Development of Plasmodium-specific liver-resident memory CD8⁺ T cells after heat-killed sporozoite immunization in mice

Malaria remains a major cause of mortality in the world and an efficient vaccine is the best chance of reducing the disease burden. Vaccination strategies for the liver stage of disease that utilise injection of live radiation-attenuated sporozoites (RAS) confer sterile immunity, which is mediated by CD8⁺ memory T cells, with liver-resident memory T cells (T_RM) being particularly important. We have previously described a TCR transgenic mouse, termed PbT-I, where all CD8⁺ T cells recognize a specific peptide from Plasmodium. PbT-I form liver T_RM cells upon RAS injection and are capable of protecting mice against challenge infection. Here, we utilize this transgenic system to examine whether nonliving sporozoites, killed by heat treatment (HKS), could trigger the development of Plasmodium-specific liver T_RM cells. We found that HKS vaccination induced the formation of memory CD8⁺ T cells in the spleen and liver, and importantly, liver T_RM cells were fewer in number than that induced by RAS. Crucially, we showed the number of T_RM cells was significantly higher when HKS were combined with the glycolipid α-galactosylceramide as an adjuvant. In the future, this work could lead to development of an antimalaria vaccination strategy that does not require live sporozoites, providing greater utility.