PREDOMINANT ROLE FOR DIRECTLY TRANSFECTED DENDRITIC CELLS IN ANTIGEN PRESENTATION TO CD8(+)T CELLS AFTER GENE GUN IMMUNIZATION

Cutaneous gene (DNA) bombardment results in substantial expression of the encoded antigen in the epidermal layer as well as detectable expre ssion in dendritic cells (DC) in draining lymph nodes (LNs). Under the se conditions, two possible modes of DC antigen presentation to naive CD8(+) T cells might exist: (a) presentation directly by gene-transfec ted DC trafficking to local lymph nodes, and (b) cross-presentation by untransfected DC of antigen released from or associated with transfec ted epidermal cells. The relative contributions of these distinct mode s of antigen presentation to priming for cytotoxic T cell (CTL) respon ses have not been clearly established. Here we show that LN cells dire ctly expressing the DNA-encoded antigen are rare; 24 h after five abdo minal skin bombardments, the number of these cells does not exceed 50- 100 cells in an individual draining LN. However, over this same time p eriod, the total number of CD11c(+) DC increases more than twofold, by an average of 20,000-30,000 DC per major draining node. This augmenta tion is due to gold bombardment and is independent of the presence of plasmid DNA. Most antigen-bearing cells in the LNs draining the site o f DNA delivery appear to be DC and can be depleted by antibodies to an intact surface protein encoded by cotransfected DNA. This finding of predominant antigen presentation by directly transfected cells is also consistent with data from studies on cotransfection with antigen and CD86-encoding DNA, showing that priming of anti-mutant influenza nucle oprotein CTLs with a single immunization is dependent upon coexpressio n of the DNAs encoding nucleoprotein and B7.2 in the same cells. These observations provide insight into the relative roles of direct gene e xpression and cross-presentation in CD8+ T cell priming using gene gun immunization, and indicate that augmentation of direct DC gene expres sion may enhance such priming.