Erythrophagocytosis in malaria: Host defence or menace to the macrophage?

Methodologies and tools to shed light on erythrophagocytosis

Red blood cells (RBC) are the most abundant circulating cell of the human body. RBC are constantly exposed to multiple stresses in the circulation, leading to molecular and structural impairments and death. The physiological process of RBC senescence or ageing is referred to as eryptosis. At the end of their lifespan, aged RBC are recognized and removed from the blood by professional phagocytes via a phenomenon called erythrophagocytosis (EP); the phagocytosis of RBC. Some genetic and acquired diseases can influence eryptosis, thereby affecting RBC lifespan and leading to hemolytic anemia. In some diseases, such as diabetes and atherosclerosis, eryptosis and EP can participate in disease progression with both professional and non-professional phagocytes. Therefore, investigating the process of EP in vivo and in vitro, as well as in different cell types, will not only contribute to the understanding of the physiological steps of EP, but also to the deciphering of the specific mechanisms involving RBC and EP that underlie certain pathologies. In this review, the process of EP is introduced and the different methods for studying EP are discussed together with examples of the experimental procedures and materials required.

Diminished organelle motion in murine Kupffer cells during the erythrocytic stage of malaria

Parasitized erythrocytes are ingested by murine hepatic macrophages during malaria infection. We non-invasively monitored how this altered the motion of intracellular phagosomes in Kupffer cells using magnetometry. Submicrometric γFe(2)O(3) particles were injected prior to malaria infection. They were cleared from the blood, primarily by Kupffer cells, and retained within their phagosomes. The mice were periodically magnetized. After removing this external magnet, the aligned iron particles created a remnant magnetic field (RMF) which then decayed (relaxation), reflecting the motion of particle-containing phagosomes. After baseline measurements of relaxation, the mice were injected intravenously with Plasmodium chabaudi-parasitized or normal murine red blood cells (RBCs). During the next 15 days, relaxation measurements, parasitaemia and haematocrit values were monitored. At 6 days post injection with 3 × 10(7) parasitized RBCs, relaxation rates had decreased. At this time, all mice had parasitaemias greater than 58 per cent and haematocrits less than 20 per cent. At day 7, while the parasitaemias were declining, the rate of relaxation continued to decrease. Throughout the experiment, relaxation remained constant in animals injected with normal RBCs. Electron microscopy revealed Kupffer cells filled with damaged and parasitized erythrocytes, and haemoglobin degradation pigment. We conclude that ingestion and metabolism of parasitized erythrocytes by liver macrophages during malaria infection decreases their organelle motion with likely consequences of compromised host defences.

Robust erythrophagocytosis leads to macrophage apoptosis via a hemin-mediated redox imbalance: role in hemolytic disorders

MP from the RES are responsible for the clearance of senescent RBC. Although the frequency of senescent RBC is low under steady-state conditions, it increases dramatically during hemolytic disorders, resulting in enhanced erythrophagocytosis. As erythrophagocytosis has been involved in MP dysfunction and as certain hemolytic disorders associate to MP apoptosis, a possible link between erythrophagocytosis and the viability of phagocytes was investigated herein. To mimic hemolytic disorders, two distinct in vitro models, artificially oxidized RBC and DSRBC, were chosen to study the erythrophagocytosis impact on the viability of J774A.1 MP. Although CRBC were weakly phagocytosed and did not affect MP viability significantly, erythrophagocytosis of oxidized RBC and DSRBC was robust and resulted in a sharp decrease of MP viability via apoptosis. Under these conditions, Hb-derived HE was shown to be involved in the induction of apoptosis. Moreover, oxidized RBC, DSRBC, and HE generated ROS species, which were responsible for the apoptosis of MP. Furthermore, HO-1, strongly induced in response to treatment with oxidized RBC, DSRBC, or HE, was shown to protect MP partially against apoptosis, suggesting that robust erythro-phagocytosis may exceed the detoxification capabilities of MP. Taken together, these results suggest that enhanced erythrophagocytosis associated to hemolytic disorders leads to MP apoptosis in vitro and may have critical implications for the control of malaria infection and for the exacerbated susceptibility to bacterial infections during hemolytic disorders.

