Despite the constantly rising number of patients suffering from OA (20), there is, thus far, no causal treatment for OA and symtomatic treatment cannot halt disease progression (21). The local low-grade inflammation in OA has been linked to cartilage degeneration and subsequent joint destruction (22). Complementary to surgical treatment strategies (23), various potentially disease modifying drugs, that may selectively disrupt inflammatory pathways present in OA, are currently being investigated (24). Of these, PRP is one of the most popular products already in clinical use. PRP was observed to be superior to other therapeutics, including hyaluronic acid, corticosteroids, and placebo, with respect to clinical outcomes and disease progression in OA (25, 26). However, response to therapy is highly heterogenous and PRP fails in a relevant proportion of affected patients. To date, reasons for absent responses to treatment remain elusive.
To the best of our knowledge, this is the first study assessing cellular and cytokine compositions of various commercially available PRP systems used in daily clinical practice and comparing these to corresponding donor blood samples.
All PRP systems resulted in a significant proportional enhancement of leukocytes. This observation is consistent with previous findings (27–34). Commercially available PRP systems result in products with varying leukocyte concentrations, that can be categorized in two groups: leukocyte-rich PRP (LR-PRP) and leukocyte-poor PRP (LP-PRP). While LR-PRP systems tend to aggregate leukocytes, LP-PRP systems reduce leukocyte concentrations compared to corresponding blood concentrations (27–34). Slow spin speeds of around 1,500 rpm are associated with an up to threefold concentration of platelets, an almost complete elimination of red blood cells (RBC), and a reduction of leukocyte concentrations (LP-PRP), whereas higher spin speeds of around 3,200 rpm are associated with an up to ninefold concentration of platelets, some loss of RBCs, and an increase in leukocyte concentrations (LR-PRP) (27). Previous studies ranked the ACP® and the Angel™ sytems among the LP-PRP systems (2, 33, 35).
The concentration of leukocytes was accompanied by an overall reduction in granulocytes and an proportional increase of lymphocytes and monocytes. Donor blood samples contained 1.1% of leukocytes, of which over 60% were granulocytes, which was reduced to at least 40% in final PRP products. The lymphocytes and monocytes proportion was increased from around 40% in donor blood to around 60% and above in the PRP samples. As previously observed (27), the nSTRIDE® APS system, categorized as an LR-PRP system, showed higher proportions of granulocytes when compared to the LP-PRP systems, ACP® and Angel™. Wakayama et al. compared the nSTRIDE® APS and the MyCells® (LP-)PRP system. Both systems concentrated lymphocytes, but only the nSTRIDE® system concentrated neutrophils in both healthy volunteers and OA patients (30). Fitzpatrick et al. compared the GPS III, SmartPrep® II, and the ACP®. They detected that neutrophils and lymphocytes were the most concentrated fractions within the leukocyte populations (33). These results were supported by a canine feasibility study that solely analyzed the nSTRIDE® APS system and observed a global concentration of leukocytes, neutrophils, lymphocytes, and monocytes (31). The aforementioned findings are not completely in accordance with our data, but only Wakayama et al. reported data on human subjects (30). As the proportion of neutrophils and monocytes directly influences the composition of pro- and anti-inflammatory cytokines in the final product (27), this may impact clinical outcomes in treated patients.
We observed considerably varying cytokine compositions between products. The concentrations of the pro-inflammatory cytokines IFN-γ and TNF-α were significantly increased in the nSTRIDE® APS when compared to donor blood samples. This was not the case for the other two systems. As PRP is mainly used for its anti-inflammatory properties, this finding appears counterintuitive. Previous studies have observed a concentration of pro-inflammatory cytokines, such as IL-1β, IL-6, and TNF-α, especially in LR-PRP products (29, 30, 34–36). Increased levels of IL-1β and TNF-α directly contribute to cartilage degradation and enhanced IL-6 production, which then enhances inflammatory responses that cause bone resorption in OA (37). Accordingly, most authors do not recommend the use of LR-PRP for OA therapy, as they expect these products to increase or, at least, maintain the low-grade inflammation present in affected joints (34, 38, 39). In laboratory studies, however, LR-PRP prevented chondrogeneous matrix degradation, increased chondrocyte cellularity, and inhibited the production of matrix metallopeptidase 13 (40–42). Interestingly, both Mariani et al. and Cole et al. observed no local changes in cytokine concentrations in synovial fluid after PRP injections (35, 43). These observations are confirmed by clinical studies, which showed overall effectiveness of products with various cytokine compositions, including LR-PRP, for OA treatment (44). While the relevance of cytokine compositions in PRP products on clinical outcomes in OA may be questioned, it is important to note that regardless of individual donor blood cytokine compositions, some PRP products will yield a high pro-inflammatory cytokine composition, which may affect immune cells already present in the affected joint.
When comparing donor blood and corresponding PRP samples, we observed a maintenance of each participant’s adaptive immune profile. This is highly relevant, since high systemic levels of distinct T cell subsets have been associated with impaired tissue healing (45, 46). These TEMRA T cells accumulate in large numbers also at the injury site and are the major local producers of pro-inflammatory cytokines (46), which are, again, linked to the age-related phenotype (47) and the development of OA (48). Further, a local downregulation of these cells led to a decreased concentration of pro-inflammatory cytokines and an improved bone regeneration in a preclinical fracture model (49). Our observations could help to understand the heterogeneous clinical effectiveness of PRP in OA therapy, as we observed that the individual adaptive immune profile is directly transferred into the PRP products. Further clinical studies are needed to examine this potential influence of patients’ individual adaptive immune system and its receptivity to regenerative therapies, such as PRP.
This study has some limitations. First, we provided data from a rather small cohort of healthy participants. Second, correlations of our results with clinical outcome data are not available. In future, prospective observational or randomized interventional studies are needed in order to analyse our findings for clinical relevance. On the other hand, this is the first study to merge the cellular and cytokine compositions of different PRP products and match these to individual donor immune profiles. This is highly relevant, as authors were previously able to highlight the influence of the individual adaptive immune capacities on tissue regeneration (46, 49). The systems used for PRP production in our study are commercially available and broadly used in clinical practice, allowing for a high comparability with other researchers’ results.