Meeting the threshold of multifunctional CD8+ TEM by γ-irradiated sporozoite vaccination, essential to ensue sterile protection against Plasmodia liver stage infection

Abstract

Abstract People in malaria endemic area develop immunity after several years of exposure to parasite, children particularly take longer to develop protective immunity. Interestingly, the protection is lot more consistent among children living in high transmission area compared to those of low transmission area, suggesting that protection may depend on the frequency of exposure and/or a load of parasite exposure. Thus, we hypothesize children experiencing multiple exposures might develop critical frequencies of anti-parasite lymphocytes ensuring the protection to Plasmodia. CD8+ T cells play a key role in mediating protection against Plasmodia liver-stage infection. In present study, we took whole sporozoite (spz) vaccination strategy and immunized C57Bl/6 mice with multiple doses of γ-spz at different time intervals. We found continuous immunization with 3 and 4 doses of γ-spz, 2 weeks apart, provides ~50% and 100% sterile protection, respectively. However, mice immunized with 3 doses of γ-spz with an interval of 90 days during 2nd – 3rd dose fail to be protected. Upon measuring the CD8+ T cell responses, we found that higher frequencies of liver-stage specific multifunctional CD8+ T cells (CD44hiKLRG-1intCD107+IFN-γ+) were accumulated in mice immunized with 4 doses of γ-spz (180–410 cells/106; Avg. 284), whereas the same among mice receiving 3 doses showed lower frequencies (50–125 cells/106; Avg. 75) before challenge, and mixed frequencies (126–551 cells/106; Avg. 340) following challenge, suggesting that protection against Plasmodia infection could be ensured either by pre-existing multifunctional memory CD8+ T cells before the infection or mobilizing the required frequencies of the same to liver during the course of infection.