Is antigen specificity the key to efficient adoptive T-cell therapy?

IRES-dependent translation of the long non coding RNA meloe in melanoma cells produces the most immunogenic MELOE antigens

MELOE-1 and MELOE-2, two highly specific melanoma antigens involved in T cell immunosurveillance are produced by IRES-dependent translation of the long « non coding » and polycistronic RNA, meloe. In the present study, we document the expression of an additional ORF, MELOE-3, located in the 5' region of meloe. Data from in vitro translation experiments and transfection of melanoma cells with bicistronic vectors documented that MELOE-3 is exclusively translated by the classical cap-dependent pathway. Using a sensitive tandem mass spectrometry technique, we detected the presence of MELOE-3 in total lysates of both melanoma cells and normal melanocytes. This contrasts with our previous observation of the melanoma-restricted expression of MELOE-1 and MELOE-2. Furthermore, in vitro stimulation of PBMC from 6 healthy donors with overlapping peptides from MELOE-1 or MELOE-3 revealed a very scarce MELOE-3 specific T cell repertoire as compared to the abundant repertoire observed against MELOE-1. The poor immunogenicity of MELOE-3 and its expression in melanocytes is consistent with an immune tolerance towards a physiologically expressed protein. In contrast, melanoma-restricted expression of IRES-dependent MELOE-1 may explain its high immunogenicity. In conclusion, within the MELOE family, IRES-dependent antigens represent the best T cell targets for immunotherapy of melanoma.

Improving the in vivo persistence, distribution and function of cytotoxic T lymphocytes by inhibiting the tumor immunosuppressive microenvironment

Adoptive cell transfer immunotherapy of malignant tumors has the problem of symbiosis between effector cells and tumor cells, a short in vivo residence time, and a poor killing efficiency of effector cells. Thus, releasing effector cells from the cancer immunosuppressive microenvironment and improving their effective time and functional status in vivo would seem to be ideal strategies for facilitating immunotherapy. Low-dose cyclophosphamide administration can effectively break immunotolerance by inhibiting regulatory T cells. In the present study, in order to verify whether the persistence, distribution and function of effector cells can be improved by inhibiting immunosuppressive microenvironment, low-dose cyclophosphamide was previously intraperitoneally injected into melanoma-bearing C57BL/6 mice, thereafter, CFSE-labeled cytotoxic T lymphocytes were transfused intravenously, and their effective time, distributive pattern, and killing efficiency in different groups were observed by measuring the fluorescence intensity and cell cycle of cytotoxic T lymphocytes distributed in various organs, in comparison with tumor growth. We found down-regulating Tregs in vivo can simultaneously reduce the levels of interleukin-10 and transforming growth factor-β. Migration and distribution of cytotoxic T lymphocytes in vivo was found to vary with time. Inhibition of immunotolerance can significantly improve the persistence, distribution, and function of cytotoxic T lymphocytes. Correspondingly, significantly higher secretion of perforin, granzyme B, IL-2, and IFN-γ in tumor tissues with decreased tumor growth was seen in the cyclophosphamide injection group than in the control group. Our study may provide useful information on the cyclophosphamide-mediated mechanism for facilitating tumor immunotherapy by inhibiting the immunosuppressive tumor microenvironment.

Adoptive transfer with high-affinity TCR to treat human solid tumors: how to improve the feasibility?

The adoptive transfer of tumor antigen-specific T cells recently achieved clinical efficacy for a fraction of melanoma patients refractory to other therapies. Unfortunately, the application of this strategy to the remaining melanoma and most other cancer patients is hampered by the difficulty to generate high-affinity tumor-reactive T cells. Two strategies are currently developed to extend the feasibility of this therapeutic approach: clinical grade tool production for MHC-peptide multimer-driven sorting of antigen-specific T cells from the endogenous peripheral T cell repertoire and de novo engineering of the missing repertoire by genetic transfer of cloned specific T cell receptor (TCR) into T cells. The expected multiplication of adoptive transfer treatments, by these strategies, and their careful evaluation should enable the cure of a number of otherwise compromised cancer patients and to gain insight into the characteristics of transferred T cells best fitted to eradicate tumor cells, in terms of antigen specificities, phenotype, and functions. In particular, identification of tumor-rejection antigens by this approach would improve the design and efficacy of all immunotherapeutic approaches.