SummaryIgG extracted from the sera of African adults immune to malaria were injected intravenously into eight Plasmodium falcipamm-infected nonimmune Thai patients. Clinical and parasitological improvement was reproducibly obtained in each case. After the disappearance of the transferred Ig, recrudescent parasites were equally susceptible to the same Ig preparation. High levels of antibodies to most parasite proteins were detected by Western blots in the receivers' sera (taken before transfer) as in the donors' Ig, thus indicating that the difference was qualitative rather than quantitative between donors and receivers. In vitro, the clinically effective Ig had no detectable inhibitory effect on either penetration or intra-erythrocytec development of the parasite. On the contrary, they sometimes increased parasite growth. In contrast, these IgG, as the receivers' Ig collected 4 d after transfer, but not those collected before transfer, proved able to exert an antibodydependent cellular inhibitory (ADCI) effect in cooperation with normal blood monocytes. Results were consistent among the seven isolates studied in vitro, as with the recrudescent parasites. Thus, the results obtained in the ADCI assay correlate closely with clinical and parasitological observations.

No abstract

SummaryThe relevance of the antibody-dependent cellular inhibition (ADCI) of Plasmodium falciparum to clinical protection has been previously established by in vitro studies of material obtained during passive transfer of protection by immunoglobulin G in humans. We here report further in vitro investigations aimed at elucidating the mechanisms underlying this ADCI effect. Results obtained so far suggest that (a) merozoite uptake by monocytes (MN) as well as by polymorphonuclear ceils has little influence on the course of parasitemia; (b) the ADCI effect is mediated by a soluble factor released by MN; (c) this or these factors are able to block the division of surrounding intraerythrocytic parasites at the one nucleus stage; (d) the critical triggering antigen(s) targeted by effective Abs would appear to be associated with the surface of merozoites, as opposed to that of infected red blood cells; (e) the MN receptor for Abs effective in ADCI is apparently Fc3'RII, and not ILl; (f) MN function is up-and down-regulated by interferon-3' and interleukin 4, respectively; and (g) of several potential mediators released by MN, only tumor necrosis factor (TNF) proved of relevance. The involvement of TNF in defense may explain the recently described increased frequency of the TNF-2 high-expression promoter in individuals living in endemic regions despite its compromising role in severe malaria.

We have previously found that the acquired protection against malaria implicates a mechanism of defense that relies on the cooperation between cytophilic antibodies and monocytes. Accordingly, an assay of antibody-dependent cellular inhibition (ADCI) of parasite growth was used as a means of selecting for molecules capable of inducing protective immunity to malaria. This allowed us to identify in the sera of clinically protected subjects an antibody specificity that promotes parasite killing mediated by monocytes. This antibody is directed to a novel merozoite surface protein (MSP-3) of a molecular mass of 48 kD. Purified IgG from protected subjects are effective in ADCI and those directed against MSP-3 are predominantly cytophilic. In contrast, in nonprotected individuals, whose antibodies are not effective in ADCI, anti-MSP-3 antibodies are mostly noncytophilic. A region in MSP-3 targetted by antibodies effective in the ADCI assay was identified and its sequence was determined; it contains an epitope not defined by a repetitive structure and does not appear to be polymorphic. Antibodies raised in mice against a peptide containing this epitope, as well as human antibodies immunopurified on this peptide, elicit a strong inhibition of Plasmodium falciparum growth in ADCI assay, whereas control antibodies, directed to peptides from other molecules, do not. The correlation between isotypes of antibodies produced against the 48- kD epitopes, clinical protection, and the ability of specific anti-MSP- 3 antibodies to block the parasite schizogony in the ADCI assay suggests that this molecule is involved in eliciting protective mechanisms.

In view of the recent demonstration that antibodies that are protective against Plasmodium falciparum malaria may act in collaboration with blood monocytes, we investigated the isotype content of sera from individuals with defined clinical states of resistance or susceptibility to malaria. Profound differences in the distribution of each immunoglobulin subclass were found. Immunoglobulin G1 (IgG1) and IgG3, two cytophilic isotypes, predominated in protected subjects. In nonprotected subjects, i.e., children and adults that have sustained a primary malarial attack, four different situations were encountered: (i) an imbalance in which IgG2, a noncytophilic class, predominated (mostly seen in primary attacks), (ii) an imbalance also concerning IgG2 but only of a given antigenic specificity, (iii) an imbalance in which mostly IgM antibodies predominated (a frequent event in children), and, less frequently, (iv) an overall low level of antimalarial antibodies. Of 33 nonimmune subjects studied, all but one had one of the above defects. The function of total immunoglobulin presenting such an isotype imbalance was studied in vitro in antibody-dependent cellular inhibition assays. IgG from protected subjects cooperated efficiently with blood monocytes, whereas IgG from nonprotected groups did not. Also, the latter could inhibit the in vitro effect of the former: in competition assays whole IgG from primary-attack cases with increased IgG2 content and IgG or IgM from children from endemic areas competed with IgG from immune adults. This led us to formulate the hypothesis that nonprotected subjects have antibodies to epitopes critical for protection, but that these antibodies are nonfunctional. These results bring some clues to the very long delay required to reach protection against malaria and clearly stress the need to investigate immune responses in both quantitative and qualitative terms.