Solid tumors “melt” from the inside after successful CD8 T cell attack

IFN-γ-dependent tumor-antigen cross-presentation by lymphatic endothelial cells promotes their killing by T cells and inhibits metastasis

Tumor-associated lymphatic vessels promote metastasis and regulate antitumor immune responses. Here, we assessed the impact of cytotoxic T cells on the local lymphatic vasculature and concomitant tumor dissemination during an antitumor response. Interferon-γ (IFN-γ) released by effector T cells enhanced the expression of immunosuppressive markers by tumor-associated lymphatic endothelial cells (LECs). However, at higher effector T cell densities within the tumor, T cell-based immunotherapies induced LEC apoptosis and decreased tumor lymphatic vessel density. As a consequence, lymphatic flow was impaired, and lymph node metastasis was reduced. Mechanistically, T cell-mediated tumor cell death induced the release of tumor antigens and cross-presentation by tumor LECs, resulting in antigen-specific LEC killing by T cells. When LECs lacked the IFN-γ receptor expression, LEC killing was abrogated, indicating that IFN-γ is indispensable for reducing tumor-associated lymphatic vessel density and drainage. This study provides insight into how cytotoxic T cells modulate tumor lymphatic vessels and may help to improve immunotherapeutic protocols.

The solid tumor microenvironment-Breaking the barrier for T cells: How the solid tumor microenvironment influences T cells: How the solid tumor microenvironment influences T cells

The tumor microenvironment (TME) plays a pivotal role in the behavior and development of solid tumors as well as shaping the immune response against them. As the tumor cells proliferate, the space they occupy and their physical interactions with the surrounding tissue increases. The growing tumor tissue becomes a complex dynamic structure, containing connective tissue, vascular structures, and extracellular matrix (ECM) that facilitates stimulation, oxygenation, and nutrition, necessary for its fast growth. Mechanical cues such as stiffness, solid stress, interstitial fluid pressure (IFP), matrix density, and microarchitecture influence cellular functions and ultimately tumor progression and metastasis. In this fight, our body is equipped with T cells as its spearhead against tumors. However, the altered biochemical and mechanical environment of the tumor niche affects T cell efficacy and leads to their exhaustion. Understanding the mechanobiological properties of the TME and their effects on T cells is key for developing novel adoptive tumor immunotherapies.

Revisiting the role of T cells in tumor regression

This note challenges the current idea that a key role of T cells in tumor regression is to directly kill tumor cells. It favors the view that TIL are keys but act indirectly by helping other immune cells to damage the tumor and its stroma.

Contact-Dependent Killing by Cytotoxic T Lymphocytes Is Insufficient for EL4 Tumor Regression In Vivo

Immunotherapies are an emerging strategy for treatment of solid tumors. Improved understanding of the mechanisms employed by cytotoxic T lymphocytes (CTL) to control tumors will aid in the development of immunotherapies. CTLs can directly kill tumor cells in a contact-dependent manner or may exert indirect effects on tumor cells via secretion of cytokines. Here, we aim to quantify the importance of these mechanisms in murine thymoma EL4/EG7 cells. We developed an agent-based model (ABM) and an ordinary differential equation model of tumor regression after adoptive transfer of a population of CTLs. Models were parameterized based on in vivo measurements of CTL infiltration and killing rates applied to EL4/EG7 tumors and OTI T cells. We quantified whether infiltrating CTLs are capable of controlling tumors through only direct, contact-dependent killing. Both models agreed that the low measured killing rate of CTLs in vivo was insufficient to cause tumor regression. In our ABM, we also simulated CTL production of the cytokine IFNγ in order to explore how an antiproliferative effect of IFNγ might aid CTLs in tumor control. In this model, IFNγ substantially reduced tumor growth compared with direct killing alone. Collectively, these data demonstrate that contact-dependent killing is insufficient for EL4 regression in vivo and highlight the potential importance of cytokine-induced antiproliferative effects in T-cell-mediated tumor control. SIGNIFICANCE: Computational modeling highlights the importance of cytokine-induced antiproliferative effects in T-cell-mediated control of tumor progression.

