INTERLEUKIN-22 SERUM LEVELS IN PATIENTS WITH RHEUMATOID ARTHRITIS IN SANA'A CITY, YEMEN

Interleukin (IL) -22 is a novel mediator of a member of IL-10 family cytokines that is produced by many different types of lymphocytes including both those of innate and adaptive immune system. This cytokine has potent proliferative and inflammatory effects on different cell lines. Recently, accumulated data has indicated that IL-22 plays an important role in the pathogenesis of rheumatoid arthritis (RA). We aimed to investigate the levels of IL-22 and its association with demographic, clinical data as well as serological markers in RA. IL-22 serum levels were measured in 45 newly diagnosed RA patients without any treatment and 45 healthy individuals as control by a manual Enzyme linked immunosorbent assay (ELISA). Correlations of IL-22 serum levels were sought with demographic, clinical data and serological parameters. IL-22 levels were significantly elevated in serum of RA patients (median= 86.89ng/ml and range = 896) compared to serum of healthy control (median=75.36ng/ml and range=459), p=.022. The IL-22 levels were correlated positively with C-reactive protein (CRP), anti-cyclic citrullinated peptide (ACCP) antibodies in RA patients. Significant higher levels of serum IL-22 in RA patients compare with those in healthy control. Highly significant association between serum levels of IL-22 and the serological markers (CRP and ACCP antibodies) in the diagnosis of RA suggest the potential levels of IL-22 as a valuable biomarker for the evaluation of disease severity in RA patients.


INTRODUCTION
Rheumatoid arthritis (RA) is a chronic inflammatory disease that represents one of the most common autoimmune-related disease. Histologically, it is characterized by prominent infiltration of inflammatory mononuclear cells, such as T cells and macrophages, and the proliferation of synovial fibroblasts 1 . In RA, it is clear that inflammatory cytokines play a key role in driving T cell activation and migration that lead to joint destruction 2 . Interleukin (IL)-22 is a novel α-helical protein, the human IL-22 encoding gene is located in the longer arm (q15) of chromosome 12 3 . It belongs to a group of cytokines called the IL-10 family which is a class of potent mediators of cellular inflammatory responses 4,5 . IL-22 differs from other cytokines of IL-10 family by being a potent proliferative and inflammatory agent for different cell lines 3, 6 . Many types of cells from lymphoid lineage can secrete IL-22, including both those of the innate and adaptive immune system. In humans, these cells include activated CD4 + T cells, CD8 + T cells 7-9 and γδ T cells 10 as well as various innate lymphoid cells such as Natural killer (NK) cells, NKT cells 11-13 lymphoid tissue inducer (LTi) and LTi-like cells 14 . Several studies have shown that IL-22 has a major role in both defense against certain microbes and the development and maintenance of chronic inflammatory diseases 15-16 . In addition, it plays an important role in mucosal tissue protection and wound healing 16,17 . Moreover, it induces proliferative and anti-apoptotic pathways in responsive cells allowing for tissue preservation 18 . The IL-22 receptor complex is composed of IL-22R1 and IL-10R2 19-21 IL-22R1 subunit is restricted to cell lineages of a non-haematopoitic origin, in particular, pancreas, kidney and liver as well as barrier surfaces such as the skin, intestine and lung 22,23 . It is important to note that the bone marrow, peripheral blood mononuclear cell, spleen, thymus do not express IL-22R 5, 24 and therefore immune cells are not targets of IL-22 23  This study was conducted to investigate the presence of IL-22 in the sera of patients with RA and healthy controls and to determine the association between the level of IL-22 and the blood parameters including C-Reactive Protein (CRP), rheumatoid factor (RF), and anti-citrullinated-peptide (ACCP) antibodies, as well as its association to demographic and clinical data in RA cases.

DATA ANALYSIS
According to data distribution, the quantitative data were expressed as median and range 33 . The demographic and clinical data were expressed as number and percentage. Independent sample T test was used for comparison between the patients and control groups. The potential correlation between variables was analyzed by the spearman rank correlation test. All statistical tests were performed by using the SPSS version 20 for windows (SPSS, Inc., Chicago, IL, USA) with 95% confidence interval. A two sided pvalue of≤ 0.05 was considered statistically significant.

RESULTS
The demographic data of healthy control and patients showed in Table 1 (Table 2). IL-22 levels in serum of RA patients were significantly higher compared to that in the healthy control (p= .022). As we expected, there were significant differences between patients and healthy control in the levels of CRP, RF, and ACCP (p=0.000) Table 3.  Correlational analysis between the serum levels of IL-22 in the patient and personal and clinical data show no significant difference. As regard serologic parameters, a significant positive correlation was found between the levels of serum IL-22 and CRP and ACCP (rho=.416 p=.004, rho=.559 p=.000, respectively), however, there was no significant correlation between levels of IL-22 and RF in RA patients (Table 4, Figure  1 and Figure 2, respectively).

DISCUSSION
IL-22 has been recently suggested to be involved in the pathogenesis of autoimmune arthritis. In current study, it was observed significantly elevated levels of IL-22 in serum of RA patients compared to healthy controls (p=.022). Our data are in accordance with previous reports that found elevation of IL-22 in serum and plasma of patients with RA 34- 37 . In consistent with current study, IL-22 mRNA was detected in synovial tissue directly as well as in synovial fluid mononuclear cells in patients with RA 6,37, 38 . As regard to the sources of IL-22 in humans, many studies reported that the higher frequency of peripheral IL-22 + CD 4+ T cells in RA patients than those in the controls 36,39 . Moreover, Zhoa et al., showed that IL-22 + CD4 + T cells were correlated positively with the disease activity in RA patients and the percentage of these cells were correlated positively with the levels of plasma IL-22 in these patients 36 . Another recent study has been shown that the synovium in RA patients is infiltrated by T lymphocytes especially Th17 which is also a source of IL-22 40 .

Figure 2: The correlation between serum levels of IL-22 and ACCP in RA patients.
Correlation analysis revealed that a significant positive correlations between levels of serum IL-22 and CRP and ACCP antibodies (rho=0.0416, p=0.004 and rho=0.559, p=0.000, respectively). In line with current result, kim et al., found a significant association between serum IL-22 and ACCP antibodies 37 .

CONCLUSION
In conclusion, obtained data indicated high levels of IL-22 in RA patients and that the strong association with ACCP antibodies suggests the potential of IL-22 and ACCP antibodies levels as predictive markers in this disease. It is also of interest that as immune cells do not express IL-22, targeting IL-22 and related signaling may be an effective therapeutic approach for treating autoimmune RA.

AUTHOR'S CONTRIBUTION
The manuscript was carried out, written, and approved in collaboration with all authors.