Controlling the Phenotype of Macrophages Promotes Maturation of Tissue-Engineered Cartilage

Tissue reactions after transplantation can affect the maturation and prognosis of the transplanted engineered tissue in regenerative medicine. Since macrophages are broadly subdivided into two major phenotypes, inflammatory (M1) and anti-inflammatory/wound healing (M2), in this study, we examined the properties of macrophages in transplantation of tissue-engineered cartilage, to clarify their effects on cartilage maturation. Human chondrocytes were embedded in a poly-L-lactic acid scaffold, which was transplanted subcutaneously on the back in athymic mice. When the constructs were analyzed by real-time polymerase chain reaction, interleukin 1 expression was detectable at 4 days, and it reached a peak at 7 days. Interleukin 6 expression was increased at 7 to 11 days, suggesting that M1 macrophages were abundant around this time. On the other hand, expression of markers for M2 macrophages occurred rather later, with Fizz and Ym1 expression peaking at around 11 to 14 days, possibly indicating that polarization of macrophages in tissue-engineered cartilage could shift from M1 to M2 around 11 days after transplantation. When cultured by using the conditioned medium of M2 macrophages, chondrocytes showed significantly increased expression of type 2 collagen, suggesting that M2 macrophages could enhance the maturation of tissue-engineered cartilage. Also, by partially depleting macrophages with clodronate liposomes in the initial period, during which M1 macrophages were dominant, more cartilage matrix accumulated in transplanted constructs at 2 weeks. It was suggested that polarization of macrophages shifted from M1 to M2 in the transplantation of tissue-engineered cartilage, and controlling the polarization could be advantageous for the maturation of transplanted engineered tissues. Impact statement In transplantation of engineered tissues, it is imperative for immune reactions to proceed in a proper and timely manner. In this study, we transplanted tissue-engineered cartilage consisting of a biodegradable polymer scaffold and chondrocytes, and examined the properties of macrophages. It was shown that the polarization of macrophages shifted from inflammatory (M1) to anti-inflammatory/wound healing (M2) around 11 days after transplantation. Partial suppression of macrophages at the early stage of transplantation, which were mainly M1 macrophages, promoted more accumulation of cartilage matrix. This study indicates a possible approach to facilitate cartilage maturation by intervening in the polarity of macrophages.