The afferent arc of the in vivo cytotoxic T-cell immunity assessed by second set rejection of ascitic allogeneic tumors was shown to be depressed by bacterial lipopolysaccharide (LPS) that was administered simultaneously with or 1 day before injection of allogeneic spleen cells as stimulators. Two different LPSs from Escherichia coli O55 and Klebsiella O3 displayed similar activities whereas dextran sulfate, concanavalin A, or poly A:U was not effective. Stimulator activities of allogeneic cells was not directly modified by LPS. Any definite suppressor activity on afferent or efferent arc of the T-cell response was not demonstrable in mice receiving LPS and allogeneic cells. Further, the LPS effect for immune depression was not diminished by whole body X-ray irradiation to the recipient at 300 R, which ablated the B-cell reactivity to LPS for polyclonal activation, or by treatment of the recipient with carrageenan, a known toxic agent to macrophages. It was suggested from these results that LPS suppresses the cytotoxic T-cell immunity by modulating responder T cells to be temporarily refractory to the allogeneic stimulus rather than by activating suppressor cells such as radiation-sensitive lymphocytes and carrageenan-sensitive macrophages.
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