Immunization of C3H/HeJ mice with 4 X 10(9) SRBC yields a whole splenic T-cell population which can, upon transfer, specifically suppress recipient direct and indirect plaque-forming cells (PFC) responses to sheep erythrocytes (SRBC). Discontinuous bovine serum albumin density gradient fractionation of these T cells demonstrated a population of low density T cells which augmented and a population of high density T cells which suppressed recipient responses irrespective of the number of T cells transferred. Moreover, infusion of admixtures of low and high density cells resulted in intermediate regulatory functions which could be predicted by knowing the regulatory capacity of each population alone. In addition to heterogeneity existing among regulatory T cells as regards amplification and suppression, it appeared that heterogeneity existed within the suppressor T population. Thus, T cells capable of inhibiting direct PFC could be distinguished from those suppressing indirect PFC by their differential localization in peripheral lymphoid tissue, differences in the dissipation of suppressive influences during incubation at 37 degrees C, and by differences in the possible requirement for adherent cell populations. While the relative frequency of both low density amplifier and high density suppressor cells increased with the dose of SRBC used for their induction, it appeared that suppressor cells might be generated in response to feedback signals from amplifier cells. These studies indicate that further delineation of heterogeneity existing within suppressor populations may be helpful in defining mechanisms required for the induction and manifestation of suppressive regulatory forces.

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