Inflammation is a driving force of tendinopathy. The oxidation of phospholipids by free radicals is a consequence of inflammatory reactions and is an important indicator of tissue damage. Here, we have studied the impact of oxidized phospholipids (OxPAPC) on the function of human tenocytes. We observed that treatment with OxPAPC did not alter the morphology, growth and capacity to produce collagen in healthy or diseased tenocytes. However, since OxPAPC is a known modulator of the function of immune cells, we analyzed whether OxPAPC-treated immune cells might influence the fate of tenocytes. Co-culture of tenocytes with immature, monocyte-derived dendritic cells treated with OxPAPC (Ox-DCs) was found to enhance the proliferation of tenocytes, particularly those from diseased tendons. Using transcriptional profiling of Ox-DCs, we identified amphiregulin (AREG), a ligand for EGFR, as a possible mediator of this proliferation enhancing effect, which we could confirm using recombinant AREG. Of note, diseased tenocytes were found to express higher levels of EGFR compared to tenocytes isolated from healthy donors and show a stronger proliferative response upon co-culture with Ox-DCs, as well as AREG treatment. In summary, we identify an AREG-EGFR axis as a mediator of a DC-tenocyte crosstalk, leading to increased tenocyte proliferation and possibly tendon regeneration.

The assessment of the degradation of biomaterials is an essential factor in the development of scaffolds for biomedical applications. The byproducts that are released from the scaffold in the implantation site as a result of its degradation may lead to the failure of the treatment. Their recognition by the immune cells as foreign bodies will lead to inflammation, which may result in the further release of degradative enzymes, as well as the oxidation, phagocytosis or encapsulation of the engineered-biomaterial. Toll-like receptors (TLRs) have been described to play a role in the direct recognition of scaffolds. However, their implication regarding the degradation-derived products of the latter has not been established, yet. Moreover, the impact of the chemical composition and morphology of the scaffolds on this interaction remains unknown. Herein, polycaprolactone (PCL)-hyaluronic acid (HA)-based electrospun scaffolds with randomly or aligned distributed fibers were degraded, the resulting products were collected and the recognition of the latter by TLRs was assessed by means of TLR reporter cells. Our results demonstrated that the degradation-derived products are capable of triggering TLRs, however, their triggering patterns differ from those of the original scaffolds. While scaffolds were mainly recognised by TLR 4, their degradation-derived products additionally triggered TLR 2/1. Moreover, this interaction changes over time and the alignment of the fibers appears to prevent the production of TLR triggering structures. Taken together, our findings highlight the importance of assessing the immune recognition of the degradation-derived products independently of the original scaffold.

This work demonstrates that scaffolds’ fiber alignment has an impact on the immune recognition of the scaffolds and presents TLR reporter cells as a simple and fast read out system for analyzing the recognition of the scaffolds by TLRs.

The soluble cytoplasmic tail of the prototypic receptor-like protein tyrosine phosphatase (PTP) CD45 (ct-CD45) is cleaved and released into the human plasma by activated phagocytes. Released ct-CD45 was found to inhibit T cell proliferation and cytokine production via engagement of Toll-like receptor 4 (TLR4). In this study, we analyzed the impact of the ct-CD45/TLR4 pathway on the function of human monocyte-derived dendritic cells (DCs). We could demonstrate that activation of DCs by ct-CD45 upregulated the expression of certain cell surface markers (e.g., CD71 and CD86) and induced IL-10 production via TLR4. Yet, in contrast to stimulation with LPS, other typical cell surface markers and cytokines were not upregulated or induced in DCs by ct-CD45. The T cell proliferation–stimulatory capacity of DCs was not modulated by ct-CD45 treatment. However, treatment of DCs with ct-CD45 modulated the cytokine profile in co-cultured T cells. While IFN-γ production induced by DCs was strongly inhibited, the release of IL-4 was increased in T cells upon stimulation with ct-CD45-treated DCs. In contrast, ct-CD45-stimulated DCs induced IL-2 and IL-10 production in co-cultured T cells comparable to untreated DCs. In summary, we could demonstrate that ct-CD45 acts as an immunoregulatory factor for DCs via a non-canonical TLR4-dependent activation pathway.