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Cancer stem cell enrichment marker CD98: A prognostic factor for survival in patients with human papillomavirus-positive oropharyngeal cancer

Open AccessPublished:December 06, 2013DOI:https://doi.org/10.1016/j.ejca.2013.11.010

      Abstract

      Purpose

      Several hypotheses have been proposed to explain the relatively good prognosis of patients with a human papillomavirus (HPV)-positive oropharyngeal squamous cell carcinoma (OPSCC) and one of these is a higher sensitivity to (chemo)radiation. Previous studies have suggested that treatment failure in OPSCC patients is caused by resistance of cancer stem cells (CSCs). The purpose of this study was to evaluate the association between the number of CSCs and prognosis in HPV-positive OPSCC patients.

      Experimental design

      All OPSCC patients (n = 711) treated between 2000 and 2006 in two Dutch university hospitals were included. Presence of HPV in a tumour tissue specimen was tested by p16-immunostaining followed by HPV DNA GP5+/6+polymerase chain reaction (PCR). The presence and intensity of tumour CSC markers CD44 and CD98 were determined by immunohistochemistry and semiquantitative scoring was performed. Overall survival (OS) and progression-free survival (PFS) rates were compared between patients with low and high CD44/CD98 expression in relation to HPV status.

      Results

      HPV-positive tumours showed a lower percentage of cells with CD44 and CD98 expression than HPV-negative tumours (p < 0.001, χ2-test). Within the group of patients with HPV-positive OPSCC, a high percentage of CD98-positive tumour cells was associated with a significantly worse 5-year OS and PFS (OS: 36.4% and PFS: 27.3%) compared to patients with a low percentage of CD98-positive cells (OS: 71.9% and PFS: 70.5%, respectively) (p < 0.001).

      Conclusions

      HPV-positive OPSCCs harbour fewer cells expressing the CSC enrichment markers CD44 and CD98. Furthermore, OS and PFS were significantly worse for patients with HPV-positive OPSCC with a high percentage of CD98-positive cells.

