Beyond inflammation: site-directed immunotherapy

Identification of interleukin-13 receptor alpha2 peptide analogues capable of inducing improved antiglioma CTL responses

Restricted and high-level expression of interleukin-13 receptor alpha2 (IL-13Ralpha2) in a majority of human malignant gliomas makes this protein an attractive vaccine target. We have previously described the identification of the IL-13Ralpha2(345-353) peptide as a human leukocyte antigen-A2 (HLA-A2)-restricted CTL epitope. However, as it remains unclear how efficiently peptide-based vaccines can induce specific CTLs in patients with malignant gliomas, we have examined whether analogue epitopes could elicit heteroclitic antitumor T-cell responses versus wild-type peptides. We have created three IL-13Ralpha2 analogue peptides by substitutions of the COOH-terminal isoleucine (I) for valine (V) and the NH(2)-terminal tryptophan (W) for either alanine (A), glutamic acid (E), or nonsubstituted (W; designated as 1A9V, 1E9V, and 9V, respectively). In comparison with the native IL-13Ralpha2 epitope, the analogue peptides 9V and 1A9V displayed higher levels of binding affinity and stability in HLA-A2 complexes and yielded an improved stimulatory index for patient-derived, specific CTLs against the native epitope expressed by HLA-A2(+) glioma cells. In HLA-A2-transgenic HHD mice, immunization with the peptides 9V and 1A9V induced enhanced levels of CTL reactivity and protective immunity against an intracranial challenge with IL13Ralpha2-expressing syngeneic tumors when compared with vaccines containing the native IL-13Ralpha2 epitope. These findings indicate highly immunogenic IL-13Ralpha2 peptide analogues may be useful for the development of vaccines capable of effectively expanding IL-13Ralpha2-specific, tumor-reactive CTLs in glioma patients.

Immunotherapy for malignant glioma: current approaches and future directions

Traditional therapies for the treatment of malignant glioma have failed to make appreciable gains regarding patient outcome in the last decade. Therefore, immunotherapeutic approaches have become increasingly popular in the treatment of this cancer. This article reviews general immunology of the central nervous system and the immunobiology of malignant glioma to provide a foundation for understanding the rationale behind current glioma immunotherapies. A review of currently implemented immunological treatments is then provided with special attention paid to the use of vaccines, gene therapy, cytokines, dendritic cells and viruses. Insights into future and developing avenues of glioma immunotherapy, such as novel delivery systems, are also discussed.

Sequential delivery of interferon-alpha gene and DCs to intracranial gliomas promotes an effective antitumor response

Effective presentation of tumor antigens by dendritic cells (DCs) is considered to be essential for the induction of antitumor T-cell responses. Apoptotic and necrotic tumors have been noted to be a robust antigen source for DCs. Because glioma cells undergo apoptosis after transfection with the type I interferon (IFN) gene and type I IFNs promote the stimulatory activity of DCs, we hypothesized that transfection of glioma cells with type I IFN genes and provision of DCs would promote particularly effective antitumor activity by both facilitating apoptosis of glioma cells and the presentation of the glioma antigens, thereby inducing specific immune responses against glioma cells. We have previously reported the proof of this hypothesis in vitro and in a subcutaneous tumor model. Here we report an extension of this approach in intracranial (i.c.) gliomas using adenoviral IFN-alpha (Ad-IFN-alpha) vector. Mice bearing day-5 i.c. GL261 glioma received sequential intratumoral (i.t.) delivery of Ad-IFN-alpha and bone marrow-derived syngeneic DCs. This treatment prolonged survival in that nine of 17 animals survived long term (> 60 days versus 0 of 10 control animals). Specific CTL activity was demonstrated following this regimen in the cervical lymph nodes, and the therapeutic efficacy was dependent upon CD8+ cells. Furthermore, these animals were protected against subsequent re-challenge with GL261 gliomas. DCs injected i.t. survived in the tumor and migrated into cervical lymph node. In vitro migration assays revealed the ability of DCs to migrate toward the tumor, suggesting that i.t. injected DCs migrate through the glioma. Taken together, this combination of gene therapy and cellular immunotherapy may be an effective future strategy for treating human gliomas.

