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 profes sional 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 we re partially MHC matched with antigen-producing muscle cells. Spleen c ells (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, r espectively. Immunization with DNA plasmids encoding the herpes simple x virus gB or the human immunodeficiency virus gp120 glycoproteins eli cited antiviral CTL activity. F-1sc --> H-2(d) chimeras responded to a n H-2(d)-restricted gp120 epitope but not an H-2(b)-restricted gB epit ope, 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 i nvolved in initiation of CTL responses. Significantly, CTL responses a gainst epitopes presented by the mismatched donor class I molecules we re elicited if F-1 bone marrow cells and sc were transferred into scid recipients before or several days to weeks after DNA immunization. Th us, bone marrow-derived APCs are sufficient for class I MHC presentati on of viral antigens after i.m. immunization with plasmid DNA. Express ion of plasmid DNA by these APCs is probably not a requirement for CTL priming. Instead, they appear to present proteins synthesized by othe r host cells.