      Keywords

      1. Introduction

      Infection with high-risk human papillomavirus (HPV) is aetiologically linked to the development of head and neck squamous cell carcinomas (HNSCCs), particularly those carcinomas that arise in the oropharyngeal region. HPV-positive oropharyngeal squamous cell carcinomas (OPSCCs) are characterised by an epidemiologic, demographical and clinical profile that deviates from that of HPV-negative OPSCCs [
      • Leemans C.R.
      • Braakhuis B.J.
      • Brakenhoff R.H.
      The molecular biology of head and neck cancer.
      ,
      • Westra W.H.
      The morphologic profile of HPV-related head and neck squamous carcinoma: implications for diagnosis, prognosis, and clinical management.
      ]. The most important difference is related to prognosis, which is markedly better for patients with an HPV-positive tumour compared to those with an HPV-negative tumour. Several hypotheses have been proposed to explain the improved outcome for patients with HPV-positive OPSCC, including an increased sensitivity to radiation and chemotherapy, differences in the role of the host immune system and the absence of field cancerisation [
      • Ang K.K.
      • Harris J.
      • Wheeler R.
      • et al.
      Human papillomavirus and survival of patients with oropharyngeal cancer.
      ,
      • Fakhry C.
      • Westra W.H.
      • Li S.
      • et al.
      Improved survival of patients with human papillomavirus-positive head and neck squamous cell carcinoma in a prospective clinical trial.
      ,
      • Kimple R.J.
      • Smith M.A.
      • Blitzer G.C.
      • et al.
      Enhanced radiation sensitivity in HPV-positive head and neck cancer.
      ,
      • Spanos W.C.
      • Nowicki P.
      • Lee D.W.
      • et al.
      Immune response during therapy with cisplatin or radiation for human papillomavirus-related head and neck cancer.
      ]. In this study, we further evaluated the role of cancer stem cells (CSCs) in HPV-positive OPSCCs.
      Previous studies suggest that treatment failure in HNSCC patients might be the consequence of therapy resistance of cancer stem cells (CSCs) [
      • de Jong M.C.
      • Pramana J.
      • van der Wal J.E.
      • et al.
      CD44 expression predicts local recurrence after radiotherapy in larynx cancer.
      ]. CSCs represent a small subpopulation of tumour cells that maintain tumour growth by fuelling the expansion of the malignant cell population infinitely [
      • Bao S.
      • Wu Q.
      • McLendon R.E.
      • et al.
      Glioma stem cells promote radioresistance by preferential activation of the DNA damage response.
      ]. CSCs can be distinguished from the bulk of the tumour based on differential expression of protein markers on the cell membrane. A large body of evidence indicates that HNSCC cells expressing high levels of the CD44 antigen possess CSC properties. CD44high HNSCC cells have been shown to initiate tumour growth in mice much more efficiently than CD44low cells, indicating that CSCs are enriched in the CD44high subpopulation of HNSCC [
      • Prince M.E.
      • Sivanandan R.
      • Kaczorowski A.
      • et al.
      Identification of a subpopulation of cells with cancer stem cell properties in head and neck squamous cell carcinoma.
      ]. Moreover, a high expression of CD44 seems associated with a poor prognosis in patients with HNSCC [
      • Lindquist D.
      • Ahrlund-Richter A.
      • Tarjan M.
      • Tot T.
      • Dalianis T.
      Intense CD44 expression is a negative prognostic factor in tonsillar and base of tongue cancer.
      ].
      Recently, we examined CD98 as a novel, putative CSC enrichment marker in HNSCC and showed that CD98high cells, in contrast to CD98low cells, are able to generate tumours in immune-deficient mice [
      • Martens-de Kemp S.R.
      • Brink A.
      • Stigter-van Walsum M.
      • et al.
      CD98 marks a subpopulation of head and neck squamous cell carcinoma cells with stem cell properties.
      ]. Studies in a multitude of cancer types showed a higher CD98 expression in progressive and metastatic tumours, which relates to a poor prognosis [
      • Esseghir S.
      • Reis-Filho J.S.
      • Kennedy A.
      • et al.
      Identification of transmembrane proteins as potential prognostic markers and therapeutic targets in breast cancer by a screen for signal sequence encoding transcripts.
      ,
      • Ichinoe M.
      • Mikami T.
      • Yoshida T.
      • et al.
      High expression of L-type amino-acid transporter 1 (LAT1) in gastric carcinomas: comparison with non-cancerous lesions.
      ,
      • Imai H.
      • Kaira K.
      • Oriuchi N.
      • et al.
      Inhibition of L-type amino acid transporter 1 has antitumor activity in non-small cell lung cancer.
      ,
      • Kaira K.
      • Oriuchi N.
      • Imai H.
      • et al.
      Prognostic significance of L-type amino acid transporter 1 (LAT1) and 4F2 heavy chain (CD98) expression in early stage squamous cell carcinoma of the lung.
      ,
      • Kaira K.
      • Takahashi T.
      • Abe M.
      • et al.
      CD98 expression is associated with the grade of malignancy in thymic epithelial tumors.
      ,
      • Sakata T.
      • Ferdous G.
      • Tsuruta T.
      • et al.
      L-type amino-acid transporter 1 as a novel biomarker for high-grade malignancy in prostate cancer.
      ].
      Recently, it was shown that a small subpopulation of cells with CSC properties could also be isolated from an HPV-positive head and neck cancer cell line [
      • Tang A.L.
      • Hauff S.J.
      • Owen J.H.
      • et al.
      UM-SCC-104: a new human papillomavirus-16-positive cancer stem cell-containing head and neck squamous cell carcinoma cell line.
      ]. As patients with an HPV-positive OPSCC respond better to treatment and have a more favourable prognosis compared to patients with an HPV-negative OPSCC, we hypothesised that HPV-positive OPSCCs, might have relatively low levels of CSCs. To test this hypothesis, we performed CD44 and CD98 immunostaining on a cohort of OPSCC patients with known HPV status [
      • Rietbergen M.M.
      • Leemans C.R.
      • Bloemena E.
      • et al.
      Increasing prevalence rates of HPV attributable oropharyngeal squamous cell carcinomas in The Netherlands as assessed by a validated test algorithm.
      ]. Furthermore, we evaluated whether CD44 and CD98 expression could be of potential relevance for predicting treatment outcome in patients with HPV-positive OPSCC.

      2. Materials and methods

      2.1 Patients and tumour samples

      To evaluate CD44/CD98 immunostaining and the relation to HPV, a test cohort was composed, which included 88 fresh-frozen, pre-treatment OPSCC samples of patients treated in the period 2008–2011. Eligible samples included histopathologically confirmed invasive squamous cell carcinoma of the oropharynx (International classification of diseases for Oncology, [ICD-10] codes C019, C051, C052, C090–C099 and C100–C109). These OPSCCs were previously tested for HPV using an HPV E6 mRNA reverse-transcriptase polymerase chain reaction (RT-PCR) [
      • Rietbergen M.M.
      • Leemans C.R.
      • Bloemena E.
      • et al.
      Increasing prevalence rates of HPV attributable oropharyngeal squamous cell carcinomas in The Netherlands as assessed by a validated test algorithm.
      ].
      To further study the association between survival and CD44/CD98 expression, all patients (n = 711) treated between 2000 and 2006 at two Dutch university hospitals were included. Patients were identified through the Dutch Cancer Registries. Patient characteristics and clinical outcome were obtained from the patient files. HPV detection was performed using pre-treatment formalin-fixed, paraffin-embedded (FFPE) biopsies [
      • Rietbergen M.M.
      • Brakenhoff R.H.
      • Bloemena E.
      • et al.
      Human papillomavirus detection and comorbidity: critical issues in selection of patients with oropharyngeal cancer for treatment De-escalation trials.
      ]. A sample was scored HPV-positive based on a positive p16INK4A-immunohistochemistry (p16-IHC) and a subsequent positive GP5+/6+ HPV DNA PCR, according to a previously validated algorithm [
      • Rietbergen M.M.
      • Leemans C.R.
      • Bloemena E.
      • et al.
      Increasing prevalence rates of HPV attributable oropharyngeal squamous cell carcinomas in The Netherlands as assessed by a validated test algorithm.
      ]. Approval for this retrospective study was obtained from the Institutional Review Board and the study adheres to the guidelines for proper secondary use of human tissue specimen (www.federa.org).