Autologous glioma cell vaccine admixed with interleukin-4 gene transfected fibroblasts in the treatment of recurrent glioblastoma: preliminary observations in a patient with a favorable response to therapy

We designed a phase I clinical trial of vaccinations with autologous glioma cells expressing transgene-derived interleukin-4 (IL-4), and treated one patient with a right temporal lobe recurrent glioblastoma. This 62-year-old man underwent craniotomy and partial tumor removal, at which time autologous tumor cells were obtained for vaccine preparation. After confirming the patient's cellular immune function by skin test, two cycles of vaccination with irradiated autologous glioma cells admixed with gene transfected fibroblasts were given intradermally. The patient demonstrated no evidence of allergic encephalitis throughout this course. Immunohistochemistry with biopsy samples taken from the vaccine sites demonstrated that the infiltration level of CD4, CD8 and CD1a positive cells increased proportionally to the amount of IL-4 produced at the each site, suggesting that there was local immune response induced at the vaccine site. While it is premature to assess effectiveness of the vaccine, this initial patient's course suggested a transient response to the vaccine, and he survived 10 months after treatment.

Brain tumor immunotherapy: an immunologist's perspective

Key concepts in brain tumor immunotherapy are reviewed. "Immunotherapy" can refer to a fully-developed, tumor-specific immune response, or to its individual cellular or molecular mediators. The immune response is initiated most efficiently in organized lymphoid tissue. After initiation, antigen-specific T lymphocytes (T cells) survey the tissues--including the brain. If the T cells re-encounter their antigen at a tumor site, they can be triggered to carry out their effector functions. T cells can attack tumor in many ways, directly and indirectly, through cell-cell contact, secreted factors, and attraction and activation of other cells, endogenous or blood-borne. Recent work expands the list of candidate tumor antigens: they are not limited to cell surface proteins and need not be absolutely tumor-specific. Once identified, tumor antigens can be targeted immunologically, or in novel ways. The immune response is under complex regulatory control. Most current work aims to enhance initiation of the response (for example, with tumor vaccines), rather than enhancing the effector phase at the tumor site. The effector phase includes a rich, interactive set of cells and mediators; some that are not usually stressed are of particular interest against tumor in the brain. Within the brain, immune regulation varies from site to site, and local neurochemicals (such as substance P or glutamate) can contribute to local control. Given the complexity of a tumor, the brain, and the immune response, animal models are essential, but more emphasis should be given to their limitations and to step-by-step analysis, rather than animal "cures".

Robust ability of IFN-gamma to upregulate class II MHC antigen expression in tumor bearing rat brains

T cells are attractive for delivering therapy to brain tumor, especially disseminated micro-tumor. However, to trigger effector function, tumor antigen must be re-presented to T cells, via major histocompatibility complex (MHC) proteins, at the tumor site. In normal brain, MHC+ antigen-presenting cells (APC) are rare, but abundant after gamma interferon (IFN-gamma) injection. Here we studied tumor-bearing brains. IFN-gamma (or buffer) was injected stereotactically into brains with established tumors from a panel of immunologically varied glioma cell lines, some expressing b-galactosidase as a micro-tumor marker. Four days later, cryostat sections were stained for tumor and MHC proteins. In phosphate-buffered saline-injected controls, class II MHC+ potential APC (microglia, macrophages) were seen only at (some) tumor sites. In rats that received IFN-gamma, class II+ potential APC were widespread, including all actual and potential micro-tumor sites and all tumor-free areas. In the same slides, neither class I nor class II MHC antigen was detected in neural cells or most tumor cells. This MHC pattern favors indirect re-presentation of tumor antigen, by tumor-adjacent APC. The robust response to IFN-gamma might also be exploited in other ways: activated microglia and macrophages can attack tumor directly, and class II+ APC may help mark micro-tumor sites.

