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Open Access Research

Daily Plasmodium yoelii infective mosquito bites do not generate protection or suppress previous immunity against the liver stage

Tzvi Pollock1, Ricardo Leitao1, Cristina Galan-Rodriguez1, Kurt A Wong12 and Ana Rodriguez1*

Author Affiliations

1 Department of Microbiology, Division of Medical Parasitology, New York University School of Medicine, New York, NY 10010, USA

2 Current Address: The Scripps Research Institute, La Jolla, CA 92037, USA

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Malaria Journal 2011, 10:97  doi:10.1186/1475-2875-10-97

Published: 18 April 2011

Abstract

Background

Human populations that are naturally subjected to Plasmodium infection do not acquire complete protection against the liver stage of this parasite despite prolonged and frequent exposure. However, sterile immunity against Plasmodium liver stage can be achieved after repeated exposure to radiation attenuated sporozoites. The reasons for this different response remain largely unknown, but a suppressive effect of blood stage Plasmodium infection has been proposed as a cause for the lack of liver stage protection.

Methods

Using Plasmodium yoelii 17XNL, the response generated in mice subjected to daily infective bites from normal or irradiated mosquitoes was compared. The effect of daily-infected mosquito bites on mice that were previously immunized against P. yoelii liver stage was also studied.

Results

It was observed that while the bites of normal infected mosquitoes do not generate strong antibody responses and protection, the bites of irradiated mosquitoes result in high levels of anti-sporozoite antibodies and protection against liver stage Plasmodium infection. Exposure to daily infected mosquito bites did not eliminate the protection acquired previously with a experimental liver stage vaccine.

Conclusions

Liver stage immunity generated by irradiated versus normal P. yoelii infected mosquitoes is essentially different, probably because of the blood stage infection that follows normal mosquito bites, but not irradiated. While infective mosquito bites do not induce a protective liver stage response, they also do not interfere with previously acquired liver stage protective responses, even if they induce a complete blood stage infection. Considering that the recently generated anti-malaria vaccines induce only partial protection against infection, it is encouraging that, at least in mouse models, immunity is not negatively affected by subsequent exposure and infection with the parasite.