Abstract
It has been suggested that dominant monoclonal responses in mice after transfer of small numbers of spleen cells are the result of strong selection for cell clones producing antibody of high affinity. We have attempted to measure the affinity of anti‐DNP (2,4dinitrophenyl) and anti‐NIP (4hydroxy5iodo3nitrophenacetyl) antibodies produced in monoclonal responses by inhibition of plaque‐forming cells (PFC) with free hapten. PFC from individual monoclonal spleen foci and from spleens after adoptive transfer of 5 × 10^5^ to 2 × 10^6^ primed spleen cells were generally inhibited at lower free antigen concentrations than the PFC found in a heterogeneous secondary response.
It was possible to compare several individual cell clones, forming monoclonal antibody to DNP, with respect to the free antigen concentration needed for 50 % plaque inhibition. These comparisons did not correlate well with the relative affinities of serum antibody estimated by the Stupp‐Farr technique.
Published work analyzing PFC from plasma cell tumor MOPC 315 forming antibody to DNP might have predicted that the curve of PFC inhibition by free hapten for a monoclonal PFC population would be steep. Usually this was not the case, and 20–80 % PFC inhibition occurred over 100‐fold concentration range of free hapten.
Some of the anti‐DNP clones formed large plaques and when these PFC were inhibited by free antigen, the plaque diameter decreased progressively as the hapten concentration increased. Large plaques consequently require more free antigen for inhibition than small plaques. The poor agreement between apparent affinity measured by plaque inhibition and that measured in serum antibody for monoclonal antibody responses may well be a result of the observed differences in plaque size between individual clones.
We therefore have strong reservations about the use of plaque inhibition by free antigen to estimate antibody affinity at the cellular level.