Preclinical optimization of a prime-and-trap malaria vaccine

Abstract

Malaria is a deadly parasitic disease that disproportionally impacts infants and children under five years old. A more effective vaccine is urgently needed to reduce the global burden of malaria. The liver stage (LS) is a critical bottleneck in the parasite lifecycle that if blocked, would prevent blood stage infection, clinical disease, and transmission. Radiation-attenuated sporozoite (RAS) vaccines are critical for sterile LS protection in mice, non-human primates (NHP), and humans by inducing antibodies and CD8+ T cells, including liver resident memory CD8+ T cells (Trm) that are critical for long-term protection. Such T cells can also be induced by a novel two-step vaccine strategy called prime-and-trap that was designed to simplify and improve upon RAS-only vaccination. Prime-and-trap combines DNA priming against theimmunodominant circumsporozoite protein (CSP) with a subsequent intravenous (IV) dose of liver-homing RAS to “trap” the activated and expanding T cells in the liver. This strategy induces durable protective CSP-specific CD8+ liver Trm cells in mice, and efforts are underway to translate this vaccine strategy to NHPs and humans. Reducing the RAS dose and/or determining if RAS must be strictly administered by the IV route would further enhance the translational potential of the prime-and-trap malaria vaccine. This dissertation provides preclinical research aimed at enhancing the efficacy of the RAS trapping component of the prime-and-trap vaccine. This dissertation aimed to optimize the translational potential of RAS trapping by co-administration with the potent glycolipid adjuvant 7DW8-5 to 1) reduce the dose, and 2) improve the administration route to be more suitable for clinical use. Additionally, since these studies revealed drastic differences between protection in male and female mice, this dissertation also explored if the addition of the 7DW8-5 adjuvant could enhance protection in male mice. First, this study found that in female mice, IV-administered freshly-dissected RAS could be replaced with the more desired but less immunogenic cryopreserved RAS and that the dose of IV-RAS could be reduced four-fold by co-administration with 7DW8-5. Next, the study found intradermal (ID) co-administration of RAS and 7DW8-5 in ultra-low volumes (2.5 μL) was completely protective and dose sparing compared to standard ID volumes (10-50 μL) and induced protective levels of CSP-specific CD8+ T cells in the liver. The finding that adjuvants and ultra-low volumes are required for ID-RAS efficacy may explain why prior reports about higher volumes of unadjuvanted ID-RAS proved less effective. Finally, the study found that male mice were not protected from any prime-and-trap vaccine regimen (IV-RAS or ID-RAS) with or without 7DW8-5. Further, the final studies found significant sex differences in cytokine expression induced by 7DW8-5 in mice. Additional studies are required to understand the mechanism of protection induced by IV- or ID-RAS and 7DW8-5 and to understand the dramatic protection differences observed between male and female mice. The ID route may offer significant translational advantages over the IV route and may improve RAS vaccine development. Taken together, this research found that the translational potential of the prime-and-trap malaria vaccine could be improved by the co-administration of 7DW8-5. These studies pave the way for further evaluation of prime-and-trap in NHPs and humans and future studies should continue to include sex as a biological variable.