Combination GITR targeting/PD-1 blockade with vaccination drives robust antigen-specific antitumor immunity

Tumor progression is facilitated immunologically by mechanisms that include low antigen expression, an absence of coimmunostimulatory signals, and the presence of regulatory T cells (Tregs), all of which act to suppress and restrict effector T cells in the tumor. It may be possible to overcome these conditions by a combination of modulatory immunotherapy agents and tumor-antigen targeting to activate and drive effective antitumor T cell responses. Here, we demonstrated that co-administration of aGITR and aPD-1 monoclonal antibodies (mAb) in combination with a peptide vaccine (Vax) in mice bearing established tumors significantly delayed tumor growth and induced complete regression in 50% of the mice. This response was associated with increased expansion and functionality of potent Ag-specific polyfunctional CD8⁺ T cells, reduced Tregs, and the generation of memory T cells. Tumor regression correlated with the expansion of tumor-infiltrating antigen-specific CD8⁺ effector memory T cells, as depletion of this cell population significantly reduced the effectiveness of the triple combination Vax/aGITR/aPD-1 therapy. These findings support the concept that dual aGITR/aPD-1 combination with cancer vaccines may be a novel strategy against poorly immunogenic tumors.

Permissive expansion and homing of adoptively transferred T cells in tumor-bearing hosts

Activated T cells expressing endogenous or transduced TCRs are two cell types currently used in clinical adoptive T-cell therapy. The ability of these cells to recognize their antigen, expand and traffic to the tumor site are the initial steps necessary for successful therapy. In this study, we used in vivo bioluminescent imaging (BLI) of Renilla luciferase (RLuc) expressing T cells to evaluate the ability of adoptively transferred T cells to survive, expand and home to tumor site in vivo. Using this method, termed RT-Rack (Rluc T cell tracking), we followed T-cell response against tumors in vivo. Expansion and homing of adoptively transferred T cells were antigen dependent, but independent of the host immune status. Moreover, we successfully detected T-cell response to small and large tumors, including autochthonous liver tumors. The adoptively transferred T cells were not ignorant or excluded in a partially tolerant host, which expressed low level of the target in the periphery. Using T cell receptor (TCR)-engineered T cells, we showed the ability of these cells to respond in tumor-bearing hosts by expanding and homing to the tumor site. In all these models, the host immune status, the nature of the tumor or of the antigen, the tumor size and the presence of the targeted antigen in the periphery did not prevent the adoptively transferred T cells from responding by expanding and homing to the tumor. However, T cells had higher expression of the inhibitory receptor PD1 and reduced functional activity when a self-antigen was targeted.

Expression of EPHRIN-A1, SCINDERIN and MHC class I molecules in head and neck cancers and relationship with the prognostic value of intratumoral CD8+ T cells

Background

Our group has previously shown that EPHRIN-A1 and SCINDERIN expression by tumor cells rendered them resistant to cytotoxic T lymphocyte-mediated lysis. Whereas the prognostic value of EPHRIN-A1 expression in cancer has already been studied, the role of SCINDERIN presence remains to be established. In the present work, we investigated the prognosis value of EPHRIN-A1 and SCINDERIN expression in head and neck carcinomas. In addition, we monitored the HLA-class I expression by tumor cells and the presence of tumor-infiltrating CD8⁺ T cells to evaluate a putative correlation between these factors and the survival prognosis by themselves or related to EPHRIN-A1 and SCINDERIN expression.

Methods

Tumor tissue sections of 83 patients with head and neck cancer were assessed by immunohistochemistry for the expression of EPHRIN-A1, SCINDERIN, HLA class I molecules and the presence of CD8⁺ T cells.

