Background/Purpose:

Alarmins, also referred to as damage associated molecular patterns (DAMPS), are endogenous molecules mobilized in response to tissue damage and are known to activate the innate immune system in the early stages of disease. The molecular mechanisms that regulate inflammatory and remodelling pathways in tendinopathy are largely unknown therefore identifying early immune effectors is essential to understanding the pathology.

S100A8 and S100A9 are low molecular weight calcium binding proteins constitutively expressed by cells of myeloid origin. Under pathological conditions they are released in other cell types in response to environmental triggers and cellular damage. Based on our previous investigations highlighting tendinopathy as an alarmin mediated pathology we sought evidence of S100A8 & A9 expression in human tendinopathy and thereafter, to explore mechanisms whereby S100 proteins may regulate release of inflammatory mediators and matrix synthesis in human tenocytes.

Methods:

Torn supraspinatus tendon (representing established pathology) and matched intact subscapularis tendon (representing ‘early pathology’) biopsies were collected from patients undergoing arthroscopic shoulder surgery. Control samples of healthy hamstring tendon were collected from patients undergoing anterior cruciate ligament (ACL) reconstruction. S100A8 & A9 expression was analyzed at transcript and protein level using quantitative RT-PCR and immunohistochemistry, respectively. Primary human tenocytes were cultured from hamstring tendon tissue obtained during hamstring tendon ACL reconstruction. The in vitro effect of recombinant human S100A8 & A9 on human tenocytes was measured using quantitative RT-PCR and release of inflammatory mediators was measured at a protein level by ELISA.

Results:

Immunohistochemical staining of tendinopathic tissues indicated the presence of S100A8 and S100A9 in tendinopathy with early diseased tissue displaying a distinct increase in S100A8 and S100A9 expression compared with control and established pathology. These findings were mirrored by data obtained at transcript level from both early and late pathology. Treating tenocytes with exogenous S100A8 & A9 significantly increased protein release of IL-6, IL-8 and CCL2 and an induction of CCL20 and CXCL10 release was observed. However, no alterations in genes associated with matrix remodelling were observed at a transcript level.

Conclusion:

We have confirmed the presence of S100A8 and S100A9 in tendinopathy and propose that S100A8 & A9 participate in early pathology by modulating the stromal microenvironment and influencing the inflammatory profile of tenocytes. S100A8 and S100A9 may participate in a positive feedback mechanism involving enhanced leukocyte recruitment and release of pro-inflammatory cytokines from tenocytes that perpetuates the inflammatory response within the tendon in the early stages of disease. This, in turn, may contribute aberrant matrix remodelling and support a detrimental transition from acute to chronic inflammation. Selectively targeting DAMP signalling in early disease provides scope for novel translational strategies in the management of tendon disorders.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/s100a8-s100a9-alarmin-mediated-inflammation-in-tendinopathy/