Boosting of DNA Vaccine-elicited Gamma Interferon Responses in Humans by Exposure to Malaria Parasites

A mixture of DNA plasmids expressing five Plasmodium falciparum pre-erythrocyte-stage antigens was administered with or without a DNA plasmid encoding human granulocyte-macrophage colony-stimulating factor (hGM-CSF) as an immune enhancer. After DNA immunization, antigen-specific gamma interferon (IFN-gamma) responses were detected by ELISPOT in 15/31 volunteers to multiple class I- and/or class II-restricted T-cell epitopes derived from all five antigens. Responses to multiple epitopes (</=7) were detected simultaneously in some volunteers. By 4 weeks after challenge with P. falciparum parasites, 23/31 volunteers had positive IFN-gamma responses and the magnitude of responses was increased from 2- to 143-fold. Nonetheless, none was protected against malaria. Volunteers who received hGM-CSF had a reduced frequency of IFN-gamma responses to class I peptides compared to those who only received plasmids expressing P. falciparum proteins before challenge (3/23 versus 3/8; P = 0.15) or after parasite challenge (4/23 versus 5/8; P = 0.015) but not to class II peptides before or after challenge. The responses to one antigen (P. falciparum circumsporozoite protein [PfCSP]) were similar among volunteers who received the five-gene mixture compared to volunteers who only received the PfCSP DNA plasmid in a previous study. In summary, DNA-primed IFN-gamma responses were boosted in humans by exposure to native antigen on parasites, coadministration of a plasmid expressing hGM-CSF had a negative effect on boosting of class I-restricted IFN-gamma responses, and there was no evidence that immunization with PfCSP DNA in the mixture reduced T-cell responses to PfCSP compared to when it was administered alone.