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132 Qiu and Smith
these vectors. Various prime-boost strategies have been developed, aiming at
amplifying the high-quality response generated during the DNA priming interval,
including the use of proteins (13,14) or live virus vectors (15,16). Such approaches
are generally successful in inducing protective immunity against microbial or
tumorigenic viruses; however, it is much more difficult to generate responses at
the magnitude of therapeutic usefulness to tumor antigens.
Unlike prophylactic vaccination aimed at priming the immune system with
an antigen that has never been encountered, tumor antigens are self-antigens that
are expressed during early development or disease stage, resulting in immune
tolerance to these antigens. Removal or silencing of high-avidity, self-reactive
T cells to ubiquitously expressed or blood-borne antigens from the repertoire in
thymus is necessary to prevent autoimmunity, a mechanism known as central
tolerance (reviewed in Ref. 17). In addition, peripheral tolerance involving a
multitude of factors contributes to the suppression of T-cell functions toward
tissue-restricted antigens. Therefore, therapeutic cancer vaccines need to overcome
the hurdle of immune tolerance to induce immune responses of high
quantity as well as high quality in order to achieve positive clinical outcomes.
While more stringent requirements for activation and effector function are necessary
for lower avidity T-cell precursors, a successful cancer vaccine should also be
capable of directing T cells to traffic out of the lymphoid organs and migrating to
the tumor sites where tumor-specific recognition and immune attack take place.
Various experimental models have been established in the last decade, and in
this chapter, we outline the pros and cons of those models emphasizing the
prime-boost immunization approaches using plasmid vectors based on understanding
of the mechanism of action and options to build superior immunization
strategies.
DNA IMMUNIZATION: MECHANISMS OF ACTION
Many studies were carried out in preclinical models, to address the mechanism
of action of DNA immunization, employing quite diverse and creative designs.
Most notably, use of bone marrow chimeric (BMC) mice, adoptive cell transfer,
strategies of identification and tracking of in situ transfected somatic cells or
APCs, manipulation of immune costimulation, and evaluation of the role of
innate immune cells, all contributed to the emergence of multiple models
explaining the induction of immunity subsequent to DNA vector administration
(Fig. 1).
The majority of earlier studies were carried out by intramuscular injection
of plasmid, which conclusively showed the in situ expression of transgene
within myocytes (18). It is estimated that hundreds, or at the maximum thousands,
of myocytes acquired transgene expression that usually lasted for days/
weeks—parameters sufficing the induction of a class I–restricted immune
response (19–22). Transplantation of ex vivo transfected myocytes and measurement
of immunity in BMC mice demonstrated the necessity of matched MHC