Cognate interactions between helper T cells and B cells. II. Dissection of cognate help by using a class II-restricted, antigen-specific, IL-2-dependent helper T cell clone

In order to determine the involvement of T-B cell contact vs lymphokine production in mediating B cell cycle entry and progression. Th cell clones 'defective' in lymphokine production were cloned. Th-3.1 is one such clone that required IL-2 to produce significant levels of IL-4 and IFN-γ. Unlike conventional Th clones, Th-3.1 induced B cell proliferation only in the presence of Ag and IL-2. In contrast to the absolute requirement of IL-2 for Th-3.1-induced B cell proliferation, IL-2 was not required for the formation of stable Th-3.1-B cell conjugates or Th-3.1-induced B cell entry into the G₁ phase of the cell cycle. In the absence of IL-2 and under conditions that promoted Th-B cell interactions, Th-3.1 induced 10 to 20% of resting B cells to enter G₁. B cell entry into the cell cycle was not inhibited by antilymphokine mAb or promoted by exogenous lymphokines, suggesting that endogenous lymphokine activity was not required for Th-3.1-induced G₀ to G₁ transition. The data suggested that the IL-2-independent induction of B cells into G₁ by Th-3.1 was a cell contact-dependent event. Direct proof that Th-3.1-B cell contact was necessary for B cell cycle entry was provided by comparative in situ analysis of the RNA synthetic activity and the RNA content of B cells that were in physical contact with Th-3.1 or not in contact with Th-3.1. In situ autoradiography of RNA synthesis illustrated that a high frequency of B cells in contact with Th-3.1 expressed heightened RNA synthetic activity, whereas 'bystander' B cells were less frequently induced into cycle. In situ laser cytometry of B cell size and total RNA content showed that B cells in physical contact with Th-3.1 had a higher RNA content and were larger than 'bystander' B cells present in the same microcultures. This model system has allowed the dissection of T cell help into IL-2-dependent and IL-2-independent phases. Early cell contact-dependent events and B cell cycle progression into G₁ were IL-2 independent, whereas the production of lymphokines (IL-4, IFN-γ) by Th-3.1 and Th-3.1-induced B cell proliferation was IL-2 dependent.