As measured by [(3)H]thymidine uptake, spleen cells of mice injected 7 d previously with a single dose of cyclophosphamide (Cy) (125 mg x kg (-1)) gave an enhanced response to dextran sulfate (DS), a diminished response to lipopolysaccharide (LPS), and a normal response to concanavalin A. Addition of syngeneic thymocytes to spleen cells inhibited the enhanced response of the cells to DS and slightly enhanced their response to LPS. Pretreatment of thymocytes by 4-hydroxyperoxycyclophosphamide (4HP-Cy) in vitro (an in vitro active derivative of Cy) abrogated the effect of thymocytes on the DS response but not on the LPS response. Pretreatment of spleen cells by small doses of 4HP-Cy (0.1-1.0 μg. ml(-1)) in vitro enhanced the capacity of the cells to respond to DS but either did not affect, or even diminished their capacity to respond to LPS. The enhancement of the DS response by 4HP-Cy treatment could not be detected using spleen cells depleted of T cells or lacking functioning T cells. 4HP-Cy doses more than 3 μg ml(-1) diminished or abolished the capacity of the spleen cells to respond to LPS as well as their capacity to respond to DS.

The results show (a) that in contrast to the LPS-reactive B-lymphocyte subset, the proliferative capacity of DS-reactive subset is negatively controlled by a Cy- and 4HP-Cy-sensitive T-cell subset and (b) that these T- suppressor cells are more sensitive to Cy and 4HP-Cy (to their respective active alkylating metabolites) than B lymphocytes and T cells carrying other immunological functions.

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