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)

Membrane-bound and soluble interleu-kin-15 (IL-15)/IL-15 receptor (R) complexes trigger differential transcription factor activation and functions on human hematopoietic progenitors. Indeed, human spleen myofibroblasts (SMFs) are characterized by a novel mechanism of IL-15 trans-presentation (SMFmb [mem-brane-bound]-IL-15), based on the association of an endogenous IL-15/IL-15R complex with the IL-15Rc chains. SMFmb-IL-15 (1) induces lineage-specific signaling pathways that differ from those controlled by soluble IL-15 in unprimed and committed normal progenitors ; (2) triggers survival and proliferation of leukemic progenitors expressing low-affinity IL-15R (M07Sb cells); (3) causes only an antiapoptotic effect on leukemic cells expressing high-affinity receptors (TF1 cells). This behavior is likely due to the IL-15R chain present on these cells that interact with the SMFmb-IL-15, inhibiting signal transducer and transcriptional activator 5 (STAT5) activation. On the other hand, the soluble IL-15/ IL-15R complex (hyper IL-15) displays a dominant pattern of action, activating only those cells expressing low-affinity IL-15R (IL-15Rc). Thus, hyper IL-15 induces antiapoptotic effects on M075b cells and the up-regulation of STAT6 activation on adult peripheral blood (PB) pre-natural killer (NK) committed progenitors. The latter effect using 100-fold concentrations of recombinant (r)-IL-15. In conclusion , SMFmb-IL-15 and soluble IL-15R/ IL-15 complexes seem to play a pivotal role in the control of the survival, proliferation and differentiation of both normal and leukemic circulating progenitors, highlighting new functions of IL-15 and of