[Inflammation and osteoarthritis-related pain]

Osteoarthritis (OA) is one of the major causes of chronic pain. Although OA has long been considered a non-inflammatory "wear and tear" disease leading to loss of articular cartilage, recent findings provide convincing evidence that inflammatory mechanisms play a pivotal role in the pathophysiology of OA. In OA mononuclear cells (e. g. T‑cells and macrophages) infiltrate the synovial membrane and the levels of pro-inflammatory cytokines in peripheral blood and synovial fluid samples are elevated. Increased release of inflammatory mediators including interleukin (IL) IL-1β, IL-6, IL-8, IL-15 und tumor necrosis factor alpha (TNF‑α) induces the expression of proteolytic enzymes such as matrix metalloproteinases resulting in cartilage breakdown. Molecular and cellular interactions between the immune and nervous system are also involved in the development of OA-related pain. Inflammatory mediators including IL-6 und TNF‑α lead to peripheral sensitization of joint nociceptors and growth factors (e. g. NGF) trigger the expression of TRPV1 channels in primary afferents. Moreover, neuropeptides reduce the threshold of nociceptors of OA joints. The current review highlights the role of inflammatory mechanisms in OA-induced joint pain considering clinical signs of inflammation and major inflammatory pathways.

A predominant Th1 polarization is present in synovial fluid of end-stage osteoarthritic knee joints: analysis of peripheral blood, synovial fluid and synovial membrane

Thorough understanding of the complex pathophysiology of osteoarthritis (OA) is necessary in order to open new avenues for treatment. The aim of this study was to characterize the CD4⁺ T cell population and evaluate their activation and polarization status in OA joints. Fifty-five patients with end-stage knee OA (Kellgren-Lawrence grades III-IV) who underwent surgery for total knee arthroplasty (TKA) were enrolled into this study. Matched samples of synovial membrane (SM), synovial fluid (SF) and peripheral blood (PB) were analysed for CD3⁺ CD4⁺ CD8^- T cell subsets [T helper type 1 (Th1), Th2, Th17, regulatory T cells] and activation status (CD25, CD69, CD45RO, CD45RA, CD62L) by flow cytometry. Subset-specific cytokines were analysed by cytometric bead array (CBA). SM and SF samples showed a distinct infiltration pattern of CD4⁺ T cells. In comparison to PB, a higher amount of joint-derived T cells was polarized into CD3⁺ CD4⁺ CD8^- T cell subsets, with the most significant increase for proinflammatory Th1 cells in SF. CBA analysis revealed significantly increased immunomodulating cytokines [interferon (IFN)-γ, interleukin (IL)-2 and IL-10] in SF compared to PB. Whereas in PB only a small proportion of CD4⁺ T cells were activated, the majority of joint-derived CD4⁺ T cells can be characterized as activated effector memory cells (CD69⁺ CD45RO⁺ CD62L^- ). End-stage OA knees are characterized by an increased CD4⁺ T cell polarization towards activated Th1 cells and cytokine secretion compared to PB. This local inflammation may contribute to disease aggravation and eventually perpetuate the disease process.

Unicompartmental and bicompartmental knee osteoarthritis show different patterns of mononuclear cell infiltration and cytokine release in the affected joints

It is still controversial which cell types are responsible for synovial inflammation in osteoarthritic (OA) joints. The aim of this study was to quantify the mononuclear cell populations and their cytokines in patients with different knee OA subtypes. Synovial membrane (SM), synovial fluid (SF) and peripheral blood (PB) were harvested from patients with unicompartmental (UC) and bicompartmental (BC) knee OA. Frequencies of mononuclear cells were assessed by flow cytometry in PB and SM. Naive SF samples were analysed for a broad variety of cytokines by multiplex analysis. SM of both groups displayed a distinct mononuclear cell infiltration, with CD14(+) macrophages being the major cell population, followed by CD4(+) T cells and only small numbers of CD8(+) T, CD19(+) B and CD16(+) CD56(+) natural killer (NK) cells. Between the two groups, SM of BC OA showed significantly higher amounts of mononuclear cells (135·7 ± 180 versus 805 ± 675 cells/mg, P = 0·0009) and higher CD4(+) T cell presence (3·4 ± 4·6 versus 9·1 ± 7·5%, P = 0·0267). SF of BC OA displayed significantly higher concentrations for a number of proinflammatory cytokines [CXCL1, eotaxin, interferon (IFN)-γ, interleukin (IL)-7, IL-8, IL-9, IL-12]. UC and BC OA show significant differences in their synovial inflammatory pattern. Whereas in UC OA CD14(+) macrophages are the predominant cell population, BC OA has a higher inflammatory profile and seems to be driven by CD14(+) macrophages and CD4(+) T cells. Inclusion of clinical information into the analysis of cellular and molecular results is pivotal in understanding the pathophysiology of OA.