The role of T-cell-mediated mechanisms in virus infections of the nervous system

T lymphocytes play a decisive role in the course and clinical outcome of viral CNS infection. Summarizing the information presented in this review, the following sequence of events might occur during acute virus infection: After invasion of the host and a few initial rounds of replication, the virus reaches the CNS in most cases by hematogeneous spread. After passage through the BBB, CNS cells are infected and replication of virus in brain cells causes activation of the surrounding microglia population. Moreover, local production of IFN-alpha/beta induces expression of MHC antigens on CNS cells, and microglial cells start to phagocytose cellular debris, which accumulates as a result of virus-induced cytopathogenic effects. Upon phagocytosis, microglia becomes more activated; they up-regulate MHC molecules, acquire antigen presentation capabilities and secrete chemokines. This will initiate up-regulation of adhesion molecules on adjacent endothelial cells of the BBB. Transmigration of activated T lymphocytes through the BBB is followed by interaction with APC, presenting the appropriate peptides in the context of MHC antigens. It appears that CD8+ T lymphocytes are amongst the first mononuclear cells to arrive at the infected tissue. Without a doubt, their induction and attraction is deeply influenced by natural killer cells, which, after virus infection, secrete IFN-gamma, a cytokine that stimulates CD8+ T cells and diverts the immune response to a TH1-type CD4+ T cell-dominated response. Following the CD8+ T lymphocytes, tissue-penetrating, TH1 CD4+ T cells contact local APC. This results in a tremendous up-regulation of MHC molecules and secretion of more chemotactic and toxic substances. Consequently an increasing number of inflammatory cells, including macrophages/microglia and finally antibody-secreting plasma cells, are attracted to the site of virus infection. All trapped cells are mainly terminally differentiated cells that are going to enter apoptosis during or shortly after exerting their effector functions. The clinical consequences and the influence of the effector phase on the further course of the infection depends on the balance and fine-tuning of the contributing lymphoid cell populations. Generally, any delay in the recruitment of effector lymphocytes to the tissue or an unbalanced combination of lymphocyte subsets allows the virus to spread in the CNS, which in turn will cause severe immune-mediated tissue effects as well as disease. If either too late or partially deficient, the immune system response may contribute to a lethal outcome or cause autosensitization to brain-specific antigens by epitope spreading to the antigen-presenting system in peripheral lymphoid tissue. This could form the basis for subsequent booster reactions of autosensitized CD4+ T cells--a process that finally will end in an inflammatory autoimmune reaction, which in humans we call multiple sclerosis. In contrast, a rapid and specific local response in the brain tissue will result in efficient limitation of viral spread and thereby a subclinical immune system-mediated termination of the infection. After clearance of virus-infected cells, downsizing of the local response probably occurs via self-elimination of the contributing T cell populations and/or by so far unidentified signal pathways. However, much of this is highly speculative, and more data have to be collected to make decisive conclusions regarding this matter. Several strategies have been developed by viruses to escape T cell-mediated eradication, including interference with the MHC class I presentation pathway of the host cell or "hiding" in cells which lack MHC class I expression. This may result in life-long persistence of the virus in the brain, a state which probably is actively controlled by T lymphocytes. Under severe immunosuppression, however, reactivation of viral replication can occur, which is a lethal threat to the host.

New animal models to probe brain tumor biology, therapy, and immunotherapy: advantages and remaining concerns

New genetic models provide better biological mimics of human tumors. The new models can give deeper insight into tumorigenesis and provide better targets for testing therapies. To use the new models most successfully, it is useful to keep in mind limitations that are harder to overcome by genetic manipulation. These include biochemical and anatomical differences between species, as well as differences in scale, both spatial and temporal. Three approaches to new genetic brain tumor models are described in the following articles. This essay provides a context, bringing out both advantages and remaining concerns. Examples are taken from work in brain tumor immunobiology and immunotherapy. The complementarity of different models, and the dichotomy between general principles and model-specific details are stressed.

Local neurochemicals and site-specific immune regulation in the CNS

Although it is often described as "immunologically privileged," the brain can display vigorous immune activity, both clinically and experimentally. The underlying control mechanisms are under active study. Here we shift attention from the brain as a whole to its diverse microenvironments. We review evidence that immune regulation in the brain is site-specific, and that local neurochemicals contribute to the site-specific control. Key points are illustrated by recent work from a rat model in which local injection of the proinflammatory cytokine, IFN-gamma, was used to modulate 2 essential aspects of the cell-mediated immune response: T cell entry from the blood, and expression of the MHC proteins that are needed to present antigen to the newly entered T cells. A growing number of neurologic disorders are known to be exacerbated by the immune/inflammatory network. Understanding the factors that influence local immune function may help explain the distribution of localized CNS damage and, more importantly, may suggest new therapeutic approaches for both desirable and unwanted responses.

