Protective immune responses to the 42-kilodalton (kDa) region of Plasmodium yoelii merozoite surface protein 1 are induced by the C-terminal 19-kDa region but not by the adjacent 33-kDa region

Abstract

Vaccination of mice with the 42-kDa region of Plasmodium yoelii merozoite surface protein 1 (MSP1(42)) or its 19-kDa C-terminal processing product (MSP1(19)) can elicit protective antibody responses in mice. To investigate if the 33-kDa N-terminal fragment (MSP1(33)) of MSP1(42) also induces protection, the gene segment encoding MSP1(33) was expressed as a glutathione S-transferase (GST) fusion protein. C57BL/6 and BALB/c mice were immunized with GST-MSP1(33) and subsequently challenged with the lethal P. yoelii YM blood stage parasite. GST-MSP1(33) failed to induce protection, and all mice developed patent parasitemia at a level similar to that in naive or control (GST-immunized) mice; mice immunized with GST-MSP1(19) were protected, as has been shown previously. Specific prechallenge immunoglobulin G (IgG) antibody responses to MSP1 were analyzed by enzyme-linked immunosorbent assay and immunofluorescence. Despite being unprotected, several mice immunized with MSP1(33) had antibody titers (of all IgG subclasses) that were comparable to or higher than those in mice that were protected following immunization with MSP1(19). The finding that P. yoelii MSP1(33) elicits strong but nonprotective antibody responses may have implications for the design of vaccines for humans based on Plasmodium falciparum or Plasmodium vivax MSP1(42).

FIG. 1.

Challenge infection with P. yoelii YM of mice immunized with MSP1₃₃ or MSP1₁₉. Three weeks after the third immunization with GST-MSP1₃₃ or -MSP1₁₉, groups of four or five C57BL/6 and BALB/c mice were inoculated intravenously with 10⁴ P. yoelii YM. Parasitemia was monitored daily from day 3. Control groups consisted of mice immunized with GST alone and naive mice. Only mice in the GST-MSP1₁₉ groups were able to survive the challenge; mice that did not survive are indicated by †.

FIG. 2.

IgG subclass of antibodies to native P. yoelii MSP1 in mice immunized with GST-MSP1₃₃ or -MSP1₁₉, as measured by IFAT. Sera from C57BL/6 and BALB/c mice immunized three times with either GST-MSP1₃₃ or -MSP1₁₉ were analyzed by IFAT for reactivity with mouse RBC infected with P. yoelii YM. IFAT reactivity for all four IgG subclasses was measured and is shown as titers, with different bar patterns representing different subclasses. One of the GST-MSP1₁₉-immunized BALB/c mice had an IgG1 titer in excess of 100,000 (262,000). IgG3 was only detected in BALB/c mice immunized with GST-MSP1₃₃. C indicates mice that cleared their infection; the number in parentheses indicates the day on which the infection was cleared. † indicates mice that died or were humanely killed, with the day of death in parentheses. Percentages indicate the highest parasitemia detected during the course of infection.