat an already established intracranial tumor. Using the Fischer 9L intracranial glioma model, we attempted to delineate the underlying mechanisms for these observations. Animals were vaccinated with irradiated 9L-glioma cells (i9L) together with interferon-γ(IFN-γ) before, at the same time, or after intracranial (IC) 9L tumor cell challenge. Survival of animals was then monitored for 60 days. In addition, developments of tumor -specific cytotoxicity and Τ lymphocyte proliferation at the various stages of vaccination were also studied. Animals vaccinated 14 and 7 days prior to intracranial tumor cell challenge showed a significant increase in survival. By contrast, vaccinations applied 5 days prior to, at the time of (i.e., day 0) or 7 days after IC tumor cell challenge failed to influence survival. Histological examination of brain tissue of vaccinated animals identified that vaccination increased mononuclear cell infiltration of the induced tumor. When spleen cells were stimulated with mitomycin C-treated i9L to generate cytotoxic lymphocytes (CTL), only CTL from animals primed with i9L plus IFN-γ killed 9L-glioma cells. CTL from non -primedrats or rats with established 9L intracranial tumor failed to affect 9L-glioma cells. Similarly, when stimulated with mitomycinC treated i9L, only spleen cells from primed animals showed an increase in proliferation. These data suggest that the tumor itself may exert a strong immunomodulatory effect at the systemic and Corresponding author.

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© 2000 American Chemical Society

Park and Mrsny; Controlled Drug Delivery ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

221 local levels. The exact mechanism of this immunomodulatory effect is currently unknown. Nevertheless, subversion of this immunosuppressive effect would be expected to enhance the therapeutic efficacy of active specific immunotherapy.

Downloaded by NATL UNIV OF SINGAPORE on May 5, 2018 | https://pubs.acs.org Publication Date: May 15, 2000 | doi: 10.1021/bk-2000-0752.ch022

Introduction The prognosis of patients with malignant glioma following conventional treatment is poor. Surgery, irradiation, and chemotherapeutic treatments rarely succeed in curing the disease. Among patients carrying a diagnosis of Glioblastoma Multiforme, average survival time is generally 12-18 months with few patients surviving beyond 2 years . The apparent lack of efficacy of conventional therapy has prompted a search for other potentially beneficial therapies. One of the more promising approaches for the treatment of brain tumors is the development of cancer vaccines. Although cerebral tissue does not possess any HLA antigens and a brain-tumor specific antigen has yet to be identified, glioma cells have been found to express low levels of MHC class I and class II molecules as well as tumor-associated antigens . These antigens are capable of inducing cell-mediated immune responses, thus rendering glioma cells susceptible to immunotherapy . These findings, when taken together with the recent observation that activated host immune cells are able to effectively infiltrate the blood-brain-barrier , suggest the feasibility of applying an intensified active immunization strategy to treat malignant brain tumors. For several decades, numerous attempts have been made to establish the use of immunotherapy for treatment of brain tumors. The approaches taken include tumor cells with adjuvant , tumor cells transduced with specific viral and allogeneic MHC genes or tumor cells transduced with cytokine genes to augment or enhance their immunogenicity. Most of these experiments, however, demonstrated that antitumor immunity was effective only when generated in the absence of viable tumor cells. These approaches are only effective against subsequent tumor cell challenge or in preventive therapy. Few studies have demonstrated success in treating established intracranial tumors, with cures being especially hard to achieve when immunizations were launched several days after tumor cell challenges . This explains why no one has been successful in prolonging survival in patients with high grade gliomas in controlled trials using manipulation of the immune system. The ability to create antitumor immunity in the presence of an established intracranial tumor is clearly of clinical relevance. By employing a Fischer 9L intracranial glioma model as our model system, the present study was instigated to delineate the mechanisms underlying the ineffectiveness of antitumor immunity in treating an established intracranial tumor. Using an assay of survival studies, histological surveys and in vitro cytotoxicity and proliferation assays, the antitumor effect of immunization before or after intracranial tumor cell challenge was studied. Our results demonstrate that a modulating effect of tumor cells on the immune system may be an important factor contributing to the failure of vaccination against the established brain tumor. 1

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222 Materials A n d Methods Cell Line and Experimental Animal 9L gliosareoma, a malignant glioma cell line syngeneic to the Fischer 344 rat, was used for the proposed studies. This cell is known to secrete transforming growth factor-beta (TGF-β), making it very similar to human gliomas . The tumor cell line was cultured in RPMI-1640 medium (Life Tech. Inc., Bethesda, MD) supplemented with 10% fetal bovine serum (FBS), and 2 mM glutamine. The cells were grown in a incubator at 37°C with a 5% C0 /air atmosphere. Fischer 344 male rats (Charles River Lab., Wilmington, MA) weighing 250 ± 20 grams were used. 9


