The initial search in electronic databases PubMed (1950 – 2019) identified 182 studies. After screening, a total of 28 studies were included into our analysis (Figure 1 – Flow chart).
The selected 28 studies: were all published between 2004 and 2018; from 12 different countries; the sample size ranged from 18 to 300 patients. The control groups were composed by healthy individuals (Control), patients with potentially malignant oral lesions (PMOL), oral lichen planus (OLP), or periodontitis (Table 1). The total number of patients, combining the 28 studies, considering each cytokine evaluated, each study group, and cytokine per study group are described in the Figure 2. IL-8 was investigated in 1245 individuals, followed by IL-6 and TNFα, investigated in 963 and 724 individuals, respectively (Figure 2A). Between the studied groups, the OC had the higher number of individuals included (1670), while 1574 healthy controls, 667 individuals with PMOD, 108 with OLP and 62 with periodontitis were included in the 28 selected articles (Figure 2B).
The cytokines identified in the frequency of appearance in these studies were: IL-8 (50%), IL-6 (50%), TNF-α (28.6%), IL-1β (21.4%), IL-10 (17.9%), IL-1α (10.7%) and IL-1, IL-1RA, IL-4 and IL-13 (3.6% each) (Figure 2). The mean cytokine expression values for each control and oral cancer groups (Table 1) were used for calculation of the expression ratio between control and cancer groups for each individual study (Figure 3). Since the oral cancer groups were used as denominators, cytokine expression ratios lower than “1”, were indicative of higher cytokine levels among cancer samples, while ratios higher than “1” indicated higher levels among controls.
Expression levels of IL-8, IL-6, IL-1β, IL-10, IL-1, IL-1RA, IL-4 and IL-13 were always higher among cancer groups in all studies evaluated (Figure 3). For TNF-α and IL-1α, salivary expression levels were usually higher among OC patients when compared to matching controls. However, Rhodus et al. 2005b (12.5% of all TNF-α studies) and Lee et al. (2018) (33.3% of all IL1α studies) observed higher TNF-α and IL1α expression in healthy controls than in OC patients, respectively (Figure 4C and 4F).
Interleukin-8
Salivary interleukin-8 (IL-8) levels in oral cancer patients were compared to controls in 14 articles (Figure 4A). The study with the smaller population of oral cancer included 05 patients (Cheng, S. et al. 2014), and the biggest one, 100 oral cancer patients (Rajkumar, K. et al. 2014). Among these 14 articles, 14 groups of oral cancer, 5 groups of potentially malignant oral disorder (PMOD) patients, 2 groups of oral lichen planus (OLP) and 2 groups of periodontitis patients were evaluated. In all these studies, IL-8 levels were significantly different between OSCC patients and controls. The values of IL-8 in control group varied from 52.1 to 1580.7 pg/ml; for the premalignant group, values between 140.3 and 1918.2 pg/ml were found; and in the oral cancer group, the values were between 283.7 and 4082.8 pg/ml [14, 15, 24–27, 16–23].
Interleukin-6
Interleukin-6 (IL-6) was evaluated in 14 articles, and in all of them the amount of IL-6 was statistically higher in oral cancer patients compared to the control group (Figure 4B). The population of the studies vary from 9 subjects in the smallest study, to 100 subjects in the biggest one. Five articles only compared oral cancer patients with a control group of non-cancer people, while 07 articles also compared the oral cancer with premalignant lesions, and 01 article compared the oral cancer patients with a non-cancer group with periodontitis. The IL-6 values for the control group vary from 0 to 16.0 pg/ml in eleven of the articles, while in two studies it was found in higher values (33.4 and 69.23 pg/ml). For the pre-malignant group, values between 0.431 and 35.3 pg/ml were found in four articles and higher values, between 43.0 and 217.8 pg/ml were found in another five studies. In the oral cancer groups, the values were from 0.707 to 435.04 pg/ml, and the values were higher than 80.0 pg/ml in eight studies [14, 18, 32–35, 20, 23, 25, 26, 28–31].
Tumor necrosis factorα (TNF-α)
TNF-α was analyzed in 08 articles and it was found in significant higher amount in patients with oral cancer, compared to controls, in 7 of these studies (Figure 4C). The study population vary from 09 to 100 subjects. Three studies only compared oral cancer patients with a healthy control group, while 05 articles compared the oral cancer patients with subjects with premalignant lesions. The TNF-α values for the control groups were from 0.013 to 11300 pg/ml. For oral cancer patients, the TNF-α values vary from 0.739 to 23100 pg/ml [23, 26, 31, 32, 36–39].
Interleukin-1b
The IL-1b was analyzed in 06 articles and a significant difference between oral cancer patients and control subjects was found in all studies (Figure 4D). The number of subjects included in the studies varied from 28 to 60. Two studies compared oral cancer patients with pre-malignant lesions, 04 studies compared oral cancer patients with healthy subjects. The IL-1b values in the control group varied from 14.1 to 354 pg/ml. In the premalignant group the variation was between 39.6 to 143 pg/ml, while in the oral cancer patients the amount varied from 101.03 to 906 pg/ml [15, 16, 18, 32, 38, 40].
Interleukin-10
Interleukin-10 (IL-10) was evaluated in 05 articles and in three of them was observed statistical difference in cancer patients compared do control subjects (Figure 4E). The studies evaluated from 22 to 78 subjects per group. All articles compared oral cancer patients to control subjects. The salivary IL-10 values were between 0.027 and 10.01 pg/ml in control groups, and between 0.037 and 14.91 pg/ml in oral cancer patients [32, 37, 41–43].
Interleukin-1a
IL-1a was evaluated in 03 articles, two compared oral cancer patients with healthy controls and one study compared with oral lichen planus (Figure 3). Two of these articles presented results with significant difference between oral cancer and controls. The IL-1a quantification varied from 135.88 to 1054.6 pg/ml in control group, was 293.6 pg/ml in oral lichen planus, and between 201.7 and 995.71 pg/ml in oral cancer patients [14, 23, 32].
Other Cytokines
The IL-1RA was found in one study that compared oral cancer patients with healthy controls (Figure 3). The salivary IL-1RA in the oral cancer group was 2831.6 pg/ml, and 1949.2 pg/ml in control group, not showing significant difference between them [43].
The IL-1 (without specification of the subunit) was studied in one article, comparing oral cancer patients with patients with premalignant lesions and a group of healthy controls (Figure 3). It was found differences in salivary IL-1 between groups. The IL-1 value found in oral cancer patients was 454.4 pg/ml, in premalignant lesions was 255.1pg/ml, and in healthy controls 173.2 pg/ml [26].
The IL-4 was studied in 01 article comparing oral cancer patients with healthy subjects (Figure 3). Oral cancer patients present 1.17 pg/ml of IL-4 in saliva, while healthy subjects 1.01 pg/ml, without statistical difference between groups [43].
The IL-13 also was analyzed in 01 study, comparing the same groups as the previous one (Figure 3). In oral cancer patients the IL-13 level was 0.76 pg/ml, and in the control groups 0.23 pg/ml, showing a difference statistically significant [43].