Factors influencing the development of an anti-factor IX (FIX) response following administration of adeno-associated virus-FIX

The present study sought to determine the impact of the route of administra tion of an adeno-associated virus (AAV) vector encoding human factor IX (hF IX) on the induction of an immune response against the vector and its xenog enic transgene product, hFIX, Increasing doses of AAV-hFIX were administere d by different routes to C57BI/6 mice, which typically demonstrate signific ant immune tolerance to hFIX, The route of delivery had a profound impact o n serum hFIX levels as well as the induction of an anti-hFIX humoral immune response. At all dose levels tested, delivery of AAV-hFIX by an intramuscu lar (IM) route induced an anti-body response against the human FIX protein and no hFIX was detected in the serum of animals even at doses of 2 x 10(11 ) DNA viral particles (vp) of AAV-hFIX, This was in stark contrast to the m ice that received AAV-hFIX by intraportal vein (IPV) administration. No ant i-hFIX inhibitors were observed in any of these mice and therapeutic levels of hFIX were detected in the serum of all mice that received doses of 2 x 10(10) vp AAV-hFIX and higher. When pre-existing neutralizing immunity to A AV was established in mice, AAV-hFIX administration by either the IM or IPV routes did not result in detectable serum hFIX. Although hFIX expression w as not observed in mice with pre-existing neutralizing immunity to AAV, an anti-hFIX response was induced in all of the animals that received AAV-hFIX by the IM route. This was not observed in the preimmune mice that received AAV-hFIX by IPV administration. These results suggest that the threshold o f inducing an immune response against a secreted transgene product, in this case the xenoprotein hFIX, is lower when the Vector is administered by the IM route even in animals with pre-existing immunity to AAV. (C) 2001 by Th e American Society of Hematology.