Experimental infection and protection against ... - TI Pharma

184 Chapter 9
strain P. falciparum-infected erythrocytes (PfRBC) or controls (Figure 3 and
supplementary figures 2+3). Whereas cellular responses to uninfected
erythrocytes (uRBC) never differed in any experiment from those to culture
medium alone, stimulation with PfRBC elicited small percentages of lymphocytes
producing IFNγ or TNFα, but not IL-2, in both groups of volunteers prior to
immunization (day I-1, Supplementary figure 2). Although no statistically
significant increment was seen in the overall percentage of cells producing
individual cytokines (IFNγ+ or TNFα+) in either group following immunization
(day C-1), a marked and significant increase was observed in the percentages of
cells producing multiple cytokines in response to PfRBC in vera (IFNγ+IL-2+
p=0.026, Figure 3a; TNFα+IL-2+ p=0.046, Figure 3b; IFNγ+TNFα+IL-2+ p=0.032,
Figure 3c). The importance of these pluripotent lymphocytes in acquired
immune protection is suggested by their higher cytokine content and possibly as
a consequence a better effector function (Figure 3d-f). The major contributors to
this increase in PfRBC-responding pluripotent lymphocytes were
[CD3+CD45RO+) memory-like T cells (p=0.025 vs. day I-1, Figure 3g and
supplementary figure 3), in particular CD4+CD8- cells (p=0.005 vs. day I-1, Figure
3h). Most noticeably, these new pluripotent lymphocytes were predominantly of
the effector memory (CD62L-CD45RO+) phenotype (p=0.005 vs. day I-1),
although there was also a small but significant (p=0.017) increase in the
numbers of responding central memory (CD62L+CD45RO+) cells in vera (Figure
3i).
Discussion
Our study shows that sterile protection to an homologous challenge with P.
falciparum malaria can be much more efficiently induced in comparison to
irradiation-attenuated sporozoite immunization. In the endemic situation, nonsterile
semi-immunity is acquired only after years of repeated natural exposure.
The improved efficiency of our approach we believe to be due to a critical
balance of exposure to pre- and intra-erythrocytic antigens. In contrast to
irradiated sporozoites that arrest early during liver-stage development [4], intact
sporozoites under CQ cover mature fully and develop into a first generation of
blood-stage parasites [11], thus presenting to the host’s immune system a
markedly broader palette of pre-erythrocytic and additionally, albeit at relatively
low-dose, erythrocytic-stage antigens. The contribution of intra-erythrocytic
antigens to the development of protective immunity is suggested by Pombo et
al., who reported that repeated intravenous injection of ultra-low densities of