Hypofractionated radiotherapy combined with lenalidomide improves systemic antitumor activity in mouse solid tumor models

Background: Hypofractionated radiotherapy (hRT) can induce a T cell-mediated abscopal effect on non-irradiated tumor lesions, especially in combination with immune checkpoint blockade (ICB). However, clinically, this effect is still rare, and ICB-mediated adverse events are common. Lenalidomide (lena) is an anti-angiogenic and immunomodulatory drug used in the treatment of hematologic malignancies. We here investigated in solid tumor models whether lena can enhance the abscopal effect in double combination with hRT. Methods: In two syngeneic bilateral tumor models (B16-CD133 melanoma and MC38 colon carcinoma), the primary tumor was treated with hRT. Lena was given daily for 3 weeks. Besides tumor size and survival, the dependence of the antitumor effects on CD8+ cells, type-I IFN signaling, and T cell costimulation was determined with depleting or blocking antibodies. Tumor-specific CD8+ T cells were quantified, and their differentiation and effector status were characterized by multicolor flow cytometry using MHC-I tetramers and various antibodies. In addition, dendritic cell (DC)-mediated tumor antigen cross-presentation in vitro and directly ex vivo and the composition of tumor-associated vascular endothelial cells were investigated. Results: In both tumor models, the hRT/lena double combination induced a significant abscopal effect. Control of the non-irradiated secondary tumor and survival were considerably better than with the respective monotherapies. The abscopal effect was strongly dependent on CD8+ cells and associated with an increase in tumor-specific CD8+ T cells in the non-irradiated tumor and its draining lymph nodes. Additionally, we found more tumor-specific T cells with a stem-like (TCF1+ TIM3- PD1+) and a transitory (TCF1- TIM3+ CD101- PD1+) exhausted phenotype and more expressing effector molecules such as GzmB, IFNγ, and TNFα. Moreover, in the non-irradiated tumor, hRT/lena treatment also increased DCs cross-presenting a tumor model antigen. Blocking type-I IFN signaling, which is essential for cross-presentation, completely abrogated the abscopal effect. A gene expression analysis of bone marrow-derived DCs revealed that lena augmented the expression of IFN response genes and genes associated with differentiation, maturation (including CD70, CD83, and CD86), migration to lymph nodes, and T cell activation. Flow cytometry confirmed an increase in CD70+ CD83+ CD86+ DCs in both irradiated and abscopal tumors. Moreover, the hRT/lena-induced abscopal effect was diminished when these costimulatory molecules were blocked simultaneously using antibodies. In line with the enhanced infiltration by DCs and tumor-specific CD8+ T cells, including more stem-like cells, hRT/lena also increased tumor-associated high endothelial cells (TA-HECs) in the non-irradiated tumor. Conclusions: We demonstrate that lena can augment the hRT-induced abscopal effect in mouse solid tumor models in a CD8 T cell- and IFN-I-dependent manner, correlating with enhanced anti-tumor CD8 T cell immunity, DC cross-presentation, and TA-HEC numbers. Our findings may be helpful for the planning of clinical trials in (oligo)metastatic patients.

Adding liposomal doxorubicin enhances the abscopal effect induced by radiation/αPD1 therapy depending on tumor cell mitochondrial DNA and cGAS/STING

BACKGROUND

Localized radiotherapy (RT) can cause a T cell-mediated abscopal effect on non-irradiated tumor lesions, especially in combination with immune checkpoint blockade. However, this effect is still clinically rare and improvements are highly desirable. We investigated whether triple combination with a low dose of clinically approved liposomal doxorubicin (Doxil) could augment abscopal responses compared with RT/αPD-1 and Doxil/αPD-1. We also investigated whether the enhanced abscopal responses depended on the mitochondrial DNA (mtDNA)/cyclic GMP-AMP synthase (cGAS)/stimulator of interferon genes (STING)/IFN-I pathway.

MATERIALS/METHODS

We used Doxil in combination with RT and αPD-1 in two tumor models (B16-CD133 melanoma and MC38 colon carcinoma) with mice bearing two tumors, only one of which was irradiated. Mechanistic studies on the role of the mtDNA/cGAS/STING/IFN-I axis were performed using inhibitors and knockout cells in vitro as well as in mice.