Results

No significant prognosis value could be attributed to these factors independently, despite a tendency of association between EPHRIN-A1 and a worse clinical outcome. No prognostic value could be observed when CD8⁺ T cell tumor infiltration was analyzed combined with EPHRIN-A1, SCINDERIN or HLA class I expression.

Conclusion

These results highlight that molecules involved in cancer cell resistance to cytotoxic T lymphocytes by themselves are not a sufficient criteria for prognosis determination in cancer patients. Other intrinsic or tumor microenvironmental features should be considered in prognostic evaluation.

Longitudinal confocal microscopy imaging of solid tumor destruction following adoptive T cell transfer

A fluorescence-based, high-resolution imaging approach was used to visualize longitudinally the cellular events unfolding during T cell-mediated tumor destruction. The dynamic interplay of T cells, cancer cells, cancer antigen loss variants, and stromal cells-all color-coded in vivo-was analyzed in established, solid tumors that had developed behind windows implanted on the backs of mice. Events could be followed repeatedly within precisely the same tumor region-before, during and after adoptive T cell therapy-thereby enabling for the first time a longitudinal in vivo evaluation of protracted events, an analysis not possible with terminal imaging of surgically exposed tumors. T cell infiltration, stromal interactions, and vessel destruction, as well as the functional consequences thereof, including the elimination of cancer cells and cancer cell variants were studied. Minimal perivascular T cell infiltrates initiated vascular destruction inside the tumor mass eventually leading to macroscopic central tumor necrosis. Prolonged engagement of T cells with tumor antigen-crosspresenting stromal cells correlated with high IFNγ cytokine release and bystander elimination of antigen-negative cancer cells. The high-resolution, longitudinal, in vivo imaging approach described here will help to further a better mechanistic understanding of tumor eradication by T cells and other anti-cancer therapies.

Comparison of vaccine-induced effector CD8 T cell responses directed against self- and non-self-tumor antigens: implications for cancer immunotherapy

It is generally accepted that CD8 T cells play a major role in tumor control, yet vaccination aimed at eliciting potent CD8 T cell responses are rarely efficient in clinical trials. To try and understand why this is so, we have generated potent adenoviral vectors encoding the endogenous tumor Ags (TA) tyrosinase-related protein-2 (TRP-2) and glycoprotein 100 (GP100) tethered to the invariant chain (Ii). Using these vectors, we sought to characterize the self-TA-specific CD8 T cell response and compare it to that induced against non-self-Ags expressed from a similar vector platform. Prophylactic vaccination with adenoviral vectors expressing either TRP-2 (Ad-Ii-TRP-2) or GP100 (Ad-Ii-GP100) had little or no effect on the growth of s.c. B16 melanomas, and only Ad-Ii-TRP-2 was able to induce a marginal reduction of B16 lung metastasis. In contrast, vaccination with a similar vector construct expressing a foreign (viral) TA induced efficient tumor control. Analyzing the self-TA-specific CD8 T cells, we observed that these could be activated to produce IFN-γ and TNF-α. In addition, surface expression of phenotypic markers and inhibitory receptors, as well as in vivo cytotoxicity and degranulation capacity matched that of non-self-Ag-specific CD8 T cells. However, the CD8 T cells specific for self-TAs had a lower functional avidity, and this impacted on their in vivo performance. On the basis of these results and a low expression of the targeted TA epitopes on the tumor cells, we suggest that low avidity of the self-TA-specific CD8 T cells may represent a major obstacle for efficient immunotherapy of cancer.

