RESEARCH SUMMARY

The main goal of our group is to study the molecular mechanisms that regulate T cell signalling in the development of autoimmune diseases and cancer. We focus on the role of the Gadd45 (growth arrest and DNA damage-inducible genes) and p38 MAPK (mitogen-activated protein kinase) families in suppression of autoimmunity and cancer.

In mammalian cells, the best-characterised mechanism for p38 activation is via a phosphorylation cascade termed the classical MAPK pathway, in which a MAPK kinase kinase (MAPKKK) phosphorylates MKK3, MKK4 and MKK6. These kinases then phosphorylate p38 on the Thr180-Tyr182 motif, enhancing substrate access to the catalytic site and increasing its activity.

We found that in T cells, p38 is activated by an alternative mechanism in response to antigen T cell receptor (TCR) signalling.

TCR ligation increases activity of the Src kinase Lck, which phosphorylates Zap70. This tyrosine kinase phosphorylates p38 on Tyr323, which in turn induces autophosphorylation at Thr180, resulting in p38 activation.

Gadd45a is a negative regulator of the alternative pathway. Gadd45a binds to p38, preventing Zap70-mediated p38 phosphorylation at Tyr323. p38 MAPK is important in the pathogenic immune response in rheumatoid arthritis (RA).

We addressed the function of the Tyr323-dependent and the classical MAPKK pathways in T cells from patients with distinct types of inflammatory arthropathies. We quantified phosphorylation of Tyr323p38 and (Thr180-Tyr182)p38 on T cells from healthy controls and patients with RA or ankylosing spondylitis (AS) to identify variables associated with p38 phosphorylation and disease activity.

In a cross-sectional study, we measured p38 phosphorylation on Tyr323 and Thr180-Tyr182 on T cells from 30 control individuals, 33 patients with AS, 30 with RA in remission and 79 with active RA.

We collected clinical characteristics and analysed the correlation between clinical variables, disease activity score (DAS) and p38 phosphorylation levels.  Multivariate regression analysis was used to identify variables associated with p38 phosphorylation on Tyr323 and Thr180-Tyr182.

We found that p38 phosphorylation on Tyr323 was higher in T cells from patients with active RA than in patients with RA in remission or with AS. Tyr323p38 phosphorylation was associated with disease activity as determined by the DAS28.

Enhanced p38 phosphorylation was linked to Lck-mediated activation of the Tyr323-dependent pathway in the absence of upstream MAPKK activation.

Our results indicate that the Tyr323-dependent pathway has a central function in T cell-mediated p38 activation in RA patients and correlates with disease activity, suggesting selective inhibition of this pathway as an attractive target for specific downregulation of p38 activity in RA patients.