Transforming growth factor beta signaling is essential for the autonomous formation of cartilage-like tissue by expanded chondrocytes.

Cartilage is a tissue with limited self-healing potential. Hence, cartilage defects require surgical attention to prevent or postpone the development of osteoarthritis. For cell-based cartilage repair strategies, in particular autologous chondrocyte implantation, articular chondrocytes are isolated from cartilage and expanded in vitro to increase the number of cells required for therapy. During expansion, the cells lose the competence to autonomously form a cartilage-like tissue, that is in the absence of exogenously added chondrogenic growth factors, such as TGF-βs. We hypothesized that signaling elicited by autocrine and/or paracrine TGF-β is essential for the formation of cartilage-like tissue and that alterations within the TGF-β signaling pathway during expansion interfere with this process. Primary bovine articular chondrocytes were harvested and expanded in monolayer culture up to passage six and the formation of cartilage tissue was investigated in high density pellet cultures grown for three weeks. Chondrocytes expanded for up to three passages maintained the potential for autonomous cartilage-like tissue formation. After three passages, however, exogenous TGF-β1 was required to induce the formation of cartilage-like tissue. When TGF-β signaling was blocked by inhibiting the TGF-β receptor 1 kinase, the autonomous formation of cartilage-like tissue was abrogated. At the initiation of pellet culture, chondrocytes from passage three and later showed levels of transcripts coding for TGF-β receptors 1 and 2 and TGF-β2 to be three-, five- and five-fold decreased, respectively, as compared to primary chondrocytes. In conclusion, the autonomous formation of cartilage-like tissue by expanded chondrocytes is dependent on signaling induced by autocrine and/or paracrine TGF-β. We propose that a decrease in the expression of the chondrogenic growth factor TGF-β2 and of the TGF-β receptors in expanded chondrocytes accounts for a decrease in the activity of the TGF-β signaling pathway and hence for the loss of the potential for autonomous cartilage-like tissue formation.

Regulation of bovine IL-12R beta 2 subunit mRNA expression in bovine lymph node cells

The beta 2 subunit of the interleukin (IL)-12 receptor (IL-12R beta 2) has been shown to play an essential role in differentiation of T helper 1 (Th1) cells in the murine and human system, and antibodies raised against IL-12R beta 2 recognized this molecule on human Th1 but not Th2 cells. However, while the cytokines secreted by clones of murine cells allowed the definition of distinct T helper cell subsets, bovine clones with polarized Th1 and Th2 cytokine profiles were rarely found. This raised important questions about the regulation of immune responses in cattle. We therefore cloned bovine IL-12R beta2 (boIL-12R beta 2) DNA complementary to RNA (cDNA) from the start codon to the 3' end of the mRNA. Comparison of boIL-12R beta 2 cDNA with human and murine IL-12R beta 2 cDNA sequences revealed homologies of 85 and 78%, respectively. The deduced protein sequence showed the hallmark motifs of the cytokine receptor superfamily including the four conserved cysteine residues, the WSXWS motif and fibronectin domains in the extracellular part as well as a STAT4 binding site in the intracellular part of the molecule. Using real-time reverse transcription-polymerase chain reaction, upregulation of mRNA expression of this molecule could be demonstrated in cultured bovine lymph node cells stimulated with phytohemagglutinin. Furthermore, cells with upregulated boIL-12R beta 2 mRNA responded with enhanced expression of interferon gamma to treatment with interleukin 12.