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Subcutaneous Vaccination 6

0.8 ml of phosphate buffered saline (PBS) containing 5 χ 10 irradiated tumor cells (10,000 rad) and 10,000 U IFN-γ (R&D Systems, Minneapolis MN) were injected subcutaneously into the right hind leg of Fischer 344 rats. After the first injection, the rats received daily subcutaneous injections of IFN-γ (10,000 U in 0.3 ml) at the original site for 5 additional days. IFN-γ was chosen as the adjuvant in these vaccination studies for two reasons. Firstly, it can increase class I and class II MHC molecule expression on tumor cells and, therefore, act to enhance the development of cytotoxic Τ cells . Secondly, it has been shown to induce a potent antitumor immune response when used as an adjuvant or in a genetically-engineered tumor vaccine . 10

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Intracranial Tumor Implantation Using a stereotactic frame (Kopf, USA) and a Hamilton syringe, 10 μΐ PBS containing 5 χ 10 tumor cells were injected into the right frontal lobe of the rat. After receiving IC injections, the animals were monitored twice a day for neurological signs and weighed every third day until the end of the experiment (60 days after tumor cell implantation). All animals were sacrificed when moribund or at Day 60. At the time of death, brains of all animals were harvested for histological evaluation. 3

Proliferation Assay And Cytotoxicity Assay At various time intervals after vaccination, mononuclear cells were obtained from the spleens of Fischer 344 rats using a previously described method . To perform the proliferation assay, isolated nonadherent mononuclear cells (responder) were incubated with mitomycin C-treated 9L tumor cells (stimulator) at a stimulatonresponder ratio of 0.4:1 in the wells of a 96-well microtiter plate (5xl0 responder cells/well). After 72 hours incubation, mononuclear cell proliferation was 13

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determined using a CellTiter 96 AQ non-radioactive cell proliferation assay kit (Promega, Madison WI). To perform the cytotoxicity assay, aliquots of isolated mononuclear cells were co-incubated with mitomycin C-treated 9L glioma cells for 5 days. 5xl0 9L tumor cells (target cells) were then incubated in 5% FBS culture media with the 5-day cultured nonadherent mononuclear cells (effector cells) at an effectontarget ratio of 10:1, for 4 hours at 37°C. At the end of the incubation, mononuclear cell-mediated cytotoxicity was determined by using the CytoTox 96 Non-Radioactive Cytotoxicity Assay kit (Promega). The data were calculated as: % Cytotoxicity = (EXR - ESR - TSR + CMB)/(TMR -TSR - VC +CMB) χ 100. {Where EXR is experimental LDH release, ESR is effector cell spontaneous LDH release, TS is target cell spontaneous LDH release, TMR is target cell maximum LDH release, VC is volume correction, and CMB is culture medium LDH background.)


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Histological detection of mononuclear cell infiltration To detect mononuclear cell infiltration at the site of IC tumor cell challenge, two series of experiments were carried out. The first involved immunization prior to tumor cell challenge (prevention experiment), and the second involved immunization after tumor cell challenge (treatment experiment). In the first study, rats in each group were injected subcutaneously with i9L and IFN-γ on day 0, challenged intracranially on day 7 and euthanized at day 12, 17, or 22. For the treatment study, rats in each group were intracranially challenged on day 0, vaccinated on day 7 and euthanized at day 12,17 or 22. The brain from each animal was removed, fixed in 10% formalin, and embedded in paraffin for histological examination. Fourmicrometer tissue sections were stained with hemotoxylin and eosin according to standard procedures and evaluated for the presence of mononuclear cell infiltration by a neuropathologist blinded to the treatment groups.

Statistical Analysis Log-rank analyses were used to determine statistical significance between the survival data of the treatment groups. Student's unpaired t-test was used to determine the differences between the various groups in proliferation assay. P< 0.05 was considered significant.

Results

Efficacy of Tumor Vaccine in Preventing Tumor Development The ability of vaccinating the animals with a mixture of i9L and IFN-γ to elicit an antitumor immune response against a subsequent tumor challenge was studied.


