Tendon injury in the horse carries a high morbidity and monetary burden. Despite appropriate
therapy,
reinjury is estimated to occur in 50-65% of cases. Although intralesional mesenchymal stem cell (MSC)
therapy has improved tissue architecture and
reinjury rates, the mechanisms by which they promote repair are still being investigated. Additionally, reevaluating our application of MSCs in
tendon injury is necessary given recent evidence that suggests MSCs exposed to
inflammation (deemed MSC licensing) have an enhanced reparative effect. However, applying MSC
therapy in this context is limited by the inadequate quantification of the temporal
cytokine profile in
tendon injury, which hinders our ability to administer MSCs into an environment that could potentiate their effect. Therefore, the objectives of this study were to define the temporal
cytokine microenvironment in a surgically induced model of equine
tendon injury using ultrafiltration probes and subsequently evaluate changes in MSC gene and
protein expression following in vitro inflammatory licensing with
cytokines of similar concentration as identified in vivo. In our in vivo surgically induced
tendon injury model, IL-1β and
IL-6 were the predominant pro-inflammatory
cytokines present in tendon ultrafiltrate where a discrete peak in
cytokine concentration occurred within 48 h following injury. Thereafter, MSCs were licensed in vitro with IL-1β and
IL-6 at a concentration identified from the in vivo study; however, only IL-1β induced upregulation of multiple genes beneficial to tendon healing as identified by
RNA-sequencing. Specifically, vascular development, ECM synthesis and remodeling,
chemokine and
growth factor function alteration, and
immunomodulation and tissue reparative genes were significantly upregulated. A significant increase in the
protein expression of
IL-6,
VEGF, and
PGE2 was confirmed in IL-1β-licensed MSCs compared to naïve MSCs. This study improves our knowledge of the temporal tendon
cytokine microenvironment following injury, which could be beneficial for the development and determining optimal timing of administration of regenerative
therapies. Furthermore, these data support the need to further study the benefit of MSCs administered within the inflamed tendon microenvironment or exogenously licensed with IL-1β in vitro prior to treatment as licensed MSCs could enhance their therapeutic benefit in the healing tendon.