      2.2 Immunohistochemical staining of CD44 and CD98

      Formalin-fixed paraffin-embedded sections of HNSCC tumour biopsies were deparaffinised and subjected to Tris/EDTA (10 mM/1 mM, pH 9.0) antigen retrieval. Primary antibodies U36 (anti-CD44v6, developed at our laboratory [
      • Van Hal N.L.
      • van Dongen G.A.
      • Stigter-van Walsum M.
      • Snow G.B.
      • Brakenhoff R.H.
      Characterization of CD44v6 isoforms in head-and-neck squamous-cell carcinoma.
      ]) and anti-CD98 (clone H-300, Santa Cruz Biotechnology), were diluted in PBS containing 2% goat serum and incubated overnight at 4 °C. Afterwards, the BrightVision +Poly-HRP-Anti Ms/Rb/Rt IgG kit (Immunologic BV, Duiven, The Netherlands) was employed according to the description of the manufacturer. The staining was developed with diaminobenzidine and H2O2 as chromogen. Sections were counterstained with haematoxylin and cover slipped with Kaiser’s glycerine. To correct for the differences in dimensions of the biopsies, decisions on CD44 and CD98 scores were based on a single area that contained viable tissue with over 70% HNSCC cells.

      2.3 Evaluation of immunohistochemical staining

      CD44 and CD98 expression were evaluated by staining intensity and by percentage of positive cells. Percentage of CD44-positive cells was evaluated semi-quantitatively, and classified according to the percentage of malignant cells: 1 (⩽10% of tumour cells stained), 2 (11–50% stained), 3 (51–75% stained) or 4 (>75% stained) [
      • Lindquist D.
      • Ahrlund-Richter A.
      • Tarjan M.
      • Tot T.
      • Dalianis T.
      Intense CD44 expression is a negative prognostic factor in tonsillar and base of tongue cancer.
      ]. Percentage of CD98-positive cells was classified differently, in accordance to previous reports, as overall staining was lower: 1 (⩽10% of tumour cells stained), 2 (11-25% stained), 3 (26–50% stained) and 4 (>50% stained) [
      • Kaira K.
      • Oriuchi N.
      • Imai H.
      • et al.
      Prognostic significance of L-type amino acid transporter 1 (LAT1) and 4F2 heavy chain (CD98) expression in early stage squamous cell carcinoma of the lung.
      ,
      • Kaira K.
      • Takahashi T.
      • Abe M.
      • et al.
      CD98 expression is associated with the grade of malignancy in thymic epithelial tumors.
      ]. The intensity of CD44 and CD98 staining was scored separately and evaluated as absent (0), weak (1), moderate (2) or strong (3) [
      • Lindquist D.
      • Ahrlund-Richter A.
      • Tarjan M.
      • Tot T.
      • Dalianis T.
      Intense CD44 expression is a negative prognostic factor in tonsillar and base of tongue cancer.
      ].
      Three investigators independently classified the percentage of positive cells and staining intensity in all cases, without prior knowledge of the clinical data. The data are presented as the mean of the three observations.

      2.4 Statistical analyses

      Probability values of <0.05 indicated a statistically significant difference. A Mann-Whitney U Test was performed to compute the CD44/CD98 expression differences between HPV-positive and HPV-negative OPSCCs. Differences in patient characteristics between HPV-positive and HPV-negative cases were assessed using the Pearson χ2-test or Student’s t-test. Bonferroni correction was used to compare subgroups for specific variables. The end-points were overall survival (OS) and progression-free survival (PFS). OS was defined as the time from date of incidence (defined as the date on which the squamous cell carcinoma was histologically confirmed) to death (any cause). PFS was defined as the time period from date of incidence to death or the first documented relapse, which was categorised as local-regional recurrence or distant metastases. Patients who developed a second primary tumour were censored at the incidence date of that tumour. Survival rates were estimated by means of the Kaplan-Meier method and associations were analysed with the log-rank test. Multivariate analyses were performed using a forward selection procedure (p was set at <0.05 to enter the model) in the Cox proportional hazards model to identify independent prognostic factors. Intraclass correlation coefficients (with 95% confidence intervals [CI]) were computed to determine interobserver variability of the three investigators for percentages of CD44 and CD98 positive cells.

