LAG3: a novel immune checkpoint expressed by multiple lymphocyte subsets in diffuse large B-cell lymphoma

Blockade of the PD-1 axis has modest efficacy in diffuse large B-cell lymphoma (DLBCL), but data regarding LAG3 are sparse. The impact of LAG3 digital gene expression was tested in 309 patients with DLBCL treated with standard chemoimmunotherapy. Cellular distribution of LAG3 protein was determined by immunohistochemistry and flow cytometry. In tumorinfiltrating lymphocytes (TILs), LAG3 expression was highest on CD4(+) regulatory T cells (Tregs) and was also highly expressed on CD8(+) T cells compared with CD4(+) non-Tregs (both P = .008). LAG3(high) TILs were enriched in PD-1 and TIM-3. LAG3 was also expressed on a proportion of malignant B cells, and these patients had significantly higher LAG3 messenger RNA in their biopsies (P = .03). LAG3(high) gene expression was associated with inferior survival in discovery/validation cohorts, independent of cell of origin and the international prognostic index. Patients who were PD-L1(high) were fivefold more likely to be LAG3 hi g h (P < .0001). Patients who were LAG3(high)/PD-L1(high) had an inferior progression-free survival (P = .011) and overall survival (P = .005) compared with patients who were LAG3(low)/PD-L/(high). Digital spatial protein analysis confirms LAG3 expression on T cells and, surprisingly, tumor-associated macrophages (TAMS) at higher levels than found on CD20(+) B cells in the tumor microenvironment. LAG3 is frequently expressed on CD4(+) Tregs and CD8(+) TILs, typically with other immune checkpoints, and is also present in a proportion of malignant B cells in DLBCL and in areas enriched for TAMS. LAG3 high expression is associated with poor outcome independent of conventional prognosticators.