Saturday, March 29, 2008

The B cell receptor

The B-cell receptor

The B-cell receptor is a multimeric complex formed by the assembly of a surface immunoglobulin homodimer and a non-covalently-bound heterodimer Igα/Igβ (CD79A/CD79B). Low expression of the B-cell receptor is the hallmark of lymphocytes in chronic lymphocytic leukaemia.[23]

The mechanisms accounting for poor expression of the B-cell receptor in chronic lymphocytic leukaemia remain elusive. With the exception of one report of mutation in CD79B,[24] no genetic defects in B-cell-receptor components have been recorded.[25] and [26] By contrast with their poor expression at the membrane level, transcription and intracellular synthesis of components of the B-cell receptor are normal,[26] and [27] but they cannot be assembled and transported from the endoplasmic reticulum to the cell surface because of a folding and glycosylation defect of the μ and CD79A chains, although not of the CD79B chain. Poor expression of the CD22 molecule in B-cell chronic lymphocytic leukaemia cells was also shown to result from a folding defect arising in CD79A.[28]

Most B-cell chronic lymphocytic leukaemia cells express CD5 and IgM/IgD and, thus, have a mantle zone-like phenotype of naive cells that, in normal conditions, express unmutated immunoglobulin genes.[29] However, 50–70% of cases of chronic lymphocytic leukaemia have somatic mutations of IGHV genes,[30] as if they had matured in a lymphoid follicle. Presence or absence of somatic mutations is associated with particular IGHV genes. Specific alleles of the IGHV1-69 gene [31] and the IGHV4-39 gene have an unmutated profile.32 Subsequently, workers have reported [32] that more than 20% of patients with chronic lymphocytic leukaemia carry stereotypic B-cell receptors.[33], [34] and [35] Of note, the IGHV3-21 gene shows strikingly homologous IGHV and IGLV gene rearrangements and is associated with poor prognosis, whether expressed in a mutated or unmutated form.[36] and [37] These results strongly suggest that a common antigen epitope is recognised by these highly homologous molecules. With respect to the epitope recognised, research has shown that unmutated chronic lymphocytic leukaemia cells express highly polyreactive antibodies, whereas most mutated ones do not.[38] and [39] Infections with encapsulated organisms might be a trigger for development of chronic lymphocytic leukaemia, and work has shown that individuals with a history of pneumonia are significantly more likely to develop chronic lymphocytic leukaemia than are people without this history, and that the risk increases with number of attacks.[40] and [41]

When stimulated through the B-cell receptor pathway, the response of chronic lymphocytic leukaemia cells is impaired. Low expression of the B-cell receptor correlates with reduced induction of protein tyrosine kinase activity and defective intracellular calcium mobilisation and tyrosine phosphorylation.[42] Individuals have differing responses to IgM ligation, related to IGHV gene status. Findings of one study showed that chronic lymphocytic leukaemia cells expressing unmutated IGHV genes had a better response in most cases to stimulation via the B-cell receptor than did cells expressing mutated IGHV genes.[43]

Unexpectedly, high amounts of ZAP70—a receptor-associated protein tyrosine kinase usually found in T cells and natural killer cells but not in normal circulating B cells—are detected in most patients with unmutated chronic lymphocytic leukaemia.[44] Chronic lymphocytic leukaemia B cells that express ZAP70 are more likely to respond to IgM crosslinking with increased tyrosine phosphorylation and calcium flux than are those that do not express ZAP70. This effect might happen for any or all of the following reasons. First, after B-cell receptor ligation, ZAP70 undergoes tyrosine phosphorylation and becomes associated with surface immunoglobulin and CD79B.[45] Second, ZAP70 mediates inhibition events that terminate the signalling response.[46] Finally, ZAP70 expression is associated with advantageous survival responses because of enhanced access to proliferation centres.[47] Altogether, expression of ZAP70 in chronic lymphocytic leukaemia allows effective IgM signalling in B cells, which might lead to an aggressive disease course.

