16–19 The innate A3G response selleck screening library is surprisingly long-lasting following immunization in macaques20,21 and this has been attributed to A3G being expressed in CD4+ CD95+ CCR7− effector memory T cells.20 Up-regulation of A3G stimulated
by CD40L is mediated by ligation of CD40 cell-surface molecules on dendritic cells22 and this is also likely to account for A3G regulation in B cells expressing CD40. However, B-cell-derived A3G in vivo has not been studied previously. The signalling pathway following engagement of CD40 by CD40L elicits phosphorylation of IκB kinase complex followed by nuclear translocation of nuclear factor-κB (NF-κB), which initiates class switch recombination by binding to the κB site on IH promoters.23,24
CD40L-bound CD40 also activates extracellular signal-regulated kinase 1/2 and p38 mitogen-activated protein kinase inducing A3G mRNA and protein expression.22 Interleukin-4 bound to IL-4 receptor induces phosphorylation of Jak1 and Jak3 kinases, followed by phosphorylation and nuclear translocation of the transcription factor signal transducer and activator of transcription (STAT6) leading to class switch recombination.24 Transforming growth factor-β is another B-cell agonist critical in switching IgM to IgA.25 We have pursued a report that appeared after we had completed the project that the AID encoding gene (Aicda) responds to activation with CD40L, IL-4 and TGF-β.26 We confirmed this using human B cells, which showed maximal activation of AID mRNA with the combined three agents selleck kinase inhibitor (2665 ± 1150), compared with TGF-β alone (80·5 ± 18) and extended it to A3G mRNA from 118 ± 45 to 495 ± 88 (P = 0·030) (data not presented). Thalidomide Flow cytometry studies also demonstrated a significant increase in AID expression by the combined TGF-β + CD40L + IL-4-stimulated B cells. The mechanism advanced26 was that region 4 of the AID encoding gene (Aicda) contains the functional binding sites for NF-κB, STAT6 and Smad
3/4, which are response elements to CD40L, IL-4 and TGF-β, respectively.26 This may lead to de-repression of silencers by B-lineage-specific and stimulation-responsive enhancers. Whether this mechanism might also apply to A3G, another deaminase belonging to the same family produced by B cells, needs to be verified. We postulate that A3G produced by B cells is transmitted to CD4+ T cells probably via exosomes, in which A3G is a major component.10 B cells are significant producers of exosomes following activation of cell-surface CD40 and IL-4 receptors27 or interaction with T cells via CD40–CD40L molecules.28 Inhibition of HIV replication has been demonstrated between monocyte-derived exosomes and CD4+ T cells.9 Alternatively, B cells might produce intercellular nanotubes which establish contact with CD4+ T cells.