Influence of cellular location of expressed antigen on the efficacy of DNA vaccination: cytotoxic T lymphocyte and antibody responses are suboptimal when antigen is cytoplasmic after intramuscular DNA immunization

We examined the role of the cellular localization of antigen on the immune response after DNA immunization of mice with three forms of ovalbumin (OVA), DNA encoding OVA which was secreted (sOVA) generated 10- to 100-fold higher IgG responses with 50- and 100-fold higher levels of IgG1 than the cytoplasmic (cOVA) or membrane bound (mOVA) forms. An IgG2a predominance was seen only in cOVA and mOVA immunized mice. Although the antibody response was CD4(+) T cell dependent, the differences in the antibody response could not be compensated for by provision of excess CD4(+) T cell help in TCR transgenic mice. Together with our hapten-carrier studies, this would indicate that membrane or intracellular localization limits the availability of antigen for B cell priming which affects the magnitude and form of the antibody response. Surprisingly, stronger cytotoxic T lymphocyte (CTL) responses were generated for sOVA or mOVA than for cOVA via intramuscular (i.m.) injection. Since a cytoplasmic antigen should have best access to the canonical class I pathway for antigen presentation, our results indicate that priming of CTL responses after i.m. DNA immunization is probably by cross-presentation of antigen by nontransfected professional antigen-presenting cells. In contrast, intradermal immunization with cOVA produced optimal CTL responses but, as with mOVA, suboptimal antibody responses. This, together with our ex vivo RT-PCR analysis showing similar mRNA levels from all three constructs 7 days post-immunization, argues against the differential CTL response for i.m. injection to be due to dose.