Induction of suppressor cells in mice by cyclophosphamide

The injection of a single sublethal dose of cyclophosphamide (CY) into adult C3H/He mice induced splenic atrophy followed by considerable hypertrophy. During the phase of splenomegaly, the in vitro reactivity of spleen cells to the mitogens phytohaemagglutin and lipopolysaccharide was drastically decreased. Furthermore, the spleen cell population from CY-treated mice contained suppressor cells capable of inhibiting the in vitro reactivity of normal lymphocytes to these mitogens. After the removal of adherent cells, the suppressive activity was completely absent from the remaining fraction. The suppressive activity was also abolished after treatment of spleen cells with anti-immunoglobulin antiserum plus complement (C). After treatment with anti-Thy 1-2 antiserum plus C, the suppressive activity was not modified. Nude mice, B mice, young and old NZB mice, also developed suppressor cells with similar functional characteristics when treated with CY. However, in Nude mice the suppressor cells were not adherent and did not bear surface Ig. After fractionation of spleen cells by velocity sedimentation, the suppressive activity was detected in the fastest fraction with a velocity over 5 mm/h.