RESUMO
Experimental studies evaluated the responses of murine cardiac graft recipients to high and low levels of lipopolysaccharide (LPS) contaminating plasmid DNA preparations. Immediately prior to transplantation, graft recipients were transfected by injecting the quadriceps muscles with plasmids that encoded the murine interleukin (IL)-4 gene and beta-galactosidase (beta-gal) gene. Graft recipients transfected with plasmids encoding only the beta-gal gene served as negative plasmid controls. Three groups of mice were transfected with plasmids containing high levels of contaminating LPS: (a) nontransplanted C57B1/6 mice, (b) C57B1/6 cardiac isograft recipients, (c) DBA/2 (H-2d)-->C57BL/6 (H-2b) cardiac allograft recipients. Unexpectedly, graft failure within 24 h was observed in IL-4 transfected isograft and allograft recipients, but not in mice transfected with the beta-gal gene alone. However, histopathological findings, for example, vascular cell adhesion moelcule-1 (VCAM-1) expression in cardiac grafts and mononuclear lung infiltration, were remarkably similar for both treatment groups and consistent with LPS-induced pathology. LPS assays were used to evaluate four different methods of plasmid purification for degree of LPS contamination. A successful strategy for reducing levels of LPS contamination was identified and transfection experiments repeated in cardiac allograft recipients receiving LPS inoculum that were minimized and standardized (6.4 EU/mouse) for all treatment groups. Despite receiving substantially lower levels of LPS, in all treatment groups there was persistent cardiac graft endothelial cell activation manifested by VCAM-1 expression and persistent, albeit less severe, lung pathology. We found that plasmid contamination with LPS was unavoidable and that even very low levels can alter immune responses in transplant recipients confounding data interpretation. Thus, it is imperative to account for LPS contamination in experiments utilizing plasmid DNA for gene transfer, especially in experimental models of immunity and inflammation.