RESULTS

Addition of a single low dose of Doxil to RT and αPD-1 strongly enhanced the RT/αPD-1-induced abscopal effect in both models. Complete cures of non-irradiated tumors were mainly observed in triple-treated mice. Triple therapy induced more cross-presenting dendritic cells (DCs) and more tumor-specific CD8+ T cells than RT/αPD-1 and Doxil/αPD-1, particularly in non-irradiated tumors. Coincubation of Doxil-treated and/or RT-treated tumor cells with DCs enhanced DC antigen cross-presentation which is crucial for inducing CD8+ T cells. CD8+ T cell depletion or implantation of cGAS-deficient or STING-deficient tumor cells abolished the abscopal effect. Doxorubicin-induced/Doxil-induced IFNβ1 markedly depended on the cGAS/STING pathway. Doxorubicin-treated/Doxil-treated tumor cells depleted of mtDNA secreted less IFNβ1, of the related T cell-recruiting chemokine CXCL10, and ATP; coincubation with mtDNA-depleted tumor cells strongly reduced IFNβ1 secretion by DCs. Implantation of mtDNA-depleted tumor cells, particularly at the non-irradiated/abscopal site, substantially diminished the Doxil-enhanced abscopal effect and tumor infiltration by tumor-specific CD8+ T cells.

CONCLUSIONS

These data show that single low-dose Doxil can substantially enhance the RT/αPD-1-induced abscopal effect, with a strong increase in cross-presenting DCs and CD8+ tumor-specific T cells particularly in abscopal tumors compared with RT/αPD-1 and Doxil/αPD-1. Moreover, they indicate that the mtDNA/cGAS/STING/IFN-I axis is important for the immunogenic/immunomodulatory doxorubicin effects. Our findings may be helpful for the planning of clinical radiochemoimmunotherapy trials in (oligo)metastatic patients.

Expansion of circulating stem-like CD8+ T cells by adding CD122-directed IL-2 complexes to radiation and anti-PD1 therapies in mice

Combination of radiation therapy (RT) with immune checkpoint blockade can enhance systemic anti-tumor T cell responses. Here, using two mouse tumor models, we demonstrate that adding long-acting CD122-directed IL-2 complexes (IL-2c) to RT/anti-PD1 further increases tumor-specific CD8⁺ T cell numbers. The highest increase (>50-fold) is found in the blood circulation. Compartmental analysis of exhausted T cell subsets shows that primarily undifferentiated, stem-like, tumor-specific CD8⁺ T cells expand in the blood; these cells express the chemokine receptor CXCR3, which is required for migration into tumors. In tumor tissue, effector-like but not terminally differentiated exhausted CD8⁺ T cells increase. Consistent with the surge in tumor-specific CD8⁺ T cells in blood that are migration and proliferation competent, we observe a CD8-dependent and CXCR3-dependent enhancement of the abscopal effect against distant/non-irradiated tumors and find that CD8⁺ T cells isolated from blood after RT/anti-PD1/IL-2c triple treatment can be a rich source of tumor-specific T cells for adoptive transfers.

Necroptosis-dependent Immunogenicity of Cisplatin: Implications for Enhancing the Radiation-induced Abscopal Effect

PURPOSE

Cisplatin is increasingly used in chemoimmunotherapy and may enhance the T cell-dependent radiation-induced abscopal effect, but how it promotes antitumor immunity is poorly understood. We investigated whether and why cisplatin is immunogenic, and the implications for the cisplatin-enhanced abscopal effect.

EXPERIMENTAL DESIGN

Cisplatin, carboplatin, and the well-known immunogenic cell death (ICD) inducer oxaliplatin were compared for their potency to enhance the abscopal effect and induce type I IFN (IFN-I) and extracellular ATP, danger signals of ICD. The hypothetical role of necroptosis and associated damage-associated molecular patterns for cisplatin-induced ICD was investigated by inhibitors and knockout cells in vitro and in two tumor models in mice. A novel necroptosis signature for tumor immune cell infiltration and therapy response was developed.

RESULTS

Cisplatin enhanced the abscopal effect more strongly than oxaliplatin or carboplatin. This correlated with higher induction of IFN-I and extracellular ATP by cisplatin, in a necroptosis-dependent manner. Cisplatin triggered receptor-interacting protein kinase 3 (RIPK3)-dependent tumor cell necroptosis causing cytosolic mitochondrial DNA (mtDNA) release, initiating the cyclic GMP-AMP synthase-stimulator of interferon genes pathway and IFN-I secretion promoting T-cell cross-priming by dendritic cells (DC). Accordingly, tumor cell RIPK3 or mtDNA deficiency and loss of IFN-I or ATP signaling diminished the cisplatin-enhanced abscopal effect. Cisplatin-treated tumor cells were immunogenic in vaccination experiments, depending on RIPK3 and mtDNA. In human tumor transcriptome analysis, necroptotic features correlated with abundant CD8+ T cells/DCs, sparse immunosuppressive cells, and immunotherapy response.

