Induction of cytotoxic T lymphocytes by intramuscular immunization with plasmid DNA is facilitated by bone marrow-derived cells

Striated muscle is the predominant site of gene expression after i.m. immunization of plasmid DNA, but it is not clear if myocytes or professional antigen-presenting cells (APCs) of hematopoietic origin present the encoded antigens to class I major histocompatibility complex (MHC)-restricted cytotoxic T lymphocytes (CTL). To address this issue, CTL responses were assessed in mice engrafted with immune systems that were partially MHC matched with antigen-producing muscle cells. Spleen cells (sc) from immunocompetent F-1 H-2(bxd) mice were infused into H-2(b) or H-2(d) mice carrying the severe combined immunodeficiency (scid) mutation, creating F-1sc --> H-2(b) and F-1sc --> H-2(d) chimeras, respectively. Immunization with DNA plasmids encoding the herpes simplex virus gB or the human immunodeficiency virus gp120 glycoproteins elicited antiviral CTL activity. F-1sc --> H-2(d) chimeras responded to an H-2(d)-restricted gp120 epitope but not an H-2(b)-restricted gB epitope, whereas F-1sc --> H-2(b) chimeras responded to the H-2(b) but not the H-2(d)-restricted epitope. This pattern of epitope recognition by the sc chimeras indicated that APCs of recipient (scid) origin were involved in initiation of CTL responses. Significantly, CTL responses against epitopes presented by the mismatched donor class I molecules were elicited if F-1 bone marrow cells and sc were transferred into scid recipients before or several days to weeks after DNA immunization. Thus, bone marrow-derived APCs are sufficient for class I MHC presentation of viral antigens after i.m. immunization with plasmid DNA. Expression of plasmid DNA by these APCs is probably not a requirement for CTL priming. Instead, they appear to present proteins synthesized by other host cells.