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134 Qiu and Smith
restriction, between bone marrow–derived APCs and T cells (20–23). However,
such identical MHC allele is not required between antigen-expressing myocytes
and T cells together (23); the ample evidence accumulated in this regard challenged
the somewhat simplistic view that in situ transfected cells are directly priming
specific T cells. Instead, there must be antigen transfer between in situ transfected
myocytes and APCs, with subsequent priming of T cells within secondary lymphoid
organs (cross-priming). Only more recently, accumulated evidence suggests
that the antigen transfer between in situ transfected somatic cells and APCs may
occur in the form of heat shock protein (HSP)-polypeptide complexes and is
facilitated by the apoptosis of transgene-expressing cells (24,25). However, it is
highly likely that there is a multiplicity of mechanisms accounting for the antigen
transfer between myocytes and APCs, and they may have different bearings on
induction of cytotoxic T lymphocytes (CTL) versus other types of immunity. For
example, engineering export sequences within the open reading frame of the
plasmid vector resulted in increased Th and B cell responses, without a similar
effect on the CTL response suggesting that antigen transfer as secreted protein,
between myocytes and APCs, results in effective handling via the exogenous, but
not the MHC class I processing and presentation, pathway (26). Overall, such
elegant studies—coupled with the scarcity or lack of transgene-expressing
APCs—contributed to a momentum behind the cross-priming/cross-processing
model.
Nevertheless, interestingly, a series of reports obtained in slightly different
experimental setups challenged the cross-priming model. For example, it was
demonstrated that intradermal plasmid injection results in coexpression of the
transgene by somatic cells and APCs and that upon adoptive transfer of
migrating APCs, an increased MHC class I–restricted immune response is elicited
(27). Mere antigen transfer between somatic cells and APCs was ruled out
by using plasmids expressing antigens encompassing nuclear import sequences
(28), in conjunction with multicolor, high-resolution cell-imaging techniques.
Innovative approaches to administer plasmid vectors to the dermis by gene gun
or other strategies showed that much lower doses were needed to elicit an immune
response, compared to more traditional intramuscular administration (29). An
emerging model shaped up, by which in situ transfected Langerhans cells, upon
migration to the draining lymph node (LN), actually prime specific T cells
utilizing the conventional processing pathway (reviewed in Ref. 30).
The apparent conundrum relative to the importance of cross-priming versus
conventional pathway of induction of MHC class I–restricted immunity by plasmid
immunization can be addressed by judging the experimental evidence in light
of the route and strategy of administration. The key parameter in this regard is the
presence and the density of competent APCs within the injected tissue, capable of
expressing the transgene, migrating to draining LN and priming specific T cells
(30,31). While both the conventional and cross-priming mechanisms take place
simultaneously, their relative importance is fundamentally different as follows:
In case of intramuscular injection, the scarcity of resident APCs determines a