Dennis W. Metzger

BACKGROUND: Porcine small intestinal submucosa (SIS) is an acellular, naturally derived extracellular matrix (ECM) that has been used for tissue remodeling and repair in numerous xenotransplantations. Although a vigorous immune response to xenogeneic extracellular matrix biomaterials is expected, to date there has been evidence for only normal tissue regeneration without any accompanying rejection. The purpose of this study was to determine the reason for a lack of rejection. METHODS: Mice were implanted s.c. with xenogeneic tissue, syngeneic tissue, or SIS, and the graft site analyzed histologically for rejection or acceptance. Additionally, graft site cytokine levels were determined by reverse transcriptase polymerase chain reaction and SIS-specific serum antibody isotype levels were determined by ELISA. RESULTS: Xenogeneically implanted mice showed an acute inflammatory response followed by chronic inflammation and ultimately graft necrosis, consistent with rejection. Syngeneically or SIS implanted mice, however, showed an acute inflammatory response that diminished such that the graft ultimately became indistinguishable from native tissue, observations that are consistent with graft acceptance. Graft site cytokine analysis showed an increase in interleukin-4 and an absence of interferon-gamma. In addition, mice implanted with SIS produced a SIS-specific antibody response that was restricted to the IgG1 isotype. Reimplantation of SIS into mice led to a secondary anti-SIS antibody response that was still restricted to IgG1. Similar results were observed with porcine submucosa derived from urinary bladder. To determine if the observed immune responses were T cell dependent, T cell KO mice were implanted with SIS. These mice expressed neither interleukin-4 at the implant site nor anti-SIS-specific serum antibodies but they did accept the SIS graft. CONCLUSIONS: Porcine extracellular matrix elicits an immune response that is predominately Th2-like, consistent with a remodeling reaction rather than rejection.

The present study demonstrates that novel nanocomposites consisting of blends of polylactic acid and carbon nanotubes effectively can be used to expose cells to electrical stimulation. When osteoblasts cultured on the surfaces of these nanocomposites were exposed to electric stimulation (10 microA at 10 Hz) for 6 h/day for various periods of time, there was a 46% increase in cell proliferation after 2 days, a 307% increase in the concentration of extracellular calcium after 21 consecutive days, and upregulation of mRNA expression for collagen type-I after both 1 and 21 consecutive days. These results provide evidence that electrical stimulation delivered through novel, current-conducting polymer/nanophase composites promotes osteoblast functions that are responsible for the chemical composition of the organic and inorganic phases of bone. Furthermore, this evidence elucidates aspects of the cellular/molecular-level mechanisms involved in new bone formation under electrical stimulation.

Fc receptors for IgG expressed on macrophages and NK cells are important mediators of opsonophagocytosis and Ab-dependent cell-mediated cytotoxicity. Phagocyte-mediated opsonophagocytosis is pivotal for protection against bacteria, but its importance in recovery from infection with intracellular pathogens is unclear. We have now investigated the role of opsonophagocytosis in protection against lethal influenza virus infection by using FcR gamma(-/-) mice. Absence of the FcR gamma-chain did not affect the expression of IFN-gamma and IL-10 in the lungs and spleens after intranasal immunization with an influenza subunit vaccine. Titers of serum and respiratory Abs of the IgM, IgG1, IgG2a, and IgA isotypes in FcR gamma(-/-) mice were similar to levels seen in FcR gamma(+/+) mice. Nevertheless, FcR gamma(-/-) mice were highly susceptible to influenza infection, even in the presence of anti-influenza Abs from immune FcR gamma(+/+) mice. NK cells were not necessary for the observed Ab-mediated viral clearance, but macrophages were found to be capable of actively ingesting opsonized virus particles. We conclude that Fc receptor-mediated phagocytosis plays a pivotal role in clearance of respiratory virus infections.