Novel diversity in IL-4-mediated responses in resting human naive B cells versus germinal center/memory B cells

Recent studies have defined several phenotypic and molecular changes associ ated with the maturation of naive human B cells within the milieu of germin al centers. Although naive B cells serve as natural precursors to germinal center (GC)/memory (M) subpopulations, little is known about the physiologi cal requirements for the survival of the naive B cell pool in the absence o f cell-cell contact or Ag-mediated activation. Because IL-4 induces express ion of several membrane receptors such as CD23 which are uniquely present o n resting human naive B lymphocytes, we hypothesized that these cells might be intrinsically programmed to respond to IL-4 in the absence of cell divi sion. Using buoyant density-dependent isolation and further enrichment by n egative/positive selection of human naive and GC/M subpopulations, we chara cterized cytokine receptor moieties on these cells and analyzed their survi val and growth in the presence of IL-4 or IL-10, Resting naive B cells expr essed significantly higher IL-4 receptor cy-chain on their cell surface tha n the combined GC/M subpopulation. The IL-10 receptor and the IL-2 receptor gammac chain were almost equally expressed on both subpopulations. When cu ltured in vitro, the addition of IL-4, but not IL-IO, protected naive B cel ls from apoptosis in the absence of activation and growth. However, IL-4 ex erted no such effect on resting GC/M B cells. These data support the hypoth esis that IL-4 plays a pivotal role in the survival and maintenance of rest ing human naive B cells.