Estimation of hapten-specific antibody-forming cell precursors in microcultures.

The immune response of mouse spleen cells to hapten-conjugated polymer of flagellin (DNP-POL, NIP-POL) was studied using a microculture system. When increasing numbers of spleen cells were added to a 'filler' cell system, negative feedback effects became apparent and resulted in the generation of progressively lower numbers of plaque-forming cells (PFC) per input cell. This feedback inhibition was shown to be antigen-specific and mediated by factors released into the culture medium. The effect precludes calculation of the frequency of PFC precursors in cultures containing spleen cells alone and complicates the analysis of tolerance using in vitro assay systems. The addition of small numbers of spleen cells to a constant number of thymocytes provided a system in which Poisson analysis could be used to determine the frequency of PFC precursors capable of being activated by hapten-POL conjugates. This system was used to estimate the frequency of anti-NIP-PFC precursors in CBA spleen cells.