Small molecule disruption of the LAG-3/HLA-II interaction: implications for modulating T-cell responses

Background Although recent advances in immunotherapy have given some remarkable results in patients with advanced cancers, they are only effective in a proportion of patients, and the search for more effective novel treatments is keenly pursued. Co-inhibitory receptor lymphocyte activation gene-3 (LAG-3) has been shown to inhibit immune responses and therefore, is an attractive target. Despite the importance of LAG-3 in multiple disease settings, including cancer, the role of LAG-3 is yet to be fully understood. However, it is possible that use of a LAG-3 inhibitor, in combination with other cancer therapies, will enhance the anti-tumour immune response. In this study, a novel screening strategy was developed and used to screen a small molecule library against LAG-3 binding to key ligand major histocompatibility complex (MHC)-II. Results AlphaScreen was developed as a screening strategy for weak protein/protein interactions (PPI). Initially the assay was developed using a model low affinity PPI: the T-cell receptor (TCR) binding to peptide-MHC targets. Having demonstrated the effectiveness of AlphaScreen for detecting very low affinity PPI, this approach was subsequently used to explore the biology of LAG-3/MHC-II interactions using a range of HLA (human leukocyte antigen)-DR and –DQ molecules. Following on from this, a high through-put small molecule library screen was employed to selectively inhibit LAG3 binding to HLA-II. From a library of 50 000 small molecules, 8 hit compounds were found. These compounds were tested in a cell toxicity assay, 6 of which were found to be nontoxic. Finally, to develop the whole cell assays further, substantial progress towards development of a LAG-3 reporter cell line was made, using a LAG-3-CD3ζ chimeric receptor in a NFAT reporter cell line. Conclusions The AlphaScreen assay was highly effective at interrogating low affinity PPI. This enabled LAG-3 binding to a range of HLA-II molecules to be examined and allowed identification of specific inhibitors of this interaction. These small molecules will be examined in the future to develop clinically relevant inhibitors