In addition to defending against invasive microbes, T cells of the mammalian adaptive immune system also provide crucial protection against cancerous cells that spontaneously emerge from healthy host tissue. Cancer cells, in turn, co-opt many of the regulatory signaling pathways used by the body to prevent excessive T cell proliferation and activation. The tumor microenvironment (TMI) is frequently a highly immunosuppressive environment, and much research has been focused on identifying new targets that can enhance T cell antitumor immunity. Novel observations from my lab has found monoamine oxidase A (MAO-A) plays an important role in maintaining this immunosuppressive TMI. In Chapter 2, I observe that MAO-A functions as a “checkpoint” molecule that regulates CD8+ T cell antitumor immunity through modulating metabolization of serotonin, a neurotransmitter previously unassociated with the immune response. In Chapter 3, I observe that MAO-A also indirectly affect T cell antitumor immunity by promoting tumor associated macrophage polarization towards an immunosuppressive phenotype through signaling with oxidative stress. These two chapters provide exciting preclinical data and clinical correlation for repurposing clinical approved MAO-A inhibitors for cancer immunotherapy. Allogeneic hematopoietic stem cell transplantation is another approach to treating cancer, in particular hematological malignancies, and relies on donor T cell recognition of minor histocompatibility antigens to target residual cancer cells. However, its application is severely limited due to the risk of graft versus host disease (GvHD). Higher invariant natural killer T (iNKT) cell load following transplantation has been clinically associated with reduced GvHD without loss of the beneficial graft versus leukemia/lymphoma (GvL) effect. However, the accessibly of iNKT cells remains a major hurdle to their research and potential clinical application. In Chapter 4, I provide two protocols for generating large amounts of mouse and human iNKT cells in vivo through TCR engineering of hematopoietic stem cells (HSC-engineered iNKT) and a murine intermediary. In Chapter 5, a protocol to generate human HSC-engineered iNKT cells in vitro is introduced, and the resultant HSC-engineered iNKT are found capable of ameliorating xeno-GvHD without loss of GvL effect when added to the allograft, providing exciting preclinical data highlighting the possibly of using HSC-engineered iNKT cells as additives to the allograft to reduce GvHD risk and improve patient outcomes.