CTL-Dependent and -Independent Antitumor Immunity Is Determined by the Tumor Not the Vaccine

Combined vaccination and immunostimulatory antibodies provides durable cure of murine melanoma and induces transcriptional changes associated with positive outcome in human melanoma patients

We have developed a recombinant adenovirus vaccine encoding dopachrome tautomerase (rHuAd5-hDCT) that produces robust DCT-specific immunity, but only provides modest suppression of murine melanoma. In the current study, an agonist antibody against 4-1BB was shown to enhance rHuAd5-hDCT efficacy and evoke tumor regression, but most tumors ultimately relapsed. The vaccine triggered upregulation of the immune inhibitory PD-1 signaling pathway and PD-1 blockade dramatically enhanced the rHuAd5-hDCT + anti-4-1BB strategy, resulting in complete regression of growing tumors in > 70% of recipients. The impact of the combined anti-4-1BB/anti-PD-1 treatment did not manifest as a dramatic enhancement in either the magnitude or functionality of DCT-specific tumor infiltrating lymphocytes relative to either treatment alone. Rather, a synergistic enhancement in intratumoral cytokine expression was observed, suggesting that the benefit of the combined therapy was a local event within the tumor. Global transcriptional analysis revealed immunological changes within the tumor following the curative vaccination, which extended beyond the T cell compartment. We identified an immune signature of 85 genes associated with clearance of murine melanoma that correlated with improved survival outcome in two independent cohorts of human melanoma patients. Our data reinforce the concept that successful vaccination must overcome local hurdles in the tumor microenvironment that are not manifest within the periphery. Further, tumor rejection following vaccination involves more than simply T cells. Finally, the association of our immune signature with positive survival outcome in human melanoma patients suggests that similar vaccination strategies may be promising for melanoma treatment.

Inferring the Impact of Regulatory Mechanisms that Underpin CD8+ T Cell Control of B16 Tumor Growth In vivo Using Mechanistic Models and Simulation

A major barrier for broadening the efficacy of immunotherapies for cancer is identifying key mechanisms that limit the efficacy of tumor infiltrating lymphocytes. Yet, identifying these mechanisms using human samples and mouse models for cancer remains a challenge. While interactions between cancer and the immune system are dynamic and non-linear, identifying the relative roles that biological components play in regulating anti-tumor immunity commonly relies on human intuition alone, which can be limited by cognitive biases. To assist natural intuition, modeling and simulation play an emerging role in identifying therapeutic mechanisms. To illustrate the approach, we developed a multi-scale mechanistic model to describe the control of tumor growth by a primary response of CD8+ T cells against defined tumor antigens using the B16 C57Bl/6 mouse model for malignant melanoma. The mechanistic model was calibrated to data obtained following adenovirus-based immunization and validated to data obtained following adoptive transfer of transgenic CD8+ T cells. More importantly, we use simulation to test whether the postulated network topology, that is the modeled biological components and their associated interactions, is sufficient to capture the observed anti-tumor immune response. Given the available data, the simulation results also provided a statistical basis for quantifying the relative importance of different mechanisms that underpin CD8+ T cell control of B16F10 growth. By identifying conditions where the postulated network topology is incomplete, we illustrate how this approach can be used as part of an iterative design-build-test cycle to expand the predictive power of the model.

CD8(+) T cell response to adenovirus vaccination and subsequent suppression of tumor growth: modeling, simulation and analysis

BACKGROUND

Using immune checkpoint modulators in the clinic to increase the number and activity of cytotoxic T lymphocytes that recognize tumor antigens can prolong survival for metastatic melanoma. Yet, only a fraction of the patient population receives clinical benefit. In short, these clinical trials demonstrate proof-of-principle but optimizing the specific therapeutic strategies remains a challenge. In many fields, CAD (computer-aided design) is a tool used to optimize integrated system behavior using a mechanistic model that is based upon knowledge of constitutive elements. The objective of this study was to develop a predictive simulation platform for optimizing anti-tumor immunity using different treatment strategies.

