Role of Specific B-Cell Receptor Antigens in Lymphomagenesis

Abstract

The B-cell receptor (BCR) signaling pathway is a crucial pathway of B cells, both for their survival and for antigen-mediated activation, proliferation and differentiation. Its activation is also critical for the genesis of many lymphoma types. BCR-mediated lymphoma proliferation may be caused by activating BCR-pathway mutations and/or by active or tonic stimulation of the BCR. BCRs of lymphomas have frequently been described as polyreactive. In this review, the role of specific target antigens of the BCRs of lymphomas is highlighted. These antigens have been found to be restricted to specific lymphoma entities. The antigens can be of infectious origin, such as H. pylori in gastric MALT lymphoma or RpoC of M. catarrhalis in nodular lymphocyte predominant Hodgkin lymphoma, or they are autoantigens. Examples of such autoantigens are the BCR itself in chronic lymphocytic leukemia, LRPAP1 in mantle cell lymphoma, hyper-N-glycosylated SAMD14/neurabin-I in primary central nervous system lymphoma, hypo-phosphorylated ARS2 in diffuse large B-cell lymphoma, and hyper-phosphorylated SLP2, sumoylated HSP90 or saposin C in plasma cell dyscrasia. Notably, atypical posttranslational modifications are often responsible for the immunogenicity of many autoantigens. Possible therapeutic approaches evolving from these specific antigens are discussed.

Keywords: B-cell receptor; antigen; antigens of infectious origin; autoreactivity; lymphoma; posttransnational modification.

Figure 1

Contribution of Moraxella catarrhalis to IgD⁺ nodular lymphocyte predominant Hodgkin lymphoma (NLPHL) pathogenesis: costimulation of IgD-positive B cells by Moraxella catarrhalis RNA polymerase beta’ (RpoC) via the Fab fragment and MID/hag via the Fc fragment of the B-cell receptor (BCR). Naive IgD⁺ B cells with a BCR specific for RpoC encounter M. catarrhalis outer membrane vesicles. Binding of RpoC to the Fab and of MID/hag to the Fc of membrane IgD induces activation of RpoC-specific IgD⁺ B cells, which is supported by CD4⁺ T cells particularly in patients with an HLA-DRB1*04 haplotype. The persistent/recurrent presence of M. catarrhalis presumably induces a germinal center (GC) reaction resulting in differentiation of memory B cells and plasmablasts and production of class-switched anti-RpoC serum antibodies and apoptosis of some GC B cells due to disadvantageous mutations. Subsequently, transformation into lymphocyte predominant (LP) cells may occur, accompanied by mutations in proto-oncogenes and tumor suppressor genes, and by chromosomal translocations (e.g. BCL6 translocations).

Figure 2

Examples of posttranslational modification of lymphoma B-cell receptor (BCR) target antigens: (A) Representative Western blot of hyper-N-glycosylated Neurabin-I and SAMD14 in patients with PCNSL first reported by Thurner et al (173). Patients with primary central nervous system lymphoma (PCNSL) and SAMD14/Neurabin-I reactive lymphoma BCRs had exclusively hyper-N-glycosylated isoforms of both antigens. (B) Representative isoelectric focusing (IEF) of hypophosphorylated Arsenite resistance protein 2 (ARS2) in diffuse large B cell lymphoma (DLBCL) first reported by Thurner et al (174). DLBCL cell lines and peripheral blood lysates of DLBCL of patients and family members. ARS2 was found to be hypo-phosphorylated in a cell line and a patient, but this phenotype of an atypical posttranslational modifications (PTM) was not inherited in a Mendelian manner. (C) Representative analysis of hyperphosphorylated SLP2 (paratarg-7), which was first reported by Preuss et al (171). Hyperphosphorylated SLP2 was detected by IEF and not by SDS-PAGE. The hyperphosphorylation introduces an additional phosphate group in the molecule leading to a different isoelectric point of the protein; the increase in molecular weight is too small to be detected by SDS PAGE. Shown are immunoblots incubated with anti-human-STOML2 (paratarg-7). P1-3: MM/monoclonal gammopathy of undetermined significance (MGUS) patients with immunoreactivity against paratarg-7; C1-3: MM/MGUS patients without SLP2 (paratarg-7) immunoreactivity; B1: healthy blood donor. Inheritance: SLP2- phosphorylation state in patient families (example). The family of a relevant patient was analyzed for its SLP2 phosphorylation state by IEF. Carriers of hyperphosphorylated SLP2 (patient and persons of risk) were indicated in red. (D) Representative SDS PAGE of sumoylated HSP90 first reported by Preuss et al (175). Sumoylation of HSP90 does not change the isoelectric point of the molecule but leads to an increase in molecular weight which is detected by SDS PAGE. HD: healthy donor; P_neg: MM/MGUS patient without immunoreactivity against HSP90-SUMO; P_pos: MM/MGUS patients with immunoreactivity against HSP90-SUMO.Inheritance: HSP90 sumoylation state in patient family (example). The family of a relevant patient was analyzed for HSP90-SUMO by SDS PAGE. Carriers of HSP90-SUMO (patient and persons of risk) are indicated in red.

