LAG3 (CD223) as a cancer immunotherapy target

Abstract

Despite the impressive impact of CTLA4 and PD1-PDL1-targeted cancer immunotherapy, a large proportion of patients with many tumor types fail to respond. Consequently, the focus has shifted to targeting alternative inhibitory receptors (IRs) and suppressive mechanisms within the tumor microenvironment. Lymphocyte activation gene-3 (LAG3) (CD223) is the third IR to be targeted in the clinic, consequently garnering considerable interest and scrutiny. LAG3 upregulation is required to control overt activation and prevent the onset of autoimmunity. However, persistent antigen exposure in the tumor microenvironment results in sustained LAG3 expression, contributing to a state of exhaustion manifest in impaired proliferation and cytokine production. The exact signaling mechanisms downstream of LAG3 and interplay with other IRs remain largely unknown. However, the striking synergy between LAG3 and PD1 observed in multiple settings, coupled with the contrasting intracellular cytoplasmic domain of LAG3 as compared with other IRs, highlights the potential uniqueness of LAG3. There are now four LAG3-targeted therapies in the clinic with many more in preclinical development, emphasizing the broad interest in this IR. Given the translational relevance of LAG3 and the heightened interest in the impact of dual LAG3/PD1 targeting in the clinic, the outcome of these trials could serve as a nexus; significantly increasing or dampening enthusiasm for subsequent targets in the cancer immunotherapeutic pipeline.

Keywords: CD223; LAG3; cancer immunotherapy; immune regulation; inhibitory receptors; monoclonal antibodies; regulatory T cells.

Figure 1. Ligand interaction and structural similarities between LAG3 and CD4

LAG3, like CD4, consists of four extracellular immunoglobulin superfamily-like domains (D1-D4). LAG3 utilizes an additional 30 amino acid loop in D1 to bind to MHC class II with greater affinity. Ligation of MHC class II, expressed by antigen presenting cells or aberrantly by melanoma cells, with LAG3 mediates an intrinsic negative inhibitory signal, in which the KIEELE motif in the cytoplasmic domain is indispensable. LAG3 is highly glycosylated with two additional ligands postulated, LSECtin, expressed on melanoma cells, and Galectin-3, expressed on stromal cells and CD8⁺ T cells in the tumor microenvironment.

Figure 2. Alignment of the LAG3 cytoplasmic domain

The cytoplasmic domain of the species indicated is shown. Boxes: Blue = potential serine phosphorylation site; Green = ‘KIEELE’ motif region; Red = ‘EP’ repetitive motif. Residues: Red = EP residues within the EP motif region; Blue = conserved residues (100%); Purple = semi-conserved residues (>85% – 25/29); Green = dominant residues (>60% – 18/29).

Figure 3. LAG3 cell surface shedding mediated by ADAM10/17 metalloproteinases

Upon TCR activation, Adam10 mRNA increases and ADAM17 enzymatic activity is enhanced by protein kinase C-θ-dependent phosphorylation. ADAM10/17 cleaves LAG3 within the connecting peptide (CP) between the membrane-proximal D4 domain and the transmembrane domain, releasing soluble LAG3.