A novel recombinant protein of IP10-EGFRvIIIscFv and CD8(+) cytotoxic T lymphocytes synergistically inhibits the growth of implanted glioma in mice

The epidermal growth factor receptor (EGFR) mutant of EGFRvIII is highly expressed on glioma cells and has been thought to be an excellent target molecule for immunotherapy. IP-10 is a potent chemokine and can recruit CXCR3(+) T cells, including CD8(+) T cells that are important for the control of tumor growth. This study is aimed at investigating the therapeutic efficacy of a novel fusion protein of IP10-EGFRvIIIscFv (IP10-scFv) in combination with glioma lysate-pulsed DCs-activated CD8(+) cytotoxic T lymphocytes (CTLs) in a mouse model of glioma. A plasmid of pET-IP10-scFv was generated by linking mouse IP-10 gene with the DNA fragment for anti-EGFRvIIIscFv, a (Gly4Ser)3 flexible linker and a His-tag. The recombinant IP10-scFv in E. coli was purified by affinity chromatography and characterized for its anti-EGFRvIII immunoreactivity and chemotactic activity. C57BL/6 mice were inoculated with mouse glioma GL261 cells in the brain and treated intracranially with IP10-scFv and/or intravenously with CTL for evaluating the therapeutic effect. The glioma-specific immune responses were examined. The IP10-scFv retained anti-EGFRvIII immunoreactivity and IP-10-like chemotactic activity. Treatment with both IP10-scFv and CTL synergistically inhibited the growth of glioma and prolonged the survival of tumor-bearing mice, accompanied by increasing the numbers of brain-infiltrating lymphocytes (BILs) and the frequency of CXCR3(+)CD8(+) T cells, enhancing glioma-specific IFN-γ responses and cytotoxicity, and promoting glioma cell apoptosis in mice. Our novel data indicate that IP10-scFv and CTL have synergistic therapeutic effects on inhibiting the growth of mouse glioma in vivo.

Oncogene-targeting T cells reject large tumors while oncogene inactivation selects escape variants in mouse models of cancer

The genetic instability of cancer cells frequently causes drug resistance. We established mouse cancer models, which allowed targeting of an oncogene by drug-mediated inactivation or monospecific CD8(+) effector T (T(E)) cells. Drug treatment of genetically unstable large tumors was effective but selected resistant clones in the long term. In contrast, T(E) cells completely rejected large tumors (≥500 mm(3)), if the target antigen was cancer-driving and expressed in sufficient amounts. Although drug-mediated oncogene inactivation selectively killed the cancer cells and left the tumor vasculature intact, which likely facilitated survival and growth of resistant clones, T(E) cell treatment led to blood vessel destruction and probably "bystander" elimination of escape variants, which did not require antigen cross-presentation by stromal cells.

Induction of insert-specific immune response in mice by hamster polyomavirus VP1 derived virus-like particles carrying LCMV GP33 CTL epitope

Hamster polyomavirus (HaPyV) major capsid protein VP1 based chimeric virus-like particles (VLPs) carrying model GP33 CTL epitope derived from Lymphocytic choriomeningitis virus (LCMV) were generated in yeast and examined for their capability to induce CTL response in mice. Chimeric VP1-GP33 VLPs were effectively processed in antigen presenting cells in vitro and in vivo and induced antigen-specific CD8+ T cell proliferation. Mice immunized only once with VP1-GP33 VLPs without adjuvant developed an effective GP33-specific memory T cell response: 70% were fully and 30% partially protected from LCMV infection. Moreover, aggressive growth of tumors expressing GP33 was significantly delayed in these mice in vivo. Therefore, HaPyV VP1-derived VLP harboring CTL epitopes are attractive vaccine candidates for the induction of insert-specific CTL immune response.

Development of an MHC class I L(d)-restricted PSA peptide-loaded tetramer for detection of PSA-specific CD8+ T cells in the mouse

Objectives

We set out to develop a prostate specific antigen (PSA) peptide-loaded tetramer for enumeration of PSA-specific CD8⁺ T cells in the Balb/c mouse model.