METHODS

To better understand the therapeutic role that cytotoxic CD8(+) T cells can play in controlling tumor growth, we developed a multi-scale mechanistic model of the biology using impulsive differential equations and calibrated it to a self-consistent data set.

RESULTS

The multi-scale model captures the activation and differentiation of naïve CD8(+) T cells into effector cytotoxic T cells in the lymph node following adenovirus-mediated vaccination against a tumor antigen, the trafficking of the resulting cytotoxic T cells into blood and tumor microenvironment, the production of cytokines within the tumor microenvironment, and the interactions between tumor cells, T cells and cytokines that control tumor growth. The calibrated model captures the modest suppression of tumor cell growth observed in the B16F10 model, a transplantable mouse model for metastatic melanoma, and was used to explore the impact of multiple vaccinations on controlling tumor growth.

CONCLUSIONS

Using the calibrated mechanistic model, we found that the cytotoxic CD8(+) T cell response was prolonged by multiple adenovirus vaccinations. However, the strength of the immune response cannot be improved enough by multiple adenovirus vaccinations to reduce tumor burden if the cytotoxic activity or local proliferation of cytotoxic T cells in response to tumor antigens is not greatly enhanced. Overall, this study illustrates how mechanistic models can be used for in silico screening of the optimal therapeutic dosage and timing in cancer treatment.

Immunotherapy-induced CD8+ T cells instigate immune suppression in the tumor

Despite clear evidence of immunogenicity, cancer vaccines only provide a modest clinical benefit. To evaluate the mechanisms that limit tumor regression following vaccination, we have investigated the weak efficacy of a highly immunogenic experimental vaccine using a murine melanoma model. We discovered that the tumor adapts rapidly to the immune attack instigated by tumor-specific CD8+ T cells in the first few days following vaccination, resulting in the upregulation of a complex set of biological networks, including multiple immunosuppressive processes. This rapid adaptation acts to prevent sustained local immune attack, despite continued infiltration by increasing numbers of tumor-specific T cells. Combining vaccination with adoptive transfer of tumor-specific T cells produced complete regression of the treated tumors but did not prevent the adaptive immunosuppression. In fact, the adaptive immunosuppressive pathways were more highly induced in regressing tumors, commensurate with the enhanced level of immune attack. Examination of tumor infiltrating T-cell functionality revealed that the adaptive immunosuppression leads to a progressive loss in T-cell function, even in tumors that are regressing. These novel observations that T cells produced by therapeutic intervention can instigate a rapid adaptive immunosuppressive response within the tumor have important implications for clinical implementation of immunotherapies.

CD4 T cell dependent tumor immunity stimulated by dendritic cell based vaccine

CD8 CTLs have been accountable for the major effector cells responsible for the rejection of tumor cells. And CD40 signaling and IL-12 have been shown to be the essential pathways involved in the activation process. Immunizing mice with dendritic cells transduced with an adenovirus expressing the human melanoma antigen gp 100, an immunization strategy of xenoimmunization, stimulated potent tumor protection dependent on effective CD4 T cells in the absence of CD8 T cells. Further studies revealed that neither CD40 signaling nor IL-12 was indispensable for the activation of dendritic and CD4 T cells in this model. Stimulation of effective antitumor immunity targeting the self-antigen did not elicit autoimmunity. The implications of this study were discussed.

Potent antitumor immunity generated by a CD40-targeted adenoviral vaccine

In situ delivery of tumor-associated antigen (TAA) genes into dendritic cells (DC) has great potential as a generally applicable tumor vaccination approach. Although adenoviruses (Ad) are an attractive vaccine vehicle in this regard, Ad-mediated transduction of DCs is hampered by the lack of expression of the Ad receptor CAR on the DC surface. DC activation also requires interaction of CD40 with its ligand CD40L to generate protective T-cell-mediated tumor immunity. Therefore, to create a strategy to target Ads to DCs in vivo, we constructed a bispecific adaptor molecule with the CAR ectodomain linked to the CD40L extracellular domain via a trimerization motif (CFm40L). By targeting Ad to CD40 with the use of CFm40L, we enhanced both transduction and maturation of cultured bone marrow-derived DCs. Moreover, we improved transduction efficiency of DCs in lymph node and splenic cell suspensions in vitro and in skin and vaccination site-draining lymph nodes in vivo. Furthermore, CD40 targeting improved the induction of specific CD8(+) T cells along with therapeutic efficacy in a mouse model of melanoma. Taken together, our findings support the use of CD40-targeted Ad vectors encoding full-length TAA for in vivo targeting of DCs and high-efficacy induction of antitumor immunity.

