MECHANISTIC AND FUNCTIONAL CHARACTERIZATION OF B AND T LYMPHOCYTE ATTENUATOR (BTLA) IN CD8 TUMOR INFILTRATING LYMPHOCYTES

This dissertation project focused on understanding the functional role of BTLA on CD8+ Tumor Infiltrating Lymphocytes (TIL) from metastatic melanoma patients. Clinical trials of adoptive T-cell therapy (ACT) using autologous ex vivo expanded TIL have demonstrated the great potential of this immunotherapy with an overall clinical response rate 40-50% for stage IV metastatic melanoma patients. We have investigated a number of biomarkers in both the infused TIL and the tumor microenvironment for their association with clinical response. Surprisingly, a subset of CD8+ TIL expressing the co-inhibitory molecule BTLA (B-and-T lymphocyte attenuator) was highly associated with clinical response, while expression of other co-inhibitory molecules such as PD-1, TIM-3, and Lag3 did not associate with response. BTLA is expressed by T cells, B cells, and NK cells and serves as a T cell differentiation maker whereby high expression of BTLA associates with less differentiated T cell phenotype. While the suppressive function of the ITIM and ITSM motifs of BTLA are well described, the Grb2 motif’s function remains understudied. In this study, we sought to determine the functional characteristics of the CD8+BTLA+TIL subset and define the contribution of the Grb2 motif of BTLA in T cell co-stimulation. We have uncovered a survival advantages of the BTLA+ subset that allows for serial killing of target tumor cells, which may explain our previous correlation between this subset and response to TIL ACT. BTLA-HVEM interaction during T cell activation led to the specific activation of SRC kinase. In addition, our results unveiled a role for the BTLA-associated Grb2-binding motif in T cell proliferation and IL-2 production following TCR engagement that was independent of the inhibitory function of ITIM/ITSM motifs. Overall, our study first unveil the dual role of BTLA as both a co-stimulatory and co-inhibitory molecule. The integration of the positive and negative signals transduced by BTLA promotes IL-2 secretion while reducing certain effector function of T cell. Altogether, the combination of both BTLA signaling and inherent attributes of less differentiated T cells could promote T cell survival, persistence, and anti-tumor function