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Both IL-4 and IL-5 demonstrate B cell growth activity. IL-5 can render a cloned neoplastic B cell line, BCL1-CL-3 cells, responsive to IL-2, whereas IL-4 has no such activity. The response to IL-5 and IL-2 proceeds in two phases: the first phase which clearly depends upon IL-5 is the obvious increase in number of high affinity IL-2R (3.1-fold) with modest increase of low affinity IL-2R (1.2-fold) and gain of the ability of facilitated IL-2-binding and internalization of IL-2, and the second phase which is induced by IL-2 in the IL-5-stimulated CL-3 cells comprises the striking increase of low affinity IL-2R (8.5-fold). Kinetic study has revealed that high affinity IL-2R expressed on CL-3 cells begins to increase at 6 h and reaches to maximum at 12 h after stimulation with IL-5 or IL-5 plus IL-2, whereas low affinity IL-2R expression increases at 18 h and becomes maximal at 24 h after stimulation of CL-3 cells with IL-5 and IL-2. However, in the presence of IL-4, IL-5 cannot induce an increase in number of high affinity IL-2R on CL-3 cells. Thus, CL-3 cells stimulated with the mixture of IL-5 and IL-4 cannot respond to IL-2, and fail to show up-regulated expression of low affinity IL-2R. IL-4 also has a capacity to modestly interfere with the action of IL-2 to up-regulate low affinity IL-2R expression on IL-5-pretreated CL-3 cells. Thus, this monoclonal B cell system provides an excellent model system to define the roles of IL-5 and IL-4 involved in the B cell differentiation and to characterize the properties of B cells competent to IL-2 stimulation and the signal transduction mechanism which operates through IL-2/IL-2R system.

The responsiveness of CD5- and CD5+ B cells of BALB/c and NZB/WF1 mice to various cytokines was examined with respect to their growth and differentiation. BALB/c splenic CD5- B cells required longer incubation with IL-2 or pretreatment with IL-4 to respond to IL-2 by DNA synthesis, whereas NZB/WF1 CD5- B cells were highly competent to IL-2. Flow cytometric analysis demonstrated that NZB/WF1 and BALB/c CD5- B cells had higher and intermediate proportions of B cells positive for IL-2R beta, respectively. On the other hand, BALB/c and NZB/WF1 splenic CD5+ B cells consisted of lower proportion of B cells positive for IL-2R beta than did their corresponding CD5- B cells and grew meagerly in response to IL-2. Peritoneal exudative CD5+ B cells of NZB/WF1 mice lacked IL-2R beta mRNA expression and failed to respond to IL-2. Although both BALB/c and NZB/WF1 CD5- B cells pretreated with anti-IgM, IL-4, and IL-5 responded to IL-2 by DNA synthesis, only BALB/c CD5- B cells developed into IgM-producing cells. Furthermore, BALB/c, but not NZB/WF1 CD5-, B cells pretreated with anti-IgM and IL-5 responded to IL-2 by IgM production without DNA synthesis. Thus, cross-talk between IL-4 and IL-2 operated in the growth responses of both BALB/c and NZB/WF1 splenic CD5- B cells, whereas cross-talk between IL-5 and IL-2 operated only in the differentiation of BALB/c CD5- B cells, providing us with another intriguing functional abnormality of NZB/WF1 B cells.

IL-5 renders BCL1-CL-3 (CL-3) cells responsive to IL-2 by increasing the number of high affinity IL-2R, whereas IL-4 prohibits such action of IL-5 to prepare CL-3 cells responsive to IL-2. Here we have found that genes for p75kDa-IL-2R and p55kDa-IL-2R are differentially regulated by IL-4 and IL-5. Nonstimulated CL-3 cells constitutively express mRNA for p75kDa-IL-2R and p55kDa-IL-2R. IL-5 stimulation principally augments the expression of p75kDa-IL-2R mRNA (4- to 8-fold), although modestly increasing the expression of p55kDa-IL-2R mRNA. Kinetic studies have revealed a maximal increase in p75kDa-IL-2R mRNA expression at 12 h and a decline thereafter, substantiating our previous kinetic study of the expression of high affinity IL-2R after the IL-5 stimulation. By contrast, IL-4 stimulation modestly increases the expression of p75kDa-IL-2R mRNA, whereas markedly reducing the expression of p55kDa-IL-2R mRNA, irrespective of whether CL-3 cells were stimulated with IL-4 alone or together with IL-5 and IL-2. Moreover, addition of IL-4 into the culture containing IL-5 and IL-2 causes striking reduction in the level of J-chain mRNA, which otherwise is markedly induced by stimulation with IL-5 and IL-2. These results clearly illustrate the differential regulation of p75kDa- and p55kDa-IL-2R-gene expression by IL-5 and IL-4, and reinforce our notion that increased expression of high affinity IL-2R induced by IL-5 is responsible for the IL-2 competent state, and decreased expression of p55kDa-IL-2R by IL-4 is responsible for IL-2 unresponsive state.

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