An Acute Increase in TGF beta Increases Permeability of 70 kDa Molecular Tracer from the Heart to Cells Inhabiting Tissue Compartments of the Osteoarthritic Guinea Pig Knee Joint

Abstract

Abstract A recent pilot study showed that a spike in the inflammatory cytokines TNF-α or TGF-β, delivered via the heart in mature Dunkin-Hartley guinea pigs with osteoarthritis, results in diminished barrier function between the vascular (blood vessels) and respective tissue compartments of bone and muscle [1]. Here we aim to probe effects of TNF-α and TGF-β on barrier function at the vascular interface in different tissue compartments of the knee joint using the same animal model, and with higher resolution imaging modalities including confocal and electron microscopy. First we quantified the intensity of a fluorescent-tagged 70 kDa tracer, similar in size to albumin, the largest transporter protein in the blood, in tissue compartments of bone (periosteum, marrow space, compact bone and epiphyseal bone) and cartilage (articular cartilage, calcified cartilage, and the interface between, i.e. the epiphyseal line), as well as at sites of tendon attachment to bone. We then examined tracer presence and intensity in the respective pericellular and extracellular matrix zones of bone and cartilage. Confirming the previous study, acute exposure to both cytokines reduced barrier function (increased permeability) at vascular interfaces with tissue compartments of the knee joint, with a significant effect in the TGF-β group. Furthermore, this increase in permeability, observed at the length scale of tissue compartments, was also observed at the cellular length scale; the observation of pericellular transport of the albumin-sized molecules to osteocytes contrasts with previous observations of barrier function in healthy, untreated animals and is indicative of reduced barrier function in pericellular regions of cytokine treated animals. The acute, cytokine-induced changes to molecular transport between and within the tissue compartments of the joint, and their respective cellular inhabitants, is of particular relevance for a systems biology understanding of articular joint physiology and interactions between the vascular, musculoskeletal and immune systems, providing impetus for further studies.