Efficient phagosomal maturation and degradation of Plasmodium-infected erythrocytes by dendritic cells and macrophages

Dendritic cells (DC) and macrophages phagocytose pathogens and degrade them in their phagosomes to allow for proper presentation of foreign antigens to other cells of the immune system. The Plasmodium parasite, causative agent of malaria, infects RBC that are phagocytosed by DC and macrophages during the course of infection. Under specific conditions, the functionality of these cells can be affected by phagocytosis of Plasmodium-infected RBC. We investigated whether phagosomal maturation and degradation of Plasmodium yoelii-infected RBC in phagosomes is affected in DC and macrophages. We show that recruitment of the phagolysosomal marker Lamp-1 and of MHC-II, as well as acidification of phagosomes, was achieved in a timely manner. Using P. yoelii-infected RBC labelled with a fluorescent dye or transgenic green fluorescent protein (GFP)-expressing parasites, we found a gradual, rapid decrease in the phagosome fluorescence signal, indicating that P. yoelii-infected RBC are efficiently degraded in macrophages and DC. We also observed that pre-incubation of DC with infected RBC did not affect phagosomal maturation of newly internalized P. yoelii-infected RBC. In conclusion, after phagocytosis, Plasmodium-infected RBC are degraded by DC and macrophages, suggesting that the process of phagosomal maturation is effectively completed in malaria.

Stages of in vitro phagocytosis of Plasmodium falciparum-infected erythrocytes by human monocytes

Monocytes/macrophages play a critical role in the defense mechanisms against malaria parasites, and are the main cells responsible for the elimination of malaria parasites from the blood circulation. We carried out a microscope-aided evaluation of the stages of in vitro phagocytosis of Plasmodium falciparum-infected erythrocytes, by human monocytes. These cells were obtained from healthy adult individuals by means of centrifugation through a cushion of Percoll density medium and were incubated with erythrocytes infected with Plasmodium falciparum that had previously been incubated with a pool of anti-plasmodial immune serum. We described the stages of phagocytosis, starting from adherence of infected erythrocytes to the phagocyte membrane and ending with their destruction within the phagolisosomes of the monocytes. We observed that the different erythrocytic forms of the parasite were ingested by monocytes, and that the process of phagocytosis may be completed in around 30 minutes. Furthermore, we showed that phagocytosis may occur continuously, such that different phases of the process were observed in the same phagocyte.

Erythrocyte variants and the nature of their malaria protective effect

The malaria threat to global health is exacerbated by widespread drug resistance in the Plasmodium parasite and its insect vector, and the lack of an efficacious vaccine. Infection with Plasmodium parasites can cause a wide spectrum of pathologies, from a transient mild form of anaemia to a severe and rapidly fatal cerebral disease. Epidemiological studies in humans and experiments in animal models have shown that genetic factors play a key role in the onset, progression, type of disease developed and ultimate outcome of malaria. The protective effect of polymorphic variants in erythrocyte-specific structural proteins or metabolic enzymes against the blood-stage of the disease is one of the clearest illustrations of this genetic modulation, and has suggested co-evolution of the Plasmodium parasite with its human host in areas of endemic disease. Here, we present a brief overview of erythrocyte polymorphisms with biological relevance to malaria pathogenesis, and current work on the mechanism(s) by which these mediate their protective effect. The recent addition of erythrocyte pyruvate kinase to this group of protective genes will also be discussed.

The pathway of hemoglobin degradation in malaria parasites

Hemoglobin degradation in malaria parasites appears to be a vital and massive undertaking during intraerythrocytic development. An ordered pathway exists that efficiently catabolizes hemoglobin to yield amino acids that the parasite uses as a nutrient source, and that co-ordinates the resultant heme into a remarkable crystalline structure called hemozoin. Daniel Goldberg and Andrew Slater explain that the enzymes involved are specific and present prime targets for chemotherapeutic intervention.

Protection against malaria by thalassaemia and haemoglobin variants

Recent epidemiological evidence has given increasing support to Haldane's 1949 hypothesis that heterozygotes for such genetic disorders as thalassaemia might be protected against malaria, hence explaining the high gene frequencies for such disorders in endemic areas. As discussed here by Yongyuth Yuthavong and Prapon Wilairat, the possible cellular mechanisms, although still unclear, are emerging from in vitro studies which increasingly point to the importance of immune clearance mechanisms in some cases (such as alpha-thalassaemia and haemoglobin E). In other situations, decreased survival of the intra-erythrocytic parasite or decreased parasite invasion of the variant red blood cells may explain the protective effect. Whatever the cellular mechanisms are, the ultimate decisive factor is the relative fitness of the infected variant host, which may not be simply extrapolated from the cellular studies.

