Therapy with autologous T cells that have been gene-engineered to express chimeric antigen receptors (CAR) or T cell receptors (TCR) provides a feasible and broadly applicable treatment for cancer patients. In a clinical study in advanced renal cell carcinoma (RCC) patients with CAR T cells specific for Carbonic Anhydrase IX (CAIX) we observed toxicities which (most likely) indicated in vivo function of CAR T cells as well as low T cell persistence and clinical response rates. The latter observations were confirmed by later clinical trials in other solid tumor types and other gene-modified T cells. To improve the efficacy of T cell therapy, we have re-defined in vitro conditions to generate T cells with young phenotype, a key correlate with clinical outcome. For their impact on gene modified T cell phenotype and function, we have tested various anti-CD3/CD28 mAb-based T cell activation and expansion conditions as well as several cytokines prior to and/or post gene transfer using two different receptors: CAIX CAR and MAGE-C2(ALK)/HLA-A2 TCR. In a total set of 16 healthy donors, we observed that T cell activation with soluble anti-CD3/CD28 mAbs in the presence of both IL15 and IL21 prior to TCR gene transfer resulted in enhanced proportions of gene-modified T cells with a preferred in vitro phenotype and better function. T cells generated according to these processing methods demonstrated enhanced binding of pMHC, and an enhanced proportion of CD8+,CD27+,CD62L+,CD45RA+ T cells. These new conditions will be translated into a GMP protocol in preparation of a clinical adoptive therapy trial to treat patients with MAGE-C2-positive tumors.