BACKGROUND: It has frequently been reported that Plasmodium vivax
suppressed Plasmodium falciparum and ameliorated disease severity in
patients infected with these two species simultaneously. The authors
investigate the hypothesis that immunological responses stimulated by
P. vivax may play a role in suppressing co-infecting P. falciparum.
METHODS: Sera, taken sequentially from one of the authors (YN) during
experimental infection with P. vivax, were added to in vitro cultures
of P. falciparum. Cross-reactive antibodies against P. falciparum
antigens, and cytokines were measured in the sera. RESULTS: Significant
growth inhibitory effects upon P. falciparum cultures (maximally 68%
inhibition as compared to pre-illness average) were observed in the
sera collected during an acute episode. Such inhibitory effects showed
a strong positive temporal correlation with cross-reactive antibodies,
especially IgM against P. falciparum schizont extract and, to a lesser
degree, IgM against Merozoite Surface Protein (MSP)-119. Interleukin
(IL)-12 showed the highest temporal correlation with P. vivax
parasitaemia and with body temperatures in the volunteer. CONCLUSION:
These results suggest the involvement by cross-reactive antibodies,
especially IgM, in the interplay between plasmodial species. IL-12 may
be one of direct mediators of fever induction by rupturing P. vivax
schizonts, at least in some subjects. Future studies, preferably of
epidemiological design, to reveal the association between
cross-reactive IgM and cross-plasmodial interaction, are warranted.