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Six rats per group were immunized with either i9L, IFN-γ, PBS or a mixture of i9L + IFN-γ. One week after immunization they were challenged with IC i9L implantation. The mean survival time (MST) of controls immunized with IFN-γ (28.5 days) was indistinguishable from animals receiving PBS (26.8 days), pointing to the need to have i9L to elicit antitumor immunity. In contrast, prolonged survival was observed in rats immunized with either i9L or i9L plus IFN-γ (MST: 45.8 days and 56.5 days, respectively; Figure 1). These results suggest that immunizing animals with either i9L or a mixture of i9L and IFN-γ resulted in generation of an antitumor immune response. However, the protective effect of i9L + IFN-γ was greater than that afforded by immunizing the animals with i9L alone. Figure 1. Effect of vaccination with i9L + IFN-χ on survival of animals challenged with intra-cranial tumor cells. Differences in survival between groups were determined by non-parametric log rank test. Ρ values refer to comparison with PBS.

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*· KM. • IFN-Y (p • 0.0005) MFN-?(p» 0.4125) ^i9L (p» 0.0116)

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Days after challenge

To test the effect of time of vaccination relative to tumor cell challenge on animal survival, groups of 5 rats were immunized with i9L + IFN-γ 14, 7, 5, 3 or 0 days prior to IC tumor cell challenge. The survival of rats immunized 5, 3 and 0 days prior to tumor challenge was not different from those receiving PBS. In contrast, rats immunized 14 and 7 days prior to tumor challenge demonstrated significant protective immunity (MST: 46.0 days and 54.2 days; ρ = 0.0018 and 0.0064 when compared to PBS), Histological examination of the surviving animals for all groups on Day 60 revealed no evidence of viable IC tumor cells. These results indicate that the optimum time period required to develop an effective antitumor immune response in rats using a subcutaneously injected tumor vaccine is 7 days or more.

Efficacy of Tumor Vaccine in Treating Established Tumor Experiments were conducted to determine whether vaccinating animals with i9L + IFN-γ is effective in extending the survival of animals with established IC 9L gliomas. In these studies, animals were immunized at various times after IC 9L glioma tumor challenge. The survival of rats receiving immunization 0 or 7 days after tumor challenge was not significantly different from those receiving PBS (control). Therefore, vaccinating animals on the same day or 7 days after IC tumor cell challenge fails to increase survival. A number of mechanisms can be proposed to explain the failure of vaccination to show an impact on survival under these circumstances. It is possible that (a) a single vaccination of the animal in the presence of viable IC tumor cells is insufficient to generate a potent antitumor immune


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response, or (b) an established brain tumor may exert an inhibitory influence on the development of an antitumor immune response. To examine the former possibility, an additional experiment was performed wherein animals received 4 vaccinations of i9L + IFN-γ on days 5, 8, 11 and 14 after IC tumor cell challenge. If our first possibility is correct, the increased number of vaccinations should enhance the immune response and extend the survival of animals. This does not appear to be the case, however, in that survival of this group of animals was similar to control (data not shown). It is clear from these studies that vaccination after IC tumor cell implantation is ineffective in altering the progression of an established tumor. The second possibility was considered in other experiments investigating potential immunomodulatory actions. The results of these experiments were outlined below.

Determination of mononuclear cell infiltration at the site of tumor cell challenge To examine if established brain tumors can exert a repressive effect on the manifestation of the antitumor immune response, the extent of mononuclear cell infiltration at the IC site of tumor cell implantation was examined following various vaccination regimens, i.e., the prevention protocol and treatment protocols. In the prevention experiments (i.e., when vaccination was applied 7 days before IC tumor cell implantation), there was extensive mononuclear cell infiltration into tumor and surrounding areas 10 days after the tumor cell challenge. Five days after this observation (i.e., 15 days post tumor cell challenge), tumors in the vaccinated animals showed marked sign of regression. Ten days after tumor cell challenge, only a few mononuclear cells were observed at the tumor edge of PBS-vaccinated rats. The tumor in this group of animals continued to grow and showed no sign of regression even 15 days after tumor cell challenge (Table I-a). When the tumor and its surrounding areas were examined in the treatment group of animals (i.e., when vaccination was initiated after tumor cell challenge), there was also extensive infiltration of the tumor by mononuclear cells, although their appearance took a slightly longer time when compared to the prevention protocol (17 days vs. 10 days). Despite this mononuclear cell infiltration, tumors in these animals appeared to continue to grow (Table I-b). As in the prevention study, far fewer mononuclear cells were observed at the tumor edge of PBS-vaccinated rats. In both the prevention and treatment studies, challenging the vaccinated animals with IC injection of PBS buffer did not result in any tumor development or mononuclear cell infiltration at the site of IC injection. Together, these data demonstrate that subcutaneous immunization prior to (prevention) or after (treatment) IC tumor cell challenge results in infiltration of the tumor and its surrounding areas by mononuclear cells. However, it is only when vaccination is applied prior to tumor cell challenge that tumor cell growth is inhibited and the tumor cells eventually eradicated. Thus, our data indicate that suppression of mononuclear cell infiltration is not the mechanism by which the tumor inhibits the antitumor immune response. Rather, the most likely explanation for the failure of vaccine strategy to inhibit tumor cell growth in animals with established tumors may be related to an immunomodulatory effect of the tumor. This effect can be in the


