JCV-specific CD4 T-cell responses appear to play a critical role in the control of JCV infection, preventing PML development. Such responses can be restored in PML survivors following effective and prolonged antiretroviral therapy.

Two functionally distinct subsets of B cells that produce Th1- and Th2-like patterns of cytokines have recently been identified. Interleukin-12 (IL-12) is a critical immunoregulatory cytokine that promotes Th1 differentiation through activation of signal transducer and activator of transcription 4 (STAT4). IL-12 has been reported to induce interferon γ (IFN-γ) production in B cells, but the relevant signaling pathways are poorly documented. Here, in human primary B cells, we found a functional IL-12 receptor (IL-12R) that internalizes following IL-12 binding. IFN-γ and, to a lesser extent, IL-12 positively regulated the IL-12Rβ2 subunit but had no effect on IL-12Rβ1. On examining the effect of IL-12 on STAT4 and T-bet (2 key factors involved in IFN-γ promoter activation), we found that IL-12 induced the phosphorylation and nuclear translocation of STAT4. IL-12-dependent constitutive STAT4 activation was also observed in the Epstein-Barr virus (EBV)-transformed B-cell line RPMI 8866 that spontaneously produces IL-12. T-bet expression has been shown to be dependent on STAT1. IL-12 had no direct effect on STAT1 activation or T-bet expression in primary B cells. In contrast, IL-12-induced IFN-γ led to STAT1 activation, strong expression of T-bet, and IFN-γ expression. IL-12 therefore initiates a cascade of events in B cells, including STAT4 activation, IL-12Rβ2 up-regulation, IFN-γ production, and T-bet up-regulation, potentially leading to Th1-like differentiation. (Blood. 2003;102:4084-4089)

B-cells can contribute to the pathogenesis of autoimmune diseases not only through auto-antibody secretion but also via cytokine production. Therapeutic depletion of B-cells influences the functions and maintenance of various T-cell subsets. The mechanisms governing the functional heterogeneity of B-cell subsets as cytokine-producing cells are poorly understood. B-cells can differentiate into two functionally polarized effectors, one (B-effector-1-cells) producing a Th-1-like cytokine pattern and the other (Be2) producing a Th-2-like pattern. IL-12 and IFN-γ play a key role in Be1 polarization, but the initial trigger of Be1 commitment is unclear. Type-I-interferons are produced early in the immune response and prime several processes involved in innate and adaptive responses. Here, we report that IFN-α triggers a signaling cascade in resting human naive B-cells, involving STAT4 and T-bet, two key IFN-γ gene imprinting factors. IFN-α primed naive B-cells for IFN-γ production and increased IFN-γ gene responsiveness to IL-12. IFN-γ continues this polarization by re-inducing T-bet and up-regulating IL-12Rβ2 expression. IFN-α and IFN-γ therefore pave the way for the action of IL-12. These results point to a coordinated action of IFN-α, IFN-γ and IL-12 in Be1 polarization of naive B-cells, and may provide new insights into the mechanisms by which type-I-interferons favor autoimmunity.