Plasmodium falciparum-specific cellular immune responses after immunization with the RTS,S/AS02D candidate malaria vaccine in infants living in an area of high endemicity in Mozambique

Abstract

Results from clinical trials in areas where malaria is endemic have shown that immunization with RTS,S/AS02A malaria vaccine candidate induces partial protection in adults and children and cellular effector and memory responses in adults. For the first time in a malaria vaccine trial, we sought to assess the cell-mediated immune responses to RTS,S antigen components in infants under 1 year of age participating in a clinical phase I/IIb trial of RTS,S/AS02D in Mozambique. Circumsporozoite protein (CSP)-specific responses were detected in approximately half of RTS,S-immunized infants and included gamma interferon (IFN-gamma), interleukin-2 (IL-2), and combined IL-2/IL-4 responses. The median stimulation indices of cytokine-producing CD4(+) and CD8(+) cells were very low but significantly higher in RTS,S-immunized infants than in infants that received the comparator vaccine. Protection against subsequent malarial infection tended to be associated with a higher percentage of individuals with CSP-specific IL-2 in the supernatant (P = 0.053) and with higher CSP-specific IFN-gamma-producing CD8(+) T-cell responses (P = 0.07). These results report for the first time the detection of malaria-specific cellular immune responses after vaccination of infants less than 1 year of age and pave the way for future field studies of cellular immunity to malaria vaccine candidates.

FIG. 1.

Timeline of immunizations and blood sample collection. Timing for the blood samples used for cellular immunity studies are indicated: BS0, preimmune; BS1, 4 weeks postimmunization; and BS2, 10.5 weeks postimmunization.

FIG. 2.

Reverse cumulative distribution of cytokine concentration in supernatants from CSP-stimulated cell cultures. Plots for preimmune (a to c) and for 4-week (d to f) and 10.5-week (g to i) postimmunization samples are shown in the top, middle, and bottom rows, respectively. Immunization groups are represented by boldface (RTS,S/AS02D) and dotted (hepatitis B) lines. Arrowheads show the concentration cutoffs for positive responses: 20 pg ml-1 for IFN-γ, 100 pg ml-1 for IL-2, and 5 pg/ml-1 for IL-4. The Wilcoxon rank-sum test was used to compare cytokine concentrations between both immunization groups. Significant differences (P < 0.01) are indicated by asterisks (**).

FIG. 3.

Frequency of CSP- and HBS-specific cytokine responses in supernatants of children immunized with RTS,S/AS02D and hepatitis B (Engerix-B) vaccines before immunization (BS0, □), 4 weeks postimmunization (BS1, ░⃞), and 10.5 weeks postimmunization (BS2, ▪). Differences in the proportions of positive responders before and after immunization (brackets) and between immunization groups (arrows) were assessed by using the McNemar chi-square and Fisher exact tests, respectively. Significant differences: *, P < 0.05; **, P < 0.001.