Babesia bovis: characterization of the T helper cell response against the 42-kDa merozoite surface antigen (MSA-1) in cattle.
The Babesia bovis major merozoite surface antigen (MSA-1) is a 42-kDa integral membrane glycoprotein previously shown to induce immunodominant antibody responses in cattle protectively immune to B. bovis and to induce neutralizing antibody. Recent studies have also shown that MSA-1 B cell epitopes common to New World strains of B. bovis are not present in either Israel or Australia strains. To understand the potential role of this protein in protective immunity, T helper cell responses specific for MSA-1 were characterized in Babesia-immune cattle. Peripheral blood mononuclear cells from immune cattle proliferated against affinity-purified recombinant MSA-1 protein expressed in Escherichia coli. MSA-1 preferentially stimulated the growth of CD4+ T cells in cell lines cultured with antigen for 4 weeks. MSA-1-reactive cell lines responded to a membrane fraction of B. bovis merozoites, suggesting recognition of the native protein. However, B. bovis-reactive T cell lines and T helper clones established by stimulation with crude parasite membrane antigen failed to respond to recombinant MSA-1, indicating that this antigen is not immunodominant for T cells. The majority of MSA-1-specific T helper clones reacted to unfractionated merozoite membrane antigen from New World B. bovis strains, but none of the clones responded to Australia B. bovis or to a Mexico strain of Babesia bigemina. Several T helper clones produced low levels of cytokines when stimulated with concanavalin A and interleukin-2. Northern blot analysis revealed the expression of interleukin-2, interleukin-4, interferon-gamma, and tumor necrosis factor-alpha messenger RNA in mitogen-stimulated T helper clones, showing that the clones examined expressed an unrestricted T helper phenotype. We conclude that the MSA-1 protein, although serologically immunodominant and capable of inducing neutralizing antibodies as well as a T helper cell response, is not an immunodominant T cell antigen. Furthermore, the parasite strain specificity of the Th clones supports previous findings of extensive polymorphism in the MSA-1 glycoprotein and suggests that like B cell epitopes, T cell epitopes reside in a nonconserved portion of the protein.
Brown
W C
WC
Department of Veterinary Pathobiology, Texas A&M University, College Station 77843.
Palmer
G H
GH
McElwain
T F
TF
Hines
S A
SA
Dobbelaere
D A
DA
D000818:Animals; D000938:Antigen-Presenting Cells / Q000276:immunology; D000953:Antigens, Protozoan / Q000276:immunology*; D000954:Antigens, Surface / Q000276:immunology; D001403:Babesia / Q000276:immunology; D016793:Babesia bovis / Q000276:immunology*; D001404:Babesiosis / Q000276:immunology*; D002417:Cattle; D002418:Cattle Diseases / Q000276:immunology*; D002460:Cell Line; D003208:Concanavalin A / Q000494:pharmacology; D007371:Interferon-gamma / Q000096:biosynthesis; D007378:Interleukins / Q000096:biosynthesis; D008213:Lymphocyte Activation; D008562:Membrane Glycoproteins / Q000276:immunology*; D006377:T-Lymphocytes, Helper-Inducer / Q000276:immunology*; D014409:Tumor Necrosis Factor-alpha / Q000096:biosynthesis
D000953:Antigens, Protozoan; D000954:Antigens, Surface; D007378:Interleukins; D008562:Membrane Glycoproteins; D014409:Tumor Necrosis Factor-alpha; D003208:Concanavalin A; D007371:Interferon-gamma