      3. Results

      3.1 CD44/CD98 expression in the test cohort

      CD44 and CD98-immunostaining were first performed on a test cohort to detect a potential difference between HPV-positive and HPV-negative cases and to evaluate the chosen cut-off values. CD44 staining intensity and percentage of CD44-positive cells were significantly higher in HPV-negative tumours (n = 63) compared to HPV-positive tumours (n = 25) (p = 0.03 and p = 0.002, respectively), as calculated by the Mann-Whitney U test. CD98 staining intensity and percentage of CD98-positive cells were also significantly higher (p < 0.001 for both) for HPV-negative tumours compared to HPV-positive tumours. Distribution curves for percentage of CD44/CD98-positive cells and CD44/CD98 intensity for this test cohort are depicted in the Supplementary data (S1).

      3.2 Patient and tumour characteristics

      Next, we investigated the relation between CD44/CD98 expression and survival in tumour biopsies obtained from a large, consecutive cohort of 711 OPSCC patients. Patient and tumour characteristics are depicted in Table 1. As published before, patients with HPV-positive tumours had less advanced tumour stages than patients with HPV-negative tumours, but a more advanced nodal stage. A higher HPV-prevalence was found in squamous cell carcinomas in the base of tongue and the tonsils compared to the other oropharyngeal subsites (p < 0.001, with Bonferroni correction) [
      • Rietbergen M.M.
      • Brakenhoff R.H.
      • Bloemena E.
      • et al.
      Human papillomavirus detection and comorbidity: critical issues in selection of patients with oropharyngeal cancer for treatment De-escalation trials.
      ].
      Table 1Patient and tumour characteristics.
      HPV-positive OPSCCHPV-negative OPSCCp-Value
      Human papillomavirus (HPV)-positive group compared to HPV-negative group.
      HPV-positive OPSCC/CD98 highp-Value
      HPV-positive patients with CD98 expression >50% compared to HPV-positive patients with CD98 expression <50%.
      All cases
      Number (percentages)Number (percentages)Number (percentages)Number (percentages)
      No. of cases150 (21.1%)561 (78.9%)11711
      Age at diagnosis
      Mean61.060.6p = 0.722
      Independent t-test.
      57.3p = 0.31
      Independent t-test.
      60.7
      Median58.859.455.659.3
      Genderp = 0.374
      Chi square.
      p = 0.93
      Chi square.
      Male106 (70.7%)375 (66.8%)8 (72.7%)481 (67.7%)
      Female44 (29.3%)186 (33.2%)3 (27.3%)230 (32.3%)
      Oropharyngeal Sub-sitep < 0.001
      Chi square.
      p = 0.23
      Chi square.
      Tonsil88 (58.7%)226 (40.3%)5 (45.5%)314 (44.2%)
      Base of tongue48 (32.0%)138 (24.6%)4 (36.4%)186 (26.2%)
      Soft palate + uvula9 (6.0%)101 (18.0%)2 (18.2%)110 (15.5%)
      Oropharynx nos5 (3.3%)96 (17.1%)0101 (14.2%)
      Smokingp = 0.37
      Chi square.
      Never44 (29.3%)17 (3.0%)p < 0.001
      Chi square.
      1 (9.1%)61 (8.6%)
      Moderate (1–24 pack years)39 (26.0%)70 (12.5%)5 (45.5%)109 (15.3%)
      Heavy (>24 pack years)66 (44.0%)468 (83.4%)5 (45.5%)534 (75.1%)
      Unknown1 (0.7%)6 (1.1%)07 (1.0%)
      T-stagep = 0.001
      Chi square.
      p = 0.93
      Chi square.
      T132 (21.3%)68 (12.1%)3 (27.3%)100 (14.1%)
      T245 (30.0%)168 (29.9%)3 (27.3%)213 (30.0%)
      T353 (35.3%)184 (32.8%)4 (36.4%)237 (33.3%)
      T420 (13.3%)139 (24.8%)1 (9.1%)159 (22.4%)
      Tx02 (0.4%)02 (0.3%)
      N-stagep < 0.001
      Chi square.
      p = 0.08
      Chi square.
      N021 (14.0%)233 (41.5%)4 (36.4%)254 (35.7%)
      N121 (14.0%)79 (14.1%)1 (9.1%)100 (14.1%)
      N297 (64.7%)218 (38.9%)4 (36.4%)315 (44.3%)
      N311 (7.3%)29 (5.2%)2 (18.2%)40 (5.6%)
      Nx02 (0.4%)02 (0.3%)
      Tumour differentiationp < 0.001
      Chi square.
      p = 0.26
      Chi square.
      Well2 (1.3%)30 (5.3%)032 (4.5%)
      Moderate41 (27.3%)407 (72.5%)5 (45.5%)448 (63.0%)
      Poor107 (71.3%)124 (22.1%)6 (54.5%)231 (32.5%)
      CD44 intensityp < 0.001
      Chi square.
      p = 0.07
      Chi square.
      Absent (0)5 (3.3%)1 (0.2%)1 (9.1%)6 (0.8%)
      Weak (1)12 (8.0%)4 (0.7%)016 (2.3%)
      Moderate (2)62 (41.3%)111 (19.8%)1 (9.1%)173 (24.3%)
      Strong (3)71 (47.3%)445 (79.3%)9 (81.8%)516 (72.6%)
      CD44 expressionp < 0.001
      Chi square.
      p = 0.66
      Chi square.
      ⩽10%9 (6.0%)4 (0.7%)1 (9.1%)13 (1.8%)
      11–50%37 (24.7%)25 (4.5%)1 (9.1%)62 (8.7%)
      51–75%32 (21.3%)29 (5.2%)2 (18.2%)61 (8.6%)
      76–100%72 (48.0%)503 (89.7%)7 (63.6%)575 (80.9%)
      CD98 intensityp < 0.001
      Chi square.
      p < 0.001
      Absent (0)76 (50.7%)32 (5.7%)0108 (15.2%)
      Weak (1)41 (27.3%)128 (22.8%)0169 (23.8%)
      Moderate (2)26 (17.3%)255 (45.5%)7 (63.6%)281 (39.5%)
      Strong (3)7 (4.7%)146 (26.0%)4 (36.4%)153 (21.5%)
      CD98 expressionp < 0.001
      Chi square.
      p < 0.001
      ⩽10%105 (70.0%)70 (12.5%)0175 (24.6%)
      11–25%22 (14.7%)151 (26.9%)0173 (24.3%)
      26–50%12 (8.0%)146 (26.0%)0158 (22.2%)
      51–100%11 (7.3%)194 (34.6%)11 (100%)205 (28.8%)
      Treatment modalitiesp < 0.001
      Chi square.
      p = 0.44
      Chi square.
      SURG + RT38 (25.3%)167 (28.5%)2 (18.2%)198 (27.8%)
      RT28 (18.7%)178 (31.7%)3 (27.3%)206 (29.0%)
      CRT45 (30.0%)158 (28.2%)4 (36.4%)203 (28.6%)
      RT + LND + RT (brachytherapy)35 (23.3%)160 (8.6%)1 (9.1%)83 (11.7%)
      Unknown4 (2.7%)17 (3.0%)1 (9.1%)21 (3.0%)
      NOS means not otherwise specified, SURG means surgery, RT means radiotherapy, CRT means chemoradiation, and LND means lymph node dissection.
      Independent t-test.
      Chi square.
      low asterisk Human papillomavirus (HPV)-positive group compared to HPV-negative group.
      low asterisklow asterisk HPV-positive patients with CD98 expression >50% compared to HPV-positive patients with CD98 expression <50%.
      With regard to differentiation, the majority of all HPV-positive tumours (71.3%) were poorly differentiated, while the majority of the HPV-negative tumours (72.5%) were moderately differentiated. The intensity and percentage of CD44 and CD98 positive cells were not statistically different in poorly differentiated tumours compared to well and moderately differentiated tumours. This concerned HPV-positive as well as HPV-negative tumours. Distribution curves for percentage of CD44/CD98-positive cells and CD44/CD98 intensity for the whole patient group are depicted in Fig. 1.
      Figure thumbnail gr1
      Fig. 1Results of the scoring percentages of (A) CD44- and (B) CD98-positive tumour cells and the intensities of the immunohistochemical staining (C and D) in the whole patient group (n = 711). The data is represented as the mean observation of the three observers. Thick horizontal lines represent the median value observed in the cohort.