CONCLUSIONS

Cisplatin induces antitumor immunity through necroptosis-mediated ICD. Our findings may help explain the benefits of cisplatin in chemo(radio)immunotherapies and develop clinical trials to investigate whether cisplatin enhances the abscopal effect in patients.

Immune cell infiltration pattern in non-small cell lung cancer PDX models is a model immanent feature and correlates with a distinct molecular and phenotypic make-up

BACKGROUND

The field of cancer immunology is rapidly moving towards innovative therapeutic strategies, resulting in the need for robust and predictive preclinical platforms reflecting the immunological response to cancer. Well characterized preclinical models are essential for the development of predictive biomarkers in the oncology as well as the immune-oncology space. In the current study, gold standard preclinical models are being refined and combined with novel image analysis tools to meet those requirements.

METHODS

A panel of 14 non-small cell lung cancer patient-derived xenograft models (NSCLC PDX) was propagated in humanized NOD/Shi-scid/IL-2Rnull mice. The models were comprehensively characterized for relevant phenotypic and molecular features, including flow cytometry, immunohistochemistry, histology, whole exome sequencing and cytokine secretion.

RESULTS

Models reflecting hot (>5% tumor-infiltrating lymphocytes/TILs) as opposed to cold tumors (<5% TILs) significantly differed regarding their cytokine profiles, molecular genetic aberrations, stroma content, and programmed cell death ligand-1 status. Treatment experiments including anti cytotoxic T-lymphocyte-associated protein 4, anti-programmed cell death 1 or the combination thereof across all 14 models in the single mouse trial format showed distinctive tumor growth response and spatial immune cell patterns as monitored by computerized analysis of digitized whole-slide images. Image analysis provided for the first time qualitative evaluation of the extent to which PDX models retain the histological features from their original human donors.

CONCLUSIONS

Deep phenotyping of PDX models in a humanized setting by combinations of computational pathology, immunohistochemistry, flow cytometry and proteomics enables the exhaustive analysis of innovative preclinical models and paves the way towards the development of translational biomarkers for immuno-oncology drugs.

Deep abscopal response to radiotherapy and anti-PD-1 in an oligometastatic melanoma patient with unfavorable pretreatment immune signature

Radiotherapy can elicit abscopal effects in non-irradiated metastases, particularly under immune checkpoint blockade (ICB). We report on two elderly patients with oligometastatic melanoma treated with anti-PD-1 and stereotactic body radiation therapy (SBRT). Before treatment, patient 1 showed strong tumor infiltration with exhausted CD8+ T cells and high expression of T cell-attracting chemokines. This patient rapidly mounted a complete response, now ongoing for more than 4.5 years. Patient 2 exhibited low CD8+ T cell infiltration and high expression of immunosuppressive arginase. After the first SBRT, his non-irradiated metastases did not regress and new metastases occurred although neoepitope-specific and differentiation antigen-specific CD8+ T cells were detected in the blood. A second SBRT after 10 months on anti-PD-1 induced a radiologic complete response correlating with an increase in activated PD-1-expressing CD8 T cells. Apart from a new lung lesion, which was also irradiated, this deep abscopal response lasted for more than 2.5 years. However, thereafter, his disease progressed and the activated PD-1-expressing CD8 T cells dropped. Our data suggest that oligometastatic patients, where a large proportion of the tumor mass can be irradiated, are good candidates to improve ICB responses by RT, even in the case of an unfavorable pretreatment immune signature, after progression on anti-PD-1, and despite advanced age. Besides repeated irradiation, T cell epitope-based immunotherapies (e.g., vaccination) may prolong antitumor responses even in patients with unfavorable pretreatment immune signature.

Adding Indoximod to Hypofractionated Radiotherapy with Anti-PD-1 Checkpoint Blockade Enhances Early NK and CD8+ T-Cell-Dependent Tumor Activity

PURPOSE

There is growing interest in combinations of immunogenic radiotherapy (RT) and immune checkpoint blockade, but clinical responses are still limited. Therefore, we tested the triple therapy with an inhibitor of the indoleamine 2,3-dioxygenase pathway, which like immune checkpoints, downregulates the antitumor immune response.

EXPERIMENTAL DESIGN

Triple treatment with hypofractionated RT (hRT) + anti-PD-1 antibody (αPD1) + indoximod was compared with the respective mono- and dual therapies in two syngeneic mouse models.

