Cognate interaction between T helper cells and B cells. VII. Role of contact and lymphokines in the expression of germ-line and mature gamma 1 transcripts.

Complete reconstitution of Th cell function for B cell growth and differentiation was provided by plasma membranes (PM) derived from activated Th (PMAct) in combination with IL-4 and IL-5 (IL-4/IL-5). IL-5 has been shown to be essential for the secretion of all Ig isotypes by resting, conventional B cells activated by PMAct and IL-4. It was shown that in the presence of PMAct/IL-4/IL-5, a high frequency of resting B cells differentiated to express IgG1. IL-4 alone transiently induced the expression of germ-line gamma 1 transcripts. PMAct alone were ineffective at inducing germ-line gamma 1 transcript expression by resting B cells suggesting that Th-B cell contact was an insufficient signal to cause a detectable increase in the steady-state levels of gamma 1 germ-line transcripts. PMAct alone or PMAct/IL-4 did not induce the appearance of transcripts for secreted mu or mature gamma 1. IL-5, in combination with PMAct/IL-4, provided the necessary signal(s) required for the expression of secreted mu and mature gamma 1 transcripts. Therefore, IL-5 appeared to be an important if not essential differentiation factor for conventional B cells that have been activated by cognate help. It appeared that IL-5 promoted the secretion of Ig by inducing the synthesis of mature Ig transcripts.