Induction <i>in vitro</i> of reversible immunosuppression and inhibition of B cell receptor regeneration by defined metabolites of cyclophosphamide

Abstract

Abstract A number of defined metabolites and derivatives of cyclophosphamide (CY) were compared with the parent compound for their capacity to induce suppression of humoral antibody responses in vivo and in vitro and to inhibit the regeneration of surface immunoglobulin (sIg) receptors on B cells subsequent to capping with anti‐Ig serum. Both functions were inhibited by exposing spleen cells in vitro to the most active compounds (4‐hydroperoxycyclophosphamide and phosphoramide mustard) in concentrations of 10 μg/ml or less. Although these inhibitory activities were found to be restricted to those molecules with demonstrable alkylating activity, they are nevertheless reversible and not dependent on events leading to cell death. Inhibition with microsomally activated CY in vitro rendered splenic B cells hyper‐susceptible to tolerance induction by levan, although in the absence of this antigen they recovered full responsiveness to it within 7–10 days. Since lethally irradiated mice repopulated with adult bone marrow cells do not regenerate normal responsiveness to polysaccharide antigens in less than 50–100 days (Howard, Moreno, Hale and Vicari, Eur. J. Immunol. 1977. 7 : 431.), it is considered that the rapid recovery by CY‐treated spleen cells involves reversal of suppression in B cells and not renewal from stem cells. Our previous proposal (Shand and Howard, Nature 1978. 271 : 255.) that impairment of B cell receptor regeneration by CY is causally related to its capacity to promote B cell tolerance induction by thymus‐independent antigens is supported by the present data.