RESULTS

The tumors did not regress following treatment with hRT + αPD1. The αPD1/indoximod combination was not effective at all. In contrast, triple treatment induced rapid, marked tumor regression, even in mice with a large tumor. The effects strongly depended on CD8⁺ T cells and partly on natural killer (NK) cells. Numbers and functionality of tumor-specific CD8⁺ T cells and NK cells were increased, particularly early during treatment. However, after 2.5-3 weeks, all large tumors relapsed, which was accompanied by increased apoptosis of tumor-infiltrating lymphocytes associated with a non-reprogrammable state of exhaustion, terminal differentiation, and increased activation-induced cell death, which could not be prevented by indoximod in these aggressive tumor models. Some mice with a smaller tumor were cured. Reirradiation during late regression (day 12), but not after relapse, cured almost all mice with a large B16-CD133 tumor, and strongly delayed relapse in the less immunogenic 4T1 model, depending on CD8⁺ T cells.

CONCLUSIONS

Our findings may serve as a rationale for the clinical evaluation of this triple-combination therapy in patients with solitary or oligometastatic tumors in the neoadjuvant or the definitive setting.

Cisplatin Facilitates Radiation-Induced Abscopal Effects in Conjunction with PD-1 Checkpoint Blockade Through CXCR3/CXCL10-Mediated T-cell Recruitment

PURPOSE

Localized radiotherapy can cause T-cell-mediated abscopal effects on nonirradiated metastases, particularly in combination with immune checkpoint blockade (ICB). However, results of prospective clinical trials have not met the expectations. We therefore investigated whether additional chemotherapy can enhance radiotherapy-induced abscopal effects in conjunction with ICB.

EXPERIMENTAL DESIGN

In three different two-tumor mouse models, triple therapy with radiotherapy, anti-PD-1, and cisplatin (one of the most widely used antineoplastic agents) was compared with double or single therapies.

RESULTS

In these mouse models, the response of the nonirradiated tumor and the survival of the mice were much better upon triple therapy than upon radiotherapy + anti-PD-1 or cisplatin + anti-PD-1 or the monotherapies; complete regression of the nonirradiated tumor was usually only observed in triple-treated mice. Mechanistically, the enhanced abscopal effect required CD8⁺T cells and relied on the CXCR3/CXCL10 axis. Moreover, CXCL10 was found to be directly induced by cisplatin in the tumor cells. Furthermore, cisplatin-induced CD8⁺T cells and direct cytoreductive effects of cisplatin also seem to contribute to the enhanced systemic effect. Finally, the results show that the abscopal effect is not precluded by the observed transient radiotherapy-induced lymphopenia.

CONCLUSIONS

This is the first report showing that chemotherapy can enhance radiotherapy-induced abscopal effects in conjunction with ICB. This even applies to cisplatin, which is not classically immunogenic. Whereas previous studies have focused on how to effectively induce tumor-specific T cells, this study highlights that successful attraction of the induced T cells to nonirradiated tumors is also crucial for potent abscopal effects.

Combination treatment with hypofractionated radiotherapy plus IL-2/anti-IL-2 complexes and its theranostic evaluation

Background

Immunogenic radiotherapy (RT) can act synergistically with immune checkpoint blockers (ICBs). However, alternatives are needed for non-responding patients and those with pre-existing or ICB-induced autoimmune symptoms. Combination of RT with IL-2 could be an alternative. But IL-2 has a short half-life, and, by binding to its high-affinity receptor, it strongly stimulates immunosuppressive CD4+ Tregs and seems to promote potentially life-threatening vascular leakage. IL-2/anti-IL-2 complexes (IL-2c), which bind to the low-affinity receptor, have been reported to circumvent these disadvantages but they have not yet been thoroughly tested in conjunction with radiotherapy.

Methods

We evaluated, in three mouse models, the antitumoral effects induced by hypofractionated RT (hRT) plus IL-2c. We also used non-invasive imaging with a newly developed PET tracer based on therapeutically active IL-2c and a PD-L1 PET tracer for the theranostic evaluation of the treatment and its side effects.

