ABSTRACT
Intramuscular injection of naked plasmid DNA is known to elicit humoral and cell-mediated immune responses against the encoded antigen (1-3). Promotion of such immunity is the result of DNA uptake by muscle cells, leading to the expression and extracellular release of the antigen, which is then taken up by antigen-presenting cells (APCs) (2,3). It is also believed (3) that some of the injected DNA is taken up directly by APCs, either locally at the injection site or after its migration into the lymph nodes. There are two main disadvantages of immunization with naked DNA. First, DNA enters only a minor fraction of muscle cells, which, at any rate, are not professional APCs. Second, injected nakedDNA is exposed to and digested by deoxyribonuclease in the interstitial fluid, thus necessitating its use in relatively large quantities.
It is not unusual that injection into regenerating muscle (previously treated with muscle-damaging agents) is required in order to enhance transfection and immunity. It has been proposed (1,4) that immunization by the use of liposomes with entrapped DNA could circumvent the need for muscle involvement and facilitate (5) instead the uptake of the DNA by APCs infiltrating the site of injection or in the lymphatics (where many liposomes will end up), at the same time protecting DNA from deoxynribonuclease attack (6). Moreover, transfection of APCs with liposomal DNA and subsequent immune responses to the expressed antigen could be promoted by the judicious choice of vesicle surface charge, size, and lipid composition, or by the coentrapment of DNA with plasmids expressing appropriate cytokines [e.g., interleukin 2 and interferon-c (IFN-c)] or immunostimulatory sequences.
