Extracellular Matrix Regulation of Breast Cancer Immune Microenvironment

Abstract

The pathological grade well defines tumor aggressiveness and, indeed, high-grade breast cancer (HGBC), regardless of molecular findings, is characterized by rapid clinical course and very poor prognosis despite standard chemotherapy regimens and specific-targeted agents. Both the cellular and the extracellular compartments of the TME can contribute to the evolution of BC by powerful immune escape and immune suppression processes. In this context structural and extracellular components of the TME, namely the extracellular matrix (ECM), has been shown to contribute to many aspects of tumor progression, exerting important regulatory functions on tumor cells. The relevance of the ECM in cancer progression is strengthened by study showing that the ECM composition is a prognostic factor able to identify patients subgroups endowed with a different clinical outcome according to the distinct enrichment in ECM genes leading to four specific ECM signatures (ECM 1-4). Among the different ECM-related signatures, only the ECM3 signature, that characterize about 35 % of HGBC, identifies aggressive tumors with epithelial mesenchymal transition (EMT) features, poor prognosis, T-cell exclusion and increased MDSCs. Data obtained in mouse models over-expressing SPARC, one key gene of the ECM3, in BC cells suggested that the ECM impact on immune infiltration and that the ECM3 is involved in building an immune suppressive environment. We evaluated whether the immune suppressive features of ECM3 tumors can be sensed in the peripheral blood (PB) of HGBC patients. To this aim, we collected 70 consecutive HGBC patients that included 22 ECM3+ and 30 ECM3- and performed multiparametric flow cytometry analysis to define different immune cell populations. This PhD thesis highlights that the influence of ECM3 signature on immune cells is relevant as it is able to modify the composition of the peripheral blood of ECM3+ patients in favor of an increase in a subset of regulatory T cells (Treg), defined as PD-1neg Treg cells. Notably, we identified PD-1neg Treg being associated to ECM3 patients and provided the mechanisms through which SPARC directly influence the expression of PD-1 on Treg and therefore directly impacting on their suppressive activity. This population together with the increased recruitment in CD33+ suppressive myeloid cells might account for the T-cell excluded microenvironment of ECM3 tumors. Furthermore, analyzing SPARC expression in ECM3 tumours along with CD33+ recruitment, we provided evidences that SPARC is expressed by MDSCs. Back to mouse model, using Sparc-deficient mice, we demonstrated that SPARC is a new MDSCs marker licensing suppressive activities in these cells. Considering the peculiar immune suppressive environment of ECM3 tumors, we evaluated whether zoledronic acid (ZA) could be adopted as a strategy to revert the immunosuppression, particularly acting on MDSCs. We observed, in a few BC available patients, a reduction of the frequency of MDSCs and we showed that FACS-sorted CD33+ cells had decreased expression of PD-L1 and STAT3 after the administration of ZA. Data suggest that the PB is a mirror of ECM3+ tumors and PD-1neg Treg could be and effective target in ECM3+ patients. Therefore, novel therapeutic strategies are an urgent clinical challenge to be addressed in these HGBC patients