Abstract 3250: B cell-produced IL-27 up-regulates PD-L1 expression in the tumor microenvironment to promote breast cancer development

The host immune response can be evaded by tumor cells, e.g., through upregulation of the PD-L1/PD-1 immune checkpoint, and - paradoxically - even hijacked to promote cancer progression. Here we showed that B lymphocytes, which can produce anti-tumoral antibodies/autoantibodies in breast cancer (BCa), played an important role in promoting murine BCa development, as B cell-deficient μMT mice displayed virtually abrogated tumor growth from transplanted syngeneic ERα+ breast adenocarcinoma cells, concomitant with significantly reduced PD-L1 expression and much increased cell death in residual tumors. In BCa that grew in wildtype mice and in tumor-associated ectopic lymph nodes, B cells were the major source of IL-27, which is the IL-27p28/EBI3 heterodimer and a pleotropic cytokine with both pro- and anti-inflammatory functions. Deficiency in IL-27 expression specifically in B cells also significantly reduced the BCa growth, indicating that the effect of B cells in BCa development was mainly through IL-27 production. This was effectively induced in B cells primed by TLR ligands, including those that would be released during uncontrolled cell death in the tumor microenvironment, and stimulated by CD154 and IL-21, as likely expressed by activated tumor-infiltrating CD4+ T cells. IL-27 in turn triggered JAK-dependent STAT1 and STAT3 phosphorylation in BCa cells, enhanced PD-L1 gene expression and promoted growth of these cells. In addition, among the 2040 genes compiled by the Ingenuity Pathway Analysis as involved in BCa tumorigenesis, those encoding PD-L1, IFN-γ and Hif-1α (cytokine and transcription factor known to induce PD-L1, respectively), IDO-1, chemokine receptor CXCR3 and its ligand CXCL10 were specifically upregulated in B cells by IL-27, which also induced an overall gene signature similar to what induced by IFN-γ, reflecting the activation of STAT1 and STAT3 by both cytokines. In humans, the two genes encoding IL-27 receptor subunits were highly expressed in the breast tissue, at levels comparable to those in lymphoid organs. As shown by The Cancer Genome Atlas (TCGA) and The Kaplan Meier Plotter datasets, the higher IL27p28 expression was associated with worse survival in BCa patients overall, who displayed increased circulating IL-27 levels as compared to healthy subjects, and in ER+ BCa patients after endocrine therapies. Indeed, IL-27 supported human BCa cell growth in the presence of tamoxifen, suggesting a role of B cell-produced IL-27 in the ac