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Introduction
Oral cancers are the 10th most common cancer worldwide, with oral tongue squamous cell carcinoma (OTSCC) having the highest incidence.1 Surgery and radiotherapy (RT), the mainstay treatments for OTSCC, impact on the patients' quality of life and the 50% 5-year survival has remained unchanged for 40 years.1
Cancer stem cells (CSCs) have been proposed to be the origin of many cancers, including oral cavity squamous cell carcinoma (OCSCC). CSCs express the embryonic stem cell (ESC) markers OCT4,2 NANOG,3 SOX2,2 SALL43 and STAT3,4 the more ‘downstream’ CSC marker CD442 and the epithelial cell marker p63,5 suggesting a diverse phenotype. The relative abundance and co-expression of these markers and their localisation within OCSCC remain unclear.
Immunohistochemistry
4 μm thick formalin-fixed paraffin-embedded sections of moderately differentiated OTSCC (MDOTSCC) samples from seven male and three female patients, aged 30–84 (mean 63.3) (see online supplementary table S1) were used for immunohistochemical (IHC) staining, as previously described.6 All sections underwent single 3,3-diaminobenzidine (DAB) IHC staining for the primary antibodies NANOG, 1:500 (cat#D73G4; Cell Signaling Technology, Danvers, Massachusetts, USA), SOX2, 1:500 (cat#PA1-094; Thermo Fisher Scientific, Santa Cruz, California, USA), CD44, 1:1500 (cat#MRQ-13; Cell Marque, Santa Cruz, California, USA), pSTAT-3, 1:100 (cat#D3A7; Cell Signaling Technology), SALL4, 1:100 (cat#6E3; Cell Marque) and OCT4, 1:30 (cat#MRQ-10; Cell Marque), Bond Ready-To-Use p63 (cat#NCL-p63; Leica, Newcastle, UK). To confirm co-expression of two proteins, representative slides of MDOTSCC (n=2) underwent immunofluorescent (IF) IHC staining using a combination of VectaFluor Excel anti-rabbit 594 (ready-to-use, cat#VEDK-1594, Vector Laboratories, Burlingame, CA, USA) and Alexa Fluor anti-mouse 488 (1:500, cat#A21202, Life Technologies) to detect combinations that included NANOG, SOX2 and pSTAT3 and VectaFluor Excel anti-mouse (ready-to-use, cat#VEDK2488, Vector Laboratories) and Alexa Fluor anti-rabbit 594 (1:500, cat#A21207, Life Technologies) to detect combinations that included OCT4 or SALL4. Positive …