The apparently anomalous expression of ZAP70 in chronic lymphocytic leukaemia cells is not accounted for completely. Heat-shock protein 90 (HSP90) is a molecular chaperone that catalyses the conformational maturation of many signalling proteins in cancer, known collectively as clients. With inhibitors of HSP90, Castro and colleagues [48] showed that ZAP70 is a client protein in tumour cells, but not in T cells, from patients with ZAP70-positive chronic lymphocytic leukaemia, suggesting that the presence of ZAP70 is an oncogenic event. On the other hand, ZAP70 is expressed at various stages of B-cell maturation and in other B-cell malignant diseases. It is present in normal pre-B cells and pro-B cells and in acute leukaemias derived from them.[49] By studying normal tonsillar cells, Nolz and coworkers [50] and Cutrona and colleagues [51] detected ZAP70-positive B cells, concentrated particularly in germinal centres. ZAP70 seems to be coexpressed with other activation markers. In chronic lymphocytic leukaemia, higher amounts of ZAP70 are expressed by lymph-node cells than by circulating cells.[52] In turn, high levels of ZAP70 expression lead to increased sensitivity to chemokine migratory signals.[53] Whether expression of ZAP70 in chronic lymphocytic leukaemia cells is a result of frequent visits to proliferation centres or is the cause of enhanced access to them is still not clear.

Another unexpected molecule expressed by a subset of chronic lymphocytic leukaemia B cells is CD38. In the B-cell compartment, CD38 is not a lineage marker, but this molecule is expressed at times during B-cell development when cell-to-cell interactions are crucial.[54] Examples include an early bone-marrow precursor cell, cells in the germinal centre, and plasma cells.[55] In chronic lymphocytic leukaemia, expression of CD38 predominates in patients with unmutated IGHV genes and is associated with poor prognosis.18 Expression of CD38 in chronic lymphocytic leukaemia B cells favours B-cell growth and survival through sequential interactions between CD38 and CD31 and between CD100 and plexin B1 (PLXNB1).[56]

The activation-induced cytidine deaminase (AICDA), a B cell-restricted enzyme needed for somatic mutation and isotype switching, is upregulated in unmutated chronic lymphocytic leukaemia cells.[57], [58] and [59] Although evidence exists that AICDA expression could be confined to a small proportion of the clone,[60] AICDA seems to be functional, since unmutated cases of chronic lymphocytic leukaemia can generate isotype-switched transcripts and proteins and mutations in the pre-switch μ region.[57] Upregulation of AICDA could be associated with loss of target specificity, resulting in mutations in non-immunoglobulin genes such as BCL6, MYC, PAX5, and RHOH, which are linked to aggressive disease.[61] and [62]


23 F Vuillier, G Dumas and C Magnac et al., Lower levels of surface B-cell-receptor expression in chronic lymphocytic leukemia are associated with glycosylation and folding defects of the mu and CD79a chains, Blood 105 (2005), pp. 2933–2940.

24 AA Thompson, JA Talley and HN Do et al., Aberrations of the B-cell receptor B29 (CD79b) gene in chronic lymphocytic leukemia, Blood 90 (1997), pp. 1387–1394.

25 B Payelle-Brogard, C Magnac, FR Mauro, F Mandelli and G Dighiero, Analysis of the B-cell receptor B29 (CD79b) gene in familial chronic lymphocytic leukemia, Blood 94 (1999), pp. 3516–3522.

26 A Alfarano, S Indraccolo and P Circosta et al., An alternatively spliced form of CD79b gene may account for altered B-cell receptor expression in B-chronic lymphocytic leukemia, Blood 93 (1999), pp. 2327–2335.

27 B Payelle-Brogard, C Magnac, A Alcover, P Roux and G Dighiero, Defective assembly of the B-cell receptor chains accounts for its low expression in B-chronic lymphocytic leukaemia, Br J Haematol 118 (2002), pp. 976–985.

28 B Payelle-Brogard, G Dumas, C Magnac, AI Lalanne, G Dighiero and F Vuillier, Abnormal levels of the alpha chain of the CD22 adhesion molecule may account for low CD22 surface expression in chronic lymphocytic leukemia, Leukemia 20 (2006), pp. 877–878. )

29 V Pascual, YJ Liu, A Magalski, O de Bouteiller, J Banchereau and JD Capra, Analysis of somatic mutation in five B cell subsets of human tonsil, J Exp Med 180 (1994), pp. 329–339.

