CD4+ T lymphocytes mediate in vivo clearance of plasmid DNA vaccine antigen expression and potentiate CD8 + T-cell immune responses

There is evidence that the limited immunogenicity of plasmid DNA vaccines is the result, at least in part, of the rapid clearance of vaccine antigen expression by antigen-specific immune responses. However, the cell types responsible for the clearance of plasmid DNA vaccine antigens are not known. Here we demonstrate that macrophages, NK cells, and CD8⁺ T cells did not significantly contribute to the DNA antigen clearance but CD4⁺ T cells played the crucial role in attenuating plasmid DNA vaccine antigen expression. Adoptive transfer experiments demonstrate that CD4⁺ T cells facilitated DNA vaccine antigen clearance in a Fas/FasL-dependent manner. Furthermore, we show that depletion of CD4⁺ T cells prevented the clearance of vaccine antigen and the appearance of a CD8⁺ T-cell immune response. Inoculation of major histocompatibility complex class II KO mice with the plasmid DNA led to persistent antigen expression and abolition of a CD8⁺ T-cell immune response. Importantly, the prolongation of antigen expression by disrupting the CD4⁺ T-cell Fas/FasL myocytes signaling led to a 3- to 5-fold increase of antigen-specific CD8⁺ T-cell responses. These data demonstrate a dominant role of CD4⁺ T cell-mediated cytotoxicity in plasmid DNA vaccine antigen clearance.