Limiting dilution analysis of the frequencies of helper and suppressor T cells in untreated and TNP-treated BALB/c mice: response as a consequence of perturbation of a stable steady state.

B-cell response and suppression of response were analysed with respect to changes in frequency or composition of regulatory elements in comparison to untreated BALB/c mice. As antigenic stimulus the hapten TNP was used. Suppression was induced by intravenous (i.v.) injection of TNP-haptenized syngeneic lymphocytes; for inducing response, mice received TNP-horse red blood cells (HRBC). As the read-out system, plaque-forming cells (PFC) were determined after either the addition of naive B cells only [determination of the frequency of helper T-cells (TH)], or the addition of naive B cells plus saturating doses of TNP-specific TH [determination of the frequency of suppressor T-cells (Ts)]. The data indicate that, in untreated animals, a network of help and suppression is pre-existing, wherein TH are more frequent (1/1471) than TS (1/4413), but TS are dominant, i.e. when the fraction of non-responding cultures (F0) (determination of help) was plotted, an inversion of the curve was seen at high numbers of cells per well; however, the fractions of responding cultures (F+) (determination of suppression) could be plotted on a straight line. Application of antigen in suppressogenic or immunogenic form resulted in a two- to four-fold increase in the corresponding regulatory population but, concomitantly, a minor increase in the frequency of the mutual counteracting population was observed. Irrespective of any immunization schedule, TH were more frequent than TS. The overall shape of the 'helper' and 'suppressor' curves with spleen cells (SC) from suppressed mice resembled--besides changes in the frequencies--those obtained with SC of untreated animals. This corresponds to the maintenance of a state of unresponsiveness as in untreated animals. However, when SC from primed or suppressed plus primed mice were analysed, a different type of curve was obtained. Suppressor curves no longer followed a ratio dominance model; instead, at high numbers of cells per well, the frequency of wells with suppressive activity decreased. Correspondingly, in helper curves, a second slope of increasing numbers of wells with helper activity was seen at a high input of cells. Hence, response cannot be explained solely by expansion/activation of TH, but obviously a third regulatory population is involved which could not be detected in untreated animals. This third regulatory population could either be non-suppressible TH or--more likely, as will be discussed--contrasuppressor cells.