AVOIDANCE OF IMMUNE-RESPONSE PROLONGS EXPRESSION OF GENES DELIVERED TO THE ADULT-RAT MYOCARDIUM BY REPLICATION-DEFECTIVE ADENOVIRUS

Background Gene delivery is a rapidly expanding field with potential a pplications to every human organ system. Recently. adenoviruses have b een used as efficient nt vectors for in vivo gene transfer into the my ocardium. These methods, however, have shown a sharp decline of gene e xpression after week. To test the hypothesis that an immune-effector m echanism is involved in this decline, we compared the results after in jection of adenovirus-5 carrying the beta-galactosidase gene (Ad beta- gal) into the left ventricular myocardium of athymic nude rats (NDRs) versus immunocompetent Sprague-Dawley rats (SDRs). Methods and Results Ad beta-gal (5.0x10(9) PFU/mL) was injected into the left ventricle o f NDRs (n = 16) and SDRs (n = 22). Hearts were harvested, embedded in paraffin, and sectioned and stained for beta-gal activity, hematoxylin and eosin and picrosirius red at 4, 21, 35, 85, and 120 days. Represe ntative samples were immunostained with antibodies directed at inflamm atory markers. beta-gal activity was quantified by digital planimetry and expressed as area of staining (%+/-SEM). Peak beta-gal activity wa s highest at 4 days, with NDRs displaying significantly greater staini ng (83+/-3.0% versus 54+/-8.0% P=.03). SDRs sustained a rapid drop in activity, such that at 35 (1+/-0.19%) and 85 (1+/-0.4%) days, only occ asional cells stained positive and by 120 days (0.3+/-0.0%), activity had been extinguished. NDRs continued to show transgene expression at all time periods (35 and 85 days, 25+/-7.1% and 7.4+/-2.7%, respective ly) and was still readily detected at 120 days. An inflammatory respon se was limited in NDRs compared with SDRs, in which there was intense mono-nuclear cell infiltration, with collagen deposition and scar form ation. Immunostaining identified the majority of these inflammatory ce lls as not being of lymphocyte lineage, although small numbers of lymp hocytes and phagocytic and activated plasma cells were identified. Con clusions Our data suggest that immune-effector mechanisms can severely affect the expression of gents delivered by adenovirus. The present m odel provides efficient gene expression for at least 120 days without significant inflammatory reaction.