      3.3 CD44 expression

      CD44 intensity as well as percentage of CD44-positive cells was compared between HPV-positive and HPV-negative tumours. In Fig. 2, examples of strong CD44-immunostaining (in an HPV-negative tumour) and weak CD44-immunostaining (in an HPV-positive tumour) are shown. The CD44 intensity, dichotomised as absent (0)/weak (1)/intermediate (2) (n = 192) versus strong (3) (n = 519), was compared between HPV-positive and HPV-negative tumours. Strong CD44-staining intensity (score = 3) was observed in 445 of 561 (79.3%) HPV-negative tumours compared to 71 of 150 (47.3%) HPV-positive tumours (p < 0.001, Pearson χ2-test). Furthermore, a high percentage of CD44-positive cells (i.e. >75% of malignant cells stained) was found in 72 of 150 HPV-positive tumours (48.0%) versus 503 of 561 HPV-negative tumours (89.7%). This was a significant difference (p < 0.001) (Table 1). The interobserver intraclass correlation coefficient (ICC) for fraction of CD44-positive cells was 0.88 (95% CI: 0.87–0.90).
      Figure thumbnail gr2
      Fig. 2(A) CD44-immunostaining of an human papillomavirus (HPV)-negative oropharyngeal squamous cell carcinoma (OPSCC). (B) CD98-immunostaining of an HPV-negative OPSCC. (C) Absence of p16INK4A-immunostaining of an HPV-negative OPSCC. (D) CD44-immunostaining of an HPV-positive OPSCC. (E) Absence of CD98-immunostaining of an HPV-positive OPSCC. (F) p16 INK4A-immunostaining of an HPV-negative OPSCC. (G) CD44-immunostaining of an HPV-positive OPSCC. (H) CD98-immunostaining of an HPV-positive OPSCC. (I) p16 INK4A-immunostaining of an HPV-positive OPSCC. Pictures were taken through a 10× objective.