Methods

A candidate MHC class I PSA peptide (HPQKVTKFML_188–197) was selected based on its ability to restimulate PSA-specific CD8⁺ T cells to secrete IFN-γ in our assays. Next, H-2L^d-restricted peptide-loaded and fluorescently labeled tetramers were produced in conjunction with the NIH Tetramer Core Facility. This tetramer was then tested for staining specificity and optimized for detection of PSA-specific CD8⁺ T cells induced by our PSA-encoding adenovirus tumor vaccine.

Results

The MHC class I PSA peptide demonstrated successful restimulation of CD8⁺ T cells isolated from mice previously vaccinated with a PSA-encoding adenovirus tumor vaccine, with no restimulation observed in control vaccinated mice. The peptide-loaded H-2L^d tetramer exhibited the desired binding specificity and allowed for detection and frequency determination of PSA-specific CD8⁺ T cells by flow cytometry.

Conclusions

We have successfully designed and validated a PSA peptide tetramer for use in the Balb/c mouse model that can be used to test PSA-based prostate cancer vaccines. Until now, PSA-specific CD8⁺ T cells in the mouse have only been detectable via cytotoxic T lymphocyte (CTL) assays or intracellular cytokine staining, which primarily assess Ag-specific functional activity, not their absolute number. This research tool provides laboratories the ability to directly quantitate CD8⁺ T cells elicited by PSA-specific immunotherapies and cancer vaccines that are tested in mouse models.

Trends in cancer immunotherapy

Modulation of the immune system for therapeutic ends has a long history, stretching back to Edward Jenner's use of cowpox to induce immunity to smallpox in 1796. Since then, immunotherapy, in the form of prophylactic and therapeutic vaccines, has enabled doctors to treat and prevent a variety of infectious diseases, including cholera, poliomyelitis, diphtheria, measles and mumps. Immunotherapy is now increasingly being applied to oncology. Cancer immunotherapy attempts to harness the power and specificity of the immune system for the treatment of malignancy. Although cancer cells are less immunogenic than pathogens, the immune system is capable of recognizing and eliminating tumor cells. However, tumors frequently interfere with the development and function of immune responses. Thus, the challenge for cancer immunotherapy is to apply advances in cellular and molecular immunology and develop strategies that effectively and safely augment antitumor responses.

Tumor rejection by local interferon gamma induction in established tumors is associated with blood vessel destruction and necrosis

It has been shown that injecting a suspension of IFN-γ-secreting tumor cells results in their rejection. This effect has been attributed to IFN-γ preventing tumor stroma formation but not to a direct effect on the cancer cells. However, it is not known, which influence IFN-γ has on tumors with an established stroma. To address this question, the plasmacytoma cell line J558L was transduced with a vector allowing doxycycline-inducible IFN-γ gene expression. After the injection of the tumor cells into mice, IFN-γ was induced at different time points. Tumors did not grow when inducing IFN-γ immediately after tumor cell inoculation, while approximately half of the tumors were rejected when IFN-γ was induced in early established tumors within 2 weeks. Induction of IFN-γ 2-3 weeks after tumor cell inoculation was less efficient (0-17% rejection). IFN-γ induction in established tumors led to a reduction of CD146(+) endothelial cells and massive necrosis. Together, we show that vascularized tumors can be rejected by local IFN-γ expression, but that rejection of established tumors was less efficient over time. This suggests that transplanted tumors became less susceptible to local IFN-γ treatment the better they are established.

Multiphoton imaging of cytotoxic T lymphocyte-mediated antitumor immune responses

The actual contribution of T lymphocytes to protection against tumors is still unclear. In vitro imaging experiments show that tumor specific cytotoxic T lymphocytes (CTLs) are competent to kill target cells by conventional cytotoxic pathways. The emergence of multiphoton imaging in the past decade now allows real time in vivo imaging of CTLs. New insights are available on the behavior of antitumor T cells during the priming phase, during their traffic within the tumor tissue, and on their interactions with tumor cells during the effector phase. Recent reports suggest that direct killing of tumor cells by CTLs is a slow process, suggesting that the ratio of effector to target cells is determinant, or that additional cytotoxic contribution by other cell types is required to induce efficient tumor rejection. This review will focus on the publications that have imaged antitumor immune responses dynamically and discuss how this new information contributes to understand the implication of CTLs in tumor rejection.

