MALAT-1: Immunomodulatory lncRNA hampering the innate and the adaptive immune arms in triple negative breast cancer

BACKGROUND: Triple negative breast cancer (TNBC) is known as hot immunogenic tumor. Yet, it is one of the most aggressive BC subtypes. TNBC evolve several tactics to evade the immune surveillance phenomena, one of which is shedding of natural killer (NK) cells activating immune ligands such as MICA/B and/or by inducing the expression of the immune checkpoints such as PD-L1 and B7-H4. MALAT-1 is an oncogenic lncRNA. MALAT-1 immunogenic profile is not well investigated. AIM: The study aims at exploring the immunogenic role of MALAT-1 in TNBC patients and cell lines and to identify its molecular mechanism in altering both innate and adaptive immune cells present at the tumor microenvironment of TNBC METHODS: BC patients (n = 35) were recruited. Primary NK cells and cytotoxic T lymphocytes were isolated from normal individuals using the negative selection method. MDA-MB-231 cells were cultured and transfected by several oligonucleotides by lipofection technique. Screening of ncRNAs was performed using q-RT-PCR. Immunological functional analysis experiments were performed upon co-culturing primary natural killer cells and cytotoxic T lymphocytes using LDH assay. Bioinformatics analysis was performed to identify potential microRNAs targeted by MALAT-1. RESULTS: MALAT-1 expression was significantly upregulated in BC patinets with a profound expression in TNBC patients compared to their normal counterparts. Correlation analysis revealed a positive correlation between MALAT-1, tumor size and lymph node metastasis. Knocking down of MALAT-1 in MDA-MB-231 cells resulted in a significant induction of MICA/B, repression of PD-L1 and B7-H4 expression levels. Enhancement of cytotoxic activity of co-cultured NK and CD8(+) cells with MALAT-1 siRNAs transfected MDA-MB-231 cells. In silico analysis revealed that miR-34a and miR-17–5p are potential targets to MALAT-1; accordingly, they were found to be downregulated in BC patients. Forcing the expression of miR-34a in MDA-MB-231 cells resulted in a significant induction in MICA/B levels. Ectopic expression of miR-17–5p in MDA-MB-231 cells significantly repressed the expression of PD-L1 and B7-H4 checkpoints. Validations of MALAT-1/miR-34a" and "MALAT-1/miR-17–5p axes were performed by a series of co-transfections and functional assessment of cytotoxic profile of primary immune cells. CONCLUSION: This study proposes a novel epigenetic alteration exerted by TNBC cells mainly by inducing the expression of MALAT-1 lncRNA. MALAT-1 mediates innate and adaptive immune suppression events partially via targeting miR-34a/MICA/B and miR-175p/PD-L1/B7-H4 axes in TNBC patients and cell lines.