The role of the immune system in tendon healing: a systematic review.

INTRODUCTION: The role of the immune system in tendon healing relies on polymorphonucleocytes, mast cells, macrophages and lymphocytes, the 'immune cells' and their cytokine production. This systematic review reports how the immune system affects tendon healing. SOURCES OF DATA: We registered our protocol (registration number: CRD42019141838). After searching PubMed, Embase and Cochrane Library databases, we included studies of any level of evidence published in peer-reviewed journals reporting clinical or preclinical results. The PRISMA guidelines were applied, and risk of bias and the methodological quality of the included studies were assessed. We excluded all the articles with high risk of bias and/or low quality after the assessment. We included 62 articles assessed as medium or high quality. AREAS OF AGREEMENT: Macrophages are major actors in the promotion of proper wound healing as well as the resolution of inflammation in response to pathogenic challenge or tissue damage. The immune cells secrete cytokines involving both pro-inflammatory and anti-inflammatory factors which could affect both healing and macrophage polarization. AREAS OF CONTROVERSY: The role of lymphocytes, mast cells and polymorphonucleocytes is still inconclusive. GROWING POINTS: The immune system is a major actor in the complex mechanism behind the healing response occurring in tendons after an injury. A dysregulation of the immune response can ultimately lead to a failed healing response. AREAS TIMELY FOR DEVELOPING RESEARCH: Further studies are needed to shed light on therapeutic targets to improve tendon healing and in managing new way to balance immune response.