      3.4 CD98 expression

      CD98-immunostaining was less intense and a lower percentage of tumour cells were stained compared to CD44-immunostaining (Table 1). CD98-intensity was dichotomised as absent (0)/weak (1) (n = 277) versus intermediate (2)/strong (3) (n = 434). Percentage of CD98-positive cells was classified as ‘high’ if >50% of tumour cells were stained, in concordance with previously published reports [
      • Kaira K.
      • Oriuchi N.
      • Imai H.
      • et al.
      Prognostic significance of L-type amino acid transporter 1 (LAT1) and 4F2 heavy chain (CD98) expression in early stage squamous cell carcinoma of the lung.
      ,
      • Kaira K.
      • Takahashi T.
      • Abe M.
      • et al.
      CD98 expression is associated with the grade of malignancy in thymic epithelial tumors.
      ]. In Fig. 2, examples of weak and strong CD98-immunostaining are depicted. CD98 expression differed significantly between HPV-positive and HPV-negative tumours. An intermediate or strong CD98-staining intensity (score 2 or 3) was observed in 401 of 561 (71.5%) HPV-negative tumours compared to 33 of 150 (22.0%) HPV-positive tumours (p < 0.001). Furthermore, HPV-positive tumours showed a significantly smaller percentage of tumour cells with CD98 expression than HPV-negative tumours; a high percentage of CD98-positive cells (i.e. >50% of malignant cells stained) was found in 11 of 150 HPV-positive tumours (7.3%) versus 194 of 561 HPV-negative tumours (34.6%) (p < 0.001) (Table 1). The ICC for percentage of CD98-positive cells was 0.83 (95% CI: 0.81–0.84).

