B cell receptor (BCR) signalling plays a critical role in the pathogenesis of chronic lymphocytic leukaemia (CLL) and is triggered via both antigen-dependent and -independent (the latter termed “cell-autonomous BCR signal”) routes (Efremov & Laurenti, 2014). Antigens which stimulate BCR signalling have not yet been fully elucidated but possible molecules include autoantigens generated during apoptosis or oxidation, whilst cell-autonomous BCR signalling involves BCR-BCR complex interactions (Efremov & Laurenti, 2014; Ten Hacken & Burger, 2014). In addition, BCR signalling activates integrin signalling thus enhancing CLL cell adhesion to microenvironment stroma and resulting in increased resistance to apoptosis (Byrd et al., 2014).
The BCR signalling complex consists of membrane immunoglobulin (for antigen recognition) and a heterodimer of CD79a and CD79b proteins. In approximately 50% of cases, the leukaemic BCR is encoded by unmutated IGHV genes correlating with more aggressive CLL, suggesting that the BCR pathway is involved during CLL development and progression (Efremov & Laurenti, 2014). In CLL, several survival pathways associated with BCR signalling are enhanced in the bone marrow and lymph node compartments, e.g. phosphatidylinositide 3-kinase (PI3K), nuclear factor-κB (NF-κB), and extracellular signal-regulated kinase (ERK) pathways (Zhang & Kipps, 2014; Byrd et al., 2014).
BCR activation by antigen binding results in kinase recruitment (e.g. spleen tyrosine kinase [SYK] and the SRC kinase LYN), which phosphorylate immunoreceptor tyrosine-based activation motifs (ITAMs) in the cytoplasmic tails of CD79a and CD79b, thereby recruiting further kinases such as Bruton’s tyrosine kinase (BTK) or PI3K. Activation of BTK and PI3K subsequently results in downstream activation of AKT/mTOR, NF-κB and/or ERK (Figure 1) (Ten Hacken & Burger, 2014; Zhang & Kipps, 2014).
BCR signalling is also enhanced by ZAP-70 protein (ζ-associated protein of 70 kD) which recruits further kinases (Figure 1). ZAP-70 is seen in around 50% of CLL cases, especially in patients with aggressive disease and unmutated IGHV. ZAP-70 may also be recruited and activated by forming a supramolecular complex with CD38, CD49d, CD44, and matrix metalloproteinase-9 (MMP-9) (Figure 1) (Zhang & Kipps, 2014).
BCR signalling may be enhanced via CD38 activation. CD38 is found on CLL cells and is an unfavourable prognostic marker associated with activation and proliferation of the disease. A macromolecular complex on CLL cells comprising CD38, CD49d, CD44 and MMP-9 has been identified which may facilitate interaction between BCR signalling and these molecules. These molecules affect cell migration, invasion and homing, thus increased signalling results in cell growth and survival (Zhang & Kipps, 2014).
Furthermore, some Toll-like receptors (TLR) which are expressed and functional in CLL, when stimulated can enhance or induce activation NF-κB (Figure 1) (Zhang & Kipps, 2014).
BCR signalling may be enhanced by CXCR4 which can also directly interact with CXCL12 to induce calcium mobilisation, activation of PI3K/AKT, ERK, and serine phosphorylation of signal transducer and activator of transcription 3 (STAT3) (Figure 1) (Zhang & Kipps, 2014). The CXCR4 chemokine receptor is highly expressed on blood-derived CLL cells but expression levels are much lower for CLL cells isolated from bone marrow or lymphatic tissue, indicative of chemokine-induced surface-receptor down-regulation. Via BCR signalling down-modulation of CXCR4 chemokine and receptor expression, and binding of chemokines and antigen to the BCR, cell proliferation is promoted (Zhang & Kipps, 2014).
Activation of BCR signalling also stimulates CLL cells to express chemokines (CCL3 and CCL4), which attract additional accessory cells such as regulatory T cells to the microenvironment. CLL cells isolated from lymph nodes express increased levels of CCL3 and CCL4 and it is therefore suggested that BCR signalling also has a role in creating the microenvironment that supports CLL cell growth and/or survival (Zhang & Kipps, 2014).