Brief communication: Resistance to Falciparum malaria in alpha-thalassemia, oxidative stress, and hemoglobin oxidation

A recent survey conducted on Vanuatu Island suggests that resistance to Plasmodium falciparum in alpha-thalassemic individuals may have an immunological basis. This study is important since it seems to undermine the current idea that red-cell genetic defects give protection against falciparum malaria by reducing intraerythrocytic growth and development of the parasite. However, the mechanisms underlying these clinical and genetic observations are not yet fully understood. Based on a review of the relevant literature, we first show that the model based on the interaction between hemoglobin (Hb) and membrane components may provide a molecular basis for the involvement of the immune response in genetic adaptation to malaria. Second, we discuss the main evolutionary implications of the model. Finally, we suggest two approaches by which anthropological studies could provide a useful way of testing the model: 1) analysis of the interactions of malaria-resistance genes with genetic polymorphisms which affect the erythrocyte redox status and 2) study of the antimalarial effects of natural products (introduced as a part of a diet or for traditional antimalarial therapy) capable of interfering with the Hb/membrane interaction.

The immunomodulator AS101 restores T(H1) type of response suppressed by Babesia rodhaini in BALB/c mice

The immunomodulator AS101 has been previously shown to confer protection upon BALB/c mice infected with the intraerythrocytic parasite Babesia rodhaini (B. rodhaini). The present study focuses on the effect of AS101 administration on the acute phase of babesial infection where T helper cell subset patterns-TH1/TH2-were assessed in heavily infected mice. Secretion of cytokines of the TH1 subset (IL-2, IFN-gamma, IL-12) and of the TH2 subset (IL-10, IL-4) as well as TGF-beta was measured following the administration of AS101 2 weeks before parasite infection. Our results demonstrate that the parasites suppress IL-2 protein and IL-12 mRNA and that AS101 upregulates their secretion: IL-2, 8 u/ml vs 34 u/ml, respectively; IFN-gamma protein, 2370 pg/ml vs 4777 pg/ml, respectively. Conversely, babesial infection results in the upregulation of IL-10 and IL-4 proteins and TGF-beta transcripts, whereas AS101 downregulates their production: IL-10, 1800 pg/ml vs 360 pg/ml, respectively; IL-4, 58.3 pg/ml vs 24.5 pg/ml, respectively. A possible escape mechanism induced by B. rodhaini is suggested, starting with IL-10 inhibition of macrophage activities leading to a suppression of the TH1 response and of IL-2 in particular. It is therefore possible that AS101 may protect infected mice by activating cellular-mediated immunity and concurrently balancing the TH subset responses. It is suggested that AS101 may be effective as an antiparasitic drug.

Vaccination against Babesia bovis: T cells from protected and unprotected animals show different cytokine profiles

Vaccination of cattle against the haemoprotozoan parasite, Babesia bovis, with the recombinant antigen 11C5 resulted in 9 of 15 cattle being protected against challenge infection. The cellular immune responses of protected and unprotected cattle were compared in order to identify differences in response. No differences were observed in the pattern of change in various blood leukocyte populations throughout challenge infection. FACScan analysis revealed an increase in the proportion of cells bearing the CD2 marker in both protected and unprotected cattle over the course of infection. There were no observable differences in the frequency of various cell-surface markers between the unprotected and protected cattle. During the period of patent parasitaemia, in vitro cultures of peripheral blood mononuclear cells (PBMC) from protected cattle produced significantly more TNF-alpha (P < 0.05) than cultures from unprotected cattle. TNF-alpha concentrations remained at pre-challenge levels until day 10, when levels in the unvaccinated control and vaccinated/unprotected animals dropped. By peak parasitaemia, TNF-alpha production in vitro was significantly greater (P < 0.05) in cultures of PBMCs from protected cattle. Interferon production showed an initial peak at day 5 in all cattle, followed by a decrease and a second peak at days 10-13 in protected cattle only, which coincided with resolution of the infection.

Does the mechanism of protection from falciparum malaria by red cell genetic disorders involve a switch to a balanced TH1/TH2 cytokine production mode?

The mechanism of protection from falciparum malaria by red cell genetic disorders still remains controversial. Decreased survival of parasites in variant red cells has previously been proposed. However, in vitro experiments were not conclusive and do not seem sufficient to explain the substantial degree of in vivo protection afforded to red cell genetic trait carriers. Evidence has recently been accumulating in favour of enhancement of the host immune response by these genetic traits. Malaria-infected variant red cells undergo modifications to their antigenicity which lead to accelerated and selective removal of early blood-stage parasites by splenic macrophages, resulting in fewer parasites reaching schizogony. Consequently there will be alterations in antigen processing, presentation and recognition which could explain the differences observed in T-cell responses between trait carriers and normal individuals. It is suggested that exposure to a lower dose of early parasite-stage antigens rather than the exoantigens of late mature schizonts could lead during primary and subsequent secondary infections to differentiation of T-helper cells into balanced TH1/TH2 subsets that promote protection, reversing the susceptibility to the fatal complications of falciparum malaria.