Results

Treatment of mice bearing established B16 melanomas with hRT + IL-2c was superior to hRT + uncomplexed IL-2 or hRT alone; IL-2c alone was not effective. hRT + IL-2c was also synergistic in mice bearing C51 colon carcinomas or 4T1 mammary carcinomas. The better antitumor response correlated with increased tumor-specific CD8+ T cells and NK cells, but not CD4+ Tregs, in the irradiated tumor and in lymphoid organs. With the new PET tracer, we visualized the whole-body distribution of IL-2c and the bound receptors in naïve mice and tumor-bearing mice. Surprisingly, the tumor uptake was non-specific and only moderate. This prompted experiments demonstrating that specific IL-2c binding in the tumor is limited by IL-2 secreted by tumor-resident effector cells and that extratumorally expanded T and NK cells can infiltrate the irradiated tumor, which suggests that systemic immune activation considerably contributed to the reduction of tumor growth. Lastly, we show that a side effect of IL-2c treatment – a quite dramatic non-specific expansion of CD8+ T and NK cells – is only transient, and we visualized the associated splenomegaly as well as side effects on liver and lung by contrast-enhanced CT and PD-L1 PET.

Conclusions

Our results show that the combination of immunogenic RT with IL-2c that are directed towards the low-affinity IL-2 receptor can be synergistic and more effective than the combination with uncomplexed IL-2. In addition, our theranostic evaluation provided insights into the mechanism of action and the side effects of IL-2c treatment.

Electronic supplementary material

The online version of this article (10.1186/s40425-019-0537-9) contains supplementary material, which is available to authorized users.

Imaging and Selective Elimination of Glioblastoma Stem Cells with Theranostic Near-Infrared-Labeled CD133-Specific Antibodies

Near-infrared photoimmunotherapy (NIR-PIT), which employs monoclonal antibody (mAb)-phototoxic phthalocyanine dye IR700 conjugates, permits the specific, image-guided and spatiotemporally controlled elimination of tumor cells. Here, we report the highly efficient NIR-PIT of human tumor xenografts initiated from patient-derived cancer stem cells (CSCs). Using glioblastoma stem cells (GBM-SCs) expressing the prototypic CSC marker AC133/CD133, we also demonstrate here for the first time that NIR-PIT is highly effective against brain tumors. The intravenously injected theranostic AC133 mAb conjugate enabled the non-invasive detection of orthotopic gliomas by NIR fluorescence imaging, and reached AC133+ GBM-SCs at the invasive tumor front. AC133-targeted NIR-PIT induced the rapid cell death of AC133+ GBM-SCs and thereby strong shrinkage of both subcutaneous and invasively growing brain tumors. A single round of NIR-PIT extended the overall survival of mice with established orthotopic gliomas by more than a factor of two, even though the harmless NIR light was applied through the intact skull. Humanised versions of this theranostic agent may facilitate intraoperative imaging and histopathological evaluation of tumor borders and enable the highly specific and efficient eradication of CSCs.

Decrease of VEGF-A in myeloid cells attenuates glioma progression and prolongs survival in an experimental glioma model

BACKGROUND

Glioblastomas are highly vascularized tumors with a prominent infiltration of macrophages/microglia whose role in promoting glioma growth, invasion, and angiogenesis has not been fully elucidated.

METHODS

The contribution of myeloid-derived vascular endothelial growth factor (VEGF) to glioma growth was analyzed in vivo in a syngeneic intracranial GL261 glioma model using a Cre/loxP system to knock out the expression of VEGF-A in CD11b + myeloid cells. Changes in angiogenesis-related gene expression profile were analyzed in mutant bone marrow-derived (BMD) macrophages in vitro. Furthermore, we studied the influence of macrophages on GL261 growth, invasiveness, and protein expression profile of angiogenic molecules as well as the paracrine effect of mutant macrophages on angiogenesis in vitro.

RESULTS

Myeloid cell-restricted VEGF-A deficiency leads to a growth delay of intracranial tumors and prolonged survival. The tumor vasculature in mutant mice was more regular, with increased pericyte coverage. Expression analysis revealed significant downregulation of VEGF-A and slight upregulation of TGFβ-1 in BMD macrophages from mutant mice. Endothelial tube formation was significantly decreased by conditioned media from mutant macrophages. The expression of angiogenesis-related proteins in GL261 glioma cells in co-culture experiments either with wild-type or mutant macrophages remained unchanged, indicating that effects observed in vivo are due to myeloid-derived VEGF-A deficiency.

CONCLUSIONS

Our results highlight the importance of VEGF derived from tumor-infiltrating myeloid cells for initiating vascularization in gliomas. The combination of antiangiogenic agents with myeloid cell-targeting strategies might provide a new therapeutic approach for glioblastoma patients.

