An examination of the TRF assay reveals a heterogeneity of TRF-like activities.

Supernatant from the cloned Mlsa,d-reactive helper T cell line L2V is a potent source of T cell replacing factor (TRF) activity. To determine whether L2V SF was representative of TRF-active supernatants, it was compared with supernatant from Con A-stimulated spleen cells (CASF) by using TRF assays. This analysis, facilitated by the use of four different antigens (R36a, TNP-R36a, SRBC, and HRBC), produced the following results. 1) L2V SF and CASF allowed responses of similar magnitude against either R36a or TNP-R36a; however, CASF always allowed responses of fivefold to 30-fold greater magnitude against SRBC than L2V SF. 2) B cells from xid and "normal" mice will give equally large responses to TNP-R36a when CASF is used as the source of TRF; however, L2V SF will only effect the responses by "normal" B cells. 3) L2V SF-driven responses to R36a and SRBC, and CASF-driven responses to R36a, follow single-hit kinetics and are IL 2 independent, whereas CASF-driven responses to SRBC follow multi-hit kinetics and are IL 2 dependent. These results indicate that the TRF assay measures a heterogeneity of TRF-like activities that can be distinguished according to the supernatant, antigen, and/or B cell responders used. The determination of whether this heterogeneity is due to the number of molecular components of a single "type" of TRF required for each antigen-induced PFC response or to the existence of distinct "types" of TRF is examined.