1. BCR signaling overview
B cell antigen receptor (BCR) is a multiprotein complex expressed on the surface of B cells which consists of a ligand binding moiety, the antigen-binding transmembrane immunoglobulin (mIg), paired with the signal transduction moiety, Ig-α/Ig-βheterodimers(CD79a/CD79b). The BCR signaling pathway is crucial for proper B-cell development, activation, proliferation, differentiation and consequently for humoral immune response.
2. BCR signaling cascade
Engagement of the BCR by antigen induces membrane movement and aggregation of BCR components that lead to phosphorylation of ITAMs in the cytoplasmic tails of CD79a and CD79b. The latter is accomplished by the SRC family kinase LYN. The phosphorylated ITAMs recruit the spleen tyrosine kinase (SYK) to the receptor, where it becomes activated by phosphorylation of tyrosines, and propagates the signal activation to downstream signaling proteins. Activation of SYK plays a critical role in BCR signaling, initiating the formation of the BCR signalosome, the adaptor proteins, such as CD19 and B-cell linker (BLNK), and Bruton's tyrosine kinases (BTK), and signaling enzymes such as PLCγ2, PI3K, and Vav. Signals emanating from those signalosomes initiate and regulate downstream signaling systems including RAS/RAF/MEK/ERK pathway, which are significant for B-cell fate decisions such as proliferation, survival, differentiation and cell death.
Activation of PLCγ2 leads to the hydrolysis of phospholipid yielding the second messengers, inositol-1,4,5-trisphosphate (IP3) and diacylglycerol (DAG), which induces the release of intracellular calcium and actives the protein kinase Cβ (PKCβ), respectively. Then, calcium-dependent signaling leads to activation of nuclear factor of activation T cell (NFAT), a calcium-dependent transcription factor downstream of the calmodulin/calcineurin pathway involved in the development of mature B cells. On the other hand, activated PKCβ induces the activation of ERK, JNK and p38. Besides, PKCβ activity also induces phosphorylation and subsequent degradation of the NF-κB inhibitor (IκB), and thus activating NF-κB, which has important regulatory functions in B-cell activation, maturation and survival.
Besides being activated via the PLCγ2 pathway in B cells following BCR stimulation, ERK can also be turned on by RAS. It has been recently reported that RasGRP3, a guanine nucleotide–exchange factor, plays an important role in coupling the BCR to RAS activation, in a manner dependent on the PLCγ2/PKCβ pathway. Once activated, RasGRP3 directly binds to RAF-1, the MAP3K in the ERK pathway. Activated RAF-1 subsequently phosphorylates MEK1/MEK2, which both in turn activate ERK1/ERK2. Phosphorylated ERK1/ERK2 form dimers, a step necessary for translocation of ERKs into the nucleus which consecutively initiate transcription of regulatory gens such as fos, jun, that regulate a subset of gens responsible for cell proliferation, survival and differentiation.
Activation of PI3K requires LYN-dependent phosphorylation of CD19, expressed on the surface of B cells during all stages of B-cell ontogeny. Activated PI3K generates PIP3, which mediates membrane recruitment and subsequent activation of many signaling proteins containing pleckstrin homology (PH) domains and is implicated in the propagation of signals downstream of PI3K. Protein kinase AKT, one of the most important downstream effectors of PI3K, can enhance the survival of cell by inactivating the proapoptotic proteins and transcription factors, such as BCL-2 and mTOR. AKT can also induce the expression of prosurvival genes and induce the degradation of IκB, thus allowing NF-κB to activate the expression of several anti-apoptotic genes. Moreover, AKT is known to have profound effects on the regulation of the apoptosis and glucose metabolism by inactivating GSK3. In general, the function of PI3K-AKT signaling pathway plays an essential role in cell proliferation, growth, metabolism, and apoptosis.