Patient-derived glioblastoma stem cells are killed by CD133-specific CAR T cells but induce the T cell aging marker CD57

The AC133 epitope of CD133 is a cancer stem cell (CSC) marker for many tumor entities, including the highly malignant glioblastoma multiforme (GBM). We have developed an AC133-specific chimeric antigen receptor (CAR) and show that AC133-CAR T cells kill AC133+ GBM stem cells (GBM-SCs) both in vitro and in an orthotopic tumor model in vivo. Direct contact with patient-derived GBM-SCs caused rapid upregulation of CD57 on the CAR T cells, a molecule known to mark terminally or near-terminally differentiated T cells. However, other changes associated with terminal T cell differentiation could not be readily detected. CD57 is also expressed on tumor cells of neural crest origin and has been preferentially found on highly aggressive, undifferentiated, multipotent CSC-like cells. We found that CD57 was upregulated on activated T cells only upon contact with CD57+ patient-derived GBM-SCs, but not with conventional CD57-negative glioma lines. However, CD57 was not downregulated on the GBM-SCs upon their differentiation, indicating that this molecule is not a bona fide CSC marker for GBM. Differentiated GBM cells still induced CD57 on CAR T cells and other activated T cells. Therefore, CD57 can apparently be upregulated on activated human T cells by mere contact with CD57+ target cells.

Effective Eradication of Glioblastoma Stem Cells by Local Application of an AC133/CD133-Specific T-cell-Engaging Antibody and CD8 T Cells

Cancer stem cells (CSC) drive tumorigenesis and contribute to genotoxic therapy resistance, diffuse infiltrative invasion, and immunosuppression, which are key factors for the incurability of glioblastoma multiforme (GBM). The AC133 epitope of CD133 is an important CSC marker for GBM and other tumor entities. Here, we report the development and preclinical evaluation of a recombinant AC133×CD3 bispecific antibody (bsAb) that redirects human polyclonal T cells to AC133(+) GBM stem cells (GBM-SC), inducing their strong targeted lysis. This novel bsAb prevented the outgrowth of AC133-positive subcutaneous GBM xenografts. Moreover, upon intracerebral infusion along with the local application of human CD8(+) T cells, it exhibited potent activity in prophylactic and treatment models of orthotopic GBM-SC-derived invasive brain tumors. In contrast, normal hematopoietic stem cells, some of which are AC133-positive, were virtually unaffected at bsAb concentrations effective against GBM-SCs and retained their colony-forming abilities. In conclusion, our data demonstrate the high activity of this new bsAb against patient-derived AC133-positive GBM-SCs in models of local therapy of highly invasive GBM.

Chloroquine or chloroquine-PI3K/Akt pathway inhibitor combinations strongly promote γ-irradiation-induced cell death in primary stem-like glioma cells

We asked whether inhibitors of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway, which is highly active in cancer stem cells (CSCs) and upregulated in response to genotoxic treatments, promote γ-irradiationγIR)-induced cell death in highly radioresistant, patient-derived stem-like glioma cells (SLGCs). Surprisingly, in most cases the inhibitors did not promote γIR-induced cell death. In contrast, the strongly cytostatic Ly294002 and PI-103 even tended to reduce it. Since autophagy was induced we examined whether addition of the clinically applicable autophagy inhibitor chloroquine (CQ) would trigger cell death in SLGCs. Triple therapy with CQ at doses as low as 5 to 10 µM indeed caused strong apoptosis. At slightly higher doses, CQ alone strongly promoted γIR-induced apoptosis in all SLGC lines examined. The strong apoptosis in combinations with CQ was invariably associated with strong accumulation of the autophagosomal marker LC3-II, indicating inhibition of late autophagy. Thus, autophagy-promoting effects of PI3K/Akt pathway inhibitors apparently hinder cell death induction in γ-irradiated SLGCs. However, as we show here for the first time, the late autophagy inhibitor CQ strongly promotes γIR-induced cell death in highly radioresistant CSCs, and triple combinations of CQ, γIR and a PI3K/Akt pathway inhibitor permit reduction of the CQ dose required to trigger cell death.

Delayed cell death associated with mitotic catastrophe in γ-irradiated stem-like glioma cells

Background and Purpose

Stem-like tumor cells are regarded as highly resistant to ionizing radiation (IR). Previous studies have focused on apoptosis early after irradiation, and the apoptosis resistance observed has been attributed to reduced DNA damage or enhanced DNA repair compared to non-stem tumor cells. Here, early and late radioresponse of patient-derived stem-like glioma cells (SLGCs) and differentiated cells directly derived from them were examined for cell death mode and the influence of stem cell-specific growth factors.

