Inbred mice infected with Plasmodium yoelii differ in their antimalarial immunoglobulin isotype response

Plasmodium-specific antibodies block in vivo parasite growth without clearing infected red blood cells

Plasmodium parasites invade and multiply inside red blood cells (RBC). Through a cycle of maturation, asexual replication, rupture and release of multiple infective merozoites, parasitised RBC (pRBC) can reach very high numbers in vivo, a process that correlates with disease severity in humans and experimental animals. Thus, controlling pRBC numbers can prevent or ameliorate malaria. In endemic regions, circulating parasite-specific antibodies associate with immunity to high parasitemia. Although in vitro assays reveal that protective antibodies could control pRBC via multiple mechanisms, in vivo assessment of antibody function remains challenging. Here, we employed two mouse models of antibody-mediated immunity to malaria, P. yoelii 17XNL and P. chabaudi chabaudi AS infection, to study infection-induced, parasite-specific antibody function in vivo. By tracking a single generation of pRBC, we tested the hypothesis that parasite-specific antibodies accelerate pRBC clearance. Though strongly protective against homologous re-challenge, parasite-specific IgG did not alter the rate of pRBC clearance, even in the presence of ongoing, systemic inflammation. Instead, antibodies prevented parasites progressing from one generation of RBC to the next. In vivo depletion studies using clodronate liposomes or cobra venom factor, suggested that optimal antibody function required splenic macrophages and dendritic cells, but not complement C3/C5-mediated killing. Finally, parasite-specific IgG bound poorly to the surface of pRBC, yet strongly to structures likely exposed by the rupture of mature schizonts. Thus, in our models of humoral immunity to malaria, infection-induced antibodies did not accelerate pRBC clearance, and instead co-operated with splenic phagocytes to block subsequent generations of pRBC.

Malaria occurs when Plasmodium parasites replicate inside red blood cells, with the number of parasitised cells (pRBC) correlating with disease severity. Antibodies are highly effective at controlling pRBC numbers in the bloodstream, and yet we know very little about how they function in vivo. Human in vitro studies predict that antibodies may function in a number of ways, including via phagocytes or different complement mechanisms. However, to date it has been challenging to explore how antibodies might control parasite numbers in vivo. Here, we have used a unique method in mice, where clearance and replication of a single cohort of pRBC was closely tracked in the presence of protective antibodies. Surprisingly, antibodies played no role whatsoever in accelerating the removal of pRBC. Instead, antibodies were highly effective at preventing parasites from progressing from one generation of pRBC to the next. This process partly depended on host phagocytes. However, we found no role for complement-mediated direct killing. Together, our in vivo data suggest in mouse models that naturally-acquired antibodies do not clear pRBC, and instead prevent transition from one red blood cell to the next.

Outcome of primary lethal and nonlethal Plasmodium yoelii malaria infection in BALB/c and IFN-γ receptor-deficient mice following chloroquine treatment

IFN-γ receptor-deficient (IFN-γR(-/-)) mice and control wild-type (WT) mice, with or without chloroquine (CQ) treatment, were infected intraperitoneally with Plasmodium yoelii 17XL (lethal) and P. yoelii 17XNL (nonlethal), and then mouse survival, parasitemia, and antibody production were investigated during the course of infection. Without CQ treatment, both IFN-γR(-/-) and WT mice were susceptible to infection showing 100 % mortality after infection with 1 × 10(5) P. yoelii 17XL-parasitized erythrocytes. The P. yoelii 17XL-infected WT mice could survive by CQ treatment at a dose of 20 mg/kg for 3 days from day 3 postinfection (pi). Malaria parasites in their bloodstream could not be detected in the surviving mice after day 13 pi. CQ treatment, however, could not rescue IFN-γR(-/-) mice infected with P. yoelii 17XL. Next, we examined the production of the parasite-specific antibodies in P. yoelii 17XL-infected, CQ-treated mice. Although the production of malaria-specific IgG1, IgG2a, IgG2b, and IgG3 antibodies was observed on days 14 and 28 pi in WT mouse sera, only IgG1 was detected on day 28 pi in IFN-γR(-/-) mouse sera. On the other hand, in the nonlethal P. yoelii 17XNL infection, WT mice could control a primary infection with 1 × 10(5) parasitized erythrocytes. Although IFN-γR(-/-) mice could not control and died with increasing parasitemia, the mice could survive by CQ treatment. Both WT and IFN-γR(-/-) mice with and without medication, which survived from P. yoelii 17XNL infection, showed the variable levels of malaria-specific IgG1, IgG2a, IgG2b, and IgG3 antibodies during the course of infection. The present data indicate that the IFN-γ receptors are needed to control the infection and parasite-specific IgG2a antibody plays an essential role in recovery from the infection of erythrocytic stages of P. yoelii 17XL or P. yoelii 17XNL parasite.