30 HW Schroeder Jr and G Dighiero, The pathogenesis of chronic lymphocytic leukemia: analysis of the antibody repertoire, Immunol Today 15 (1994), pp. 288–294.

31 TJ Kipps and DA Carson, Autoantibodies in chronic lymphocytic leukemia and related systemic autoimmune diseases, Blood 81 (1993), pp. 2475–2487.

32 N Chiorazzi and M Ferrarini, B cell chronic lymphocytic leukemia: lessons learned from studies of the B cell antigen receptor, Annu Rev Immunol 21 (2003), pp. 841–894.

33 K Stamatopoulos, C Belessi and C Moreno et al., Over 20% of patients with chronic lymphocytic leukemia carry stereotyped receptors: pathogenetic implications and clinical correlations, Blood 109 (2007), pp. 259–270.

34 BT Messmer, E Albesiano and DG Efremov et al., Multiple distinct sets of stereotyped antigen receptors indicate a role for antigen in promoting chronic lymphocytic leukemia, J Exp Med 200 (2004), pp. 519–525.

35 F Ghiotto, F Fais and A Valetto et al., Remarkably similar antigen receptors among a subset of patients with chronic lymphocytic leukemia, J Clin Invest 113 (2004), pp. 1008–1016.

36 G Tobin, U Thunberg and A Johnson et al., Somatically mutated Ig V(H)3-21 genes characterize a new subset of chronic lymphocytic leukemia, Blood 99 (2002), pp. 2262–2264.

37 M Thorselius, A Krober and F Murray et al., Strikingly homologous immunoglobulin gene rearrangements and poor outcome in VH3-21-using chronic lymphocytic leukemia patients independent of geographic origin and mutational status, Blood 107 (2006), pp. 2889–2894.

38 O Pritsch, C Magnac, G Dumas, C Egile and G Dighiero, V gene usage by seven hybrids derived from CD5+ B-cell chronic lymphocytic leukemia and displaying autoantibody activity, Blood 82 (1993), pp. 3103–3112.

39 M Herve, K Xu and YS Ng et al., Unmutated and mutated chronic lymphocytic leukemias derive from self-reactive B cell precursors despite expressing different antibody reactivity, J Clin Invest 115 (2005), pp. 1636–1643.

40 T Hamblin, Is chronic lymphocytic leukemia a response to infectious agents?, Leuk Res 30 (2006), pp. 1063–1064.

41 O Landgren, JS Rapkin and NE Caporaso et al., Respiratory tract infections and subsequent risk of chronic lymphocytic leukemia, Blood 109 (2007), pp. 2198–2201.

42 F Michel, H Merle-Beral, E Legac, A Michel, P Debre and G Bismuth, Defective calcium response in B-chronic lymphocytic leukemia cells: alteration of early protein tyrosine phosphorylation and of the mechanism responsible for cell calcium influx, J Immunol 150 (1993), pp. 3624–3633.

43 S Lanham, T Hamblin, D Oscier, R Ibbotson, F Stevenson and G Packham, Differential signaling via surface IgM is associated with VH gene mutational status and CD38 expression in chronic lymphocytic leukemia, Blood 101 (2003), pp. 1087–1093.
44 A Rosenwald, AA Alizadeh and G Widhopf et al., Relation of gene expression phenotype to immunoglobulin mutation genotype in B cell chronic lymphocytic leukemia, J Exp Med 194 (2001), pp. 1639–1647.

45 L Chen, J Apgar and L Huynh et al., ZAP-70 directly enhances IgM signaling in chronic lymphocytic leukemia, Blood 105 (2005), pp. 2036–2041.

46 S Gobessi, L Laurenti, PG Longo, S Sica, G Leone and DG Efremov, ZAP-70 enhances B-cell-receptor signaling despite absent or inefficient tyrosine kinase activation in chronic lymphocytic leukemia and lymphoma B cells, Blood 109 (2007),

47 SJ Richardson, C Matthews and MA Catherwood et al., ZAP-70 expression is associated with enhanced ability to respond to migratory and survival signals in B-cell chronic lymphocytic leukemia (B-CLL), Blood 107 (2006), pp. 3584–3592.

48 JE Castro, CE Prada and O Loria et al., ZAP-70 is a novel conditional heat shock protein 90 (Hsp90) client: inhibition of Hsp90 leads to ZAP-70 degradation, apoptosis, and impaired signaling in chronic lymphocytic leukemia, Blood 106 (2005), pp. 2506–2512.

