Dose-Dependent Effects of Platelet-Rich Plasma Powder on Chondrocytes In Vitro: Letter to the Editor

Abstract

We read with great interest the article by Hahn et al 1 titled, ''Dose-Dependent Effects of Platelet-Rich Plasma Powder on Chondrocytes In Vitro.'' The article by Hahn et al 1 is a descriptive study, in a 2-dimensional (2-D) in vitro model (n=3), assessing the chondrogenic status of cartilage cells (chondrocytes) in response to different platelet-rich plasma (PRP) treatment regimens. The authors assessed changes in procollagen type 1 and procollagen type 2 concentrations in the culture supernatants, as well as colorimetric quantification of proteoglycan and glycosaminoglycan. As suggested by Hahn et al, we would have expected that the article would have clarified some practical questions underlying the optimal dose of PRP and frequency of injections helping to define a clinical protocol (ie, dose, number of applications, and timing) to treat patients with symptomatic knee osteoarthritis (OA). However, we would like to remark on 2 issues that warrant consideration because they can question the validity of their model and the conclusions. First, we respectfully point out that 2-D cultures and high cell density do not mimic cartilage physiology. Indeed, from the moment the chondrocytes begin to attach to culture plastic, spread out, and begin to divide, they change their gene expression and go on to progressively lose their chondrogenic phenotype. This is associated with a loss of their rounded shape, loss of expression of chondrocyte marker genes (eg, COL2A1, which encodes the collagen type II alpha 1 chain, and the transcription factor SOX9), and increased expression of fibroblastic genes (eg, COL1A1, which encodes the collagen type I alpha 1 chain). In the current article, the authors examine the ability of PRP powder to stimulate chondrocytes to maintain their phenotype and synthesize molecules that are specific of the extracellular matrix of hyaline cartilage neutralizing 2-D culture conditions. Regarding changes of cell shape, when cell densities are too high, and cells overlap, it is not possible to properly assess differences in morphology (see Figure 4 in the article). Second, if extracellular matrix (ECM) protein measurements are not normalized per cell number, changes of ECM molecules in the conditioned media (ng/mL) are merely an increase in cell numbers, confirming only the dose-dependent mitogenic effects of PRP in 2-D plastic cultures.