      3.5 Associations between CD44/CD98 expression and survival

      There was no significant difference in survival between the different treatment groups neither for patients with HPV-negative tumours (p = 0.204) nor for patients with HPV-positive tumours (p = 0.277), after correcting for age and stage of disease.
      A univariate survival analysis was performed to evaluate factors potentially associated with OS and PFS. The analyses were subdivided for patients with HPV-positive and HPV-negative tumours. Prognostic factors entered in the univariate analysis were: age, nodal stage, tumour stage, gender, number of pack years, CD44 intensity, percentage of CD44-positive cells, CD98 intensity and percentage of CD98-positive cells. In HPV-negative patients, age, tumour size and nodal stage were all individually associated with OS and PFS outcomes. However, neither CD44 expression nor CD98 expression were of significant importance (Table 2). In HPV-positive patients, age, tumour size and percentage of CD98-positive cells were prognostic factors for OS and PFS.
      Table 2Univariate models for overall and progression-free survival.
      HPV-positive OPSCCHPV-negative OPSCC
      Hazard ratio (95% Confidence Interval)p ValueHazard ratio (95% Confidence Interval)p Value
      Overall survival
      Age (per increase of 1 year)1.05 (1.02–1.08)p < 0.0011.02 (1.01–1.04)p < 0.001
      Gender (male versus female)0.79 (0.41–1.52)p = 0.470.81 (0.65–1.02)p = 0.07
      Tumour size (T3-4 versus T1-2)1.85 (1.03–3.35)p = 0.041.78 (1.42–2.22)p < 0.001
      Nodal stage (N2-3 versus N0-1)0.80 (0.43–1.47)p = 0.472.28 (1.84–2.82)p < 0.001
      Pack Yearsp = 0.33p = 0.21
      PY: 1–24 PY versus 0 PY1.56 (0.67–3.61)1.28 (0.60–2.76)
      PY: >24 PY versus 0 PY1.75 (0.83–3.68)1.60 (0.80–3.22)
      CD44 intensity (score 3 versus score 0/1/2)1.51 (0.79–2.87)p = 0.210.92 (0.73–1.15)p = 0.46
      CD44 percentage (⩽75% versus >75%)0.78 (0.44–1.40)p = 0.420.87 (0.62–1.22)p = 0.42
      CD98 intensity (score 2-3 versus score 0–1)1.51 (0.79–2.87)p = 0.210.92 (0.73–1.15)p = 0.46
      CD98 percentage (⩽50% versus >50%)2.92 (1.30–6.54)p = 0.0090.86 (0.69–1.07)p = 0.18
      Progression-free survival
      Age (per increase of 1 year)1.05 (1.02–1.08)p < 0.0011.02 (1.01–1.03)p = 0.006
      Tumour size (T3-4 versus T1-2)1.84 (1.04–3.27)p = 0.041.52 (1.21–1.91)p < 0.001
      Gender (male versus female)1.00 (0.56–1.84)p = 1.000.81 (0.64–1.02)p = 0.08
      Nodal stage (N2-3 versus N0-1)0.82 (0.45–1.51)p = 0.532.16 (1.73–2.69)p < 0.001
      Pack Yearsp = 0.38p = 0.63
      PY: 1–24 PY versus 0 PY1.65 (0.75–3.61)1.01 (0.49–2.10)
      PY: >24 PY versus 0 PY1.58 (0.77–3.24)1.17 (0.60–2.28)
      CD44 intensity (score 3 versus score 0/1/2)1.62 (0.87–3.02)p = 0.130.93 (0.73–1.18)p = 0.54
      CD44 percentage (⩽75% versus >75%)0.86 (0.49–1.51)p = 0.600.97 (0.67–1.40)p = 0.87
      CD98 intensity (score 2-3 versus score 0–1)1.62 (0.87–3.02)p = 0.130.93 (0.73–1.18)p = 0.93
      CD98 percentage (⩽50% versus >50%)3.57 (1.66–7.68)p = 0.0010.90 (0.71–1.13)p = 0.36
      HPV-positive patients with a high percentage of CD98-positive cells (i.e. >50% of malignant cells stained) had significantly worse 5-year OS and PFS rates (36.4% and 27.3%, respectively) compared to patients with a low percentage of CD98-positive cells (71.8% and 69.8%, respectively) (Fig. 3). The percentage of CD44-positive cells did not correlate with OS and PFS survival rates in HPV-positive patients.
      Figure thumbnail gr3
      Fig. 3(A) 5-year overall survival (OS) curves for patients with an human papillomavirus (HPV)-positive oropharyngeal squamous cell carcinoma (OPSCC) and high percentage of CD98-positive cells (i.e. >50% of malignant cells stained; red line) versus patients with a low to intermediate percentage of CD98-positive cells (i.e. ⩽50% of malignant cells stained; blue line) (p = 0.006). (B) 5-year progression-free survival (PFS) curves for patients with an HPV-positive OPSCC and high percentage of CD98-positive cells (i.e. >50% of malignant cells stained; red line) versus patients with a low to intermediate percentage of CD98-positive cells (i.e. ⩽50% of malignant cells stained; blue line) (p < 0.001). (C) 5-year OS curves for patients with an HPV-negative OPSCC and high percentage of CD98-positive cells (i.e. >50% of malignant cells stained; red line) versus patients with a low to intermediate percentage of CD98-positive cells (i.e. ⩽50% of malignant cells stained; blue line) (p = 0.176). (D) 5-year PFS curves for patients with an HPV-negative OPSCC and high percentage of CD98-positive cells (i.e. >50% of malignant cells stained; red line) versus patients with a low to intermediate percentage of CD98-positive cells (i.e. ⩽50% of malignant cells stained; blue line) (p = 0.693). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
      Multivariate analysis was performed to estimate the association of all the analysed factors with OS and PFS. Age (⩽55 years), tumour size (T1-2) and nodal stage (N0-1) were independent prognostic factors for OS and PFS in HPV-negative patients. In HPV-positive patients, smoking (⩽24 pack years), tumour size (T1-2) and a low percentage of CD98-positive cells (i.e. <50% of stained tumour cells) were independent prognostic factors for OS and PFS (Table 3).
      Table 3Multivariate models for overall and progression-free survival.
      HPV-positive OPSCCHPV-negative OPSCC
      Hazard ratio (95% Confidence Interval)p ValueHazard ratio (95% Confidence Interval)p Value
      Overall survival
      Age (per increase of 1 year)1.06 (1.03–1.09)p < 0.0011.03 (1.02–1.05)p < 0.001
      Tumour size (T3-4 versus T1-2)2.22 (1.19–4.16)p = 0.011.51 (1.21–1.90)p < 0.001
      Nodal stage (N2-3 versus N0-1)2.33 (1.86–2.91)p < 0.001
      CD98 percentage (<50% versus ⩾50%)0.23 (0.10–0.53)p = 0.001
      Progression-free survival
      Age (per increase of 1 year)1.05 (1.03–1.08)p < 0.0011.03 (1.01–1.04)p = 0.001
      Tumour size (T3-4 versus T1-2)2.28 (1.26–4.15)p = 0.0071.32 (1.05–1.67)p = 0.02
      Nodal stage (N2-3 versus N0-1)2.18 (1.73–2.74)p < 0.001
      CD98 percentage (<50% versus ⩾50%)0.20 (0.09–0.45)p < 0.001

