Restricted idiotypic profile of anti‐phosphorylcholine antibodies induced by carrier‐specific helper T cell clones

The primary antibody response of C57BL/6 mice to phosphorylcholine (PC) is dominated by immunoglobulins of the T15 idiotype family. Using monoclonal antibodies that define individual idiotopes (Id) of T15, it has been shown that the antibody plaque‐forming cells (PFC) arise from heterogeneous population of B cells that may express some but not all T15 Id. In the present study, we tested the ability of antigen (keyhole limpet hemocyanin, KLH)‐specific CD4⁺ T cell clones (T_h) to provide help for various T15 B cell subsets in response to PC‐KLH in vitro. Four independently derived, KLH‐specific T_h clones were found to provide quantitative help (80–120 PFC/culture of 10⁶ B cells) which was comparable to KLH‐primed, unselected T cells. However, the response in the presence of the cloned T cells was idiotopically restricted: T15 idiotopes B24–44 and B36–75 were expressed on 60% to 90% PFC, whereas Id AB1–2 and B36–82 were not present (0%–20% PFC). All of these Id were highly expressed in cultures with KLH‐primed, unselected splenic T cells. Evidently, the cloned T_h cells provided help to only a subset of idiotypically distinct B cells. Bulk spleen T cells from unprimed donors by themselves did not provide any help for the primary response to PC‐KLH. However, when these cells were added to the cultures containing B cells and KLH‐specific T_h clones, the repertoire of the response was fully restored, including the AB1–2/B36–82 Id (50%–80% PFC). These results suggest the existence of an auxiliary helper mechanism that may be required for activation of some B cells in addition to the antigen‐specific T_h cells.