Artificial cells as a novel approach to gene therapy - Artificial Cells, Cell Engineering and Therapy - page 253

unencapsulated cells showed only transient anti-FIX antibody response, and at
much reduced levels (Van Raamsdonk, 1999). Hemophilic mice treated with
microcapsules enclosing FIX-secreting C2C12 myoblasts showed a temporary
partial correction of activated thromboplastin time (aPTT) for approximately 3
weeks. Plasma FIX levels rose to a maximum on day 2, decreased to low levels
until day 24 and then were unmeasurable after that. The declining FIX levels
coincided with emergence of anti-hFIX antibody by day 14, increasing greater
than tenfold by day 28. The time course, and the fact that capsules removed from
the animals still secreted FIX, indicated the decline in measurable plasma FIX
and the loss of aPTT correction were due to neutralizing antibodies (Van
Raamsdonk et al., 2002).
Similar loss of efficacy associated with immune response was also observed
when B domain-deleted factor VIII was delivered to the transgenic hemophilia
A mice, in which case the immune response was exacerbated by the adenoviral
vector used in its delivery (Sarkar et al., 2000). While it was initially reported
that this strain of mice did not produce antibodies to the human FIX (Kung et al.,
1998), most of the mice receiving FIX from microcapsules did produce a
detectable antibody response. In our pilot studies, in rare cases, this development
of an antibody response did not occur (unpublished data), but the reason for this
anergy is unknown. When this anergy occurred, however, the correction of aPTT
times was maintained for almost 2 months in the relevant animals (unpublished
data). The reason for the differences in immune response to the transgene
product is poorly understood but such variability has also been observed in the
delivery of factor VIII (Connelly et al., 1998; Sarkar et al., 2000).
There are multiple strategies available to try to overcome the development of
neutralizing antibodies, such as immunosuppressive drugs or oral tolerization. In
our laboratory, we used monoclonal anti-CD4 antibodies to deplete CD4-
positive T cells. In the treated hemophilic mice, the anti-hFIX antibody response
was totally suppressed to beyond day 28, accompanied by a significant decrease
in activated thromboplastin times from hemophilic levels. After 38 days of
implantation, the microcapsules could be recovered almost quantitatively from
the intraperitoneal cavity and continued to secrete FIX at pre-implantation levels
(Van Raamsdonk et al., 2002).
Delivery of coagulation factors is probably one of the most susceptible
treatments to the neutralizing effect of these antibodies, thus adding another
level of complexity to any replacement therapy. This is likely because the
biochemistry of the coagulation cascade demands a pool of pre-formed factors to
circulate in the blood. This creates an opportune environment for the immune
system to generate antibodies and for those antibodies to bind to and neutralize
the factors. In contrast, ERT of lysosomal disorders, as discussed later, is
minimally affected by neutralizing antibodies, even though a high proportion of
ERT recipients form them.