Which of the following statements regarding transforming growth factor b (TGF-b) are true?

a. TGF-b expression is autoregulated

b. TGF-b enhances collagen synthesis

d. TGF-b may inhibit or promote cellular proliferation

TGF-b appears to be one of the key cytokines in control of tissue repair. TGF-b is strongly chemotactic for neutrophils, T cells, monocytes, and fibroblasts. TGF-b activates inflammatory cells to elaborate fibroblast growth factor, TNFa, IL-1 and increase their synthesis of extracellular matrix proteins. TGF-b also induces both the infiltrating cells and resident cells to produce more TGF-b. This auto-induction amplifies its biological effects at the site of injury and may play an important role in the development of chronic fibrosis in a variety of pathologic states. TGF-b enhances collagen synthesis as well. Lastly, TGF-b may function as a mitogen or growth inhibitor for a wide variety of cell types, including selected cell types of mesenchymal origin. Whether TGF-b stimulates or inhibits proliferation depends on the presence of other growth factors, the concentration of TGF-b, and the cell density. Thus, at low doses, TGF-b stimulates the proliferation of densely plated human marrow fibroblasts, but is inhibitory at high concentrations.