Therefore, it was possible that PL2-3 IC elicited a strong TLR9 signal not easily regulated by FcγRIIB. Although TLR9-expressing AM14 cells respond more robustly to DNA fragments enriched for CG dinucleotides than to CG-poor DNA fragments 14, 25, CG-poor DNA fragments can still be bound by TLR9 26. To extend our analysis

to weak TLR9 ligands, normally incapable of promoting AM14 MK-8669 datasheet cell cycle entry, we decided to use IC that contained defined dsDNA fragments derived from CG-poor portions of the genome. CG-poor dsDNA is the prevalent class of DNA found in the mammalian genome, and representative sequences such as sentrin-specific peptidase 1 (SenP1), a 557 fragment containing only four CG dinucleotides routinely induce minimal activation of AM14 B cells 14. CGneg, a sequence completely devoid of CG dinucleotides, was constructed to examine TLR9 specificity, and also fails to promote AM14 B-cell proliferation 11. By contrast, Clone 11 is a 573 bp long dsDNA fragment corresponding to a CG-rich unmethylated sequence found in the promoter region of the murine preribosomal RNA gene complex. Such CG-rich regions, denoted CpG islands, comprise about 2% of the mammalian genome 27. IgG2a IC incorporating Clone 11 are potent activators of AM14 B cells 14. To determine whether IC containing CG-poor

dsDNA fragments could SB203580 activate R2− AM14 B cells, we used IC consisting of 1D4 bound to Bio-SenP1 or Bio-CGneg. As a control for CG-rich DNA, we used 1D4 bound to Bio-Clone 11 IC. Similar to the results obtained with PL2-3, Clone 11 IC-activated R2+ and R2− AM14 B cells had almost identical dose–response curves. However, the R2− AM14 B cells proliferated significantly better than the R2+ AM14 B cells when stimulated with SenP1 or CGneg IC (Fig. 3A). These results indicate that FcRIIB does indeed regulate B-cell responses to endogenous TLR9 ligands; however, its regulatory capacity is only revealed with weak TLR9 ligands. To verify that the enhanced R2− AM14 B-cell response to SenP1 IC was still TLR9-dependent,

we tested the effect of the TLR9 inhibitor, oligodeoxynucleotide (ODN) INH-18, and the control (non-inhibitory) ODN, INH-48 28. The R2− AM14 B-cell responses to SenP1 IC were blocked by INH-18 but not by INH-48 (Fig. 3B). These Clomifene results demonstrate that in the absence of FcγRIIB-mediated inhibition, AM14 B cells respond to otherwise nonstimulatory DNA through a TLR9-dependent mechanism. AM14 B cells respond to RNA-containing IC through coengagement of the BCR and TLR7. TLR7-dependent AM14 B-cell responses to RNA IC are modest when compared with TLR9-dependent responses to CG-rich DNA IC, but can be significantly enhanced by addition of IFNα 18. To determine whether the absence of FcγRIIB promoted AM14 B-cell responses to RNA IC, we stimulated R2+ and R2− AM14 cells with increasing concentrations of the RNA-specific IgG2a mAb BWR4 29.