Materials and methods

Primary SLGCs were propagated in serum-free medium with the stem-cell mitogens epidermal growth factor (EGF) and fibroblast growth factor-2 (FGF-2). Differentiation was induced by serum-containing medium without EGF and FGF. Radiation sensitivity was evaluated by assessing proliferation, clonogenic survival, apoptosis, and mitotic catastrophe. DNA damage-associated γH2AX as well as p53 and p21 expression were determined by Western blots.

Results

SLGCs failed to apoptose in the first 4 days after irradiation even at high single doses up to 10 Gy, but we observed substantial cell death later than 4 days postirradiation in 3 of 6 SLGC lines treated with 5 or 10 Gy. This delayed cell death was observed in 3 of the 4 SLGC lines with nonfunctional p53, was associated with mitotic catastrophe and occurred via apoptosis. The early apoptosis resistance of the SLGCs was associated with lower γH2AX compared to differentiated cells, but we found that the stem-cell culture cytokines EGF plus FGF-2 strongly reduce γH2AX levels. Nonetheless, in two p53-deficient SLGC lines examined γIR-induced apoptosis even correlated with EGF/FGF-induced proliferation and mitotic catastrophe. In a line containing CD133-positive and -negative stem-like cells, the CD133-positive cells proliferated faster and underwent more γIR-induced mitotic catastrophe.

Conclusions

Our results suggest the importance of delayed apoptosis, associated mitotic catastrophe, and cellular proliferation for γIR-induced death of p53-deficient SLGCs. This may have therapeutic implications. We further show that the stem-cell culture cytokines EGF plus FGF-2 activate DNA repair and thus confound in vitro comparisons of DNA damage repair between stem-like and more differentiated tumor cells.

Non-invasive in vivo imaging of tumor-associated CD133/prominin

Background

Cancer stem cells are thought to play a pivotal role in tumor maintenance, metastasis, tumor therapy resistance and relapse. Hence, the development of methods for non-invasive in vivo detection of cancer stem cells is of great importance.

Methodology/Principal Findings

Here, we describe successful in vivo detection of CD133/prominin, a cancer stem cell surface marker for a variety of tumor entities. The CD133-specific monoclonal antibody AC133.1 was used for quantitative fluorescence-based optical imaging of mouse xenograft models based on isogenic pairs of CD133 positive and negative cell lines. A first set consisted of wild-type U251 glioblastoma cells, which do not express CD133, and lentivirally transduced CD133-overexpressing U251 cells. A second set made use of HCT116 colon carcinoma cells, which uniformly express CD133 at levels comparable to primary glioblastoma stem cells, and a CD133-negative HCT116 derivative. Not surprisingly, visualization and quantification of CD133 in overexpressing U251 xenografts was successful; more importantly, however, significant differences were also found in matched HCT116 xenograft pairs, despite the lower CD133 expression levels. The binding of i.v.-injected AC133.1 antibodies to CD133 positive, but not negative, tumor cells isolated from xenografts was confirmed by flow cytometry.

Conclusions/Significance

Taken together, our results show that non-invasive antibody-based in vivo imaging of tumor-associated CD133 is feasible and that CD133 antibody-based tumor targeting is efficient. This should facilitate developing clinically applicable cancer stem cell imaging methods and CD133 antibody-based therapeutics.

Analysis of direct and cross-presentation of antigens in TPPII knockout mice

Tripeptidyl peptidase II (TPPII) is an oligopeptidase forming giant complexes in the cytosol that have high exo-, but also, endoproteolytic activity. Immunohistochemically, the complexes appear as distinct foci in the cytosol. In part controversial biochemical and functional studies have suggested that TPPII contributes, on the one hand, positively to Ag processing by generating epitope carboxyl termini or by trimming epitope precursors, and, on the other, negatively by destroying potentially antigenic peptides. To clarify which of these roles is predominant, we generated and analyzed TPPII-deficient mice. Cell surface levels of MHC class I peptide complexes tended to be increased on most cell types of these mice. Although presentation of three individual epitopes derived from lymphocytic choriomeningitis virus was not elevated on TPPII-/- cells, that of the immunodominant OVA epitope SIINFEKL was significantly enhanced. Consistent with this, degradation of a synthetic peptide corresponding to the OVA epitope and of another corresponding to a precursor thereof, both being proteasomally generated OVA fragments, was delayed in TPPII-deficient cytosolic extracts. In addition, dendritic cell cross-presentation of phagocytosed OVA and of OVA internalized as an immune complex was increased to about the same level as direct presentation of the Ag. The data suggest a moderate, predominantly destructive role of TPPII in class I Ag processing, in line with our finding that TPPII is not induced by IFN-gamma, which up-regulates numerous, predominantly constructive components of the Ag processing and presentation machinery.

