On May 2, 2019 Medigene AG (FSE: MDG1, Prime Standard), a clinical stage immuno-oncology company focusing on the development of T cell immunotherapies, reported that Dr. Maja Buerdek, Director Cellular Tools, gave an oral presentation on in vitro assays to evaluate potential TCR-mediated toxicity against neuronal cells at the American Society of Gene & Cell Therapy (ASGCT) (Free ASGCT Whitepaper) Annual Meeting in Washington, DC (Press release, MediGene, MAY 2, 2019, View Source [SID1234535565]).
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Before TCR-transgenic T cells (TCR-Ts) enter clinical studies for adoptive immunotherapy of cancer, they need to be systematically tested for possible toxic effects against healthy tissues. Toxicity can be due either to on-target/off-tumor or off-target recognition by TCR-Ts.
Medigene has developed physiologically relevant 2-dimensional and 3-dimensional in vitro models to assess potential TCR-T-mediated toxicities against a variety of healthy tissues, whereby neuronal toxicity represents a special case due to paucity of non-malignant cell lines and fresh brain tissue samples. Therefore, assessments for neurotoxicity were made using inhibitory GABA neurons and astrocytes derived from induced pluripotent stem cells (iPSCs). Neither neuronal cell type was found to express PRAME antigen as determined by direct qPCR studies in vitro. Furthermore, extensive in silico data of PRAME RNA and protein expression as well as in vitro studies of RNA in a specialized panel of 24 brain tissues failed to detect PRAME expression. Thereby, these cell types theoretically should not represent direct targets for PRAME-specific TCR-Ts through on-target/off-tumor recognition since they lack specific endogenous PRAME expression. Nevertheless, neuronal cells could potentially still be recognized via PRAME-specific TCR-Ts due to off-target toxicity, based on TCR cross-reactivity for an undefined target.
Importantly, cell surface expression of HLA-A2 on neurons, a precondition for recognition by HLA-A2-restricted PRAME-specific TCR-Ts, could only be measured after treatment with interferon gamma, mimicking a pro-inflammatory environment. Therefore, to demonstrate general susceptibility to recognition and killing via TCR-Ts, and to directly assess both on target/off-tumor and off-target toxicity, the neuronal cells were pre-treated with interferon gamma and loaded exogenously with PRAME peptide and then tested with PRAME-specific TCR-Ts. The neuronal cells were recognized and killed, demonstrating that specific recognition could be detected in this 2D assay system. Importantly, non-peptide-loaded neuronal cells survived in the presence of PRAME-specific TCR-Ts in 2D co-cultures assaying for specific cytokine release and killing. Thus, both cell types were not found to be susceptible to on-target/off-tumor toxicity, as predicted, nor were they susceptible to off-target toxicity.
Since neuronal cells could show different characteristics in vivo based on tissue-like structures, 3D neuro-spheroids were generated and utilized in co-culture assays with PRAME-specific TCR-Ts. The results confirmed those described for the 2D system, whereby 3D neuro-spheroids were not killed by the PRAME-specific TCR-Ts, whereas exogenous loading of 3D neuro-spheroids with PRAME peptide led to an efficient killing of the neurons by the PRAME-specific TCR-Ts.
Dr Maja Buerdek, Director Cellular Tools at Medigene, commented: "Medigene has added these new functional in vitro assays for evaluation of TCR-T toxicity against neuronal cells to enlarge our extensive preclinical toolbox that allows for systematic assessment of TCR-T specificity, safety and pre-clinical efficacy in vitro. The PRAME-specific TCR-Ts analyzed in these studies showed a favorable safety pattern against both iPSC-derived astrocytes and GABA neurons in vitro using 2D and 3D co-culture models."