To investigate whether the MHC-Ⅰ humanized mouse model (AAD mice) combined with the Plasmodium berghei transgenic strain expressing Plasmodium falciparum circumsporozoite protein (PfCSP,Pb-PfCSP) can be used to establish an evaluation model for PfCSP-based vaccines.

AAD mice and C57BL/6J mice (6 to 8 weeks old, body weight 20 to 35 g, 4 mice per group) were challenged with graded doses (200, 500, and 1 000) of Pb-PfCSPsporozoites and wild-type P.berghei (Pb) sporozoites via tail vein injection. The susceptibility of the 2 mouse strains to Pb and Pb-PfCSP infection was compared by monitoring the onset time of erythrocytic-stage parasitemia and the dynamic changes in parasitemia levels. Flow cytometry was performed to analyze the frequency and absolute number of major hepatic immune cell subsets in the 2 mouse strains, so as to preliminarily explore the potential mechanisms underlying the differential susceptibility to malaria parasites. For vaccine efficacy evaluation, 4 AAD mice per group were immunized with either CI-679-attenuated Pb-PfCSP sporozoite vaccine or LNP-encapsulated mRNA vaccine encoding PfCSP (PfCSP@LNP), with unimmunized AAD mice as the control group. After immunization, all mice were challenged with Pb-PfCSP sporozoites, and the protective efficacy was evaluated. The frequency and number of CSP-specific tissue-resident memory T cells (Trm) in the liver were detected by flow cytometry, and CSP-specific antibody titers in serum were measured by enzyme-linked immunosorbent assay (ELISA).

Pb-PfCSP sporozoites at all tested doses successfully infected both AAD and C57BL/6J mice. Compared with C57BL/6J mice, AAD mice exhibited higher susceptibility to both Pb and Pb-PfCSP infection, as evidenced by earlier onset of erythrocytic-stage parasitemia and significantly higher parasitemia levels in the mid-stage of infection (P<0.05). Regardless of infection status, the frequency and absolute number of hepatic natural killer T (NKT) cells in AAD mice were significantly lower than those in C57BL/6J mice (P<0.05), which might be one of the reasons for the increased susceptibility of AAD mice to malaria parasite infection. After challenge with Pb-PfCSP sporozoites, PfCSP@LNP-immunized AAD mice exhibited delayed erythrocytic-stage parasitemia onset compared with the control group, and detectable titers of CSP-specific antibodies were observed in their serum; however, PfCSP-specific Trm cells were not detected in the liver. Similarly, AAD mice immunized with the nitroquine-attenuated Pb-PfCSP sporozoite vaccine only achieved a protective efficacy of 16.6%, and PfCSP-specific Trm cells were not detected either.

The Pb-PfCSP-infected AAD mouse model can be used for evaluating the efficacy of PfCSP-based vaccines in inducing CSP-specific antibody responses. However, this model requires further optimization for assessing PfCSP-specific CD8⁺ T cell response induction.