Adoptive T cell therapy: Highly effective PRAME-specific T cell receptor identified

Researchers led by Nadja Sailer from Medigene Immunotherapies GmbH in Planegg, Germany, have taken an important step forward in the treatment of melanoma and other solid tumors: They found a highly effective PRAME-specific T cell receptor for adoptive T cell therapy  (1).
Peptides from peptides&elephants helped them rule out any off-target toxicity. A single dose of accordingly modified T cells – together with a chimeric receptor that overrides the hostile tumor microenvironment – was sufficient to eliminate a hard-to-treat melanoma xenograft in mice.

Developing adoptive T cell therapies to treat solid tumors is challenging. The scientists have to:

  • identify suitable target antigens.
  • generate T cell receptors that have high sensitivity and specificity for the antigen.
  • find a way to bypass the hostile microenvironment of the tumor.
  • identify recipient T cells that incorporate the generated T cell receptors and efficiently kill tumor cells.

The cancer/testis antigen Preferentially Expressed Antigen in Melanoma (PRAME) is a promising target for the development of effective adoptive T cell therapies: It is highly expressed in several solid tumors such as melanoma, non-small cell lung cancer, multiple sarcoma subtypes, and epithelial ovarian cancer, but in normal tissue it is almost exclusively confined to the testis.
Sailer and collegues have managed to identify a PRAME-specific T cell receptor that is highly effective. They used their well-established high-throughput process to generate and characterize specific T cell receptors.

Tumor microenvironment drives T cells to exhaustion

However, the scientists still had to overcome another hurdle: The immunosuppressive networks in the tumor significantly reduce the efficacy of T cells. For example, T cells are constantly exposed to antigen-depleted metabolic factors such as glucose, inhibitory cytokines such as IL-10 and the PD-L1 expression of tumor and stromal cells. Since T cells in the tumor microenvironment often express large amounts of the PD-1 receptor, encountering PD-L1-expressing tumor cells effectively inhibits T cell effector functions. In this way, the immunosuppressive networks can drive T cells to exhaustion.

Chimeric PD1-41BB receptor prevents T cell exhaustion

To overcome the hostile microtumor environment, Sailer and colleagues converted the inhibitory PD-1 signal in the T cells into an active signal: They generated a chimeric receptor that combines the extracellular domain of PD-1 and the signaling domain of 4-1BB. 4-1BB is a stimulatory signaling module that has already been used in several chimeric antigen receptors. These antigen receptors have been shown to be clinically highly effective in immunotherapy.

Off-target toxicity tested with peptides from peptides&elephants

To detect off-target toxicity of the PD1-41BB receptor, the researchers identified homologous peptides with up to four differences to the original SLL epitope (SLLQHLIGL) of PRAME. Using the prediction tool Expitope 2.0®, they assessed the potential of the homologous peptides to bind to HLA-A2. peptides&elephants supplied the selected peptides and the researchers tested whether transduced and untransduced CD8+ T cells could recognize the peptides. To do this, they loaded the peptides in high concentrations onto T2_PD-L1 cells and co-cultured them with T cells expressing the PRAME T cell receptor. The T cells also partially expressed the PD1-41BB receptor.
The result: Fourteen peptides stimulated IFN-γ release above background level, regardless of the presence of PD1-41BB. But only one peptide was recognized at a level comparable to the PRAME-SLL peptide. The scientists then used cloning experiments to test whether the target cells processed the recognized peptides and presented them in sufficient quantities. As this was not the case, they could rule out that any of the homologous peptides stimulated a response from the receptor-modified T cells.

One dose eradicated melanoma xenograft in mouse model

To test the effectiveness of their receptor combination, the scientists added the PD1-41BB receptor to CD8+ T cells expressing the transgenic PRAME T cell receptor. Subsequent repeated exposure to tumor cells increased IFN-γ secretion, improved cytotoxic capacity, and prevented T cell exhaustion.
Treatment trials in a mouse model were also promising: a single dose of the T cell receptor-modified T cells with PD1-41BB was sufficient to eliminate a hard-to-treat melanoma xenograft. In contrast, T cell receptor-modified T cells without PD1-41BB were unable to clear the PD-L1-positive tumors.

1) Sailer N et al.: T-Cells Expressing a Highly Potent PRAME-Specific T-Cell Receptor in Combination with a Chimeric PD1-41BB Co-Stimulatory Receptor Show a Favorable Preclinical Safety Profile and Strong Anti-Tumor Reactivity. Cancers (Basel). 2022 Apr 14;14(8):1998. doi: 10.3390/cancers14081998. 

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