There is currently no exact specific marker to determine BD's inflammatory activity. Hence, it was revealed whether IL-18, sCD40, PF4V1, and NGAL inflammation biomarkers in the blood of individuals with OABD and OIBD would be useful in the diagnosis of the disease. Cytokines play a key role in the pathophysiology of BD [40]. In this current study, it was discovered that IL-18, a cytokine, increased in ocular active and inactive Behçet patients in a statistically significant way as compared to the control group values. The increase in IL-18 was more pronounced in patients with OABD than in the other two groups. The source of the IL-18 increases reported here may be monocyte and macrophage cells activated due to BD. This is because inflammation is one of the main symptoms in BD, and in this case, it has been reported that monocytes and macrophages produce a series of pro-inflammatory cytokines [6, 40, 41]. It has been previously reported that the amount of circulating IL-18 increases during BD regardless of ocular manifestation [10]. Considering a previously reported report with available data [10], it can be suggested that increases in IL-18 may be indicative of inflammatory ocular lesions. The 95% confidence interval under the curve in the ROC curve analysis recorded for the first time in our study, as well as the associated p-value, suggest that IL-18 may be a useful parameter in diagnosing active and inactive BD. Moreover, in previous studies, some cytokine releases of active and inactive Behçet patients and healthy individuals were compared, and it was reported that some cytokines such as IL-6, IL-12, and IFN-γ were significantly increased in active and inactive Behçet patients according to the values of healthy individuals [42]. Furthermore, IL-18Rα is produced in almost all immune cells and some non-immune cells [43]. Therefore, IL-18 has the potential to be an important laboratory parameter in the future in determining the inflammation processes of ocular BD, as it has roles in immune complex-mediated pathology, potentially affecting both congenital and adaptive immune responses [6].
In this study, sCD40 values in the blood of individuals with active and inactive ocular BD were studied and compared with the control group. A significant increase was found in CD40 ocular active and inactive Behçet patients compared to control values. The highest increase was recorded in individuals with OABD. CD40 plays a role in other immune and vascular pathologies, including BD. In particular, previously published data reported that platelet-derived CD40 levels were higher in Behçet's patients than in healthy control groups [19]. These available data, which are identified for the first time in OABD and OIBD, indicate that ocular inflammation may be a significant biomarker in evaluating disease activity, according to our comprehensive available study. This is because CD40 is an innate transmembrane molecule that plays an important role in adaptive immunity [44]. Hence, the increase in CD40 in ocular BD may also be an ocular reflection of systemic disease. In other words, it is predicted that CD40, which is increased in ocular BD, may have roles in the physiopathology of ocular BD. The 95% confidence interval under the curve in the ROC curve analysis recorded in our study, as well as the related p-value, suggest that CD40 may be a useful additional parameter for diagnosing OABD and OIBD.
In this study, it was found that PF4V1 amounts in OABD and OIBD were significantly increased when compared with the values of the control group, and this increase was even higher in OABD. Increased amounts of the PF4V1 molecule in OABD may be an indication that the platelets (thrombocytes) interact with leukocytes (monocytes, neutrophils, dendritic cells, T-cells) and progenitor cells to encourage the migration of inflammatory cells to the lesion areas, resulting in the release of a large number of inflammatory cytokines and ultimately an inflammatory environment in the lesion area [45]. Thus, the increase in IL-18 and sCD40 values noted in the upper section of the discussion in OABD and OIBD may have been due to an increase in leukocyte migration caused by platelets (thrombocytes) activation in cases of inflammation [45]. For instance, activated monocytes and macrophages are the key cellular source of IL-18 [46]. Similarly, it has been reported that leukocyte migration causes thrombocyte activation due to inflammation, which results in the release of inflammatory cytokines (such as IL-6) [47]. Our findings also show that PF4V1 can be a successful biomarker candidate for determining the activity of ocular BD based on its ROC values.
In this study, when the OABD and OIBD and control group values were compared in terms of NGAL amounts, a statistically significant increase was found in the case of ocular BD. Moreover, NGAL values were significantly even higher in OABD. Tissue distribution and expression in tissues exposed to microorganisms such as neutrophils, bone marrow and trachea, lung, stomach, salivary gland, and colon [48] indicate that NGAL plays significant roles in inflammatory responses. In neutrophils, NGAL secretion is regulated by activation of infection and inflammation [49]. The possible reason for the further increase of NGAL amounts in OABD and OIBD in our study may be due to the sequestration of neutrophil chemoreactants due to inflammation in the eye, and the amount may be increased to reduce the neutrophil-dependent inflammatory response. It has been previously reported that NGAL amounts are increased in other inflammatory diseases such as Rheumatoid Arthritis (RA) [33] and inflammatory bowel disease [26]. On the other hand, in a thesis study conducted in 2012, it was recorded that the amount of NGAL in BD decreased,50 and these results are inconsistent with the NGAL results of our ocular BD study, which we recorded here. The same researcher also reported that there was no statistically significant difference between the serum NGAL values of Behçet's patients with ocular involvement and healthy individuals [50]. However, they did not include the OABD group in their study. Besides, this researcher stated that Behçet's patients used one or more agents of colchicine, systemic steroids, or immunosuppressive [50]. However, the use of any medication in our patients was the exclusion criterion from the study. In addition, there is no other study evaluating NGAL results in Behçet’s eye disease in the literature. Therefore, the contrast between the aforementioned study and our study in terms of NGAL values may be due to drug use [50]. Our ROC curve analysis results also indicate that NGAL values in OABD and OIBD may be associated with the etiology of this disease and may be an inflammatory biomarker.
Moreover, in this study, only the systemic activity score was evaluated in ocular active and inactive patients using BDCAF. BDCAF scores of the patients included in the study were lower. Our BDCAF score results reported here are consistent with the results of Balbaba et al., where Serum Cortistatin levels were reported in patients with OABD and OIBD.7 As Balbaba et al. suggested in their studies [7], we predict that it may be more valuable to evaluate IL-18, sCD40, PF4V1, and NGAL amounts, particularly in periods when systemic findings indicate a higher BDCAF score.
This study has also some limitations as every study has. The main limitation of this study is its cross-sectional design and relatively small sample size. In several previous studies, it was tried to be associated with the disease by examining a single biological molecule. Looking at a molecule is no longer appropriate for disease diagnosis and treatment in the proteomic era we live in. Although 4 disease-related molecules were studied in this study, we think that the number of molecules was still insufficient, and we accept that this is another limitation of our study. In the future, studying a large number of molecules associated with diseases will save journals pages and time. Furthermore, our broad exclusion criteria prevented us from enrolling more patients in this study.
In conclusion, when all of the evidence from this study were combined, these molecules have increased in the peripheral blood of patients with OABD and OIBD, and they are low-cost, easily measurable new biomarkers that are possibly related to the disease's pathogenesis and can help diagnosis. At the same time, higher detection of these parameters in OABD can be a guide in distinguishing between OABD and OIBD and may contribute to the monitoring of OABD. According to our ROC curve results, the superiority of IL-18, sCD40, PF4V1, and NGAL biomarkers to each other in diagnosing OABD and OIBD was not significant. However, it is also clear that broader and more comprehensive studies are needed to more accurately demonstrate the functions of these IL-18, sCD40, PF4V1, and NGAL biomarkers.