Dietary testosterone suppresses protective responsiveness to Plasmodium chabaudi malaria

This study investigates the effect of orally administered testosterone on serum testosterone levels and immune responses including outcome of Plasmodium chabaudi malaria. Female C57BL/10 mice were fed on a diet impregnated with 17 alpha-methyl-testosterone for 3 weeks. This raised the circulating testosterone levels from 0.28 ng/ml to 2.69 ng/ml on the average. In these mice, blood-stage infections of P. chabaudi resulted in a lethal outcome, whereas protective immunity developed in about 80% of mice fed on control diet without testosterone. Dietary 17 alpha-methyl-testosterone reduced the capacity of peritoneal cells to generate reactive oxygen intermediates after stimulation with C3b-coated zymosan and phorbol-myristate-acetate. Also, mice fed on dietary 17 alpha-methyl-testosterone responded to heat-killed Salmonella typhimurium with a higher increase in serum TNF, whereas the induced increase in the production of IL-10 by spleen cells was largely suppressed and no effect was found with respect to the production of IFN-gamma and IL-4. Our data indicate that the method of oral administration of 17 alpha-methyl-testosterone raises circulating testosterone to levels that impair protective immune responses to P. chabaudi malaria.

Flow cytometry detection of surface antigens on fresh, unfixed red blood cells infected by Plasmodium falciparum

The immunofluorescence detection of parasite-specific antigens on the surface of red blood cells infected by Plasmodium falciparum parasites is usually performed by visual detection under a fluorescence microscope. We describe here a technique permitting the analysis of surface immunofluorescence labelling by flow cytometry. Infected red blood cells are selected on the basis of their parasitic DNA and RNA content by Hoechst and Thiazole Orange vital dyes. Cytometric analysis of these labels, as well as general erythrocyte characteristics assessed by analysis of forward and side scatter allows the selection of viable intact infected erythrocytes from other blood cells. The integrity of these selected erythrocytes was confirmed by the absence of labelling with antibodies directed against internal components such as spectrin. This technique permits the detection of specific surface immunofluorescence staining on red blood cells infected with mature stages of P. falciparum by antibodies in sera from hyperimmune Saimiri monkeys. Using Thiazole Orange dye for detection of parasitised cells, this analysis was performed on a FACSscan apparatus equipped with a single laser.

Effects of antimalarials and protease inhibitors on plasmodial hemozoin production

Malarial hemozoin may play an important role as a target for antimalarial drugs and in disease pathogenesis. A new assay for hemozoin was developed in which the hemozoin was separated from cells by filtration. Trophozoites have substantially more hemozoin than rings, but there are relatively small differences between chloroquine-sensitive and chloroquine-resistant strains. The effects of hemozoin content of chloroquine and artemisinin, two antimalarial drugs, and E64 and Pepstatin A, two protease inhibitors, were measured. At concentrations at which hypoxanthine incorporation was unaffected, the hemozoin content of rings was decreased by E64, but not by the other three compounds. Artemisinin and Pepstatin A also had little effect on the hemozoin content of trophozoites. Chloroquine and E64 inhibited trophozoite hemozoin formation, but inhibited hypoxanthine uptake to a similar or greater extent. When either rings or trophozoites were exposed to several higher concentrations of chloroquine, hemozoin content was diminished, but significantly less than hypoxanthine uptake. Various concentrations of E64, in contrast, inhibited hemozoin production by both rings and trophozoites significantly more than hypoxanthine incorporation, suggesting that hemozoin production may be directly affected by E64.

Immunoglobulin G2a isotype may have a protective role in Plasmodium berghei NK65 infection in immunised mice

All CBA mice that had been immunised by means of four successive inoculations of Plasmodium berghei NK65, each inoculation being followed by chemotherapy, survived an intravenous challenge inoculation of parasite, with 4/12 mice developing patent parasitaemia that resolved within 2 weeks. In contrast, all non-immunised control mice died before the 10th day post-challenge. Examination of sera for antibodies revealed that the immunised mice, all of which survived the challenge, had significantly high anti-plasmodial whole IgG, IgG1, and IgG2a titres before the challenge. A 16-fold rise in IgG2a titre alone was recorded on the 5th day post-challenge, with a further boosting of the titre to 4096 being observed on day 10. In comparison, the titres of Ig isotypes in the non-immunised control mice that succumbed to the challenge remained below 4. Specific IgG subclasses, in particular IgG2a, could be involved in the humoral immune protection against this rodent parasite.

