The calcium pump SERCA2a (sarcoplasmic reticulum calcium ATPase 2a), which plays 
a central role in cardiac contraction, shows decreased expression in heart 
failure (HF). Increasing SERCA2a expression in HF models improves cardiac 
function. We used direct cardiac delivery of adeno-associated virus encoding 
human SERCA2a (AAV6-hSERCA2a) in HF and normal canine models to study safety, 
efficacy, and the effects of immunosuppression. Tachycardic-paced dogs received 
left ventricle (LV) wall injection of AAV6-hSERCA2a or solvent. Pacing continued 
postinjection for 2 or 6 weeks, until euthanasia. Tissue/serum samples were 
analyzed for hSERCA2a expression (Western blot) and immune responses (histology 
and AAV6-neutralizing antibodies). Nonpaced dogs received AAV6-hSERCA2a and were 
analyzed at 12 weeks; a parallel cohort received AAV-hSERCA2a and 
immunosuppression. AAV-mediated cardiac expression of hSERCA2a peaked at 2 weeks 
and then declined (to ~50%; p<0.03, 6 vs. 2 weeks). LV end diastolic and end 
systolic diameters decreased in 6-week dogs treated with AAV6-hSERCA2a (p<0.05) 
whereas LV diameters increased in control dogs. Dogs receiving AAV6-hSERCA2a 
developed neutralizing antibodies (titer ≥1:120) and cardiac cellular 
infiltration. Immunosuppression dramatically reduced immune responses (reduced 
inflammation and neutralizing antibody titers <1:20), and maintained hSERCA2a 
expression. Thus cardiac injection of AAV6-hSERCA2a promotes local hSERCA2a 
expression and improves cardiac function. However, the hSERCA2a protein level is 
reduced by host immune responses. Immunosuppression alleviates immune responses 
and sustains transgene expression, and may be an important adjuvant for clinical 
gene therapy trials.