CD8A and neoplasm: The repression of tumour growth by PTPN2‐deficient CD8+ HER‐2 CAR T cells occurred despite tumours harbouring an immunosuppressive microenvironment (Topalian et al, 2015; Popovic et al, 2018), with increased immunosuppressive myeloid‐derived suppressor cells (MDSCs) (Fig 4C) and Tregs (Fig 4D) and the increased expression of immunosuppressive cytokines, including transforming growth factor β (Tgfb) and IL‐10 (Il10) (Fig 4E), when compared to normal mammary tissue.