Role of the cervical lymphatics in the Th2-type hierarchy of CNS immune regulation

CNS immune regulation is intimately dependent on the dynamics of cerebral extracellular fluid circulation. Animal models indicate that following the introduction of antigen into the CNS, normal circulation of interstitial and cerebrospinal fluids provides the opportunity for (a) delivery of CNS-derived antigen to lymphoid organs, as well as, (b) retention of immunologically significant amounts of antigen within the CNS. Thus, even in the absence of disease, CNS-derived antigen can induce antigen-specific activation of naive lymphocytes in lymphoid organs and specific reactivation of lymphoblasts that have migrated into the CNS. The initial peripheral immune response to CNS-derived antigen is induced in cervical lymph nodes and is characterized by a strong antibody response, no delayed-type hypersensitivity, and only priming for cytotoxic T-cell responses. This Th-2 type hierarchy of immune regulation is reinforced within the antigen-stimulated CNS where specific B lymphoblasts are permitted to develop their effector function but cell-mediated immunity is inhibited. Developing a paradigm for CNS immune regulation is important in understanding how CNS disorders in humans are induced, perpetuated, and may be manipulated.

Site-specific control of T cell traffic in the brain: T cell entry to brainstem vs. hippocampus after local injection of IFN-gamma

Although it is known that neurotransmitters and neuropeptides can affect immune function in vitro, less is understood about how the neural environment affects immune function in the brain. Previously, we showed that regulation of parenchymal class II MHC after local injection of IFN-gamma is site-specific. In this companion study, we defined the effect of local IFN-gamma on the entry of class II-restricted T cells to the brain parenchyma. To activate endogenous T cells, adult CDF rats were immunized with a normal neural antigen (MBP). Two weeks later, the proinflammatory cytokine IFN-gamma (100 to 10,000 U/site) was injected stereotaxically into two neurochemically and anatomically distinct sites, the hippocampus (area CAI) and brainstem (nucleus of the solitary tract). Monoclonal R73 was used to detect T cells on cryostat sections. The greatest difference was seen 48 h after 300 U IFN-gamma was injected at each site, when there were several-fold more parenchymal T cells in the brainstem than in the hippocampus. Most parenchymal T cells were CD4+ /class II-restricted. Thus, parenchymal T cell entry and parenchymal class II up-regulation show the same hierarchy (brainstem >> hippocampus) after local IFN-gamma injection, although T cell entry was more sensitive to the IFN-gamma dose. We suggest that the local regulatory environment contributes to site-specific immune regulation, and discuss implications for the distribution of MS plaques and other aspects of local immune control. Further, in interpreting the many previous studies of cytokine-mediated immune changes in the CNS, the possibility of site-specific differences should be considered.

Site-specific immune regulation in the brain: differential modulation of major histocompatibility complex (MHC) proteins in brainstem vs. hippocampus

Although neurotransmitters and neuropeptides are known to affect immune function in vitro and in non-neural tissues, little is known about how the local mix of neurochemicals affects immune function in the brain. Here, we study local modulation of the class II major histocompatibility complex (MHC) proteins, which present antigen to T cells in a key pathway for cell-mediated immune activity. Two sites that are well-separated anatomically and have very different neuroregulatory environments, the brainstem and hippocampus, were compared. The class II-upregulating cytokine, gamma interferon (IFN-gamma, 0.1 to 10,000 U/site), was injected stereotaxically into the hippocampus and contralateral brainstem of adult Charles-derived Fischer rats. Four days later, monoclonal antibody staining was used to detect class II MHC proteins on cryostat sections, followed by computer-assisted image analysis. As compared to hippocampus, the brainstem showed enhanced class II expression at lower IFN-gamma doses, and reached a higher plateau. Site-specific class II modulation was also seen within the layers of the hippocampus, and among other brain sites. Injection of marker protein to visualize the spread of injected protein, plus injection of IFN-gamma into alternative sites, suggested that preferential flow cannot explain all of the site-specific effects. We suggest that the local neuroregulatory environment and/or intrinsic differences among target microglia are likely to play a role. Implications for the distribution of pathological changes, such as multiple sclerosis plaques, and for local immunotherapy are discussed.