Analysis of IL-17⁺cells in facet joints of patients with spondyloarthritis suggests that the innate immune pathway might be of greater relevance than the Th17-mediated adaptive immune response

Spondyloarthritis (SpA) comprises ankylosing spondylitis (AS), reactive arthritis, arthritis/spondylitis with inflammatory bowel disease and arthritis/spondylitis with psoriasis. Inflammatory back pain, a similar pattern of peripheral joint involvement with an asymmetrical arthritis predominantly of the lower limbs and the possible occurrence of sacroiliitis, spondylitis, enthesitis and uveitis are typical clinical features in this group of diseases [1]. SpA can be split into two categories, SpA with predominant axial involvement and SpA with predominant peripheral joint involvement, with both forms overlapping in about 20% to 30% of cases [1]. It has been suggested that SpA is a T-cell-driven disease [1]. In vitro analysis [2, 3] and in situ analysis [4, 5] have shown that both CD4⁺ T cells and CD8⁺ T cells might be involved in the pathogenesis of SpA.

CD4⁺ effector T cells have been classified in the past as either Th1 cells predominantly secreting IFNγ or Th2 cells secreting IL-4. More recent work could define an IL-17-producing CD4⁺ T-cell subtype termed Th17 cells [6, 7]. The first evidence of a potential role of this cytokine was reported in human autoimmune diseases such as rheumatoid arthritis (RA) [8] and multiple sclerosis [9].

Numerous studies have been published recently investigating the frequency of Th17 cells in the peripheral blood (PB) of SpA patients in comparison to patients with other inflammatory joint diseases and controls. Contradictory results have been reported on the basis of either flow cytometry or ELISA [10‐13].

In the present study, we investigated the presence and phenotype of IL-17⁺ cells by analysing three different compartments in patients with SpA. Our results in PB and synovial fluid (SF) obtained from flow cytometric analysis were compared to results obtained from patients with RA (PB and SF) and controls (PB). Additionally, we performed an extensive analysis of IL-17⁺ cells in bone specimens from AS patients. To our knowledge, this study is the first to describe in situ analysis of IL-17 expression in bone tissue samples.

In this study, we analysed the frequency of IL-17⁺ cells in three different compartments of patients with spondyloarthritides. The most prominent finding was a significantly higher number of IL-17⁺ cells at the primary site of inflammation in the subchondral bone marrow of affected facet joints [5] in AS patients compared to OA patients. Facet joints from patients with other inflammatory rheumatic diseases, such as RA patients, would have been of interest for comparison in this analysis, but such surgical procedures are rarely performed in RA patients. Interestingly, IL-17⁺ cells were almost similarly distributed among the MNC and PNC populations, with a slight predominance in the PNC population. Surprisingly, immunofluorescence double-staining in situ showed that the clear majority of the IL-17⁺ cells were found among the CD15⁺ neutrophils (24.25 ± 10.36/HPF) and among the MPO⁺ cells of the myeloid lineage (35.84 ± 13.04/HPF), while CD3⁺ T cells (0.51 ± 0.49/HPF) and mast cells (2.28 ± 1.16/HPF) constituted only a small proportion of IL-17⁺ cells. Staining for other cell types (B cells, NK cells and erythrocyte precursors) could exclude these cells as other sources of IL-17. However, we cannot exclude that, in the early phase of the disease, such a finding might be different because our current results were obtained in patients with advanced AS.

These data suggest that IL-17⁺-secreting cells other than the Th17 cells are of relevance in local inflammation in AS. Investigators in two recent studies on synovial membranes from patients with RA [20] or peripheral SpA, including psoriatic arthritis (PsA) [21], also showed that IL-17-producing cells other than Th17 cells are of relevance. In both RA and PsA patients, mast cells were the major source of IL-17, while Th17 cells were rather rare among the IL-17-producing cells, similar to the findings in our study.

There have previously been some indirect hints that Th17 cells might play a role in the pathogenesis of SpA. An extensive genotype analysis performed recently revealed that AS is closely linked to polymorphisms in the IL-23 receptor gene [22], suggesting that Th17 might be of relevance, although the functional consequence of this IL-23 polymorphism has not been clarified. Furthermore, in HLA-B27/human β₂-microglobulin-transgenic rats, a possible animal model of SpA, HLA-B27 misfolding and the unfolded protein response resulted in a strongly activated IL-23/IL-17 axis in the colon of B27/β₂-microglobulin-transgenic rats with SpA-like disease [23].

