Superior Antimalarial Immunity after Vaccination with Late Liver Stage-Arresting Genetically Attenuated Parasites

Abstract

Summary While subunit vaccines have shown partial efficacy in clinical trials, radiation-attenuated-sporozoites (RAS) remain the “gold-standard” for sterilizing protection against Plasmodium-infection in human vaccinees. The variability in immunogenicity and replication introduced by the extensive, random DNA damage necessary to generate RAS could be overcome by genetically attenuated parasites (GAP) designed via gene deletion to arrest at defined points during liver-stage development. Here, we demonstrate the principle that late-liver-stage arresting GAP induce larger and broader CD8 T cell responses that provide superior protection in inbred and outbred mice compared to RAS or early-arresting GAP immunizations. Late-liver-stage arresting GAP also engender high-levels of cross-stage and cross-species protection and complete protection when administered by translationally-relevant intradermal or subcutaneous routes. Collectively, our results underscore the potential utility of late-liver stage arresting GAP as broadly protective next-generation live-attenuated malaria vaccines and support their potential as a powerful model for identifying antigens to generate cross-stage protection.

A T-cell response to a liver-stagePlasmodiumantigen is not boosted by repeated sporozoite immunizations

“…Our findings showed that CPS immunization induced the largest increase in peripheral CD8α LO /CD11a HI T cells-indicative of recent antigenic stimulation (37), followed by FabB/f and then RAS (Fig. 1), consistent with earlier data comparing GAP and RAS approaches (27). Thus, it appears that more diverse responses are achieved by GAP and CPS approaches compared with RAS immunization, and we identified two reproducibly positive pools herein.…”

“…Thus, there is a need to identify individual immunogenic antigens so that they can be evaluated for inclusion in multicomponent subunit vaccines. Using a unique HTS system for CD8 + T-cell antigen identification (32), we, like others (28), detected many low frequency T-cell responses to sporozoite vaccinations, with a trend toward more diversity in less-attenuated parasites (e.g., CPS and GAP > RAS), as has been reported (27). We identified a peptide from the L3 ribosomal protein as a CD8 + T-cell target in all sporozoite vaccination regimens and following immunization with heat-treated infected erythrocytes.…”

“…This phenomenon is thought to mimic the situation in hyperendemic field settings, where humans sometimes receive multiple malaria-infected mosquito bites daily. When the interval between immunizations is extended to a week or more as in our studies, we and others (27) find that endogenous CSP-specific CD8 + T cells in fact increase in frequency after repeated sporozoite immunizations. Thus, CSP-specific T cells do not appear to be affected by self-regulation using our vaccination approach.…”

“…GAP and CPS approaches express a wider array of proteins than RAS. Accordingly, late-arresting GAPs were shown to induce more diverse CTL responses than RAS vaccination in mice (27). Thus, proteins expressed de novo in the liver [e.g., by CPS and some GAPs like 3-oxoacyl-ACP synthase I/II; PY04452 (FabB/F)-deficient parasites] and in the red blood cell (RBC) stages (e.g., by CPS) have received attention as candidate antigens.…”

TAP‐mediated processing of exoerythrocytic antigens is essential for protection induced with radiation‐attenuated Plasmodium sporozoites

“…This was indeed reflected in the inability of TAP −/− mice to be protected at challenge, which supports our hypothesis concerning the need for TAP-dependent processing. It was shown that late LS Ags induce larger and broader CD8 + T-cell responses and better LS immunity [54]. However, it is unlikely that late LS Ags were expressed in the short time frame that is needed to mobilize previously induced CD8 + T cells before the release of merozoites into the blood.…”