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form of an inhibitory factor secreted by the tumor, which abrogates the antitumor activity of stimulated mononuclear cells. Table I. Mononuclear cell infdtration of tumor


Group 1 2 3

Vaccine i9L+IFN-γ i9L+IFN^ PBS

Challenge 9L PBS 9L 8

b. Treatment

Mononuclear cell infiltration Days after Challenge 5 10 15

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Tumor size

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a. Prevention*

Days after Challenge 5 10 15

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Mononuclear cell infiltration Days after Challenge 12 17 22

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Tumor size

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Days after Challenge Group Challenge Vaccine 12 17 22 ++ +++ +++ + +++ +++ 1 9L i9L+IFN^ 2 PBS i9L+IFN^ ++ ++ 3 9L PBS + ++ +-HFor the prevention experiments, rats were immunized with i9L + IFN-γ subcutaneously 7 days prior to IC challenge. Animals were euthanized 5, 10, or 15 days thereafter. For the treatment experiments, rats were immunized 7 days after IC challenge and euthanized 12, 17, or 22 days thereafter. Tumors were quantitated by a blinded observer as follows: (-) no tumor, (+) 4 mm in maxmum dimension. Mononuclear cell infiltration: (-) almost none, (+) scarce at edge of tumor, (++) more at edge and few in tumor, (+++) more at edge and in tumor. Two animals were investigated in each group at each time point. a

b

Role of immunomodulatory effect on antitumor immune response To examine if established brain tumors can exert an immunomodulating effect, the cytotoxic and proliferative activity of mononuclear cells obtained from the spleens of immunized or non-immunized animals was studied. If an established tumor exerts an immunomodulatory effect on Τ lymphocytes, mononuclear cells obtained from animals that received vaccinations 7 days or 14 days prior to Τ cell harvest should exhibit greater cytotoxicity towards target 9L cells than those receiving vaccination 5 days, 3 days, 0 days prior to harvest. Similarly, mononuclear cells obtained from animals vaccinated 7 days prior to Τ cell harvest should also show greater proliferation than those obtained from the non-vaccinated group at all stimulator to responder ratios. In both cases, our data clearly demonstrate that mononuclear cells from animals vaccinated prior to tumor cell challenge exhibited greater cytotoxic and proliferative activity towards target 9L glioma cells (Figure 2 and 3). In contrast, minimal cytotoxic or proliferative activity was observed in effector cells (as in cytotoxicity studies) and responder cells (as in proliferative studies) obtained from animals that received no immunization but were challenged


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intracranially with 9L glioma cells or from animals vaccinated 7 days after IC tumor cell challenge. These data agree with the results seen in our earlier survival experiments and support the premise that the antitumor immunity seen is the result of the development of tumor specific cytotoxic Τ cell. Further, our data suggest that established tumor might exert an inhibitory effect on the proliferation and cytotoxic activity of mononuclear cells in response to systemic vaccine therapy. • Control • Vaccine day 0 and used day 0 • Vaccintion day 0 and used day 3 - Vaccination day 0 and used day 5 • Vaccination day 0 and used day 7 Vaccination day 0 and used day 14 - Challenge day 0 and used day 14 - Challenge day 0, Vaccination day 7 and used day 14

Figure 2. Cytotoxicity analysis of spleen cells from primed rats and unprimed rats. For these studies, rats (n=2) either received vaccination with i9L + IFN-γ on day 0, IC tumor cells challenge on day 0, IC tumor cell challenge on day 0 followed by vaccination with i9L + IFN-γ on day 7, or PBS on day 0 (controls). Effector cells from animals vaccinated 7 da and 14 days before harvest displayed greater cytotoxicity than those harvested from anima receiving vaccination at 0, 3, and 5 days prior to harvest. Effector cells from animals challenged IC with 9L glioma cells alone or vaccinated 7 days after IC tumor cell challenge showed minimal cytotoxicity. A - Vaccination day -7 Β - Challenge day 0 and without vaccine C - Challenge day 0 and vaccination day 0 D-No vaccination and challenge |θ.6