Nonetheless, our results and the studies of RA patients [20] and peripheral SpA patients [21] indicate that T cells might have been overestimated as the source of IL-17 in these chronic inflammatory diseases and that an innate immune response in the context of IL-17 might be of relevance. Interestingly, a high frequency of IL-17⁺ mast cells and IL-17⁺ neutrophils, as well as a low frequency of Th17 cells, was also described in the biopsies of skin lesions of psoriasis patients [24]. An analysis of patients with ulcerative colitis revealed an elevated number of Th17 cells located in the lamina propria of inflammatory lesions [25], but the relative number of Th17 cells in comparison to other IL-17⁺ cells was not analysed. On the basis of the results of our investigation, however, we cannot exclude the possibility that Th17 cells are of any relevance in AS. Although the frequency was relatively low, it was higher than in the control group and might be sufficient to orchestrate an immune response.

In our study, mast cells as a source of IL-17 were much less frequent than in RA patients [20] and psoriasis patients [21], and also compared to neutrophils and their precursors. The IL-17 receptor A is highly expressed in hematopoietic cells [26]. Whether the positive staining of neutrophils and MPO⁺ precursor cells is due to autocrine secretion or is caused by binding of IL-17 at the IL-17 receptor could not be determined by our staining. Interestingly, it has been well described that T-cell-derived IL-17 is an important growth factor for granulopoiesis in humans [27]. Although the involvement of IL-17⁺ neutrophils in inflammatory processes has also been reported [26, 28, 29], though not yet in patients with SpA, further confirmation that IL-17 is produced by neutrophils by other methods, such as by in situ hybridisation, would be warranted. Very recently, Li et al. [30] presented impressive data showing that IL-17-producing neutrophils participated in innate immune responses in a mouse model of kidney reperfusion injury. Staining for MPO, which is produced during myeloid differentiation in the bone marrow by neutrophils and their precursors [31], is rather specific for this cell lineage, and these precursor cells appear in the shape of MNCs. Nonetheless, a more detailed characterisation of the MPO⁺ precursor cells found in our present study will be of interest in the future.

Because AS patients are considerably younger than OA patients, an age-matched subanalysis was possible in only three AS patients and three OA patients, confirming a clearly higher number of IL-17⁺ cells in AS patients than in OA patients. Nonetheless, the lack of a larger age-matched control group, which will also not easily be found in possible follow-up investigations, can be seen as a limitation of our study.

In PB and SF of SpA patients, we only looked for the frequency of Th17 cells and not for other IL-17-producing cells. Here we observed no significant differences compared to RA patients, OA patients or healthy controls with regard to PB and compared to RA with regard to SF. These results confirm our immunohistological analysis of the bone that Th17 cells do not seem to play an important role in AS. Previous investigations reported a higher number of Th17 cells in the PB of SpA patients [10, 11], while reduced levels of IL-17 were found in the SF of SpA patients in another study. Our data also indicate that the analysis of IL-17-producing cells in inflamed tissue might be more informative than in PB or SF.

It is currently unclear whether the overexpression of IL-17 in inflammatory lesions of different autoimmune diseases indicates an extraordinary pathogenic role of this cytokine during inflammatory processes or whether this is just a secondary reaction [32]. However, the good clinical response in trials with the anti-IL-17 antibody secukinumab in RA patients [33], psoriasis patients and AS patients [34] suggests that IL-17 might indeed play a role in these diseases. The observed rather rapid clinical response in these trials might be a further hint that direct inhibition of soluble IL-17 is more important than modulation of a T-cell response. Interestingly, a similar discussion of the effect of TNF blockers, mainly neutralisation of soluble TNF or modulation of T-cell responses, has been ongoing over the past few years, but the issue has not been resolved yet [35].

Our study suggest an important role for IL-17 during inflammatory processes in SpA patients. Our data also indicate that the innate immune pathway, mostly mediated through neutrophils and less via mast cells, might play a relevant role during inflammatory processes in AS patients, while Th17 cells seem to be of less importance.