The Jak‐STAT pathway is the primary signal transduction path for a number of cytokines known to have important roles in inflammatory arthritis. IFNγ and IL6 signal through STAT1, IL4 (thought to have an anti‐inflammatory role) signals through STAT6, and IL12 (critical in Th1 mediated inflammation) and IFNα signal through STAT4.5,19
To date, much of the work on these signalling pathways has been based on murine and in vitro models. We have documented Jak and STAT protein expression in vivo in RA, SpA, and OA and compared their expression with that in a normal cohort.
STAT1 expression was increased in inflammatory arthritis in comparison with OA and normal controls. Cells expressing STAT1 were predominantly fibroblasts (CD55+) but colocalisation with leucocytes (CD45+) and macrophages (CD68+) was also found.
The primary activator of STAT1 is thought to be IFNγ but IL6, IL10, and IFNα/β may also contribute to its activation.19
Yokota et al
found that IL6 was the primary activator of STAT1 in synovial fluid cells consisting predominantly of neutrophils, suggesting the presence of cell lineage specific activation pathways.20
We have reviewed the current understanding of the physiological role of STAT1 in arthritis elsewhere,21
but to summarise, human tissue and rodent arthritis models suggest that its action is predominantly anti‐inflammatory and that modulation of STAT1 function, either directly or through suppression of cytokine signalling (SOCS‐1), may have therapeutic applications.
Two published reports have described STAT1 expression in synovial tissue. Our findings concur with those of van der Pouw Kraan et al
They used microarray techniques and real time polymerase chain reaction to demonstrate increased expression of STAT1 and genes known to be regulated by STAT1 in active RA. Kasperkovitz et al
reported increased STAT1 expression in the synovium in 12 patients with RA compared with OA and reactive arthritis (ReA).7
Furthermore, they showed that the ratio of STAT1 to pSTAT1 expression in the RA group remained the same, suggesting that STAT1 activation may increase concomitantly with STAT1 expression. In contrast, we found that STAT1 expression was significantly increased in both SpA and RA. However, no mention of disease activity in the ReA group was made and in most cases the entire control group was used for the comparison with the RA group, rather than considering ReA and OA separately.
IL4, a prototypical Th2 cytokine, is the primary activator of STAT6 and it has been suggested that altering the Th1/Th2 balance in inflammatory arthritis through STAT6 may modulate disease expression.22
In support of this, Müller‐Ladner et al
found IL4 STAT (IL4 primarily signals through STAT6) expression in 8/10 RA tissues in both early (<12 months) and late (>2 years) disease.10
IL4 STAT was found in macrophages, fibroblasts, and also in follicular infiltrates. In contrast, very weak expression was seen in 3/3 patients with OA.10
Using a proteoglycan‐induced murine arthritis model and knockout models, Finnegan et al
found that IL4, signalling through STAT6, was necessary to attenuate the severity of joint inflammation. Its anti‐inflammatory role appeared to function through the regulation of IL12 production.11
Among our patient cohort, STAT6 protein was widely expressed, with representation in all tested cell lineages (fibroblast, macrophage, and lymphocyte). The only statistically significant findings were an increase in sublining expression in RA and OA in comparison with normal tissue. Our cellular distribution concurs with that of Müller‐Ladner's group,10
but we did not note any marked difference in expression between arthritis types. Although STAT6 may have an anti‐inflammatory role, the widespread expression we observed suggests that it may not be an ideal immunomodulatory target. However, we have only observed STAT6 protein expression. STAT6 phosphorylation may be widely skewed between arthritis groups, supporting a role for STAT6 modulation as a therapeutic target.
