Elevated BTLA Expression Correlates with an Immunosuppressive Microenvironment and Defines Dysfunctional Circulating...

Background: Successful translation of cancer immunotherapy is underscored by the efficacy of PD-1/PD-L1 and CTLA-4 inhibitors in the treatment of various malignancies. However, their limited efficacy in glioma indicates alternative immune escape mechanisms. We investigated B and T Lymphocyte Attenuator (BTLA), a co-inhibitory receptor structurally and functionally analogous to PD-1, to determine if it constitutes a key, unaddressed mechanism of immune escape and a novel therapeutic target in glioma. Methods: We analyzed BTLA expression and function within the tumor microenvironment of a Moroccan cohort (n = 44). This was complemented by multiparameter flow cytometry on peripheral blood from glioblastoma (GBM) patients (n = 8) to assess circulating T cell profiles. Findings were corroborated using independent transcriptomic datasets from TCGA and CGGA cohorts. Single-cell RNA-seq and citeSeq identified specific BTLA-expressing cell populations. Results: Elevated BTLA expression was significantly associated with aggressive features and poor overall survival in glioma patients. Mechanistically, BTLA levels were positively correlated with pro-tumorigenic factors, immune infiltration, and immunosuppressive checkpoints. Single-cell and citeSeq analyses revealed that BTLA was primarily expressed by exhausted T cells and conventional type 1 dendritic cells (cDC1) within the GBM microenvironment. Crucially, this phenotype was translated systemically; BTLA defined dysfunctional circulating CD8⁺ and CD4⁺ T cells characterized by diminished IFN-γ production, alongside reduced granzyme B and perforin in CD8⁺ T cells. Conclusions: Our findings indicate that BTLA may represent a relevant pathway associated with an immunosuppressive glioma microenvironment. The therapeutic potential of targeting this pathway, particularly in combination with PD-1/PD-L1 blockade, warrants further investigation.