The investigator notes that IL-4 regulates lymphocyte homeostasis by protecting cells from apoptosis and also by acting as a costimulant for cellular proliferation. Previous studies by this investigator demonstrated that in the IL-3-dependent myeloid progenitor cell type 32D, IL-4 signal transduction involves a member of the insulin receptor substrate (IRS) family. In preliminary studies from this application, the investigator has found that normal B-cells express low levels of IRS-2 message but none for IRS -1. Additional evidence is provided for IL -4 induced tyrosine phosphorylation of IRS -2 in small resting B- cells and in LPS or CD40 ligand stimulated blasts. This phosphorylated IRS -2 molecule co-precipitates with the p85 subunit of PI-3 kinase in both resting and activated B-cells. In contrast, phosphorylated IRS-2 was not found to be associated with Grb2 in resting B-cells, but only in those cells activated via LPS or CD40 ligand. The investigator interprets these results to indicate that polyclonal B-cell activators can alter IL-4 induced signal transduction pathways and suggests the hypothesis that the IRS/PI-3 kinase pathway is linked to the IL -4 induced protection from apoptosis, while the IRS/Grb2 pathway may be linked to proliferation. The overall goal of this application is to determine the role of IRS-mediated signaling pathways in regulating B- cell survival and proliferation. Three specific aims are presented to approach this goal. The first is to determine the basis for IL -4 induced signaling differences in resting versus activated B-cells. The second specific aim for this application is to determine the role of IRS/PI-3 kinase and the IRS/Grb2 pathways in signaling B-cell survival and proliferation by expression of IRS-2 on splenic B-cell survival and proliferation. Collectively, the investigator hopes that the studies described in this application will increase our understanding of how IL- 4 signals, its biologic responses, and specifically how it may use the IRS family of signal transducers to serve these functions.