The course of a primary infection of Plasmodium yoelii 17XL in both 129S1 and IFN-γ receptor-deficient mice

In the present study, we found that 129S1 mice are resistant to the infection with Plasmodium yoelii 17XL, which is highly virulent and causes lethal infection in various strains of mice. In contrast, IFN-γ receptor-deficient (IFN-γR(-/-)) mice on the 129S1 background were much more susceptible than 129S1 mice with intraperitoneal infection with 1 × 10(5) parasitized erythrocytes. The mortality in 129S1 and IFN-γR(-/-) mice was 11.6 and 79.4 %, respectively. Following inoculation of the parasites, both 129S1 and IFN-γR(-/-) mice showed a progressive increase in parasitemia. Growth rate of malaria parasites at the early stages of infection in the IFN-γR(-/-) mice was faster than that in 129S1 mice, and this difference in growth rate might cause the earlier death of IFN-γR(-/-) host from day 8 of infection than that of 129S1. In surviving mice of both strains, however, malaria parasites in their bloodstream began to decrease in number right after a peak of parasitemia and were not detectable by a microscopic examination during the observation period. Next, we investigated the cytokine and antibody production in 129S1 and IFN-γR(-/-) mice during infection. An analysis of cytokines showed that serum IFN-γ and IL-4 levels elevated significantly from day 1 and day 4 of infection, respectively, in both 129S1 and IFN-γR(-/-) mice when compared with the levels from the uninfected controls. Following the infection, significantly higher levels of malaria-specific IgG1 and IgG2a antibodies in the infected 129S1 mice were detected from day 15, and these elevations were coincident with the decrease of parasitemia. On the other hand, the levels of malaria-specific antibodies in IFN-γR(-/-) mice had a tendency to elevate on day 21 but did not reach statistical significance. The present data indicate that IFN-γR plays an essential role in mediating the early immune mechanisms induced by the infection of erythrocytic stages of P. yoelii 17XL parasite, leading to host survival.

Plasmodium yoelii blood-stage antigens newly identified by immunoaffinity using purified IgG antibodies from malaria-resistant mice

As the search for an effective human malaria vaccine continues, understanding immune responses to Plasmodium in rodent models is perhaps the key to unlocking new vaccine strategies. The recruitment of parasite-specific antibodies is an important component of natural immunity against infection in blood-stage malaria. Here, we describe the use of sera from naturally surviving ICR mice after infection with lethal doses of Plasmodium yoelii yoelii 17XL to identify highly immunogenic blood-stage antigens. Immobilized protein A/G was used for the affinity-chromatography purification of the IgGs present in pooled sera from surviving mice. These protective IgGs, covalently immobilized on agarose columns, were then used to isolate reactive antigens from whole P. yoelii yoelii 17XL protein extracts obtained from the blood-stage malaria infection. Through proteomics analysis of the recovered parasite antigens, we were able to identify two endoplasmic reticulum lumen proteins: protein disulfide isomerase and a member of the heat shock protein 70 family. Also identified were the digestive protease plasmepsin and the 39 kDa-subunit of eukaryotic translation initiation factor 3, a ribosome associated protein. Of these four proteins, three have not been previously identified as antigenic during blood-stage malaria infection. This procedure of isolating and identifying parasite antigens using serum IgGs from malaria-protected individuals could be a novel strategy for the development of multi-antigen-based vaccine therapies.

Suppression of lethal Plasmodium yoelii malaria following protective immunization requires antibody-, IL-4-, and IFN-gamma-dependent responses induced by vaccination and/or challenge infection

