Mouse models of glioblastoma: lessons learned and questions to be answered.

Glioblastoma is the most common primary brain tumour in adults. While many patients achieve disease remission following treatment with surgical resection, radiation therapy and chemotherapy, this remission is brief and invariably followed by tumour recurrence and progression. Recent work using mouse models of the disease, coupled with data generated by The Cancer Genome Atlas, have given us new insights into the mechanisms that underlie gliomagenesis and result in glioblastoma heterogeneity. These findings suggest that the treatment of glioblastoma will require a more nuanced understanding of their biology and the employment of targeted therapeutic approaches. In this review, we will summarize the current state of mouse modeling in glioma, with a focus on how these models may inform our understanding of this disease and its treatment.

Clonotype and repertoire changes drive the functional improvement of HIV-specific CD8 T cell populations under conditions of limited antigenic stimulation.

Persistent exposure to cognate Ag leads to the functional impairment and exhaustion of HIV-specific CD8 T cells. Ag withdrawal, attributable either to antiretroviral treatment or the emergence of epitope escape mutations, causes HIV-specific CD8 T cell responses to wane over time. However, this process does not continue to extinction, and residual CD8 T cells likely play an important role in the control of HIV replication. In this study, we conducted a longitudinal analysis of clonality, phenotype, and function to define the characteristics of HIV-specific CD8 T cell populations that persist under conditions of limited antigenic stimulation. Ag decay was associated with dynamic changes in the TCR repertoire, increased expression of CD45RA and CD127, decreased expression of programmed death-1, and the emergence of polyfunctional HIV-specific CD8 T cells. High-definition analysis of individual clonotypes revealed that the Ag loss-induced gain of function within HIV-specific CD8 T cell populations could be attributed to two nonexclusive mechanisms: 1) functional improvement of persisting clonotypes; and 2) recruitment of particular clonotypes endowed with superior functional capabilities.

Upregulation of PD-1 expression on HIV-specific CD8+ T cells leads to reversible immune dysfunction.

The engagement of programmed death 1 (PD-1) to its ligands, PD-L1 and PD-L2, inhibits proliferation and cytokine production mediated by antibodies to CD3 (refs. 5,6,7). Blocking the PD-1-PD-L1 pathway in mice chronically infected with lymphocytic choriomeningitis virus restores the capacity of exhausted CD8(+) T cells to undergo proliferation, cytokine production and cytotoxic activity and, consequently, results in reduced viral load. During chronic HIV infection, HIV-specific CD8(+) T cells are functionally impaired, showing a reduced capacity to produce cytokines and effector molecules as well as an impaired capacity to proliferate. Here, we found that PD-1 was upregulated on HIV-specific CD8(+) T cells; PD-1 expression levels were significantly correlated both with viral load and with the reduced capacity for cytokine production and proliferation of HIV-specific CD8(+) T cells. Notably, cytomegalovirus (CMV)-specific CD8(+) T cells from the same donors did not upregulate PD-1 and maintained the production of high levels of cytokines. Blocking PD-1 engagement to its ligand (PD-L1) enhanced the capacity of HIV-specific CD8(+) T cells to survive and proliferate and led to an increased production of cytokines and cytotoxic molecules in response to cognate antigen. The accumulation of HIV-specific dysfunctional CD8(+) T cells in the infected host could prevent the renewal of a functionally competent HIV-specific CD8(+) repertoire.

T-cell exhaustion in HIV infection.

Generation of memory T cells, which mediate immunity against microbes and cancers, relies, for optimal activity, on the interactions of multiple cell types that are highly regulated through the expression of soluble factors and negative and positive receptors. Their disruption will lead to aberrant immune responses, which can result in the invasion of the host by foreign pathogens. In chronic viral infections including HIV and hepatitis C virus, persistence of antigen and lack of CD4 help (HIV) disrupt memory T-cell function and induce defects in memory T-cell responses, which have been defined as T-cell exhaustion. In this review, we examine the molecular mechanisms involved in such T-cell dysfunction. Better understanding of these mechanisms will assist in the development of novel therapies to prevent the immune damage mediated by HIV infection.

Programmed death 1: a critical regulator of T-cell function and a strong target for immunotherapies for chronic viral infections.

PURPOSE OF REVIEW: The intricate balance between positive and negative signals delivered by accessory molecules is crucial to generate efficient immune responses while maintaining tolerance and preventing autoimmunity. Of these molecules, programmed death 1 has been described as a negative regulator of T-cell activation. This review will focus on current knowledge about PD-1 regulation in different diseases and discuss its potential benefits for the development of novel immune therapies. RECENT FINDINGS: PD-1 has recently been shown to be upregulated on HIV-specific CD8 T cells, whereas the PD-1 expression level was significantly correlated with viral load. Blockade of the PD-1/PD-L1 interaction enhanced the capacity of HIV-specific CD8 and CD4 T cells to proliferate or secrete cytokines and cytotoxic molecules. Future manipulations of this pathway could rescue the function of exhausted CD8 and CD4 T cells. SUMMARY: The engagement of PD-1 with its ligands induces inhibitory signals as it blocks T-cell receptor-induced T-cell proliferation and cytokine production. The PD-1 pathway plays a crucial role in the maintenance of peripheral tolerance and the pathogenesis of cancer and chronic viral infections. Understanding the mechanisms by which PD-1 interferes with T-cell functions will pave the way for novel therapeutic immune interventions to treat these diseases.

Generation and maintenance of human memory cells during viral infection.

Long-term maintenance of memory T cell response is the hallmark of immune protection and hence the holy grail of most vaccine development studies. Persistent memory cells, developed after either viral infection or vaccination, ensure the generation of an antiviral response upon reexposure to the pathogen. During acute viral infections, as in the case of measles and influenza viruses, strong T cell effector functions, which eradicate the virus and protect patients against reexposure, are achieved by the generation of persistent protective memory cells. However, in chronic infections, T cells drastically lose effector functions before acquiring a memory phenotype. Chronic infections can be categorized into infections where viremia is controlled and protective memory cells are maintained as in the case of EBV and CMV infections, or where the virus persists and memory cells are exhausted and disrupted as in the case of human immunodeficiency virus infection. In this review, we will discuss the different phenotypical and functional characteristics of memory cells subsets, the importance of the role they play during acute and chronic infections, and the mechanisms behind their effectiveness and persistence.