Adjuvant effect of HER-2/neu-specific adenoviral vector stimulating CD8⁺ T and natural killer cell responses on anti-HER-2/neu antibody therapy for well-established breast tumors in HER-2/neu transgenic mice

Approximately one third of patients with advanced human epidermal growth factor receptor 2 (HER-2)/neu-positive breast cancer respond to trastuzumab monotherapy, a humanized anti-HER-2/neu antibody. However, de novo and acquired antibody resistance is one of the major limitations of trastuzumab therapy warranting the search for other therapeutic strategies. One of the most remarkable features of adenovirus (AdV)-based vaccine is its ability to induce exceptionally high and sustained frequencies of transgene product-specific CD8(+) T-cell responses. In this study, we constructed two recombinant AdVs (AdV(OVA) and AdV(HER-2)) expressing ovalbumin (OVA) and HER-2/neu, and assessed AdV-induced antigen-specific cellular immune responses and preventive/therapeutic antitumor immunity. We demonstrate that AdV(OVA) stimulates efficient OVA-specific CD8(+) cytotoxic T lymphocyte (CTL) and natural killer responses, leading to preventive long-term immunity against OVA-expressing BL6-10ova melanoma in wild-type C56BL/6 mice. We further demonstrate that AdV(HER-2) stimulates HER-2/neu-specific CD8(+) CTL responses, leading to a significant reduction in breast carcinogenesis in transgenic FVBneuN mice (P<0.05), but has little therapeutic effect on pre-existing Tg1-1 tumor even at early stage (15 mm(3)). In contrast, the anti-HER-2/neu antibody therapy is capable of completely inhibiting Tg1-1 tumor growth at early stage, but fails to eradicate well-established Tg1-1 breast tumor (100 mm(3)). Interestingly, a combinatorial immunotherapy of anti-HER-2/neu antibody with AdV(HER-2) vaccine was capable of curing 4 of 10 studied mice bearing well-established Tg1-1 breast tumors and significantly delaying in death of the remaining six tumor-bearing mice (P<0.05). Taken together, our results suggest an adjuvant effect of AdV(HER-2) on anti-HER-2/neu antibody therapy for well-established breast tumor in transgenic FVBneuN mice, and this combinatorial immunotherapy of trastuzumab with AdV(HER-2) vaccine may be used as a new therapeutic strategy for treatment of advanced HER-2/neu-positive breast cancer.

Modeling anti-tumor Th1 and Th2 immunity in the rejection of melanoma

Recent experiments indicate that CD4(+) Th2 cells can reject skin tumors in mice, while CD4(+) Th1 cells cannot (Mattes et al., 2003; Zhang et al., 2009). These results are surprising because CD4(+) Th1 cells are typically considered to be capable of tumor rejection. We used mathematical models to investigate this unexpected outcome. We found that neither CD4(+) Th1 nor CD4(+) Th2 cells could eliminate the cancer cells when acting alone, but that tumor elimination could be induced by recruitment of eosinophils by the Th2 cells. These recruited eosinophils had unexpected indirect effects on the decay rate of type 2 cytokines and the rate at which Th2 cells are inactivated through interactions with cancer cells. Strikingly, the presence of eosinophils impacted tumor growth more significantly than the release of tumor-suppressing cytokines such as IFN-gamma and TNF-alpha. Our simulations suggest that novel strategies to enhance eosinophil recruitment into skin tumors may improve cancer immunotherapies.