The Role of Endoplasmic Reticulum-Associated Aminopeptidase 1 in Immunity to Infection and in Cross-Presentation

Endoplasmic reticulum-associated aminopeptidase 1 (ERAP1) is involved in the final processing of endogenous peptides presented by MHC class I molecules to CTLs. We generated ERAP1-deficient mice and analyzed cytotoxic responses upon infection with three viruses, including lymphocytic choriomeningitis virus, which causes vigorous T cell activation and is controlled by CTLs. Despite pronounced effects on the presentation of selected epitopes, the in vivo cytotoxic response was altered for only one of several epitopes tested. Moreover, control of lymphocytic choriomeningitis virus was not impaired in the knockout mice. Thus, we conclude that lack of ERAP1 has little influence on antiviral immunohierarchies and antiviral immunity in the infections studied. We also focused on the role of ERAP1 in cross-presentation. We demonstrate that ERAP1 is required for efficient cross-presentation of cell-associated Ag and of OVA/anti-OVA immunocomplexes. Surprisingly, however, ERAP1 deficiency has no effect on cross-presentation of soluble OVA, suggesting that for soluble exogenous proteins, final processing may not take place in an environment containing active ERAP1.

Differential proteasomal processing of hydrophobic and hydrophilic protein regions: contribution to cytotoxic T lymphocyte epitope clustering in HIV-1-Nef

HIV proteins contain a multitude of naturally processed cytotoxic T lymphocyte (CTL) epitopes that concentrate in clusters. The molecular basis of epitope clustering is of interest for understanding HIV immunogenicity and for vaccine design. We show that the CTL epitope clusters of HIV proteins predominantly coincide with hydrophobic regions, whereas the noncluster regions are predominantly hydrophilic. Analysis of the proteasomal degradation products of full-length HIV-Nef revealed a differential sensitivity of cluster and noncluster regions to proteasomal processing. Compared with the epitope-scarce noncluster regions, cluster regions are digested by proteasomes more intensively and with greater preference for hydrophobic P1 residues, resulting in substantially greater numbers of fragments with the sizes and COOH termini typical of epitopes and their precursors. Indeed, many of these fragments correspond to endogenously processed Nef epitopes and/or their potential precursors. The results suggest that differential proteasomal processing contributes importantly to the clustering of CTL epitopes in hydrophobic regions.

Antitumor effect of the human immunodeficiency virus protease inhibitor ritonavir: induction of tumor-cell apoptosis associated with perturbation of proteasomal proteolysis

Ritonavir is an HIV protease inhibitor used in the therapy of HIV infection. Ritonavir has also been shown to inhibit the chymotrypsin-like activity of isolated 20S proteasomes. Here, we demonstrate that ritonavir, like classical proteasome inhibitors, has antitumoral activities. In vitro, ritonavir strongly reduced the rate of proliferation of several tumor cell lines and induced their apoptosis. Nontransformed cell lines and terminally differentiated bone-marrow macrophages were comparatively resistant to the apoptosis-inducing effect. In vivo, ritonavir, administered p.o. for a week at doses of 6-8.8 mg/mouse/day, caused significant growth inhibition (76-79% after 7 days of treatment) of established EL4-T cell thymomas growing s.c. in syngeneic C57BL/6 mice. Unexpectedly, we found that ritonavir activates the chymotrypsin-like activity of isolated 26S proteasomes, in strong contrast to its effect on isolated 20S proteasomes. The net effect of low micromolar concentrations of ritonavir on the chymotrypsin-like activity in cells and cell lysates was a weak inhibition, consistent with marginal alterations of polyubiquitinated proteins, marginal alterations in acid-soluble proteolytic peptide levels, and a small accumulation of the tumor suppressor protein p53, in cells treated with ritonavir. In contrast, we found a relatively strong accumulation of the cyclin-dependent kinase inhibitor p21(WAF-1), a sign of deregulation of cell-cycle progression typical for apoptosis induction in transformed cells by classical proteasome inhibitors. We demonstrate that p21 accumulation in the presence of ritonavir is attributable to the inhibition of proteolytic degradation. Accumulation of p21 most likely reflects a selective inhibition of proteasomes, in line with the atypical degradation of p21, which does not require ubiquitination. These findings suggest that selective perturbation of proteasomal protein degradation may play a role in the antitumoral activities of ritonavir.