Our most intriguing finding is the demonstration of multiple cells of a dendritic phenotype (CD1a+, CD22−, CD68−, CD3−, CD55−) in rheumatoid synovial tissue, expressing large amounts of STAT4, STAT6, and Jak3. This staining pattern was unique to patients with seropositive RA, suggesting a differing pathogenic process than for SpA and the seronegative RA group. In addition, the location of these cells was consistent with previous work documenting the distribution of CD1a+ cells in the rheumatoid synovium.23
Dendritic cells are the major antigen presenting cells in rheumatoid synovium.24
The specific pattern of Jak3, STAT4, and STAT6 expression gives an indication of the stage of differentiation of these cells. Recent work on purified dendritic cell lines has shown that Jak3 is phosphorylated in response to CD40 ligation by CD154 on activated T cells, a critical component in dendritic cell maturation. Therefore, dendritic cells in their earlier stages of activation might be expected to express large amounts of Jak3 and its downstream transcription factors, STAT6 (and possibly, STAT5a/b). Monocyte derived dendritic cells triggering through CD40 also show expression of costimulatory molecules and production of IL12, which acts in a paracrine and autocrine fashion.12,25
IL12 signals through STAT4 and so dual cellular expression of Jak3, STAT6, and STAT4 is consistent with dendritic cells that have recently undergone activation. Although the presence of costimulatory molecules (such as CD80 and CD86) would support this hypothesis, their absence may only mean that the changes in Jak3, STAT6, and STAT4 expression occur at an early stage, before the production of other costimulatory molecules.
Overall expression of Jak3 and STAT4 was increased in the sublining but not the lining regions of the inflammatory arthritis groups when compared with OA. The absence of any significant difference in lining expression of Jak3 and STAT4 between the inflammatory and OA groups is somewhat surprising. Although lining tissue from patients with RA and SpA was more hypertrophied, the percentage of lining cells expressing STAT4 and Jak3 in the OA group was similar, possibly reflecting low grade inflammatory change in the OA synovial lining.26
There is little published work documenting the expression of Jak3 or STAT4 in arthritis and there are no studies documenting synovial distribution in a series of arthritides. Only one study has reported the presence of Jak3 in inflammatory arthritis and they used microarray techniques on peripheral blood monocytes.27
Selected genes were further assessed by reverse transcription‐polymerase chain reaction assay. Among other changes, Jak3 expression was increased in patients with psoriasis but not other inflammatory arthritis groups. Although these findings are intriguing, they cannot be extrapolated to changes at the synovial level.
STAT4 has been demonstrated in type A synovial macrophages in rheumatoid tissue taken at the time of joint replacement surgery,9
but the expression of STAT4 in early and active human inflammatory arthritis has not, to our knowledge, been documented. Studies of proteoglycan‐induced arthritis in STAT4 knockout mice have demonstrated an attenuated form of arthritis, suggesting that STAT4 is important in perpetuating inflammation.11
We postulate that STAT4/Jak3/STAT6 dendritic cell expression in seropositive RA reflects a unique pathogenic process, and Jak3/STAT4 expression may be a useful marker for identifying RA at the synovial level. Further large scale studies would explain whether the characteristic histological picture seen in these patients has any clinical application.
Our findings identify alternative pathways to modulate dendritic cell maturation in RA. Jak3, in particular, is an attractive target. Its expression is largely limited to haematopoietic cell lines and in addition to its roles in dendritic cell activation, it is also responsible for signalling of multiple cytokines important in lymphocyte activation through haematopoietin receptors (namely, IL2, IL7, IL9, and IL15)5
. Jak3 inhibition would have the potential to alter dendritic cell maturation at a time when the inflammatory process is still evolving and therefore may provide long term disease modulating activity. An orally bioavailable Jak3 antagonist has been found to reduce transplant rejection in two animal models.28
In conclusion, we have documented the differential expression of Jak‐STAT signalling pathways in four groups of synovial tissue. STAT1 expression was up regulated in inflammatory arthritis, consistent with its role in IFNγ and IL6 signalling. Our preliminary work shows that Jak3 and STAT4 are uniquely and prominently expressed by dendritic cells in RF positive patients with RA. These findings suggest a pathogenic process unique to seropositive RA. Intense Jak3, STAT6, and STAT4 protein expression is likely to correspond with an early stage of dendritic cell activation. Their characteristic staining profile raises the possibility of an alternative marker which could be used to identify dendritic cells in frozen synovial tissue and may also provide a method for identifying RA at the level of the synovium. Our findings suggest that Jak3 inhibition may be an appropriate therapeutic target in inflammatory arthritis and especially seropositive RA. Animal model studies may further define any therapeutic roles of a Jak3‐specific inhibitor in the treatment of RA.