Immunotherapy can reject intracranial tumor cells without damaging the brain despite sharing the target antigen

Although vaccines targeting tissue differentiation Ags represent a promising strategy for cancer immunotherapy, the risk of triggering autoimmune damage to normal tissues remains to be determined. Immunizing against a melanoma-associated Ag, dopachrome tautomerase (DCT), which normal melanocytes and glial cells also express, allowed concurrent analysis of autoimmune consequences in multiple tissues. We show that vaccination with recombinant adenovirus expressing DCT elicited a strong CTL response in C57BL/6 mice, leading to protection against intracranial challenge with B16-F10 melanoma cells. Both histological analysis and behavioral testing indicated that there was no evidence of neuropathology in vaccinated animals and long-term survivors. Although vitiligo or demyelination could be induced by additional stimuli (i.e., surgery or inflammation) in DCT-vaccinated mice, it did not extend beyond the inflammatory area, suggesting that there is self-regulatory negative feedback in normal tissues. These results demonstrate that it is possible to vaccinate against a tumor embedded in a vital organ that shares the target Ag.

Processing of tumor antigen differentially impacts the development of helper and effector CD4+ T-cell responses

CD4(+) T cells contribute to the antitumor T-cell response as both effectors that promote tumor rejection and helpers that facilitate the activation of other antitumor effector cells, such as CD8(+) T cells. Maximal engagement of both effector and helper CD4(+) T-cell responses is a desirable attribute of cancer vaccines. We have employed the B16F10 murine melanoma model and a series of recombinant adenovirus (Ad) vaccines expressing mutant forms of the tumor antigen, dopachrome tautomerase, to investigate the relationship between antigen processing and the antitumor CD4(+) T-cell response. Our results have revealed an unexpected dichotomy in the generation of helper and effector CD4(+) T-cell responses where CD4(+) T effector responses are dependent upon protein processing and trafficking, whereas CD4(+) T helper responses are not. The results have important implications for strategies aimed at augmenting antigen immunogenicity by altering intracellular processing and localization.

Vesicular stomatitis virus as a novel cancer vaccine vector to prime antitumor immunity amenable to rapid boosting with adenovirus

Vesicular stomatitis virus (VSV) has proven to be an effective vaccine vector for immunization against viral infection, but its potential to induce an immune response to a self-tumor antigen has not been investigated. We constructed a recombinant VSV expressing human dopachrome tautomerase (hDCT) and evaluated its immunogenicity in a murine melanoma model. Intranasal delivery of VSV-hDCT activated both CD4(+) and CD8(+) DCT-specific T-cell responses. The magnitude of these responses could be significantly increased by booster immunization with recombinant adenovirus (Ad)-hDCT, which led to enhanced efficacy against B16-F10 melanoma in both prophylactic and therapeutic settings. Notably, the interval of VSV/Ad heterologous vaccination could be shortened to as few as 4 days, making it a potential regimen to rapidly expand antigen-specific effector cells. Furthermore, VSV-hDCT could increase DCT-specific T-cell responses primed by Ad-hDCT, suggesting VSV is efficient for both priming and boosting of the immune response against a self-tumor antigen.

CD4+ T-cell-mediated anti-tumor immunity can be uncoupled from autoimmunity via the STAT4/STAT6 signaling axis

Previous reports have suggested that autoimmune sequelae may be an unavoidable consequence of successful immunization against tumor-associated antigens, which are typically non-mutated self-antigens. Using a melanoma model, we demonstrated that CD4(+) T-cell-mediated anti-tumor immunity and autoimmunity could be separated by modulating the STAT4/STAT6 signaling axis. Our results have revealed an unexpected dichotomy in the effector phase following cancer vaccination where anti-tumor immunity is mediated via a STAT6 and IL-4-dependent pathway, whereas autoimmune pathology is mediated via STAT4 through a mechanism that relies partially on IFN-gamma. Our results offer a possibility to elicit specific anti-tumor responses without triggering unwanted tissue autoimmune diseases.