49 M Crespo, N Villamor and E Gine et al., ZAP-70 expression in normal pro/pre B cells, mature B cells, and in B cell acute lymphoblastic leukemia, Clin Cancer Res 12 (2006), pp. 726–734.

50 JC Nolz, RC Tschumper, BT Pittner, JR Darce, NE Kay and DF Jelinek, ZAP-70 is expressed by a subset of normal human B-lymphocytes displaying an activated phenotype, Leukemia 19 (2005), pp. 1018–1024.

51 G Cutrona, M Colombo and S Matis et al., B lymphocytes in humans express ZAP-70 when activated in vivo, Eur J Immunol 36 (2006), pp. 558–569.

52 J Boelens, J Philippe and F Offner, B cells from lymph nodes express higher ZAP-70 levels than B-CLL cells from peripheral blood, Leuk Res 31 (2007), pp. 719–726.

53 SJ Richardson, C Matthews and MA Catherwood et al., ZAP-70 expression is associated with enhanced ability to respond to migratory survival signals in B-cell chronic lymphocytic leukemia (B-CLL), Blood 107 (2006), pp. 3584–3592.

54 F Malavasi, A Funaro, S Roggero, A Horenstein, L Calosso and K Mehta, Human CD38: a glycoprotein in search of a function, Immunol Today 15 (1994), pp. 95–97.

55 S Deaglio, K Mehta and F Malavasi, Human CD38: a (r)evolutionary story of enzymes and receptors, Leuk Res 25 (2001), pp. 1–12.

56 S Deaglio, T Vaisitti and L Bergui et al., CD38 and CD100 lead a network of surface receptors relaying positive signals for B-CLL growth and survival, Blood 105 (2005), pp. 3042–3050.

57 P Oppezzo, F Vuillier and Y Vasconcelos et al., Chronic lymphocytic leukemia B cells expressing AID display dissociation between class switch recombination and somatic hypermutation, Blood 101 (2003), pp. 4029–4032.

58 P Oppezzo, G Dumas and AI Lalanne et al., Different isoforms of BSAP regulate expression of AID in normal and chronic lymphocytic leukemia B cells, Blood 105 (2005), pp. 2495–2503.

59 H McCarthy, WG Wierda and LL Barron et al., High expression of activation-induced cytidine deaminase (AID) and splice variants is a distinctive feature of poor prognosis chronic lymphocytic leukemia, Blood 101 (2003), pp. 4903–4908.

60 E Albesiano, BT Messmer, RN Damle, SL Allen, KR Rai and N Chiorazzi, Activation induced cytidine deaminase in chronic lymphocytic leukemia B cells: expression as multiple forms in a dynamic, variably sized fraction of the clone, Blood 102 (2003), pp. 375–382.

61 SS Sahota, Z Davis, TJ Hamblin and FK Stevenson, Somatic mutation of bcl-6 genes can occur in the absence of V(H) mutations in chronic lymphocytic leukemia, Blood 96 (2000), pp. 1089–1095.

62 L Reininger, C Bodor and A Bognar et al., Richter's and prolymphocytic transformation of chronic lymphocytic leukemia are associated with high mRNA expression of activation-induced cytidine deaminase and aberrant somatic hypermutation, Leukemia 20 (2006), pp. 1089–1095.


Anonymous said...

A few years ago, my onc doc was very excited about a drug that was supposed to work against ZAP-70 positive cells. He said it might even be a good thing to be ZAP-70 positive.

Now, I never hear about such a drug.

Was there ever a drug that would work against ZAP-70 positive CLL cells? What happened to it, if there was?

(I have lost all enthusiasm when I hear 'doctors are excited about such and such.' Most of these 'exciting' drug amount to exactly zero.

I think it is cruel to get patient's hopes up when there is absolutely nothing behind the 'excitement'.)

Terry Hamblin said...

You are thinking about the geldanamycin derivatives, 17-AAG and 17-DAG which are still in clinical trials. See

Anonymous said...

This is the first time i've heard that a history of pneumonia is associated with a higher risk of developing CLL. Why is this never listed as a risk factor for CLL.

John Liston