Two-photon imaging of intratumoral CD8+ T cell cytotoxic activity during adoptive T cell therapy in mice

CTLs have the potential to attack tumors, and adoptive transfer of CTLs can lead to tumor regression in mouse models and human clinical settings. However, the dynamics of tumor cell elimination during efficient T cell therapy is unknown, and it is unclear whether CTLs act directly by destroying tumor cells or indirectly by initiating the recruitment of innate immune cells that mediate tumor damage. To address these questions, we report real-time imaging of tumor cell apoptosis in vivo using intravital 2-photon microscopy and a Förster resonance energy transfer-based (FRET-based) reporter of caspase 3 activity. In a mouse model of solid tumor, we found that tumor regression after transfer of in vitro-activated CTLs occurred primarily through the direct action of CTLs on each individual tumor cell, with a minimal bystander effect. Surprisingly, the killing of 1 target cell by an individual CTL took an extended period of time, 6 hours on average, which suggested that the slow rate of killing intrinsically limits the efficiency of antitumor T cell responses. The ability to visualize when, where, and how tumor cells are killed in vivo offers new perspectives for understanding how immune effectors survey cancer cells and how local tumor microenvironments may subvert immune responses.

Rapid accumulation of adoptively transferred CD8+ T cells at the tumor site is associated with long-term control of SV40 T antigen-induced tumors

We previously established a model to study CD8(+) T cell (T(CD8))-based adoptive immunotherapy of cancer using line SV11 mice that develop choroid plexus tumors in the brain due to transgenic expression of Simian Virus 40 large T antigen (Tag). These mice are tolerant to the three dominant T(CD8)-recognized Tag epitopes I, II/III and IV. However, adoptive transfer of spleen cells from naïve C57BL/6 (B6) mice prolongs SV11 survival following T(CD8) priming against the endogenous Tag epitope IV. In addition, survival of SV11 mice is dramatically increased following transfer of lymphocytes from Tag-immune B6 mice. In the current study, we compared the kinetics and magnitude of Tag-specific T(CD8) accumulation at the tumor site following adoptive transfer with a high dose of either Tag-immune or naïve donor cells or decreasing doses of Tag-immune lymphocytes. Following adoptive transfer of Tag-immune cells, epitope I- and IV-specific T(CD8) accumulated to high levels in the brain of SV11 mice, peaking at 5-7 days, while epitope IV-specific T(CD8 )derived from naïve donors required three weeks to achieve peak levels. A similar delay in the peak of epitope IV-specific T(CD8) accumulation was observed when tenfold fewer Tag-immune donor cells were administered, reducing control of tumor progression. These results suggest that efficient and prolonged control of established autochthonous tumors is associated with high-level early accumulation of adoptively transferred T cells. We also provide evidence that although multiple specificities are represented in the Tag immune donor lymphocytes, epitope IV-specific donor T(CD8) play a predominant role in control of tumor growth.

Decreased tumor surveillance after adoptive T-cell therapy

The effect of cancer immunotherapy on the endogenous immune response against tumors is largely unknown. Therefore, we studied immune responses against murine tumors expressing the glycoprotein (GP) and/or nucleoprotein of lymphocytic choriomeningitis virus (LCMV) with or without adoptive T-cell therapy. In nontreated animals, CTLs specific for different epitopes as well as LCMV-GP-specific antibodies contributed to tumor surveillance. Adoptive immunotherapy with monoclonal CTLs specific for LCMV-gp33 impaired the endogenous tumor-specific antibody and CTL response by targeting antigen cross-presenting cells. As a consequence and in contrast to expectations, immunotherapy enhanced tumor growth. Thus, for certain immunogenic tumors, a reduction of tumor-specific B- and T-cell responses and enhanced tumor growth may be an unwanted consequence of adoptive immunotherapy.

