Abstract
We thank the authors for their interest in and points of view on our recent study, “Platelet-Rich Plasma Does Not Inhibit Inflammation or Promote Regeneration in Human Osteoarthritic Chondrocytes In Vitro Despite Increased Proliferation,” 1 published in this journal. We fully agree with their statement that the clinical use of platelet-rich plasma (PRP) for knee osteoarthritis (OA) on a large scale should be supported by qualitative in vitro and in vivo research, especially to gain more understanding of the mechanisms and effects of PRP.
Indeed, as the authors highlighted, our in vitro study was performed with chondrocytes derived from tissue that was obtained from patients that underwent total knee replacement because of end-stage OA. The authors correctly conclude that we have demonstrated that the PRP we made did not show any effect on these OA chondrocytes in terms of inhibiting inflammation or promoting regeneration in vitro. Subsequently, the authors conclude that this study further proves the fact that PRP does not act in advanced OA stages but does so in early OA. We agree with the authors that there is literature suggesting PRP is effective in early OA 2 and there are recommendations PRP should be used in patients with symptomatic knee OA with Ahlback grades I to III or Kellgren-Lawrence grades I to III. 3 On the other hand, we would like to be more careful drawing a conclusion, as there are still many open questions regarding the efficacy of PRP, such as the optimal selection of the PRP and the patient. Moreover, most in vitro studies have been performed with chondrocytes obtained from redundant material after total joint replacement. Also, the study performed by Spreafico et al., 4 in which promoting effects on proliferation and cartilage tissue formation were observed, was performed with chondrocytes of macroscopically normal cartilage from total hip replacement surgeries due to OA. Although our samples are a mixture of mostly macroscopically healthy cartilage and some more fibrillar tissue, it is more likely that other factors, such as the PRP composition (6 times vs. 2 times platelet concentration, 126 vs. 60 ng TGF-beta/mL) play a more prominent role in explaining differences between the studies. Therefore, we strongly advocate for a uniform reporting system of the characteristics of PRP used in studies. 5
We are aware of the fact that we used cartilage tissue from patients with advanced OA for the chondrocyte isolation, and that the degree of cartilage degradation varies between the different locations in the knee, as this is also clearly reported in the experimental methods section of our article. OA is a whole joint disease and all joint tissues may be involved to some extent. Significant changes in chondrocyte behavior in very early models and stages of OA have been found previously, indicating a notable effect of the OA joint on the cells.6,7 This suggests that these chondrocytes will perform differently than cells derived from a true healthy joint and are comparable to the cells used in our study. Regarding our isolation protocol, we have to further clarify certain issues, as the authors are concerned that our cartilage is not representative for OA. In this study, we have pooled the cartilage from all parts of the joint (condyles, tibial plateaus), in order to obtain a chondrocyte population that represents the status of the cartilage in the whole OA joint. Severely degraded/fibrillated cartilage is difficult to isolate from an OA knee and was, therefore, excluded. The authors recommend a comparison of effects of PRP on chondrocyte cultures from different grades of OA. We completely agree that such an experimental design would provide excellent insight into how PRP influences chondrocytes when used in various grades of OA and could back up further studies identifying what stages of OA and maybe even which patient populations could be most effectively treated with PRP.
The authors emphasize that clinical effectiveness of PRP in early knee OA in terms of pain relief and functional improvement is best attributed to its anti-inflammatory effects and that previous studies have identified the synovium as the primary site of action of the anti-inflammatory effect. We believe that this is an important observation and should be addressed in further ex vivo and in vivo studies. Indeed, many pro-inflammatory events take place in the synovium and it plays a big role in pain. However, in our in vitro study, we focused solely on the effects of PRP on OA chondrocytes, which also produce pro-inflammatory cytokines and matrix degrading enzymes in OA. 8
With regard to the authors’ remark about the degradation rate of the gels, we are aware of the fact that calcium activates platelets and growth factor concentrations in activated PRP or platelet lysate is increased, which we have shown in a previous study 9 as well as in the current study. 1 The aim of this particular section of the current study was to investigate the capacity of our PRP gels to be used as a biomaterial for cartilage tissue engineering purposes. In general, studies clotting PRP to create a 3D biomaterial use CaCl2 to generate a hydrogel that is manageable and is able to incorporate cells.10-12 The findings of this section of the article do not relate to intraarticular injections of PRP and do not provide an answer whether activated or not activated PRP should be used for injection nor for effects of single versus multiple injections. As the authors mention, there are in vivo and clinical studies performed that focus on these questions.2,13
As per authors’ remark, the platelets are indeed about 2 times concentrated in the PRP we used in our study compared to the whole blood. It is well known that PRP varies a lot in composition and there are also commercially available and clinically used PRPs with a 2-time higher platelet concentration compared to the whole blood.14-17 More knowledge is required on the influence of platelet concentration, as a recent in vitro study reported a dose- and time-dependent effect of a standardized PRP lyophilizate on chondrocytes. 18 In addition, thorough evaluation and characterization of any PRP product is extremely important, as we have clearly highlighted in the discussion of our study. As aforementioned, we highly support a uniform reporting system of the characteristics of PRP used in studies. 5
To conclude, we do not agree with the authors’ statement that the title of our recent study is a negative reflection on PRP. The title of our study clearly states that we have performed an in vitro study in OA chondrocytes. In our view, our article clearly highlights the specific experimental setups and conditions used for in vitro testing; thus, the data reported does not create the impression that PRP is ineffective in knee OA and this conclusion cannot be drawn from our study. The study investigated the effects of PRP on one cell type derived from a pathological tissue, with clearly reported characterizations of the PRP. Under no circumstance we want to extrapolate these results to clinical effects. Therefore, we stand with the conclusions in our in vitro study that PRP showed a nonfavorable effect on chondrocytes derived from OA cartilage.