Melanoma patients harbor tumor mutation-specific PD1+ CD8 T cells in the peripheral blood

Steven Rosenberg's research group at National Institutes of Health (NIH) continues to define and refine condition for T cell-based cancer-specific immunotherapy. (for example, introduction of high-throughput personalized screening strategy capable of evaluating T cell reactivity to neo-antigens presented on all of the HLA restriction elements of the individual).

This time, their group showed that peripheral blood of melanoma patients harbors small but distinct PD1+ CD8 T cell population enriched in specificities for cancer associated neo-antigens.

If one compares CD8 T cells from PBMC v Tumor sites (TIL), blood derived T cells contains few PD1⁺/PD1^high CD8 T cells.

However, when the authors has expanded in vitro those sorted PD1⁺ CD8 T cells and co-cultured them with autologous dendritic cells expressing tumor neo-antigens (as tandem minigenes, TMG), they could identify circulating neoantigen-reactive CD8 T cells in three of the four melanoma patients evaluated.

Then the authors re-constructed blood PD1⁺/PD1^high CD8 T cell TCR specificity by (a) pairing the sequences encoding the two most-dominant TCR-α and TCR-β sequences, (b) cloning them into retroviral vectors and (c) transducing autologous PBMC. This TCR construct could [for example] detect neo-antigens derived from mutations in the genes MAGE family member A6 (MAGEA6).

Importantly, both PD1⁺/PD1^high CD8 T cells enriched from peripheral blood or T lymphocytes transduced with retroviruses expressing neo-antigen-specific TCRs could detect autologous tumor cell lines.

Finally, the authors found that blood and tumor site derived PD1⁺ CD8 T cell showed high degree of overlap in their TCR specificity [to tumor neo-antigens], suggesting that analysis of peripheral PD1⁺ CD8 T cells from cancer patients could reveal TCR specificities of tumor infiltrated lymphocytes.

In summary, this study is another evidence that cancer immunotherapy holds great promise in providing cancer antigen-tailored treatments. Identification of cancer neo-antigen specific T cells (TCRs) as shown in this study, would accelerate development of tumor-specific TCR constructs and could contribute in overcoming precursor limitation inherent to endogenous T cell clones.

David Usharauli