Lung cancer remains the leading cause of cancer deaths worldwide, with non-small cell lung cancer (NSCLC) accounting for ~80% of all cases. While advances in immunotherapy have contributed to improved survival of NSCLC patients, only 20% respond to treatment. A major factor contributing to this anti-tumour immunity is the presence of immunosuppressive regulatory T (Treg) cells within the tumour microenvironment. Treg cell infiltration is a well-established negative prognostic factor for NSCLC patients, however, completely eradicating Treg cells would unleash catastrophic autoimmunity. Understanding the differences between Tregs in tumours and healthy tissues is critically needed to develop novel therapies targeting tumour-associated Treg cells.
CyTOF data of primary human lung tumours and matched non-malignant tissue has revealed a significant shift in the expression of pro-survival proteins in tumour-associated Treg cells compared with Treg cells in the healthy lung. Specifically, MCL-1 upregulation in tumour-infiltrating Treg cells was observed in all early-stage NSCLC patients. Flow cytometric analyses of effector tumour Treg cells in the K-RasG12D;p53fl/fl mouse model of lung cancer also revealed increased MCL-1 expression compared to wild-type lungs. Furthermore, combination treatment with anti-PD1 and MCL-1 inhibitor (S63845) showed prolonged survival compared with anti-PD-1-only therapy.
We investigated how inhibition of MCL-1 and anti-PD1 therapy regulated the spatial localisation and numbers of tumour-infiltrating T cells in K-RasG12D;p53fl/fl lung tumours using OPAL multiplex immunofluorescence. A significant reduction of Ki67+ PD1+ effector Treg cells was observed in tumour tissues after MCL-1 inhibition, consistent with previous flow cytometry data. However, MCL-1 inhibition also depleted the infiltration of functional effector CD8+ and CD4+ cells into tumours, highlighting the need to explore new drug targets that target specifically tumour-associated Treg cells. However, further validation is necessary to evaluate the effects of combination anti-PD1 and S63845 treatment on the depletion of tumour-associated Treg cells and infiltration of functional immune cells. Nevertheless, we anticipate our preliminary work will provide a foundation to uncover new avenues to targeting tumour Treg cells and improve patient outcomes to immunotherapy.