No abstract

T cells are important in the immune response to malaria, both for their cytokines and their help for antibody production. To look at the relative importance of these roles, a T-cell receptor (TCR) transgenic mouse has been generated carrying a TCR specific for an epitope of the merozoite surface protein 1 (MSP-1) of the malaria parasite, Plasmodium chabaudi. In adoptive transfer experiments, malaria-specific CD4 ؉ T cells expand and produce interferon ␥ (IFN-␥) early in infection, but the population contracts quickly despite prolonged persistence of the parasite. MSP-1-specific CD4 ؉ cells can protect immunodeficient mice from lethal infection; however, the parasite is only completely cleared in the presence of B cells showing that T helper cells are critical. Levels of malariaspecific antibody and the speed of their production clearly correlate with the time of resolution of infection, indicating that a critical threshold of antibody production is required for parasite clearance. Furthermore, T cells specific for a shed portion of MSP-1 are able to provide help for antibody to the protective region, which remains bound to the infected erythrocyte, suggesting that MSP-1 has all of the components necessary for a good vaccine. IntroductionT helper cells are essential for a protective immune response to the blood stages of the rodent malaria parasite, Plasmodium chabaudi chabaudi (AS). 1 There is debate, however, over the extent to which T cells protect via Th1 cytokine-mediated mechanisms or by the antibodies that they help to produce. Both mechanisms dominate the T-cell response to P chabaudi in turn, with an early Th1-type cytokine response, which switches later in infection to one that provides effective help for malaria-specific antibody production and produces less interferon ␥ (IFN-␥). 2,3 Increased T-helper cell activity in the later stages 3 is clearly beneficial because experiments in B cell-deficient mice demonstrate that B cells and antibodies are required for complete clearance of parasites, 4-6 although the requirements and specificity of T-cell help and antibody for rapid clearance or immunity to reinfection are not known.To determine the precise role of malaria-specific T helper cells and the potential of an important vaccine antigen, merozoite surface protein 1 (MSP-1), in the protection from and clearance of malaria infection, we have generated a T-cell receptor (TCR) transgenic mouse with a TCR specific for P chabaudi MSP-1. This molecule is expressed on the invasive merozoite surface, 7 and its C-terminal domain can induce a protective immune response. [8][9][10][11] However, this domain is not efficiently processed by antigenpresenting cells and is a less effective inducer of CD4 ϩ T-cell responses than other parts of MSP-1. 10,12,13 For more effective vaccination strategies it would be important to know whether CD4 ϩ T-cell help can be generated from other parts of MSP-1, which are more readily processed and presented on antigenpresenting cells. The MSP-1-specific TCR of our transgenic mouse,...

A primary infection of mice with Plasmodium chabaudi chabaudi (AS) is characterized by a rapid and marked inflammatory response. Typically, IL‐12, TNF‐α  and IFN‐γ  are produced in the spleen, and are transiently present in plasma. The cells involved in this early response are unknown. Here we show that dendritic cells derived from GM‐CSF‐stimulated mouse bone marrow cultures produce TNF‐α  within 30 min of exposure to P.c.chabaudi schizonts. IL‐6, IL‐12p40 and p70 follow this. The production of these cytokines was not dependent on the presence of T cells or NK cells and did not require CD40. Incubation of dendritic cells with P.c.chabaudi schizonts also resulted in up‐regulation of MHC class II, CD40 and CD86 but not CD80. In contrast to some strains of the human parasite, P. falciparum, P.c. chabaudi (AS) did not inhibit the up‐regulation of MHC class II, CD86 or CD40 induced by LPS. Therefore, the erythrocytic stages of P.c.chabaudi are able to activate dendritic cells directly. The consequences of such an interaction could be rapid activation of TH1 cells and induction of immunity, and in the event of a large response also induction of TNF‐α  associated pathology.

In this study we have investigated the antibody and CD4 T-cell responses to the well-characterized malaria vaccine candidate MSP-1 during the course of a primary Plasmodium chabaudi chabaudi (AS) infection. Specific antibody responses can be detected within the first week of infection, and CD4 T cells can be detected after 3 weeks of infection. The magnitude of the CD4 T-cell response elicited during a primary infection depended upon the region of MSP-1. In general, the highest precursor frequencies were obtained when a recombinant MSP-1 fragment corresponding to amino acids 900 to 1507 was used as the antigen in vitro. By contrast, proliferative and cytokine responses against amino acids 1508 to 1766 containing the C-terminal 21-kDa region of the molecule were low. The characteristic interleukin 4 (IL-4) switch that occurs in the CD4 T-cell population after an acute blood stage P. c. chabaudi infection was only consistently observed in the response to the amino acid 900 to 1507 MSP1 fragment. A lower frequency of IL-4-producing cells was seen in response to other regions. Although the magnitudes of the immunoglobulin G antibody responses to the different regions of MSP-1 were similar, the isotype composition of each response was distinct, and there was no obvious relationship with the type of T helper cells generated. Interestingly, a relatively high antibody response to the C-terminal region of MSP-1 was observed, suggesting that T-cell epitopes outside of this region may provide the necessary cognate help for specific antibody production.

The C-terminal fragment of merozoite surface protein-1 (MSP-1) of the mouse malaria parasite Plasmodium chabaudi chabaudi (AS) stimulates a weak CD4 T cell response when compared to the response to a more structurally simple region of the molecule. The tertiary structure of the C-terminal region of MSP-1 is maintained by five disulfide bonds. A peptide from this region could only be processed and loaded onto newly synthesized MHC class II molecules, whereas a peptide from the structurally simple region was available for loading onto recycling MHC class II. CD4(+) T cell hybridomas took longer to recognize an epitope derived from the disulfide-bonded region whether native parasite or recombinant MSP-1 antigen was used. Reduction of disulfide bonds in the C-terminal region subsequently allowed peptides to be loaded onto recycling MHC class II and greatly enhanced the rapidity of the T cell response. These data demonstrate that differential processing occurs intramolecularly in MSP-1, which may be responsible for the observed weak CD4 T cell responses against this region. The consequences of this in vivo may be that limited T cell help is available for protective antibody production which has important implications for designing vaccines based on MSP-1.