The Effects of Antigen Valency and CpG ODN on B Cells

Abstract

B cells express toll-like receptor 9 (TLR9), that recognizes microbial DNA containing unmethylated cytosyl guanosyl (CpG) sequences, induces innate immune responses and facilitates antigen-specific adaptive immunity. Studies indicate that in addition to stimulating innate immunity, TLR9 ligands can induce apoptosis in TLR9 expressing cancer cells. To understand the mechanism for TLR9-induced apoptosis, we compared the effects of CpG containing oligodeoxynucleotides (CpG ODN) on mouse primary, splenic B cells and a mouse lymphoma B cell line, CH27. CpG ODN stimulated the proliferation of primary B cells but inhibited cell proliferation and induced apoptosis in CH27 lymphoma B cells in a sequence-specific, TLR9-dependent fashion. While CpG ODN induced sustained activation of NF-B and increase in c-myc protein levels in primary B cells, NF-B activation was transient in the lymphoma B cells. These data suggest that the differential effects of CpG DNA on primary and lymphoma B cells occur due to differences in NF-B activation. The CpG ODN-induced impaired NF-B activation in the lymphoma B cells results in an imbalance between NF-B and c-myc activities, inducing apoptosis in TLR9-expressing B lymphoma cells.

The B cell antigen receptor (BCR) binds to antigens in their native form. The BCR can distinguish subtle differences in antigen structure and trigger differential responses. Here, we analyzed the effects of antigen valency on the functions of the BCR using three different antigen systems - anti-BCR antibody -based antigens, phosphorylcholine (PC)-based antigens, and hen egg lysozyme (HEL)-based antigens. While both paucivalent and polyvalent antigens induced the redistribution of surface BCR into microdomains, polyvalent antigen-induced BCR microdomains persisted. Significantly, this trend was consistently observed in all three antigen systems studied. Ganglioside GM1, tyrosine-phosphorylated proteins and phosphorylated ERK colocalized with BCR microdomains, suggesting these function as surface signaling microdomains. Co-receptor, CD19 and MHC class II molecules, but not CD45 and transferrin receptor, concentrated in the BCR surface microdomains. Prolonged BCR caps were also concomitant with a reduction in BCR movement to late endosomes/lysosomes. Thus, antigen valency influences B cell responses by modulating the stability of BCR-signaling microdomains and BCR-mediated antigen transport.