Figure 3. Proliferation analysis of spleen cells from primed rats and unprimed rats. §0.2 For these studies, animals either received vaccination with i9L + IFN-γ on day 0, IC tumor cell challenge on day 0 followed by Β c Treatment subcutaneous injection of PBS on day 7, IC tumor cell challenge on day 0 followed by i9L + IFN^vaccination on day 7, or vaccination with PBS on day 0 (control). Seven days after the various treatments, mononuclear cells w obtained from the spleens of the treated animals for proliferation analysis. Mononuclear cells from animals vaccinated 7 days prior to sacrifice showed more proliferation than those from either the controls, the animals challenged intracranially with 9L glioma cells, or animals tha received vaccination 7 days after IC tumor cell challenge. Data represent mean +S.E. from experiments. *P< 0.001 relative to MTS conversion of other treatment group. |o.4

Discussion We have demonstrated that subcutaneous vaccination with i9L + IFN-γ can generate a potent protective antitumor effect against subsequent 9L glioma IC


228 challenge in the Fischer 344 rat. Thus, our current studies extend those findings by Jean et al. , who reported similar results using a mixture of IL-12 and irradiated tumor cells for immunizing animals. The ability to generate an antitumor immune response by subcutaneous administration of irradiated tumor cells and cytokines circumvents the need for genetically engineered tumor cells to express particular cytokines. Unfortunately, despite the simplistic nature of this approach, we were unable to use this technique to extend the survival of animals with already established brain tumors. In the present study, development of antitumor immune response followed a specific time course. Our data indicated that at least 7 days are required for the immune system to develop an effective antitumor immune response, with the optimal response being obtained when vaccination was applied 7 days prior to tumor cell challenge. This was reflected in increased survival in rats immunized at least 7 days prior to IC tumor cell challenge. The reason for a loss of antitumor effects when vaccination occurs within 5 days of tumor challenge is probably both quantitative and qualitative. At shorter periods of immunization (such as 3 days), the buildup of immune effector cells is probably not reaching a critical level to stage a successful attack on tumor cells. Tumor-induced immunosuppression has been well documented in patients with malignant glioma and in tumor-bearing animals . Accordingly, tumor cells may release an immunomodulating factor, which functions to suppress the antitumor immune response. The release of such a factor may render the already poorly primed T-cells to a state of anergy, allowing the injected tumor cells to escape the assault by the cytotoxic Τ cells. Application of vaccine at the time of or after tumor cell challenge also failed to exert an antitumor effect. Under these conditions, there would be no primed T-cells to begin with, and the tumor-derived suppressive factor may either inhibit the development of antitumor immune response or exert modulating influence on the antitumor immune response. The most compelling data supporting the contention that the tumor cells are releasing a factor that modulates rather than inhibits the development of antitumor response came from our histological studies wherein mononuclear cell infiltration of the tumor occurred whether vaccination occurred 7 days prior to or 7 days after IC tumor cell challenge. Hence, recruitment of mononuclear cells to the tumor appeared to be independent of the time of vaccination relative to tumor cell implantation. Had a tumor-derived factor been suppressing development of the anti-tumor response, one would have expected reduced mononuclear cell infiltration of the tumor under conditions that failed to affect tumor development, i.e., vaccination after tumor cell challenge. The observation that mononuclear cells are present at the tumor site but ineffective in eliciting tumor repression (as was the case for vaccination after tumor cell challenge) leads to the intriguing possibility that the tumor cells may be locally repressing the cytotoxic activity of infiltrating mononuclear cells. Another significant finding in our studies is the possibility that tumor cells may also be exerting a systemic immunosuppressive effect. Mononuclear cells isolated from the spleen of immunized animals exhibit cytotoxic and proliferative activity when exposed to immunogenic tumor cells. Our studies demonstrated that these activities are significantly reduced when the mononuclear cells are obtained from animals with established brain tumors. In vitro and in vivo studies in our laboratory


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229 are currently in progress to identify and subsequently neutralize this immunosuppressive factor. In summary, experiments reported herein suggest that established brain tumors have a strong repressive effect on the immune system at the systemic and local levels. This activity may be sufficient to diminish the activity of active immunotherapy to destroy an established tumor. It is anticipated that inhibition of this tumor suppressive factor should enhance the efficacy of active immunotherapy in the treatment of brain tumors.


Acknowledgements We gratefully acknowledge the University of Colorado Cancer Center, Milheim Foundation for Brain Tumor Research, Pharmaceutical Research and Manufacturers of America Foundation for funding support. The authors also like to thank Drs. Bette DeMaster for perfonning the histological analysis, Pan Zhaoxing for statistical analysis and David Thompson for his helpful criticism.

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