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Differential Binding of Escherichia coli Enterotoxins LT-IIa and LT-IIb and of Cholera Toxin Elicits Differences in Apoptosis, Proliferation, and Activation of Lymphoid Cells

Witebsky Center for Microbial Pathogenesis and Immunology,¹ Department of Microbiology and Immunology,² Department of Oral Biology, The University of Buffalo, State University of New York at Buffalo, Buffalo, New York³

Received 2 September 2004/ Returned for modification 18 October 2004/ Accepted 27 December 2004

Cholera toxin (CT), LT-IIa, and LT-IIb are potent adjuvants which induce distinct T-helper (Th)-cell cytokine profiles and immunoglobulin G (IgG) subclass and IgA antibody responses. To determine if the distinct immune regulatory effects observed for LT-IIa, LT-IIb, and CT are elicited by binding of the enterotoxins to their cognate ganglioside receptors, the lineages of lymphoid cells that interact with the three enterotoxins and their effects on various lymphocyte responses in vitro were evaluated. Binding patterns of LT-IIa, LT-IIb, and CT to several lymphoid cell populations were distinctive for each enterotoxin. LT-IIa and CT, but not LT-IIb, induced apoptosis in CD8⁺ T cells. LT-IIa(T34I), a mutant with no detectable binding to gangliosides, did not induce apoptosis. Blockade of GM₁ on the surface of CD8⁺ T cells by LT-IIa(T14I), a mutant that binds only to GM₁ but does not induce apoptosis, did not inhibit induction of apoptosis by LT-IIa. Mitogen-induced proliferation of CD8⁺ T cells was abrogated by treatment with CT, while resting CD8⁺ T cells which were sensitive to LT-IIa-induced apoptosis became more resistant to apoptosis after mitogen activation. Exposure to CT, but not to LT-IIa or LT-IIb, inhibited mitogen-driven CD4⁺ T-cell proliferation and expression of CD25 and CD69. In mitogen-stimulated B cells, CT, but not LT-IIa or LT-IIb, enhanced expression levels of CD86, while only CT induced B-cell differentiation into plasma cells. Thus, LT-IIa, LT-IIb, and CT exhibit distinguishable immunomodulatory properties which are likely dependent upon their capacities to recognize different ganglioside receptors on lymphocytes.

Editor: J. D. Clements

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