During persistent activation of immune cells, immune checkpoints are upregulated and inhibit the cell function. While potentially desirable for autoimmunity and excessive immune activation, immune checkpoints are harmful during cancer and potentially harmful during infection. Immune checkpoint inhibitors have revolutionized the treatment of several forms of cancer. Lymphocyte activation gene-3 (LAG3) is among the most promising immune checkpoints for targeting in cancer due to its efficacy and selective expression. However, much about LAG3 remains unknown. The goal of this thesis was to provide a greater understanding of LAG3 expression and mechanism of action.

Because LAG3 research is typically focused on T cells, LAG3 is consequently often ignored on other cells. Also, the LAG3 mechanism of action is mostly uncharacterized due in part to the lack of conserved inhibitory motifs and a consistent model of study. This thesis found that a high frequency of monocytes express LAG3. This expression was confirmed by two separate and frequently used antibodies for LAG3 detection. However, these antibodies detected significantly different fractions of cells expressing LAG3. These antibodies were then tested on LAG3-negative or LAG3-positive Jurkat cells where it was shown that the polyclonal antibody bound non-specifically and with less sensitivity than the monoclonal antibody.

Next, LAG3 expression on T cells was shown to be downregulated when cells were activated in the presence of toll-like receptor (TLR) agonists. Suggesting a mechanism of downregulating LAG3 when an infection is active.

To study the LAG3 mechanism, a model of LAG3 activity was created using transduced Jurkat cells. This model reflected LAG3-mediated inhibition of IL-2 after activation with superantigen. This model was used to identify MAPK pathway as a branch of the TCR signaling pathway influenced by LAG3 expression.

This study sheds light on LAG3 expression and mechanisms of LAG3 expression and function, helping to inform further research into LAG3.