Figure 3

Paratarg-7-specific T-helper cells in myeloma/monoclonal gammopathy of undetermined significance (MGUS) patients with a SLP2-specific paraprotein as a new type of epitope spreading. (A) Representative IFN-g ELISPOTs first reported by Neumann et al. showed by in vitro stimulation for SLP2(paratarg-7)-specific TH1 cells in 9/14 patients specific responses to the antigen. 8/9 patients had a significant (p << 0.05.) stronger response against the phosphorylated variant of the SLP2-peptides used for stimulation (red columns) compared to the peptides derived from the non-phosphorylated wild type (blue columns) (209). (B) T-helper cells from 5/12 patients showed a significant stronger TH2 response against the modified peptides compared to the wild type peptides. Again, these are the results of in vitro stimulation of myeloma/MGUS patients’ T-helper cells with a paratarg-7/SLP2-specific paraprotein using overlapping 15 amino acids long peptides covering the first 30 amino acids of the SLP2 sequence. Wild-type peptides and peptides with a phosphorylated Ser17 of the posttranslational modifications (PTM) variant were used. Subsequently, the culture supernatant was analyzed by ELISA for the TH2 cytokine IL-5 (209). (C) T-helper cells with a T-cell receptor (TCR) specific for Ser17-phosphorylated version of SLP2 are primed by antigen-presenting cells, equipped with the corresponding permissive MHC-II molecules offering all necessary costimulatory signals for full maturation. (D) Subsequently, T-helper cells with these properties (phosphospecific SLP2-reactive) can stimulate all B cells presenting a phosphorylated Ser17 epitope. The specificity of the receptor of these B cells for the hyperphosphorylated isoform of the antigen is not important. Thus, B cells are also stimulated whose BCRs bind the unmodified wild type of SLP2. This form of epitope spreading comprises the same amino acid of the antigen, but with the difference of posttranslational modification. Therefore, this type of spreading is vertically oriented. For some other posttranslationally modified antigens, the lymphoma BCRs are specific for modified isoform/variant depending on the PTM, i.e. HSP90-SUMO.

Figure 4

Schematic overview of the development of normal B cells and their malignant counterparts. Arrows: Normal B-cell differentiation; dotted arrows: normal B-cells as cell of origins of specific lymphomas. This scheme is an adaption of the scheme from Küppers et al., 2005, with added B-cell receptor (BCR) antigens identified in the meantime.

Figure 5

Usage of lymphoma B-cell receptor (BCR) antigens for targeting lymphoma. (A) Forward targeting: Classical antibody therapy cannot differentiate between malignant and benign B cells. mAbs against B cell surface antigens such as CD20 bind to their target present on all B cells and activate complement, antibody-dependent cell-mediated cytotoxicity or direct cell death. The same is also true for antibodies against receptors with immunomodulatory functions. Independently of this, tumor cell death could also be induced antibody-independently by drugs that interfere with critical signaling pathways (such as ibrutinib, which interferes with BTK, a step in the BCR signaling pathway). Ag, antigen. (B) Reverse targeting: The BAR (BCR antigen for reverse targeting) concept is based on the highly specific interaction of a BCR found exclusively on malignant B cells with its highly specific target antigen; benign B cells do not possess this BCR. Synthetic conjugates of BCR antigen with a toxin (BAR toxin) bind exclusively to the malignant cells, are internalized and release the toxin that kills the cell. (C) CARs: Conventional CAR with CD19 scFv/CD28 4-1BB CD3ζ CAR backbone (left); the anti CD19 scFv was exchanged by the frequent BCR antigen of MCL resulting in the construct SAMD14/neurabin-I/CD28 4-1BB CD3ζ CAR backbone (middle) or combined with anti CD19 scFv (right).