Neoadjuvant vaccination provides superior protection against tumor relapse following surgery compared with adjuvant vaccination

Tumors that recur following surgical resection of melanoma are typically metastatic and associated with poor prognosis. Using the murine B16F10 melanoma and a robust antimelanoma vaccine, we evaluated immunization as a tool to improve tumor-free survival following surgery. We investigated the utility of vaccination in both neoadjuvant and adjuvant settings. Surprisingly, neoadjuvant vaccination was far superior and provided approximately 100% protection against tumor relapse. Neoadjuvant vaccination was associated with enhanced frequencies of tumor-specific T cells within the tumor and the tumor-draining lymph nodes following resection. We also observed increased infiltration of antigen-specific T cells into the area of surgery. This method should be amenable to any vaccine platform and can be readily extended to the clinic.

High-dose chemotherapy augments the efficacy of recombinant adenovirus vaccines and improves the therapeutic outcome

We have investigated the therapeutic potential of a prototypic melanoma vaccine based on recombinant adenovirus expressing human dopachrome tautomerase in the B16F10 murine melanoma model. We found that in the presence of a tumor, the magnitude of T-cell immunity evoked by the vaccine was significantly reduced. This impairment was compounded by defects in cytokine production and degranulation within the tumor-infiltrating lymphocytes (TILs). We showed that the combination of vaccination with high-dose cyclophosphamide was able to skew the response toward the target antigen and enhanced both the quantity and quality of antigen-specific CD8+ and CD4+ T-cell responses in tumor-bearing mice, which resulted in the inhibition of tumor growth. Furthermore, when tumor-specific antigens were targeted by the vaccine, the combination therapy could actually produce tumor regression, which appeared to result from the high frequency of antigen-specific T cells. These data show that recombinant adenovirus vaccines are compatible with conventional high-dose chemotherapy and that the combined treatment results in improved therapeutic outcomes relative to either agent individually.

Delivery strategies of melanoma vaccines: an overview

For many years, various cancer vaccines have been widely evaluated, however clinical responses remain rare. In this review, we attempt to address the question of which delivery strategies and platforms are feasible to produce clinical response and define the characteristics of the strategy that will induce long-lasting antitumor response. We limit our analysis and discussion to microparticles/nanoparticles, liposomes, heat-shock proteins, viral vectors and different types of adjuvants. This review aims to provide an overview of the specific characteristics, strengths and limitations of these delivery systems, focusing on their impacts on the development of melanoma vaccine. To date, only adoptive T-cell transfer has shown promising clinical outcomes compared to other treatments.

Gentherapie

Die Gentherapie ist eine junge Wissenschaft, die Nukleinsäuren zur Therapie einsetzt (Hengge u. Bardenheuer 2004). Die somatische Gentherapie befasst sich mit der Behandlung von somatischen (Körper-)Zellen (⧁ Tab. 4.1.1), wobei das therapeutische Gen ein im Organismus benötigtes Protein kodiert.

Enhanced antitumor immunity elicited by dendritic cell vaccines is a result of their ability to engage both CTL and IFN gamma-producing NK cells

Activation of cytotoxic T lymphocytes (CTLs) is a primary goal of many cancer vaccination therapies. We have evaluated two vaccination platforms, dendritic cells (DCs) and recombinant adenoviruses (rAds), for their ability to elicit CTL response and antitumor protection. Although rAd was more potent in CTL priming, DC vaccination provided greater protective and therapeutic antitumor activity. Subsequent analyses ruled out the possibility that the two vaccines elicit qualitatively distinct CTL, and demonstrated instead that DCs could better engage natural killer (NK) cells as an additional effector mechanism. We demonstrated that, although both DCs and rAd can stimulate rapid NK expansion, only DC-activated NK cells are able to produce interferon-gamma (IFN gamma) and mediate antitumor protection. Moreover, our data showed that exogenously delivered DCs preferentially engaged the Mac-1(high)CD27(high) NK subset, thereby suggesting that this NK population plays a predominant role in NK:DC interaction. Interestingly, at least 3 days were required for DC-triggered NK cells to acquire effector functions, indicating that a similar priming process operates between T cells and NK cells. Our results suggest that the nature of the vaccine platform can determine the relative involvement of NK and T cells in antitumor immunity, and that methods to augment NK function should be included in vaccination strategies in order to complement CTL-mediated control of tumor growth.