Do autochthonous tumors interfere with effector T cell responses?

The assumption that autochthonous tumors interfere with the effector T cell (T(E)) response implies that they first induce functional T cells. However, if T(E) are generated, they usually remain functional, persist life-long as memory cells and prevent tumors. This holds true for some virus-induced tumors and is associated with evolutionary pressure. In contrast, models that allow monitoring of tumor antigen-specific T cells suggest that spontaneous autochthonous tumors either sneak through or induce T(E) too late when the tumor has developed resistance to T(E) or induce tolerance. This can be explained by the absence of evolutionary pressure.

Targeted inflammation during oncolytic virus therapy severely compromises tumor blood flow

Oncolytic viruses (OVs) are selected or designed to eliminate malignancies by direct infection and lysis of cancer cells. In contrast to this concept of direct tumor lysis by viral infection, we observed that a significant portion of the in vivo tumor killing activity of two OVs, vesicular stomatitis virus (VSV) and vaccinia virus is caused by indirect killing of uninfected tumor cells. Shortly after administering the oncolytic virus we observed limited virus infection, coincident with a loss of blood flow to the interior of the tumor that correlated with induction of apoptosis in tumor cells. Transcript profiling of tumors showed that virus infection resulted in a dramatic transcriptional activation of pro-inflammatory genes including the neutrophil chemoattractants CXCL1 and CXCL5. Immunohistochemical examination of infected tumors revealed infiltration by neutrophils correlating with chemokine induction. Depletion of neutrophils in animals prior to VSV administration eliminated uninfected tumor cell apoptosis and permitted more extensive replication and spreading of the virus throughout the tumor. Taken all together, these results indicate that targeted recruitment of neutrophils to infected tumor beds enhances the killing of malignant cells. We propose that activation of inflammatory cells can be used for enhancing the effectiveness of oncolytic virus therapeutics, and that this approach should influence the planning of therapeutic doses.

Engineered CD8+ cytotoxic T cells with fiber-modified adenovirus-mediated TNF-alpha gene transfection counteract immunosuppressive interleukin-10-secreting lung metastasis and solid tumors

T-cell suppression derived from tumor-secreted immunosuppressive interleukin (IL)-10 becomes a major barrier to CD8+ T-cell immunotherapy of tumors. Tumor necrosis factor-alpha (TNF-alpha) is a multifunctional cytokine capable of activating T and dendritic cells (DCs) and counteracting IL-10-mediated DC inhibition and regulatory T-cell-mediated immune suppression. In this study, we constructed a recombinant adenovirus (MF)AdVTNF with fiber-gene modified by RGD insertion into the viral knob's H1 loop and a melanoma cell line B16(OVA/IL-10) engineered to express ovalbumin (OVA) and to secrete IL-10 (2.2 ng/ml/10(6) cells/24 h). We transfected OVA-specific CD8+ T cells with (MF)AdVTNF, and found a fivefold increase in transgene human TNF-alpha secretion (4.3 ng/ml/10(6) cells/24 h) by the engineered CD8+ T(TNF) cells transfected with (MF)AdVTNF, compared to that (0.8 ng/ml/10(6) cells/24 h) by CD8+ T cells transfected with the original AdVTNF without viral fiber modification. The engineered CD8+ T(TNF) cells exhibited enhanced cytotoxicity and elongated survival in vivo after adoptive transfer. TNF-alpha derived from both the donor CD8+ T cells and the host cells plays an important role in donor CD8+ T-cell survival in vivo after adoptive transfer. We also demonstrated that the transfected B16(OVA/IL-10) tumor cells secreting IL-10 are more resistant to in vivo CD8+ T-cell therapy than the original B16(OVA) tumor cells without IL-10 expression. Interestingly, the engineered CD8+ T(TNF) cells secreting transgene-coded TNF-alpha, but not the control CD8+ T(control) cells without any transgene expression eradicated IL-10-secreting 12-day lung micrometastasis in all 10/10 mice and IL-10-secreting solid tumors ( approximately 5 mm in diameter) in 6/10 mice. Transfer of the engineered CD8+ T(TNF) cells further induced both donor- and host-derived memory CD8+ T cells, leading to a stronger long-term antitumor immunity against the IL-10-secreting B16(OVA/IL-10) tumor cell challenges. Therefore, CD8+ T cells engineered to secrete TNF-alpha may be useful when designing strategies for adoptive T-cell therapy of solid tumors.

