Germinal center-derived lymphomas: The darkest side of humoral immunity

Abstract

One of the unusual features of germinal center (GC) B cells is that they manifest many hallmarks of cancer cells. Accordingly, most B-cell neoplasms originate from the GC reaction, and characteristically display abundant point mutations, structural genomic lesions, and clonal diversity from the genetic and epigenetic standpoints. The dominant biological theme of GC-derived lymphomas is mutation of genes involved in epigenetic regulation and immune receptor signaling, which come into play at critical transitional stages of the GC reaction. Hence, mechanistic studies of these mutations reveal fundamental insight into the biology of the normal and malignant GC B cell. The BCL6 transcription factor plays a central role in establishing the GC phenotype in B cells, and most lymphomas are dependent on BCL6 to maintain survival, proliferation, and perhaps immune evasion. Many lymphoma mutations have the commonality of enhancing the oncogenic functions of BCL6, or overcoming some of its tumor suppressive effects. Herein, we discuss how unique features of the GC reaction create vulnerabilities that select for particular lymphoma mutations. We examine the interplay between epigenetic programming, metabolism, signaling, and immune regulatory mechanisms in lymphoma, and discuss how these are leading to novel precision therapy strategies to treat lymphoma patients.

Keywords: epigenetic deregulation; germinal center; immune surveillance; lymphomagenesis; precision therapy; signal transduction.

Figure 1

Cell fate decisions in the germinal center (GC) and derived lymphomas. Transient GCs are formed by antigen‐activated mature B cells. (1) In the dark zone, GC B cells proliferate and undergo somatic hypermutation to mutate their immunoglobulin genes. (2) GC B cells transit from the dark zone to the light zone after having divided a determined number of times. (3) In the light zone, cells are selected based on their affinity for the encountered antigen, through their interaction with follicular dendritic cells (FDCs) and follicular T‐helper cells (T_FH). (4) After positive selection, cells can concomitantly activate MYC and mTORC1 anabolic programs to grow in size and recycle back to the dark zone for further mutation and clonal expansion. (5) Selected cells can also exit the GC and differentiate into plasma cells (PC) via the intermediary plasmablast (PB) stage, or (6) differentiate into memory B (MB) cells. (7) Cells that are not selected in the light zone or that are damaged during somatic hypermutation in the dark zone undergo apoptosis. Mutual exclusion of BCL6 expression or activity with MYC/mTORC1 reflects commitment to proliferation vs anabolism and recycling. With IRF4/PRDM1 it reflects maintenance of GC identity vs GC exit through differentiation into PC. The putative relation of GC‐derived B‐cell lymphomas and DLBCL subtypes with their respective GC cell‐of‐origin, based on transcriptional and genetic profiles and other characteristics, is shown. Note that C1/BN2 cases resemble marginal zone B cells and that the C5/MCD cases are similar to the extranodal forms of DLBCL

Figure 2

Proposed epigenetic driver mechanisms in GC B‐cell lymphomas. In the GC reaction, there is transient suppression of enhancers and promoters of genes that regulate immune signaling pathways, antigen presentation, and checkpoints, which revert back to the active state when GC B cells are signaled to exit the GC reaction. Lymphomas arise from failure of GC exit signals to restore expression of these genes through several proposed epigenetic mechanisms: (A) EZH2 is induced in GC B cells and converts H3K4me3 active promoters (green) to H3K4me3/H3K27me3 bivalent promoters (yellow) for transient repression of target genes, which is reversed upon GC exit. EZH2 mutations in lymphoma cause accumulation and permanent silencing (red) of bivalent promoters. (B) CREBBP maintains active enhancers (green) marked by H3K27Ac in mature B cells. In GC B cells, these enhancers are transiently toggled off (yellow) by HDAC3 through H3K27 deacetylation and then restored upon GC exit signaling. In lymphomas with CREBBP loss‐of‐function mutation, HDAC3 is now unopposed to maintain aberrant silencing (red) of these enhancers. (C) KMT2D maintains enhancer activity (green) in mature B cells through H3K4me1 and possibly H3K4me3. In the GC, these enhancers become demethylated, possibly through the actions of KDM1 or KDM5 histone demethylases and these enhancers are reactivated in B cells exiting the GC reaction. KMT2D loss‐of‐function mutations result in persistent demethylation of enhancers and failure of the respective genes to respond to signals. (D) B‐cell enhancers are decorated by the 5hmC activating mark (green) in a TET2‐dependent manner, which is retained (green) in the GC reaction (unlike the histone marks from B or C). TET2‐loss‐of‐function mutation results in failure to maintain enhancer 5hmC and loss of enhancer activating mark H3K27Ac with corresponding repression of the respective genes (red)

