IV inoculated parasites reach the buy BVD-523 liver within minutes (26), whereas sporozoites inoculated into the skin slowly trickle out of the inoculation site over a period of 1–3 h (27). Our results indicate that the lower parasite liver load after ID inoculation is unlikely to be explained by a delayed arrival
of sporozoites in the liver. Comparison of the parasite liver load at 35 h post-ID injection was still ±15 times lower compared to the parasite liver load at 30 h post-IV injection (Figure 2). Despite differences between parasites species, including among others infectivity (28) or host cell preference (29–31), our data in P. berghei parallel previous results in P. yoelii studies (25). Therefore, the relatively low level of parasites capable of reaching the liver after ID injection is likely a common feature among Plasmodium species.
CD8+ T cell responses are known to be essential for protection induced by attenuated live sporozoite immunization in rodent models. Our data corroborate previous studies on P. berghei RAS-induced immunity showing expansion of CD8+ memory T cells, mainly in the liver, together with high IFNγ production in IV immunized Pritelivir cell line mice (12–15). The low immune responses observed after ID immunization likely follow the low parasite liver load. RAS ID and subcutaneous immunization of human volunteers also show low protection levels, and in nonhuman primates and mice subcutaneous or ID immunization lead to lower (-)-p-Bromotetramisole Oxalate IFNγ responses compared to IV sporozoite immunization
(18). Despite the differences in phenotyping and gating strategy, CD8+ effector (memory) T cells (CD44hi CD62L-) and not central memory T cells (CD44hi CD62L+) are identified as induced T-cell subset. In another study using the P. yoelii model, major CD8+ T cell responses were generated in the draining lymph nodes after infected mosquito bites or ID inoculation of sporozoites. Although parasite liver load was reduced, complete protection defined as impediment of blood-stage infection was not evaluated (32). We did not test the regional lymph nodes response and cannot exclude a possible contribution but our data clearly demonstrate that ID inoculation is inefficient in inducing protection. In addition, a measure of sporozoite load in regional lymph nodes following ID inoculation would have been informative. Unfortunately, in vivo visualization of PbGFP-Luccon is not possible because of a relatively low luciferase expression at the sporozoite stage (22). Next to cellular components, antibody responses can contribute to protection by whole sporozoite immunization (8). Our data suggest that induced functional antibodies may contribute to protection but are more likely related to exposure.