###### Abstract  
Cancer cells, especially cancer stem cells (CSCs), are known for their therapeutic resistance and ability to induce a cancer relapse even many years after successful treatment. The quest for a novel protocol utilizing some commonly used non-oncologic drugs that would improve patients outcomes seems to be the right solution. Aspirin (ASA) is one of such eminent drugs. Our study demonstrated that ASA may exert synergistic effect with the anti-Fas antibody on CSCs of colorectal cancer cell lines. We found that such compound treatment inhibited the pro-cancerous effect of anti-Fas stimulation and decreased pathogenicity, survival and CD133-positive cells' count. Additionally, ASA with anti-Fas antibody may have a positive impact on dendritic cells' functions. Our innovative study explored simultaneous usage of two biologically active compounds which haven't been considered in such combination to assess their significance in colorectal cancer cell biology.  
cancer stem cells; colorectal cancer; anti-Fas; aspirin; dendritic cells +
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clinical trials assess whether ASA can increase disease-free survival in cancer patients (trial identifiers: NCT02467582; NCT02301286; NCT02945033; NCT03170115 at clinicaltrials.gov). Numerous results evidenced that the final therapeutic efficacy of ASA depends on the other chemotherapeutic drug applied in combination with this compound. The observation that ASA presented synergistic activity with anti-PD-L1 antibody (Ab) in the treatment of human tumors [7] laid the foundation for a clinical trial of their combined use in ovarian cancer patients (NCT02659384).  
Fas (Fas, also known as CD95 molecule), a member of the tumor necrosis factor (TNF) receptor family, has been extensively studied for its proapoptotic function [8; 9]. Fas receptor, Fas-mediated apoptotic pathway can be triggered by the caspase cascade activation, including a caspase-3 (one of the effector caspases). However, Fas signaling was also associated with non-apoptotic activities in cancer cells [10; 11; 12; 13; 14; 15]. It was experimentally estimated that the level of Fas in cancer cell membranes necessary for their survival is 1000 times lower than the level required for its pro-apoptotic signaling [11]. Fas-mediated non-apoptotic activity is involved in a variety of signaling pathways independent of the death-promoting track [11; 14; 16; 17; 18; 19]. The mechanisms regulating whether Fas triggers pro- or non-apoptotic signals remain to be fully explained.  
The preliminary assumption is that ASA and anti-Fas antibody (Ab) exert synergistic effect targeting cancer stem cells (CSCs) derived from human CRC cell lines. However, the final effect depends on the cancer cell line used for the analysis [20]. The current manuscript presents the results of experiments evaluating this original hypothesis. The literature and our previous data motivated us to thoroughly analyze Fas signaling functions in CRC progression [20]. The detailed role of Fas signaling for CSCs features and viability are still not fully evident. Since the CSCs vulnerability to Fas ligand (FasL) was demonstrated by the [PERSON] group [21], the issues of harnessing Fas to CSC elimination is appealing.  
Our study aims to present the potential therapeutic activity of ASA administrated simultaneously with anti-Fas Ab in CRC cell lines. The comprehensive analysis of various effects, including CRC cells phenotype change, spherogenicity or cellular viability, that can be associated with treatment effectiveness, was conducted.