3. Negative regulation
The magnitude and duration of the BCR signaling is tightly governed by a dynamic equilibrium between activating and inhibiting mechanisms. Negative regulation of BCR signaling is a very complex process in which LYN, CD22 (the negative BCR co-receptor), and SH2 domain-containing tyrosine phosphatase-1 (SHP-1) are each limiting elements. Signalosomes triggered by the BCR can be modulated by transmembrane receptors such as FcγRIIb1, CD19, and CD22, which associate with the BCR either constitutively or in a ligand dependent. Their inhibitory function relies on their cytoplasmic domain referred to as the immunoreceptor tyrosine-based inhibition motif (ITIM), which is phosphorylated by LYN upon BCR stimulation. Phosphorylated ITIMs recruit phosphatases, such as Src homology 2 (SH2) domain-containing phosphatase-1 (SHP-1) or SH2 domain-containing inositol phosphatase (SHIP), thus arresting BCR signalosomes. Therefore, LYN is responsible for both positive and negative regulation of BCR signaling. Recently, a new immunoregulatory function has been attributed to ITIMs, which can propagate inhibitory signals under specific configurations termed inhibitory ITAM (ITAMi).
4. Relationship with diseases
BCR signaling plays an important pathogenic role in chronic lymphocytic leukemia (CLL) and B cell lymphomas, based on structural restrictions of the BCR as well as BCR-dependent survival and growth of the malignant B cells. Several prognostic markers in CLL, such as IgVH mutational status, zeta-chain-associated protein kinase 70 (ZAP-70) and chemokine (C-C motif) ligand 3 (CCL3), are associated with (auto) antigen binding and function of the BCR, suggesting a relation between enhanced BCR signaling and worse prognosis. Also, BCRs in CLL patients are characterized by a biased usage of IGHV and IGLVk/l genes, which differs from those of normal B cells. Oftentimes, specific IGHVs partner with specific IGHD-Js and specific IGLVs with specific IGLJs, leading to remarkably similar, stereotyped heavy chain complementarity determining region 3 (HCDR3) and smIgs. These researches support the concept of antigen-driven selection and expansion of CLL clones, and they suggest that recurrent binding of restricted sets of antigenic epitopes are linked to the selection of those normal B cell clones entering the CLL pathogenetic process. More evidence for the importance of BCR signaling in CLL comes from comparative GEP data that revealed BCR signaling as the most prominent pathway activated in CLL cells isolated from lymphatic tissues. Along the same lines, cells from those patients with the worse outcome (e.g., unmutated CLL (U-CLL)) display GEPs identifying activation of genes downstream of the BCR.
5. Therapeutic targeting
The significant role of the BCR signaling in promoting B-CLL cell survival has prompted new therapeutic strategies targeted to signalosome elements. In particular, the most advanced BCR-signaling targets are SYK, BTK and PI3Kδ.
R788 (fostamatinib disodium, FosD) is the only SYK inhibitor in clinical use to date. It is a prodrug that is rapidly convertedinto bioactive form, R406, in vivo. Promising results have been obtained in B-CLL using R788 in a Phase I/II study; PCI-32765, the first human BTK inhibitor, can inhibit the BCR, the chemokine receptor-mediated signaling pathways and the secretion of chemokines (CCL3 and CCL4) mediated by BCR activity. It has shown encouraging clinical results in patients with B-cell malignancy, particularly in patients with B-CLL. Besides, GS-1101 (Idelalesib), previously called CAL-101, is a potent and highly selective PI3Kδ inhibitor that is the first PI3Kδ inhibitor in clinical use. GS-1101 induces apoptosis in B cell lines and primary cells from patients with different B cell malignancies, including CLL, MCL, and multiple myeloma.
Over the decades BCR signaling has been studied extensively. Deciphering such a complex signaling pathway composed of hundreds of proteins, stimulating dozens of cell reactions has given a significant insight into understanding the biology of lymphoid malignancies. Following those basic discoveries, a number of new drugs targeting pathologic BCR signaling have appeared and many of them are showing excellent clinical properties.