Immunization with Plasmodium yoelii merozoite surface protein (PyMSP)-8 protects mice from lethal malaria but does not prevent infection. Using this merozoite surface protein-based vaccine model, we investigated vaccine- and infection-induced immune responses that contribute to protection. Analysis of prechallenge sera from rPyMSP-8-immunized C57BL/6 and BALB/c mice revealed high and comparable levels of Ag-specific IgG, but differences in isotype profile and specificity for conformational epitopes were noted. As both strains of mice were similarly protected against P. yoelii, we could not correlate vaccine-induced responses with protection. However, passive immunization studies suggested that protection resulted from differing immune responses. Studies with cytokine-deficient mice showed that protection was induced by immunization of C57BL/6 mice only when IL-4 and IFN-gamma were both present. In BALB/c mice, the absence of either IL-4 or IFN-gamma led to predictable shifts in the IgG isotype profile but did not reduce the magnitude of the Ab response induced by rPyMSP-8 immunization. Immunized IL-4-/- BALB/c mice were solidly protected against P. yoelii. To our surprise, immunized IFN-gamma-/- BALB/c mice initially controlled parasite growth but eventually succumbed to infection. Analysis of cytokine production revealed that P. yoelii infection induced two distinct peaks of IFN-gamma that correlated with periods of controlled parasite growth in intact, rPyMSP-8-immunized BALB/c mice. Maximal parasite growth occurred during a period of sustained TGF-beta production. Combined, the data indicate that induction of protective responses by merozoite surface protein-based vaccines depends on IL-4 and IFN-gamma-dependent pathways and that vaccine efficacy is significantly influenced by host responses elicited upon infection.

Acute and chronic phases of Toxoplasma gondii infection in mice modulate the host immune responses

Murine antibody responses to soluble proteins are generally restricted to the immunoglobulin G1 (IgG1) isotype. When mice were infected with Toxoplasma gondii Beverley and concomitantly immunized with a soluble unrelated protein antigen, a modification in the isotypic distribution of antibodies directed against this nonparasite antigen was observed, with a preferential production of IgG2a. Interestingly, when mice were immunized with a soluble protein antigen during the chronic phase (day 40) of infection with T. gondii Beverley, a similar modification in the isotypic distribution of antiprotein antibodies was observed.

Immunization against the murine malaria parasite Plasmodium yoelii using a recombinant protein with adjuvants developed for clinical use

Mice vaccinated with a recombinant protein containing the two EGF-like modules of Plasmodium yoelii merozoite surface protein-1 in liposomes or combined with the formulations SBAS2.1 and SBAS2, were protected against a lethal malaria infection. The protection achieved with these adjuvants developed for clinical use was as good as or better than that achieved with Freund's adjuvant. A parasite-specific response was needed for protection. Analysis of the immunoglobulin sub-class response showed that MSP-1-specific IgG1, and to a lesser extent IgG2a and IgG2b, were induced, suggesting that these antibodies were important for protection. Mice passively immunized with serum or purified IgG from vaccinated mice had delayed onset of parasitemia and were able to control the infection.

Protective immunity against Plasmodium yoelii malaria induced by immunization with particulate blood-stage antigens

The Plasmodium yoelii murine model was used to test several combinations of blood-stage antigens and adjuvants for the ability to induce immunity to blood-stage malaria. Upon fractionation of whole blood-stage antigen into soluble and insoluble components, only the particulate antigens (pAg) induced protective immune responses. Of a number of adjuvants tested, Quil A was the most effective. Immunization with pAg plus Quil A induced solid protection against nonlethal and lethal P. yoelii challenge infection. Analysis of cytokine production revealed mRNA for Th1-type cytokines (interleukin 2 [IL-2] and gamma interferon) as well as Th2-type cytokines (IL-4 and IL-10) in the spleens of both protected and susceptible animals. The data suggested that the protective pAg response was associated with the earlier production of cytokine mRNA with a Th2 phenotype somewhat favored. Immunization of B-cell-deficient JHD mice indicated that the protection against P. yoelii induced by pAg immunization was B cell dependent. Although immunization with pAg plus Quil A increased the levels of antigen-specific antibodies of all four immunoglobulin G (IgG) isotypes, protection correlated most closely with the presence of IgG1 and IgG2b antibodies. Sera from pAg-plus-Quil A-immunized animals recognized only a limited subset of six to eight distinct P. yoelii antigens, primarily associated with the pAg fraction. These results provide the basis for the identification and characterization of potential vaccine antigens, selected solely for their ability to immunize against blood-stage malaria.

