Rosette forming cells in non-immunized mice

ANTIBODIES which are produced by developing cells can be detected and enumerated in the spleen of immunized animals either as plaque forming cells (PFC)1,2, or as rosette forming cells (RFC)3-5. In immunized mice the formation of plaques and rosettes has recently been shown to be attributable to two distinct populations of lymphoid cells6. Both PFC and RFC techniques also detect a "background" level of activity in the spleens of normal non-immunized animals. Using sheep red cells (SRC) as the test antigen, a background level of 0.05-0.5 PFC/106 splenocytes has been determined in non-immunized mice1,7. A much higher background level of 102-103 RFC/106 splenocytes8 has, however, been shown by the rosette method. This high background number of RFC could reflect (a) antibody forming cells; (b) cells passively coated with cytophilic antibody9; or (c) non-immunological activity.