Glycosphingolipids as novel targets for T-cell suppression by the B subunit of recombinant heat-labile enterotoxin.
Heat-labile enterotoxin subunit B (LTB) is a noncatalytic protein derived from Escherichia coli that binds to ganglioside GM1, a glycosphingolipid on the surface of mammalian cells. In this study, the effects of recombinant LTB (rLTB) on murine lymphocytes were examined in vitro. T and B cells readily bound fluorescein isothiocyanate-labeled rLTB. CD8+ T cells bound twice as much as CD4+ T cells and B cells. Exposure of T-cell subsets and B cells to rLTB abrogated mitogen-driven proliferation. CD8+ T cells were more susceptible to rLTB than either CD4+ T cells or B cells. There were differences in the sensitivity of lymphocytes from various strains of mice to rLTB. This was attributed to qualitative and quantitative differences in the CD4+ T cells. rLTB induced apoptosis in both T-cell subsets, but the level was significantly higher in CD8+ T cells. Apoptosis peaked at around 8 h after exposure to rLTB and incubation at 37 degrees C. Binding to ganglioside GM1 was essential for suppression, since rLTB/G33D, a mutant which does not bind GM1, failed to inhibit proliferation or induce apoptosis. Naive T cells, which were acutely sensitive to rLTB, became more resistant after activation. Conversely, activated T cells regained their sensitivity to rLTB when they reverted back to a resting state. A 1-h pulse with rLTB was sufficient to inhibit T-cell proliferation and cytotoxic-T-lymphocyte generation in primary mixed lymphocyte reaction cultures. CD8+ T cells were preferentially depleted in these cultures. rLTB also induced functional modifications in T cells as indicated by inhibition of gamma interferon secretion after polyclonal activation. Thus, rLTB may have immunomodulatory properties independent of its ability to induce apoptosis.
Truitt
R L
RL
Department of Pediatrics, Cancer Center, Medical College of Wisconsin, Milwaukee 53226, USA. rtruitt@mcw.edu
Hanke
C
C
Radke
J
J
Mueller
R
R
Barbieri
J T
JT
D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D001427:Bacterial Toxins / Q000494:pharmacology*; D016207:Cytokines / Q000096:biosynthesis; D004768:Enterotoxins / Q000494:pharmacology*; D029968:Escherichia coli Proteins; D005677:G(M1) Ganglioside / Q000502:physiology*; D007166:Immunosuppressive Agents / Q000494:pharmacology*; D008213:Lymphocyte Activation / Q000187:drug effects; D007959:Lymphocyte Culture Test, Mixed; D051379:Mice; D008806:Mice, Inbred AKR; D008807:Mice, Inbred BALB C; D008810:Mice, Inbred C57BL; D008811:Mice, Inbred DBA; D011994:Recombinant Proteins / Q000494:pharmacology; D013601:T-Lymphocytes / Q000187:drug effects*
D001427:Bacterial Toxins; D016207:Cytokines; D004768:Enterotoxins; D029968:Escherichia coli Proteins; D007166:Immunosuppressive Agents; D011994:Recombinant Proteins; D005677:G(M1) Ganglioside; C027538:heat-labile enterotoxin, E coli