G Bendle

Prospects for immunotherapy of malignant disease

The majority of T cell‐recognized tumour antigens in humans are encoded by genes that are also present in normal tissues. Low levels of gene expression in normal cells can lead to the inactivation of high‐avidity T cells by immunological tolerance mechanisms. As a consequence, low‐avidity T cell responses in patients are often inadequate in providing tumour protection. Recently, several technologies have been developed to overcome tolerance, allowing the isolation of high‐affinity, HLA‐restricted receptors specific for tumour‐associated peptide epitopes. Furthermore, transfer of HLA‐restricted antigen receptors provides an opportunity to empower patient T cells with new tumour‐reactive specificities that cannot be retrieved from the autologous T cell repertoire.

CD8α/α homodimers fail to function as co-receptor for a CD8-dependent TCR

In this study, we have started to dissect the molecular basis of CD8 dependence of a high and low avidity CTL clone specific for the same peptide epitope. Using anti‐CD8α and anti‐CD8β antibodies, we found that cytotoxicity and IFN‐γ production by high but not by low avidity CTL was strongly CD8 dependent. We isolated the TCR genes of both types of CTL clones and used retroviral gene transfer to analyse the function of these TCR in primary T cells of wild‐type and CD8β‐deficient mice. Both TCR triggered antigen‐specific killing in wild‐type T cells, and blocking experiments showed that CD8 dependence/independence co‐transferred with the TCR into primary T cells, indicating that it was dictated by the TCR itself. Gene transfer experiments into CD8β‐deficient T cells revealed that only the TCR derived from the CD8‐independent CTL clone elicited antigen‐specific cytotoxicity, while the CD8‐dependent TCR was non‐functional in the absence of the CD8β‐chain. These data indicate a striking difference between CD8α/β heterodimers and CD8α/α homodimers as only the former were able to provide co‐receptor function for the CD8‐dependent TCR.

Broadly expressed tumour?associated proteins as targets for cytotoxic T lymphocyte-based cancer immunotherapy

T cell-based antigen-specific immunotherapy targeting self-proteins aberrantly expressed in many tumours offers the potential for widely applicable cancer immunotherapy, but carries the risk of autoimmunity. Immunological tolerance represents an inherent limitation of cancer vaccines targeting such broadly expressed tumour-associated proteins. Therefore, strategies to circumvent T cell tolerance have been developed and, when combined with T cell receptor (TCR) gene transfer technology, can generate highly avid tumour-reactive patient cytotoxic T lymphocytes (CTLs) specific for peptide epitopes of tumour-associated proteins. This review analyses the level of tolerance to broadly expressed tumour-associated proteins in the autologous T cell repertoire, assesses strategies that have been developed to circumvent T cell tolerance to such antigens, and evaluates the prospects for effective immunotherapy targeting broadly expressed tumour-associated proteins.

Generation and characterization of transgenic mice expressing a T-cell receptor specific for the tumour-associated antigen MDM2

T‐cell‐based antigen‐specific immunotherapy targeting tumour‐associated antigens offers the potential for cancer immunotherapy. However, the majority of identified tumour‐associated antigens are also expressed at low levels in normal tissues and mechanisms of tolerance induction are likely to affect the quality of T‐cell responses to such antigens. In this study a T‐cell receptor transgenic model was developed to determine the magnitude of T‐cell tolerance to the tumour‐associated antigen murine double minute‐2 (MDM2), a widely expressed protein that is found at elevated levels in many tumours. The analysis of transgenic mice showed that thymic deletion was responsible for purging large numbers of MDM2‐specific T cells from the repertoire. However, some T cells with specificity for MDM2 were able to escape thymic deletion and persisted in the peripheral T‐cell pool. Functional analysis revealed that these T cells displayed defects in antigen‐driven expansion. This functional impairment of the MDM2‐specific T cells was maintained following adoptive transfer of the T cells into hosts that were unable to present the T‐cell‐receptor‐recognized antigen. This study demonstrates that thymic deletion and the functional impairment of T cells present in the periphery both operate to establish T‐cell tolerance to the tumour‐associated antigen MDM2. Furthermore, the tolerant phenotype was stable and did not require continuous MDM2 peptide presentation in normal tissues.