The commercialization of the first ever Chimeric Antigen Receptor (CAR) T cell therapies, Kymriah™ and Yescarta™, in 2017 proved to be a major advancement in personalized cancer treatment. The success of such adoptive T cell therapies has led to the rapid advance in growth of CAR-T therapy development from the 2nd generation anti-CD19 CAR’s, which while efficacious showed issues with long-term persistence, to novel 3rd generation anti-CD19 CAR’s which show improved clinical efficacy. Future development of CAR-T therapies will seek to overcome variations in responses and unpredictable toxicity in patients, which can be partially attributed to interpatient heterogeneity of CAR-T products. An interesting approach for overcoming CAR-T cell heterogeneity is to select specific CAR-T subpopulations. Three key gamma-chain cytokines, interleukin (IL)-2, IL-7, and IL-15 have been shown independently to play a role in the proliferation and differentiation of T cells. This dissertation demonstrated the impact of all three cytokines on various combinations on four T cell subsets (CD4+ naïve, CD4+ memory, CD8+ naïve and CD8+ memory) and determined optimal concentrations for proliferation and differentiation which may be translated to future CAR-T manufacturing. IFN-γ is a key moderator of cell-mediated immunity, released in large amounts in activated CD8+ T cells and exerts a variety of pro-inflammatory functions in these cells, suggesting that different populations with varying cytolytic potential exist and that IFN-γ can be used as an efficacy marker for CD8+ CAR-T cells. Single-cell expression studies of CAR-T cells have shown that the levels of cytokine expression are highly variable, with expression of these cytokines reflecting an activated state that will correlate with anti-tumor activity in patients. Thus, a ‘cytokine-optimized’ xiii CAR-T product would balance the levels of cytokine production for optimal activation and efficacy yet avoid the pitfalls of overstimulation. On the basis that cytokine expression is key, I have developed a method to assess CAR-T cells within a polyclonal population, based on their CAR expression levels. I designed a fusion protein that contains an IFN-γ scFv fused to the CD19 extracellular domain and have characterized it using a multitude of protein and cell-based assays. Size identification of the fusion protein using SDSPAGE gel electrophoresis confirmed the fusion protein produced was of the estimated molecular mass, including evaluation of the glycosylation patterns. Several different ELISA assays confirmed specific binding of the novel fusion protein to various physiologically relevant concentrations of IFN-γ. Further ELISA assays confirmed the binding of the CD19 ECD to the common anti-CD19 scFv, FMC63, through use of an FMC63-Fc protein. Binding of the fusion protein to anti-CD19 CAR expressing HEK293T and T cells was confirmed using flow cytometry and the sensitivity range was evaluated. This fusion protein can successfully identify individual CAR-T expressing cells, with work continuing to evaluate the identification of cell-specific IFN-γ secretion. Further, this fusion protein has the potential for use in cell therapy analytics (to determine the percentage of population that are high secreting) and as a process step, to isolate desired subpopulations with improved safety and efficacy.