Antigenic variation of parasite-derived antigens on the surface of Babesia bovis-infected erythrocytes

The hemoparasite Babesia bovis antigenically alters the bovine erythrocyte membrane surface by expression of isolate-specific, parasite-derived polypeptides. To determine whether antigenic variation also occurred on the infected erythrocyte surface, a calf was infected once with parasitized erythrocytes carrying the C9.1 clonal line of B. bovis. In vitro cultures then were established periodically from the peripheral blood and analyzed with sequentially collected sera from the same animal. The surface reactivity of infected erythrocytes cultured from the infected animal varied over time, on the basis of reactivity in live cell immunofluorescence, surface immunoprecipitation, and panning assays. Subclones C8 and H10, established from day 41 cultures, were analyzed immunochemically. A loss of immunoreactivity was observed in antigens corresponding to the 113- and 128-kDa parasite-derived antigens of clone C9.1, demonstrating epitopic variation in these antigens; the immunochemical recognition of these antigens paralleled the results of live cell immunofluorescence and panning assays. Concomitant size polymorphism suggested polypeptide structural variation of these antigens as well. Calves infected by inoculation of infected blood or by injection of cloned parasites from in vitro cultures rapidly developed antibodies which cross-reacted among the clonal variant lines, suggesting the presence of common as well as unique epitopes. These results demonstrate that antigenic variation occurs on the surface of B. bovis-infected erythrocytes and that the parasite-derived antigens of 113 and 128 kDa compose at least a part of the antigens undergoing variation.

Identification of two Th1 cell epitopes on the Babesia bovis-encoded 77-kilodalton merozoite protein (Bb-1) by use of truncated recombinant fusion proteins

Previous studies have demonstrated the serologic and T-cell immunogenicity for cattle of a recombinant form of the apical complex-associated 77-kDa merozite protein of Babesia bovis, designated Bb-1. The present study characterizes the immunogenic epitopes of the Bb-1 protein. A series of recombinant truncated fusion proteins spanning the majority of the Bb-1 protein were expressed in Escherichia coli, and their reactivities with bovine peripheral blood mononuclear cells and T-cell clones derived from B. bovis-immune cattle and with rabbit antibodies were determined. Lymphocytes from two immune cattle were preferentially stimulated by the N-terminal half of the Bb-1 protein (amino acids 23 to 266, termed Bb-1A), localizing the T-cell epitopes to the Bb-1A portion of the molecule. CD4+ T-cell clones derived by stimulation with the intact Bb-1 fusion protein were used to identify two T-cell epitopes in the Bb-1A protein, consisting of amino acids SVVLLSAFSGN VWANEAEVSQVVK and FSDVDKTKSTEKT (residues 23 to 46 and 82 to 94). In contrast, rabbit antiserum raised against the intact fusion protein reacted only with the C-terminal half of the protein (amino acids 267 to 499, termed Bb-1B), which contained 28 tandem repeats of the tetrapeptide PAEK or PAET. Biological assays and Northern (RNA) blot analyses for cytokines revealed that following activation with concanavalin A, T-cell clones reactive against the two Bb-1A epitopes produced interleukin-2, gamma interferon, and tumor necrosis factors beta and alpha, but not interleukin-4, suggesting that the Bb-1 antigen preferentially stimulates the Th1 subset of CD4+ T cells in cattle. The studies described here report for the first time the characterization, by cytokine production, of the Th1 subset of bovine T cells and show that, as in mice, protozoal antigens can induce Th1 cells in ruminants. This first demonstration of B. bovis-encoded Th1 cell epitopes provides a rationale for incorporation of all or part of the Bb-1 protein into a recombinant vaccine.

