5). Regulatory elements that predispose for TLR-mediated RAG-1 promoter activation include binding sites for interferon regulatory factors, activating AZD9291 nmr protein-1, signal transducer and activator of transcription and myc transcription factors.[39] Interestingly, in murine B cells myc is induced upon TLR9 stimulation via protein kinase B (PKB)/Akt.[41] Accordingly, PKB/Akt-mammalian target of rapamycin signalling is indispensible for CpGPTO-induced human B-cell blast formation, proliferation, IL-6 production and differentiation,[17] and may therefore
directly or indirectly contribute to re-expression of RAG as proposed in ref. [42]. Moreover, nuclear factor-κB signalling, another important effector pathway of TLR9, is considered an important regulator of RAG expression[43] and a decisive promoter of secondary LC rearrangement.[44] TLR9-induced RAG re-expression and LC rearrangement may, therefore, result from coordinated PKB/Akt and NF-κB signalling. In a previous study we demonstrated that stimulation with CpGPTO selectively drives IgM+ B cells into a prolonged proliferative state.[17] As shown in Fig. 7(c) the Ku-0059436 mw presence of the TLR9-specific
CpG motif is critical for the induction of proliferation. This proliferative burst may, however, also counteract RAG-2 expression, because RAG-2 expression was lately shown to correlate with expression of p27kip, a cell cycle inhibitor.[6] However,
ongoing DNA synthesis requires post-replicative DNA repair, and availability of Ku70/80 and other NHEJ enzymes could facilitate RAG-dependent receptor revision. Taking into consideration that PKB/Akt induces proliferation and simultaneously blocks receptor editing via inactivation of FOXO transcription factors in pre-B cells,[45] we suggest that initial PKB/Akt-dependent proliferation triggers RAG-1 expression while gradual ceasing of proliferation and onset of differentiation may evoke FOXO-mediated RAG-2 expression. Finally, receptor revision, i.e. secondary Dichloromethane dehalogenase LC rearrangement, may be accomplished in a nuclear factor-κB-dependent manner.[44] Future investigations will have to prove this model correct. In this study we reasoned that a BCR signal must precede receptor revision and therefore postulated that CpGPTO either activates or mimics BCR signalling. This hypothesis was supported by the finding that inhibition of syk and lyn kinases, molecules essential for proximal BCR signalling, affects the response to CpGPTO (Fig. 7). However, these assays cannot distinguish whether the kinases are recruited as a consequence of BCR engagement by CpGPTO, act downstream of TLR9 (thereby circumventing the requirement for BCR engagement) or synergistically interconnect both pathways. Previous reports indicate that, indeed, both PTO-modified ODN and multivalent DNA aptamers engage surface IgM.