B-Cell Receptor Signaling in Diffuse Large B-Cell Lymphoma: Tonic Alone in the Germinal Center B-Cell Subtype, Plus Self Antigen-Induced in the Activated B-Cell Subtype

Introduction. Targeting BCR signaling with the BTK inhibitor ibrutinib is clinically effective against most B-cell lymphomas, including the activated B-cell (ABC) subtype of diffuse large B-cell lymphoma (DLBCL), but not the germinal center B-cell (GCB) subtype. Active BCR signaling in GCB-DLBCL was suggested by studies with a Syk inhibitor and our previous studies using BCR knockout (KO). We addressed these questions: why is the BCR active in DLBCL, and how does it signal in GCB-DLBCL?

Methods. We used CRISPR/Cas9 technology to modify selected genes by KO or homologous recombination-mediated knock-in (KI). For some genes KI was used to express a fluorescent protein (FP; e.g., GFP) instead of the targeted gene (KI/KO), or to modify the targeted gene together with KI of an FP, for detection of modified cells.

Results. In GCB lines (OCI-Ly7 and OCI-Ly19) and ABC lines (U2932 and HBL-1), we simultaneously replaced the hypervariable region (HVR) exons of both immunoglobulin heavy (IgH) and light chains (IgL) with HVR sequences from normal B cells recognizing tetanus toxoid (TT). GFP and CFP respectively marked KI of IgH and IgL HVRs, and KI of the endogenous HVR sequences in each line served as controls. In CFP+/GFP+ cells, the TT specific BCR (TT-BCR) was expressed at similar or higher levels than the endogenous BCR (endo-BCR) and was functional, as shown by calcium flux in response to TT. The TT-BCR maintained growth of GCB lines (Fig. 1), indicating that they use “tonic”, antigen-independent BCR signaling. Other features of tonic signaling were confirmed in more GCB lines: 1) the toxicity of BCR KO, which eliminates AKT S473 phosphorylation, was rescued by PTEN KO or expression of constitutively active AKT (mAKT), showing that BCR signaling serves principally to activate PI3K/AKT; and 2) KO of SYK or CD19, or truncation or ITAM mutation of the cytoplasmic tail of CD79A, none of which affect surface BCR levels, were as toxic as BCR KO but were non-toxic in BCR/PTEN double-KO cells.

In contrast, the TT-BCR was as growth-slowing as BCR KO to the ABC line U2932 (Fig. 1), and substantially toxic to HBL-1, indicating that BCR signaling is self antigen-dependent in ABC-DLBCL. Reversion of somatic hypermutations in the U2932 HVRs was also as growth-slowing as BCR KO (Fig. 1), suggesting that self-antigen reactivity developed during BCR affinity maturation. Tonic signaling by the TT-BCR provided a detectable benefit (as compared to BCR KO) in PTEN-expressing HBL-1, whereas there was no difference between TT-HVR BCR and BCR KO in PTEN-deficient U2932. The surface TT-BCR level was higher than the endo-BCR level in ABC lines, and dropped with TT stimulation, suggesting that endo-BCRs in ABC lines undergo constant antigen stimulation with BCR internalization. The presumed self-antigen in ABC lines seems to be cell line-specific, since HVRs from ABC lines TMD8 and HBL-1 did not rescue growth of U2932. BCR KO in ABC lines was also not rescued by PTEN KO or mAKT. In cells whose BCRs were labeled by KI to fuse GFP to CD79A, super-resolution microscopy showed macro-clustering of BCR complexes at the surface of ABC line HBL-1, not seen in GCB lines (Fig. 2).

Several findings suggested the clinical potential of targeting tonic BCR signaling in DLBCL: 1) clinical trial-stage inhibitors of SYK (P505-15) and PI3K (idelalisib) were toxic to GCB lines (less so with PTEN KO); 2) GCB lines (6/8) were sensitized by BCR KO to an in vitro CHOP-like regimen; 3) P505-15 or idelalisib sensitized GCB lines (3/3) to CHOP in vitro; and 4) evidence of tonic signaling in ABC line HBL-1 after removing antigen-driven signaling by HVR replacement.

Conclusion. The BCR provides antigen-independent tonic signals to activate PI3K/AKT in GCB-DLBCL and antigen-dependent signaling in ABC-DLBCL. Targeting of B-cell specific tonic signling alone or in combination could be clinically effective in both types of DLBCL.

 

Fig. 1. Effect of BCR KO or HVR replacement in OCI-LY19 (A) and U2932 (B) cell lines. Endogenous IgH and IgL HVRs were replaced with HVR pairs (TT3 and/or TT6) recognizing tetanus toxoid, reverted to undo the effect of SHM, or restored with original HVRs.

 

Fig. 2. Representative super-resolution images of BCR localization in live DLBCL cells. BCR labeled by CD79A-GFP fusion, surface membrane by CellMask staining. (bars = 5µm)