Response to Comment on “γδ T Cells Are Required for the Induction of Sterile Immunity during Irradiated Sporozoite Vaccinations”

We welcome the comments from McKenna and Tsuji on our recent publication demonstrating a requirement for γδ T cells to induce sterile immunity during irradiated sporozoite vaccinations. In our studies, Malian adults who were resistant to Plasmodium falciparum infection after receiving the irradiated PfSPZ Vaccine had higher Vδ2 γδ T cell levels compared with susceptible vaccinees or placebo controls (1), and γδ T cells were required during irradiated Plasmodium berghei sporozoite vaccination to induce sterile immunity in C57BL/6 mice. The latter results were consistent whether we used γδ knockout mice or we depleted γδ T cells using the GL3 mAb prior to vaccinations. In contrast, depletion of γδ T cells just prior to challenge with nonirradiated P. berghei parasites did not impair vaccine-induced protection, whereas depletion of CD8⁺ T cells just before challenge completely ablated protection. Only a subset of γδ T cells appear to be required for induction of CD8 T cells, because depletion of the Vγ4 subset had no impact on acquisition of protective immunity. Thus, γδ T cells were required during vaccination to induce protective CD8 T cells, but their effector function did not appear to be required to mediate sterile immunity at the time of challenge.

Previous studies cited by McKenna et al. also examined the role of γδ T cells during irradiated sporozoite vaccinations of mice (2, 3), but they differed by using irradiated Plasmodium yoelii sporozoites to vaccinate C57BL/6 mice, an approach that has not been shown to elicit sterile immunity. Indeed, neither prior study reported sterile protection but instead reported decreases in liver-stage parasite burden. For example, in the study by Tsuji et al. (2), adoptive transfer of a single clone (291-H4) derived from vaccinated αβ T cell-deficient mice into naive mice led to a significant reduction but not complete elimination of liver-stage P. yoelii parasites. In contrast, our studies clearly demonstrate sterile immunity, as no blood-stage parasites ever emerge. The discrepant inferences from these studies may be due to the different endpoints used to define vaccine-induced protection as either partial or sterile immunity in two different malaria models.

A second difference in studies, highlighted by McKenna et al., has to do with the quantity and preparation of isolated sporozoites used for vaccination. In our experience, three inoculations of 10⁴ irradiated P. berghei sporozoites elicit sterile protection in C57BL/6 mice. These results are highly reproducible and consistent in all our experiments to date.

A third possible explanation suggested by McKenna et al. for the discrepant results is that the GL3 mAb fails to fully deplete gd T cells but instead renders them “invisible” by reducing surface levels of the target marker (as reported in Ref. 4). As we highlighted in our Discussion, we cannot discount this latter possibility; however, it is worth noting that the GL3 depletion during vaccination did suffice to impair induction of protective CD8 T cells. Further studies are warranted to clarify the effects of the GL3 mAb on γδ T cells.