Figure 3

Metabolic dysregulation in GC‐derived B‐cell lymphoma. In the mitochondrion, the tricarboxylic acid (TCA) cycle produces reduced NADH and FADH2 that are used by complexes I to V of the electron transport chain (ETC) to generate ATP through oxidative phosphorylation (oxphos). The TCA can be fueled by fatty acid‐ or pyruvate‐derived acetyl‐CoA. Alternatively, glutamine can be used to generate alpha‐ketoglutarate (aKG). DLBCLs have developed dependency on SIRT3 to replenish the TCA cycle (also known as anaplerosis). SIRT3 stimulates glutaminolysis by activating the glutamine dehydrogenase (GDH). Glucose can be converted into pyruvate through glycolysis or used through the pentose phosphate pathway (PPP) to generate ribose 5‐phosphate (R5P) and NADPH. Some GC‐derived B‐cell lymphomas depend on PP2A and G6PD, a key PPP enzyme, to switch glucose carbon usage from glycolysis to the PPP. This provides antioxidant protection and supports ribonucleotide biosynthesis in proliferating cells. Pyruvate can also be converted into lactate as part of the “aerobic glycolysis” that tumor cells use to “bypass” the TCA cycle, to generate some ATP and to create biomass. This is known as the Warburg effect and can be induced by MYC and HIF1‐alpha stabilization in DLBCL and FL. Finally, mTORC1 activation in the GC happens downstream of T cell‐positive selection signals via the PI3K/Akt/mTOR pathway or downstream of nutrient signaling via activation of RagA/C and the v‐ATPase. These components either carry gain‐of‐function (GOF) mutations, are hyper‐activated, or are expressed at high levels in GC‐derived B‐cell lymphomas, resulting in mTORC1 constitutive activation. mTORC1 is the master regulator of anabolism and while constitutive activation is detrimental to GC B cells, it appears to favor lymphoma growth. G6PD, glucose‐6‐phosphate dehydrogenase; ROS, reactive oxygen species; v‐ATPase, vacuolar ATPase proton pump

Figure 4

Aberrant BCR signaling in GC‐derived B‐cell lymphomas. BCR signaling is essential for mature B‐cell survival. Most GC‐derived lymphomas hijack BCR and/or PI3K signaling, including: BL, FL, the ABC‐DLBCL subtypes C5 and MCD, and the GCB‐DLBCL subtypes C3 and EZB. Depicted are the most frequent mutations that promote aberrant BCR/PI3K signaling activation in these lymphomas. Alterations that affect indirect modulators of the PI3K pathway such as PTEN and MIR17HG are more reminiscent of antigen‐independent “tonic” BCR signaling than antigen‐dependent “chronic‐active” BCR signaling. *MYD88 is mutated in ABC‐DLBCL but not in FL. Concurrent mutations of CD79A/B and MYD88 are most common in C5 and MCD lymphomas and less so in other lymphomas. BL, Burkitt lymphoma; FL, follicular lymphoma; DLBCL, diffuse large B‐cell lymphoma

Figure 5

Immune surveillance evasion. Germinal center‐derived B‐cell lymphomas have evolved ways to evade antitumor immunity including escaping from immune recognition, inhibiting immune effector cells and inducing a tumor‐supportive environment. Escape: Loss of MHC II expression caused by CIITA or CREBBP inactivation prevents antigen presentation to CD4+ T cells. Loss of the MHC I component B2M and loss of CD58 prevent interaction with cytotoxic CD8+ T cells and natural killer (NK) cells. Inhibition: Some DLBCLs (C1) harbor gains of the PD‐L1/PD‐L2 locus. Increased PD‐L1/L2 levels result in increased interaction with T cells via PD‐1, which induces T‐cell anergy. Modification: In FL, high CD70 levels induce differentiation of CD4+ T cells into immunosuppressive T regulatory cells that inhibit cytotoxic CD8+ cells. DLBCLs instead carry CD70‐inactivating mutations that might contribute to reduced interactions with antitumor T and NK cells, which also express CD27. TNFRSF14 normally interacts with BTLA (B and T lymphocyte attenuator), which regulates B‐cell expansion in the GC. Loss of TNFRSF14 might therefore contribute to uncontrolled proliferation. It was also linked to increased recruitment of tumor‐supportive follicular T‐helper (T_FH) cells and follicular dendritic cells (FDCs)