Targeted and untargeted CD137L fusion proteins for the immunotherapy of experimental solid tumors

INTRODUCTION

CD137L is a member of the tumor necrosis factor superfamily that provides a costimulatory signal to T cells. In this study, two novel CD137L fusion proteins were produced and compared with the CD137 agonist antibody 2A.

MATERIALS AND METHODS

Murine CD137L was linked to the COOH terminus of either the Fc fragment of immunoglobulin (untargeted version) or TNT-3 (targeted version), an antibody that binds to necrotic regions of tumors. Groups of mice bearing established Colon 26 tumors were then treated daily x 5 with each fusion protein or 2A to determine their immunotherapeutic potential.

RESULTS

Both fusion proteins retained CD137L activity in vitro and TNT-3/CD137L showed tumor-binding activity by biodistribution analysis in tumor-bearing mice. The fusion proteins also produced similar responses in vivo at the 1 nmol per dose range and showed a 60% (TNT-3/CD137L) or 40% (Fc/CD137L) survival of treated mice at 150 days after tumor implantation, similar to the effects of 2A. Morphologic and immunohistochemical analyses showed massive central necrosis and infiltration of granzyme B-positive cells in necrotic areas and viable peripheral regions of treated tumors. Finally, cell depletion studies showed that CD137L-mediated tumor regression was CD8(+) T cell dependent.

CONCLUSIONS

From these studies, it was determined that both targeted and untargeted CD137L fusion proteins showed effective antitumor activity, but that the targeted version was more potent. Therefore, the use of the natural CD137 ligand is a promising approach to the treatment of solid tumors by virtue of its ability to produce physiologic costimulation within the tumor, limiting side effects often seen with agonist antibody therapies.

In vivo imaging of cytotoxic T cell infiltration and elimination of a solid tumor

Although the immune system evolved to fight infections, it may also attack and destroy solid tumors. In most cases, tumor rejection is initiated by CD8⁺ cytotoxic T lymphocytes (CTLs), which infiltrate solid tumors, recognize tumor antigens, and kill tumor cells. We use a combination of two-photon intravital microscopy and immunofluorescence on ordered sequential sections to analyze the infiltration and destruction of solid tumors by CTLs. We show that in the periphery of a thymoma growing subcutaneously, activated CTLs migrate with high instantaneous velocities. The CTLs arrest in close contact to tumor cells expressing their cognate antigen. In regions where most tumor cells are dead, CTLs resume migration, sometimes following collagen fibers or blood vessels. CTLs migrating along blood vessels preferentially adopt an elongated morphology. CTLs also infiltrate tumors in depth, but only when the tumor cells express the cognate CTL antigen. In tumors that do not express the cognate antigen, CTL infiltration is restricted to peripheral regions, and lymphocytes neither stop moving nor kill tumor cells. Antigen expression by tumor cells therefore determines both CTL motility within the tumor and profound tumor infiltration.

Challenges for cancer vaccine development

The first generation of human cancer vaccines has been tested in phase III clinical trials, but only a few of these have demonstrated sufficient efficacy to be licensed for clinical use. This article reviews some of the mechanisms that could contribute to these limited clinical responses, and highlights the challenges faced for development of future vaccines.