Investigation of the role of the p38 MAPK α and δ isoforms in nonresponse to tumour necrosis factor blockade in the synovium of rheumatoid arthritis patients

Background

Thickening of the synovial membrane with proliferation of macrophage-like and fibroblast-like synoviocytes is observed in affected joints of rheumatoid arthritis (RA) patients, as well as extensive synovial infiltration of inflammatory cells. TNF plays a key role in driving the pathogenesis and persistence of RA. Blockade of TNF, using current biologics, has had profound therapeutic effects; nevertheless, between 30% and 40% of RA patients do not respond to this treatment. The mechanisms of response and nonresponse to biologic therapy remain unclear. p38 MAPK is present in the rheumatoid synovium and thought to play a role in the pathogenesis of RA, suggested by evidence that p38 is required for TNF-induced inflammatory processes. There are four p38 MAPK isoforms, α, β, γ and δ, which are found in varying levels in inflammatory cells. Although most research has to date focused on p38α, it has been demonstrated in the synovium that the functional protein produced from the L1 retrotransposable element can specifically induce p38δ. This suggests a possible second p38-mediated pathway of joint destruction in RA that may not be blocked by anti-TNF therapy. This study aimed to correlate the expression of p38δ in the rheumatoid synovium with clinical response and nonresponse to the anti-TNF biologic, infliximab (Remicade).

Methods

We examined the expression of p38 MAPK isoforms (α and δ) pre and post infliximab therapy in the synovium of RA patients, to compare clinical response with nonresponse. All patients entered into this study have failed at least two disease-modifying drugs, and fulfil the revised 1987 ACR criteria for RA. The effect of TNF blockade in five responders and three non-responders on the p38 MAPK α and δ isotypes was studied; expression and activation (phospho-p38) in synovial biopsies of responders and nonresponders was measured by immunohistochemistry using a semiquantitative scoring system, at baseline and approximately week 16 of therapy. A therapeutic effect was determined by patients achieving an ACR20 response at week 14, and the Wilcoxon signed rank test was performed to assess significance of changes in expression levels post treatment.

Results

In responders, p38α, p38δ and phospho-p38 expression were all significantly decreased post infliximab treatment in the synovial sublining layer (P < 0.05). A similar trend was seen in the synovial lining layer of responders, with a decrease in expression of p38α and p38δ and phospho-p38 post infliximab treatment in four of five responders, although these results did not reach statistical significance. p38δ and phospho-p38 in the sublining layer and phospho-p38 in the lining layer increased, or remained high, in all three nonresponders tested, and this tendency was the opposite to that seen in the responders.

Conclusion

In the RA synovium, expression and activation of p38α and p38δ in the sublining layer correlated with response, and increased p38δ expression and activation was associated with nonresponse. A flow cytometric-based investigation of p38 isoform expression/activation in peripheral blood mononuclear cells has been initiated.