The time course of selected malarial infections in cytokine-deficient mice

Murine malarial parasites have long been characterized by their requirement for either antibody-mediated immunity (AMI) or cell-mediated immunity (CMI) for suppression of acute parasitemia, with Plasmodium yoelii reportedly requiring AMI for suppression and P. chabaudi requiring CMI. To assess this characterization in terms of the current T(H1)/T(H2)-CMI/AMI hypothesis, we infected gene-targeted "knockout" mice lacking either a type-1 cytokine (IL-2 or IFN-gamma) or a type-2 cytokine (IL-4 or IL-10) with one or the other species of Plasmodium. We observed that type-1 cytokine-deficient mice developed exacerbated malaria with either P. yoelii or P. chabaudi, compared with that seen in heterozygote controls. Moreover, type-2 cytokine knockout mice showed a similar time course of infection with either parasite compared with that seen with their controls. We conclude that the mechanism of resolution of these well characterized malarial infections cannot be linked definitely to these T(H1)- and T(H2)-associated cytokines as predicted by the T(H1)/T(H2)-CMI/AMI hypothesis.

Trypanosoma cruzi infection in mice induces a polyisotypic hypergammaglobulinaemia and parasite-specific response involving high IgG2a concentrations and highly avid IgG1 antibodies

Trypanosoma cruzi infection in BALB/c mice induced a reversible polyisotypic hypergammaglobulinaemia, with particularly high levels of IgG2a, IgM and IgE. Hypergammaglobulinaemia started during the acute phase of infection and persisted during chronic disease until 11-13 weeks post-infection (w.p.i.), when immunoglobulin levels, with the exception of IgE, returned near normal values. Parasite-specific antibodies counted for 14 to 23% of gammaglobulinaemia, in acute and chronic infection respectively. The titres of IgM antibodies rose from two w.p.i. IgA, IgE and IgG subclass antibodies built up gradually over the time of parasite clearance (i.e., between three and six w.p.i.). All antibody isotypes, including IgM reached significant and stable titres throughout chronic infection. IgG2a, IgG1 and IgM antibodies had constantly higher titres than the other antibody isotypes. The dominance of IgG2a antibodies was due to their high plasma concentrations, around 70% of all antibodies available in the chronic infection. IgG1 had the highest functional avidity, whereas its concentration corresponded to only 10% of the whole antibody fraction. These results indicate that T. cruzi infection in mice induces a polyisotypic humoral immune response, dominated by some antibody isotypes, with major differences in concentrations and functional avidities. This could be of crucial importance in determining the outcome of infection.

Plasmodium falciparum malaria: evidence for an isotype imbalance which may be responsible for delayed acquisition of protective immunity

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.

Antibody response in Plasmodium vinckei malaria after treatment with chloroquine and adjuvant interferon-gamma

The antibody response of mice infected with Plasmodium vinckei after treatment with chloroquine either alone or in combination with interferon-gamma (IFN-gamma) was determined. Sequential serum samples were drawn from BALB/c mice receiving either 240 micrograms chloroquine on the day of infection or 120 micrograms chloroquine plus 10(4) units IFN-gamma daily for 11 days beginning on day 3 prior to infection. Mice treated with additional IFN-gamma showed an early induction of IgG2a response and a reduction in IgG1 antibodies as detected by the immunofluorescence technique at between 10 and 16 days after infection as compared with mice treated with chloroquine alone. Thus, IFN-gamma may partly exert its antimalarial activity via the induction of IgG2a antibody formation. At 4-6 weeks after infection, when mice from both groups resisted homologous re-infection, the predominant antibody isotypes found in both groups were IgG1 and IgG2a. Serum samples obtained from mice in both treatment groups at 6 weeks after infection were used for serum transfer experiments. When parasitised erythrocytes were preincubated with such immune serum, a retardation of the course of parasitaemia by 2 days was observed.

Plasmodium yoelii: antibody response in resistant and susceptible mouse strains

The numbers of antigen-reactive antibody-secreting cells, levels of parasite antigen-specific serum antibodies and numbers of red blood cells staining positive for surface immunoglobulin were determined for susceptible and resistant mouse strains following infection with Plasmodium yoelii 17x. As a control, these parameters also were measured using antigen prepared from normal red blood cells. The relatively susceptible C57BL/6 mice produced more antigen-specific antibody-secreting cells and had higher levels of immunoglobulin positive red blood cells than did DBA/2 mice, but the DBA/2 mice had more antigen-specific IgG in their sera. Both mouse strains possessed cells secreting antibody reactive with soluble normal red blood cell antigen; however, C57BL/6 mice had more IgG positive unparasitized RBC than did DBA/2 mice. Despite possessing fewer antibody positive normal RBC, DBA/2 mice had significantly higher levels of serum antibodies that reacted with soluble red blood cell antigen. These data indicate that levels of serum antibody may not reflect the amounts of antibody produced and that use of any single assay to assess the magnitude of the antibody response may give rise to misleading results.