Intradermal immunization of mice with radiation-attenuated sporozoites of Plasmodium yoelii induces effective protective immunity

Abstract

BackgroundIntravenous injection of mice with attenuated Plasmodium berghei sporozoites induces sterile immunity to challenge with viable sporozoites. Non-intravenous routes have been reported to yield poor immunity. Because intravenous immunization has been considered to be unacceptable for large scale vaccination of humans, assessment was made of the results of intradermal immunization of mice with Plasmodium yoelii, a rodent malaria parasite whose infectivity resembles that of human malaria.MethodsMice were immunized with two injections of isolated, radiation-attenuated P. yoelii sporozoites, either by intravenous (IV) or intradermal (ID) inoculation. In an attempt to enhance protective immunogenicity of ID-injections, one group of experimental mice received topical application of an adjuvant, Imiquimod, while another group had their injections accompanied by local "tape-stripping" of the skin, a procedure known to disrupt the stratum corneum and activate local immunocytes. Challenge of immunized and non-immunized control mice was by bite of sporozoite-infected mosquitoes. Degree of protection among the various groups of mice was determined by microscopic examination of stained blood smears. Statistical significance of protection was determined by a one-way ANOVA followed by Tukey's post hoc test.ResultsTwo intravenous immunizations produced 94% protection to mosquito bite challenge; intradermal immunization produced 78% protection, while intradermal immunization accompanied by "tape-stripping" produced 94% protection. There were no statistically significant differences in degree of protective immunity between immunizations done by intravenous versus intradermal injection.ConclusionsThe use of a sub-microlitre syringe for intradermal injections yielded excellent protective immunity. ID-immunization with large numbers of radiation-attenuated P. yoelii sporozoites led to levels of protective immunity comparable to those achieved by IV-immunization. It remains to be determined whether an adjuvant treatment can be found to substantially reduce the numbers of attenuated sporozoites required to achieve a strong protective immunity with as few doses as possible for possible extension to immunization of humans.

Extrahepatic Exoerythrocytic Forms of Rodent Malaria Parasites at the Site of Inoculation: Clearance after Immunization, Susceptibility to Primaquine, and Contribution to Blood-Stage Infection

“…or i.d. (5,30), induces sterile protective immunity in rodents, primates, and humans and is currently the gold standard to which other immunization protocols are compared (17). It is possible that extrahepatic EEFs, due to their location in an immune-privileged site or to the homing of effector T cells primarily to the liver, may resist killing by immune effector cells.…”

Reduced Plasmodium berghei sporozoite liver load associates with low protective efficacy after intradermal immunization

“…Moreover, protective efficacy following IV immunizations in mice is attributed to liver CD8+ effector memory T cells and high levels of IFNγ production (12)(13)(14)(15). However lower levels of protection are induced following intradermal (ID) sporozoite immunization with either P. berghei genetically attenuated parasites (GAP) (16) or P. yoelii RAS (17). In a recent clinical study, subcutaneous or ID immunization with irradiated P. falciparum sporozoites also showed suboptimal immune responses and protective efficacy in humans (18).…”

Looking under the skin: the first steps in malarial infection and immunity

“…One study reported lower protective efficacy after intradermal compared with intravenous immunization with P. berghei RAS or DAP 140 . However, other studies using P. yoelii DAP 95 and RAS 141 have recorded similar protection levels for intradermal and intravenous immunization. Some researchers who used P. yoelii 95 or P. berghei 140,142 have found the parasite load in the liver to be lower after intradermal versus intravenous injection of RAS, suggesting that protective efficacy is related to the ability of sporozoites to reach the liver 116,34 .…”