Bulky pcDNA plasmid encoding Leishmania- and Sandfly-derived antigens resulted in transient Th1 response post Leishmania major infection in BALB/c mice.

Leishmaniasis is a common parasitic infection in tropical regions worldwide, and yet there is no effective human vaccine available. The results of leishmanization in animal models have emphasized the role of antigen persistence after recovery from the disease. It is argued that alternative vaccine formulations other than leishmanization, will not succeed in controlling the natural infection without antigen stability. The use of DNA constructs is one of the proposed solutions for longer-term antigen expression. DNA plasmids could carry longer sequences coding multiple antigens. However, one major regulatory concern associated with conventional plasmids is the antibiotic resistance genes on the backbone besides the difficulty in the delivery of large constructs into the cells. Our goal in this study was to clone and express two different antigens from Leishmania (L.) major (LmsTI1, LeIF) along with the sand fly antigens (PpSP15) in the conventional pcDNA3.1 plasmid and the new generation plasmid NTC9385R known as nanoplasmid, to evaluate the protectivity of a multi-protein construct against L. major. Irrespective of the protein combination and final arrangement of the components which was fully analyzed prior to immunization, multi-valent pcDNA plasmid immunization resulted in a weak Th1 polarization but not durable protection in BALB/c mice. Meanwhile, the preparation of multi-valent nanoplasmid was a complex task and failed where a fragment larger than the backbone was inserted. The take away message of this study is that optimization of electroporation parameters for large plasmids or substitution with appropriate new generation of plasmids respecting the fragment size can promisingly pave the way toward multi-valent DNA vaccines and avoid DNA cocktails for complex organisms.