Immune escape associated with functional defects in antigen-processing machinery in head and neck cancer

Recent revival of interest in the role of immune surveillance in the pathogenesis and control of malignant diseases has focused attention on escape mechanisms used by tumor cells to evade immune recognition. Defects in the host's tumor antigen-specific immune responses and abnormalities in tumor cell expression of HLA class I molecules and tumor antigen are known to contribute to tumor progression. However, the mechanism(s) responsible for the lack of tumor cell recognition by functional HLA class I antigen-restricted, tumor antigen-specific CTLs despite expression of the restricting HLA class I allele and targeted tumor antigen by tumor cells remain(s) unexplained. In squamous cell carcinomas of the head and neck (SCCHN), this type of tumor escape is a rule rather than the exception. Here, we discuss evidence pointing to functional defects in the antigen-processing machinery as one mechanism underlying resistance of SCCHN cells to recognition and lysis by HLA class I antigen-restricted, tumor antigen-specific CTL. In addition, based on the restoration by IFN-gamma of SCCHN cell sensitivity to recognition by these CTL, we suggest strategies that may improve the clinical course of the disease by enhancing susceptibility of malignant cells to immune recognition.

GM-CSF-secreting cancer immunotherapies: preclinical analysis of the mechanism of action

Granulocyte-macrophage colony-stimulating factor (GM-CSF)-secreting tumor cell immunotherapies have demonstrated long-lasting, and specific anti-tumor immune responses in animal models. The studies reported here specifically evaluate two aspects of the immune response generated by such immunotherapies: the persistence of irradiated tumor cells at the immunization site, and the breadth of the immune response elicited to tumor associated antigens (TAA) derived from the immunotherapy. To further define the mechanism of GM-CSF-secreting cancer immunotherapies, immunohistochemistry studies were performed using the B16F10 melanoma tumor model. In contrast to previous reports, our data revealed that the irradiated tumor cells persisted and secreted high levels of GM-CSF at the injection site for more than 21 days. Furthermore, dense infiltrates of dendritic cells were observed only in mice treated with GM-CSF-secreting B16F10 cells, and not in mice treated with unmodified B16F10 cells with or without concurrent injection of rGM-CSF. In addition, histological studies also revealed enhanced neutrophil and CD4+ T cell infiltration, as well as the presence of apoptotic cells, at the injection site of mice treated with GM-CSF-secreting tumor cells. To evaluate the scope of the immune response generated by GM-CSF-secreting cancer immunotherapies, several related B16 melanoma tumor cell subclones that exist as a result of genetic drift in the original cell line were used to challenge mice previously immunized with GM-CSF-secreting B16F10 cells. These studies revealed that GM-CSF-secreting cancer immunotherapies elicit T cell responses that effectively control growth of related but antigenically distinct tumors. Taken together, these studies provide important new insights into the mechanism of action of this promising novel cancer immunotherapy.

TAP expression reduces IL-10 expressing tumor infiltrating lymphocytes and restores immunosurveillance against melanoma

