Characterization of T-cell immune responses in clinical trials of the candidate RTS,S malaria vaccine

Abstract

The candidate malaria vaccine RTS,S has demonstrated 45.7% efficacy over 18 months against all clinical disease in a phase-III field study of African children. RTS,S targets the circumsporozoite protein (CSP), which is expressed on the Plasmodium sporozoite during the pre-erythrocyte stage of its life-cycle; the stage between mosquito bite and liver infection. Early in the development of RTS,S, it was recognized that CSP-specific cell-mediated immunity (CMI) was required to complement CSP-specific antibody-mediated immunity. In reviewing RTS,S clinical studies, associations between protection and various types of CMI (CSP-specific CD4⁺ T cells and INF-γ ELISPOTs) have been identified, but not consistently. It is plausible that certain CD4⁺ T cells support antibody responses or co-operate with other immune-cell types to potentially elicit protection. However, the identities of vaccine correlates of protection, implicating either CSP-specific antibodies or T cells remain elusive, suggesting that RTS,S clinical trials may benefit from additional immunogenicity analyses that can be informed by the results of controlled human malaria infection studies.

Keywords: AS01; AS02; NK cell; Plasmodium; RTS; S; adjuvant; cell-mediated immunity; malaria; vaccine.

Figure 1.

(A) Structure of the RTS,S antigen. Schematic description of the antigen. The RTS,S antigen contains recombinant forms of circumsporozoite protein (CSP) - NANP repeat regions (R) and T-cell epitope domain (T) linked to hepatitis B surface antigen (HBsAg; S) - , as well as HBsAg alone (S). The T-cell epitope domain is further subdivided into characterised epitopes; TH2R, Region II, TH3R and CS-T3. (B) Scanning electron micrograph of a preparation of (low electron density) RTS,S virus-like particles (scale bar = 100 nm).

Figure 2.

Models for the initiation of NK-cell activation and the interactions between a CSP-specific CD4⁺ T cell, an antigen-presenting cell (APC) and an NK cell. Direct interactions are marked by cognate receptor-ligand interactions, indirect interactions via the production of cytokines are marked by black arrows, and effector mechanisms due to IFN-γ or cytotoxic molecules are marked by large grey-shaded arrows. (A) After vaccination, APCs take up RTS,S antigen and, in the draining lymph node, present processed RTS,S-derived peptides via HLA-II T-cell receptor (TCR) interactions. From these interactions and from CD40-CD40L interactions, CD4⁺ T cells are stimulated to produce IL-2. This IL-2 then activates NK cells and helps B cells to proliferate and produce antibodies, as well as inducing T-cell proliferation through a positive feedback loop. (B) Upon re-encounter with CSP in the draining lymph nodes, (derived from RTS,S or sporozoites), APC present CSP derived peptides to CS-specific CD4⁺ T cells. NK cells, in the proximity of IL-2 secreted by CD4⁺ T cells are activated and start secreting IFN-γ. This IFN-γ may signal to the APC to produce IL-12 and IL-18, which in turn further promotes IFN-γ production by NK cells in a positive feedback loop. The IFN-γ produced by NK cells may further activate CD4⁺ T cells. Death of infected cells can then be induced by NK cells through released IFN-γ or degranulating cytotoxic molecules. (C) In the liver, sporozoites traverse from the sinusoidal capillary lined with liver sinusoidal endothelial cells (LSECs) through (a few) Kupffer cells (KC) before infecting a hepatocyte (HC) (dashed line). CSP peptides are presented by Kupffer cells to memory or activated CD4⁺ T cells, which start secreting IL-2. This IL-2 activates liver NK cells, which are further activated by IL-12 secreted by the Kupffer cells. The NK cells then also secrete IFN-γ and cytotoxic degranulation molecules. Circulating CSP-specific antibodies induced by RTS,S/AS01, recognize the CSP shed by traversing sporozoites on the surface of hepatocytes and NK cells are further activated through binding of those antibodies to the FcgRIII receptors on NK cells.