Lymphoid neogenesis in chronic inflammation

Aims

To investigate the relationship between 'lymphoid chemokines' CXCL13 and CCL21 and the developmental dynamics of organized lymphoid tissue in rheumatoid synovitis, by determining their local production sites in the context of progressively enlarging perivascular cell aggregates.

Materials and methods

Forty synovial samples from 20 rheumatoid arthritis patients were studied. The organizational features of cell aggregates were analysed by staining serial sections for T cell, B cell and follicular dendritic cell (FDC) markers and peripheral node addressin (a group of vascular adhesion molecules constitutively expressed by high endothelial venules in secondary lymphoid tissues). The dimensional grading analysis was performed according to the radial cell count from the central blood vessel to the point of widest infiltration: Grade 1 (two to five cells), grade 2 (six to 10 cells) and grade 3 (>10 cells). CXCL13 and CCL21 expression was analysed by immunohistochemistry and in situ hybridization with radiolabelled CXCL13-specific and CCL21-specific RNA probes.

Results

In 17 patients grade 2 and/or grade 3 aggregates were demonstrated. Fully formed follicular-like aggregates, with detectable centrally located CD21⁺ dendritic cell clusters and T/B segregation, were identified exclusively in 7/20 patients (35%) of grade 3 aggregates. CXCL13 was observed in 7.66% grade 1, 39.9% grade 2 and 82.6% grade 3 aggregates, while CCL21 expressing cells were observed in 3.06% grade 1, 15.93% grade 2 and 43.55% grade 3. CXCL13 and CCL21 were expressed within lymphocytic clusters in 18/20 and 15/20 patients, respectively, both at protein level and mRNA level, in mature CD21⁺ FDC containing T/B cell follicular-like structures as well as in not organized T cell/B cell aggregates devoid of CD21⁺ FDC networks.

Conclusions

This study demonstrates for the first time, by in situ hybridization, the site of production of CXCL13 and CCL21 in the context of synovial ectopic lymphoid tissue. The morphological and grading analysis provides evidence of the independence, in the synovial inflammatory microenvironment, between the ectopic production of lymphoid chemokines and the presence of a mature CD21⁺ FDC-rich/germinal centre lymphoid-like structures. This supports the hypothesis, similar to that shown in animal models, of a potential role of these factors in prefollicular stages of synovial lymphoid neogenesis.