Many immune therapeutic strategies are under development for melanoma to treat metastatic disease and prevent disease reoccurrence. However, human melanoma cells are often deficient in antigen processing and this appears to play a role in their expansion and escape from immunosurveillance. For example, expression of the transporters associated with antigen processing (TAP1 and TAP2) is down‐regulated in the mouse melanoma cell line B16F10. This results in a lack of tumor‐associated antigen processing, low surface expression of MHC Class I molecules and low immunogenicity. We observe that restoration of TAP1 expression by transfection resurrects the processing and presentation of viral antigens, and the melanoma‐associated antigen, TRP‐2. Immunization with irradiated B16F10/rTAP1 transfected cells generates CTLs that are capable of killing B16F10/rTAP1 transfected targets and B16F10 targets deficient in TAP1. Furthermore, B16F10/rTAP1 transfectants grow at a significantly slower rate in mice than B16F10 cells. In an experimental model that closely recapitulates the clinical situation, treatment of B16F10 tumors in mice with a vaccinia virus vector expressing TAP1 also significantly decreases tumor growth in vivo. Furthermore, tumors treated with vaccinia TAP1 had significantly reduced numbers of immunosuppressive, CD3⁺/IL‐10 positive, tumor infiltrating lymphocytes. Therefore, TAP1 expression restores both antigen presentation and immunogenicity in B16F10 melanoma cells and concomitantly reduces immunosuppressive IL‐10 production at the local tumor site, thereby increasing immunosurveillance mechanisms against tumors. © 2007 Wiley‐Liss, Inc.

Attenuation of the Glucocorticoid Response during Ad5IL-12 Adenovirus Vector Treatment Enhances Natural Killer Cell–Mediated Killing of MHC Class I–Negative LNCaP Prostate Tumors

Tumor cells can evolve to evade immune responses by down-modulating surface MHC class I expression and become refractory to T cell-directed immunotherapy. We employed a strategy to bypass this escape mechanism using a recombinant adenovirus vector expressing interleukin-12 (Ad5IL-12) to target natural killer (NK) cell-mediated killing of human prostate tumors in NOD.scid mice. Fluorescence-activated cell sorting analysis revealed that LNCaP tumor cells bear negligible levels of MHC class I molecules; yet, they express MICA/B molecules, ligands for the NKG2D receptors found on NK cells. Transduction of LNCaP cells with the Ad5IL-12 vector prevented tumor formation in NOD.scid mice, indicating that NK cells alone can conduct tumor immunosurveillance and mediate protection. Intratumor injection of the Ad5IL-12 vector to established LNCaP tumors in NOD.scid mice resulted in a significant delay of tumor growth mediated by NK cell killing activity. The dependency of NK cells in this protective response was shown by the complete loss of Ad5IL-12 therapeutic efficacy on LNCaP tumors established in NOD.Cg-Rag1(tm1Mom)Prf1(tm1Sdz) congenic mice, which are devoid of NK cell activity. More pronounced attenuation of tumor growth and enhanced NK killing activity was observed when pharmacologic adrenalectomy with mitotane was done in combination with Ad5IL-12 vector treatment. The Ad5IL-12 vector treatment also induced killing of MICA/B-negative MHC class I-positive PC3 tumors formed in NOD.scid mice. Together, these results indicate that a targeted NK cell response could provide a generic approach for cancer immunotherapy, and that enhancing the NK cell response via control of cortisol levels may provide an additional therapeutic avenue in cancer.

Recombinant Adenovirus Vaccines Can Successfully Elicit CD8+ T Cell Immunity under Conditions of Extreme Leukopenia

We have examined the efficacy of vaccination with recombinant adenovirus under conditions of extreme leukopenia in lethally irradiated mice reconstituted with autologous bone marrow. The expansion of antigen-specific CD8(+) T cells following immunization of lethally irradiated hosts paralleled the recovery of total CD8(+) T cells. Surprisingly, the numbers of antigen-specific CD8(+) T cells in lethally irradiated mice beyond 6 weeks postimmunization were comparable to the numbers found in nonirradiated controls. CD8(+) T cells elicited in the lethally irradiated hosts were functionally indistinguishable from those elicited in normal hosts. Antigen expression and presentation persisted for a longer period of time in the draining lymph nodes of irradiated mice compared to those of nonirradiated animals, suggesting that antigen presentation mechanisms were intact during the reconstitution period. Experiments employing allogeneic bone marrow demonstrated that radioresistant host antigen-presenting cells were responsible for antigen presentation during the process of immune reconstitution. These results demonstrate clear compatibility of adenovirus vaccines and cytotoxic therapy. Furthermore, these observations provide novel insights into the mechanisms of CD8(+) T cell activation following adenovirus immunization.