Bovine helper T cell clones recognize five distinct epitopes on Babesia bovis merozoite antigens

Helper T cell clones from two Babesia bovis-immune cattle were characterized for use in identification of potentially protective immunogens of B. bovis merozoites. Proliferation assays with 11 CD4+ clones revealed a differential pattern of response to soluble cytosolic antigen, membrane-enriched antigen, detergent extracts of the membrane-enriched antigen, soluble culture supernatant exoantigen, and different geographical isolates of B. bovis as well as Babesia bigemina parasites. When the data were combined, the clones could be grouped according to five different patterns of response. One group recognized only the membrane-enriched fraction of New World and Australian parasites. Four remaining groups recognized antigens found in the cytosolic as well as the membrane-enriched fraction, and clones representative of each group were used to identify cytosolic antigens fractionated by anion-exchange chromatography with the use of fast-performance liquid chromatography. One clone (C97.3C3), which responded to all B. bovis isolates and to B. bigemina, recognized a single peak of activity that eluted with 0.25 M NaCl and contained protein bands of 70 and 75 kDa. The remaining clones were stimulated by a second antigenic peak that eluted between 0.35 and 0.45 M NaCl and contained protein bands of 42, 47, 56, and 84 kDa. The majority of the clones produced interferon, whereas tumor necrosis factor alpha/tumor necrosis factor beta production was less frequent. These studies provide the basis for using helper T cell clones to identify potentially protective immunogens of B. bovis and delineate a minimum of five helper T cell epitopes recognized by two immune cattle.

Oxidative damage of erythrocytes infected with Plasmodium falciparum. An in vitro study

The extent of reduced glutathione, activity of glutathione peroxidase, amount of membrane lipid peroxidation products, and the extent of hemoglobin release from host erythrocytes during in vitro Plasmodium falciparum growth was studied. Highly synchronized parasite cultures were studied to examine the alterations caused by different growth stages of the parasite. There was a moderate increase in the reduced glutathione content as the parasite matured, which was significant only in schizont-rich erythrocyte lysates (p < 0.05) whereas the activity of glutathione peroxidase was significantly low in all the parasitized red blood cells (ring-infected RBC, p < 0.005; trophozoite- and schizont-infected RBC, p < 0.001). The lipid peroxidation product, malonyldialdehyde, of the host red cells increased gradually to more than fourfold in schizont-rich cells as compared with normal erythrocytes (p < 0.001). The hemoglobin release from cultured cells was significantly higher in all parasitized red cell cultures as well as in uninfected cells kept in in vitro, as compared with normal erythrocytes. The consequence of such changes induced by the malarial parasites in the host red cells in the pathogenesis of erythrocyte destruction and anemia of P. falciparum malaria is discussed.

Testosterone-induced diminution of two peptides in spleen cells from testosterone-immunosuppressed B10 mice

High-performance liquid chromatography (HPLC) is used to detect testosterone (T)-sensitive peptides in spleen cells isolated from female C57BL/10 mice immunosuppressed against Plasmodium chabaudi malaria by T treatment. Two peaks with retention times of about 25 min and 34 min, respectively, were identified to be diminished by about 52% and 47%, respectively, in spleen cells from T-treated mice compared to those from untreated mice. Amino acid sequencing revealed that the 24 min peak consisted of the dipeptide Met-Phe and the 34 min peak contained a degradative fragment of the alpha-chain of hemoglobin. Our data suggest that the immunosuppressive T treatment of B10 mice induces a perturbation of erythrophagocytosis in spleens.

Babesia bovis: Bovine helper T cell lines reactive with soluble and membrane antigens of merozoites

Babesia bovis-specific T cell lines were established from cattle infected with either tick-derived or cultured parasites by stimulation of peripheral blood mononuclear cells with a crude parasite membrane fraction. Induction and enrichment of CD4+ T cells occurred over time. All cell lines responded vigorously and in a dose-dependent, MHC-restricted manner to intact merozoites, and to soluble and membrane fractions derived from merozoites by homogenization and high-speed centrifugation. Solubilization of the membrane fraction with nondenaturing zwitterionic or nonionic detergents yielded antigenic extracts which also stimulated the T cells. However, a differential response was observed, in that cell lines from one animal proliferated vigorously to the detergent extracts of the membrane fraction, whereas cell lines from a second animal proliferated only weakly to these extracts. SDS-PAGE analysis revealed common protein bands of 90 and 22 kDa in the various immunogenic fractions. Cell lines from the animal infected with cultured parasites also responded to parasite culture supernatant "exoantigens" and to the related parasite, Babesia bigemina. We conclude that antigens present in merozoite membranes and soluble parasite extracts preferentially stimulate CD4+ T cells from cattle immune to Babesia bovis. The differential pattern of response of T cell lines from different cattle suggests that more than one protein or epitope is immunodominant for T cells.