      4. Discussion

      Over the past decades, CSCs have been identified in multiple solid tumours and were implicated to play a role in resistance to anticancer treatments [
      • Hong S.P.
      • Wen J.
      • Bang S.
      • Park S.
      • Song S.Y.
      CD44-positive cells are responsible for gemcitabine resistance in pancreatic cancer cells.
      ,
      • Rich J.N.
      Cancer stem cells in radiation resistance.
      ]. As CSCs generally divide slowly and are rich in DNA repair enzymes and detoxification mechanisms, treatment failure likely occurs due to ineffective killing of these CSCs [
      • Reya T.
      • Morrison S.J.
      • Clarke M.F.
      • Weissman I.L.
      Stem cells, cancer, and cancer stem cells.
      ]. As many studies have shown that patients with an HPV-positive OPSCC respond better to (chemo) radiation than patients with an HPV-negative OPSCC, we hypothesised that a possible explanation for this could be lower percentages of CSCs in HPV-positive OPSCCs. In this study, we evaluated the immunostaining patterns of two CSC markers, CD44 and CD98, in both HPV-positive and HPV-negative OPSCCs. Expression of both CD44 and CD98 was significantly lower in patients with an HPV-positive OPSCC. This suggests that HPV-positive tumours may have lower numbers of CSCs, which may be reflected by a better therapy response. Therefore, we evaluated whether CD44 and/or CD98 expression could be of potential relevance for predicting treatment in HPV-positive patients. CD44 expression was not associated with survival neither in HPV-positive patients nor in HPV-negative patients. This is likely due to the fact that CD44 was abundantly expressed in almost all tumours.
      Several studies have shown that CD44 expression might be used as an outcome predictor in head and neck cancer [
      • de Jong M.C.
      • Pramana J.
      • van der Wal J.E.
      • et al.
      CD44 expression predicts local recurrence after radiotherapy in larynx cancer.
      ,
      • Lindquist D.
      • Ahrlund-Richter A.
      • Tarjan M.
      • Tot T.
      • Dalianis T.
      Intense CD44 expression is a negative prognostic factor in tonsillar and base of tongue cancer.
      ], but we cannot confirm this observation. Although CD44 seems an interesting marker for CSC enrichment in HNSCC, it is not optimal. In several studies it was shown that CD44 protein expression is not only present on the basal cells (the compartment where the stem cells are assumed to reside) but also on the suprabasal cells, both on normal and malignant squamous tissue [
      • Van Hal N.L.
      • van Dongen G.A.
      • Stigter-van Walsum M.
      • Snow G.B.
      • Brakenhoff R.H.
      Characterization of CD44v6 isoforms in head-and-neck squamous-cell carcinoma.
      ,
      • Herold-Mende C.
      • Seiter S.
      • Born A.I.
      • et al.
      Expression of CD44 splice variants in squamous epithelia and squamous cell carcinomas of the head and neck.
      ,
      • Mack B.
      • Gires O.
      CD44s and CD44v6 expression in head and neck epithelia.
      ,
      • Soukka T.
      • Salmi M.
      • Joensuu H.
      • et al.
      Regulation of CD44v6-containing isoforms during proliferation of normal and malignant epithelial cells.
      ,
      • van Zeeburg H.J.
      • van Beusechem V.
      • Huizenga A.
      • et al.
      Adenovirus retargeting to surface expressed antigens on oral mucosa.
      ]. Subtle differences in CD44 expression might by detected by FACS-sorting, but quantification of CD44-immunostained tissue sections is difficult considering the profuse CD44 expression throughout normal and malignant squamous tissues. In contrast, CD98 expression was restricted to cells in the basal layer and was expressed on a more restricted cell population than CD44, making CD98 more distinctive than CD44. In HPV-negative patients, no association was seen between CD98 expression and survival. However, in patients with HPV-positive OPSCC, OS and PFS were significantly worse for patients with a high percentage of CD98-positive tumour cells.
      At this moment, de-escalation trials are being performed for patients with an HPV-positive OPSCC. We suggest it might be useful to evaluate the presence of CSC markers, such as CD98, in addition to HPV-status, as a stratification marker. Consequently, patients with an HPV-positive OPSCC, but a high fraction of CD98-positive tumour cells, might better not be selected for de-escalating oncological treatment. Although a challenging and attractive idea, the use of CD98 as a prognostic marker should first be preceded by a thorough validation phase in prospective clinical trials.
      The observation that CD98 expression was not associated with survival in HPV-negative patients, suggests that the molecular characteristics of these cells might be more relevant than the mere numbers. This requires additional investigation and characterisation of these cells in responding and non-responding tumours.
      In conclusion, most HPV-positive OPSCCs show low expression levels of the CSC markers CD44 and CD98 compared to HPV-negative OPSCCs. Furthermore, OS as well as PFS was markedly better for HPV-positive patients with a low fraction of CD98-positive tumour cells compared to HPV-positive patients with high fraction of CD98-positive tumour cells. In the future, we might use CD98 expression as an additional prognostic marker for selection of HPV-positive patients in clinical trials.

      Role of the funding source

      This study was supported by a grant from the Dutch Cancer Society (KWF-VU2009-4531). The funding source did not contribute to the study.

      Conflict of interest statement

      None declared.

      Appendix A. Supplementary data

      • Supplementary data

        Results of the scoring the percentage of (A) CD44- and (B) CD98-positive tumour cells and the intensities of the immunohistochemical stainings (C and D) in the test cohort (n = 88). The data is represented as the mean observation of the three observers. Thick horizontal lines represent the median value observed in the cohort.

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