The results of the present study indicated that buffy coat is appropriate for cartilage regeneration. It contains several MSCs, growth factors, and cytokines. Therefore, it can differentiate into engineered cartilage without TGF-β1 in vitro. After addition of HA, it promotes the cartilage regeneration capability of buffy coat, and the problem that buffy coat is difficult to adhere to the cartilage defect could be solved as well.
In addition, TGF-β1 at the range of 0.1-25 ng/mL is often essential for proper regulation of MSC proliferation, differentiation, and ECM synthesis, while a higher concentration may inhibit the process [16,17]. All the values of TGF-β1 obtained are, in fact, lower than the expected ones. According to a research performed by Veselý et al. [18], FGF-2 is normally undetectable in serum or with very low values; and the experimental value obtained by the buffy coat was 1.326 ng/ml. In a recently conducted study on a rabbit, a low dose of FGF-2 (10 μg/mL) soaked in a highly porous scaffold showed improved healing of osteochondral lesions, whereas higher dose of FGF-2 (100 μg/mL) or scaffold without FGF-2 demonstrated an inverse response [19].
The chondrogenic differentiation of MSCs is induced by IGF-1, while it is enhanced when IGF-1 and TGF-β1 are used in combination. As the value of IGF-1 was found to be as low as 0.195 ng/ml in the present study, we hypothesized that such a low concentration of IGF-1 might not be able to suppress the chondrogenic differentiation of MSCs.
Moreover, PDGF released from platelets stimulates chemotaxis and mitosis of fibroblasts, synthesis of collagen, as well as remodeling of the extracellular matrix. In an experiment carried out by Schmidt et al. [20], the effect of PDGF increased proliferation of chondrocytes with concentrations ranging from 4.7 to 300 ng/mL with a maximum number of cells at 75 ng/ml. In the current research, the concentration of PDGF, 1.514 ng/ml, is neither very high nor very low than Schmidt et al.’s findings, thus it is reasonable to hypothesize that the PDGF present in the isolated buffy coat has enough potential for proliferation of chondrocytes.
Generally, VEGF is not found in adult human articular cartilage under physiological conditions and has been reported as one of the most significant factors that can cause arthritis. VEGF in the minimum amount can trigger the formation of a new blood vessel and increase blood flow to the site of injury, thereby protecting the proliferated MSCs [21]. Another growth factor, EGF stimulates epithelization and decreases the healing process, in which a high level of EGF causes thinning, loss of integrity, and degradation of the articular cartilage [22]. Since the concentration of EGF is insignificant in the isolated buffy coat, it is reasonable to hypothesize that the chances of losing the integrity and degradation of the articular cartilage are very low, while its’ presence can stimulate angiogenesis of endothelial cells and mitosis of mesenchymal stem cells.
Additionally, IL-1β and TNF-α are present in joints where cartilage is undergoing repair or regeneration due to injury or disease. In an in vitro study carried out by Schuerwegh et al. [23], it was uncovered that very high concentrations of IL-1 (100 ng/mL) and TNF-α induced apoptosis of bovine chondrocytes. In addition, no effect of IL-6 was noted on the function of cartilage with respect to chondrocyte apoptosis, viability, and proliferation. To our knowledge, inflammatory cytokines are inevitable after tissue injury or disease, hence, we hypothesized that the presence of very low concentration (0.219 ng/ml) of inflammatory cytokines by the buffy coat is negligible [23].
Furthermore, Behrendt et al. found that the effect of IL-10 was highly tangible at 100 pg/ml compared with 1000 pg/ml, which prevented posttraumatic cell death, matrix degradation, and chondrocyte dedifferentiation in mechanically injured cartilage [24]. The value obtained in the present research was as low as 1.969 ng/ml, which is near to the concentration of IL-10 reported by Behrendt et al. [24].
A number of previous studies found that IL-4 has an inhibitory effect on degradation of proteoglycans in the articular cartilage, and also reduces the variation in the production of proteoglycans that are visible in the course of OA [25]. The concentrations of IL-4 obtained in the current experiment was 1.115 ng/ml, which might inhibit the effect of degradation of injured cartilage.
Platelet-rich plasma (PRP) research has become more prevalent in recent years. It is a safe treatment for autologous blood products, containing high concentrations of platelets that can be treated by minimally invasive arthroscopy. It has also been reported that the repaired cartilage by PRP is fibrocartilage in nature, while the frequency and duration of treatment have not been reported yet [26,27]. Therefore, treatment by PRP needs to be further studied to eliminate the existing clinical barriers.
It is noteworthy that BMC contains a variety of biologically active molecules and cells, including platelets, lymphocytes, neutrophils, and monocytes, in addition to MSC. A number of scholars demonstrated that BMC is a product of PRP plus MSC. In fact, BMC and PRP contain different cytokines and growth factors. It was reported that BMC contains several platelets, total monocytes, and CD34 positive cells [28]. Cassano et al. [28] confirmed that BMC is rich in interleukin 1 receptor antagonist (IL-1ra), which is a natural IL-1 receptor antagonist, and it can treat autoimmune diseases and rheumatoid arthritis. Scholars also expressed that buffy coat is a BMC without red blood cell (RBC), that is, there is no RBC that hinders adhesion and proliferation of MSC, thus, buffy coat has a superior cartilage regeneration capability [10]. When buffy coat was extracted, we accidentally scraped numerous RBC samples, in which the quality of tissue-engineered cartilage was not eventually satisfactory. Horn et al. also reported that RBCs interferes with adhesion and proliferation of MSCs [27].
The concentration of MSC in bone marrow extracted from rabbit iliac crest was previously reported equal to 607.8/ml [9]. Thus, there are approximately 1216 MSCs and 4.6 × 106 MNCs in 2 mL of bone marrow. The MSC concentration in bone marrow is very low. Therefore, the same amount of MSCs cannot be inoculated, although the same number of MSCs and MNCs must be inoculated. After PGA plus HA, the buffy coat was easily fixed on the defected cartilage. We used PGA to cover cartilage defects, similar to a membrane. And it was disclosed that cartilage regeneration is generally stable and the appearance is acceptable after coverage of PGA. Siciari et al. demonstrated that a cell-free PGA-HA scaffold has an appropriate cartilage regeneration effect [29].
The present in vivo study revealed that some individuals with poor cartilage regeneration had hyper osteogeny around the joints and had traumatic arthritis as well. These may be related to surgery or individual differences. However, the cartilage regeneration effect in buffy coat+HA group was found to be very satisfactory and stable. When the bone marrow was pumped out for a long period of time, the bone marrow was easily solidified in the syringe. When the buffy coat was extracted with such bone marrow, the buffy coat likely contained several RBCs or MSCs. The same situation was also observed when the ambient temperature was high.
However, the buffy coat was extracted by Ficoll gradient centrifugation, Ficoll couldn’t be used in clinical practice. Thus, it is essential to develop a new method for extracting buffy coat over the Ficoll gradient centrifugation, which is the key to the application of buffy coat in the future.
In the current research, we developed a new method for extracting buffy coat without Ficoll. The buffy coat extracted by the new method and the buffy coat extracted by Ficoll had no difference in growth factor and cytokine content, and the MSC content was known as well. Therefore, our next study is to apply the new method to extract the buffy coat to regenerate cartilage for clinical application of buffy coat.