The transcriptional program during germinal center reaction - a close view at GC B cells, Tfh cells and Tfr cells

Abstract

The germinal center (GC) reaction is a key process during an adaptive immune response to T cell specific antigens. GCs are specialized structures within secondary lymphoid organs, in which B cell proliferation, somatic hypermutation and antibody affinity maturation occur. As a result, high affinity antibody secreting plasma cells and memory B cells are generated. An effective GC response needs interaction between multiple cell types. Besides reticular cells and follicular dendritic cells, particularly B cells, T follicular helper (Tfh) cells as well as T follicular regulatory (Tfr) cells are a key player during the GC reaction. Whereas Tfh cells provide help to GC B cells in selection processes, Tfr cells, a specialized subset of regulatory T cells (Tregs), are able to suppress the GC reaction maintaining the balance between immune activation and tolerance. The formation and function of GCs is regulated by a complex network of signals and molecules at multiple levels. In this review, we highlight recent developments in GC biology by focusing on the transcriptional program regulating the GC reaction. This review focuses on the transcriptional co-activator BOB.1/OBF.1, whose important role for GC B, Tfh and Tfr cell differentiation became increasingly clear in recent years. Moreover, we outline how deregulation of the GC transcriptional program can drive lymphomagenesis.

Keywords: BOB.1/OBF.1; GC B cells; Tfh cells; Tfr cells; germinal center (GC); lymphoma; transcriptional regulation.

Figure 1

Transcriptional control of GC B cells. During GC initiation, TFs IRF4, IRF8 and MEF2B induce the expression of BCL-6. BOB.1/OBF.1 is also involved in the initiation of BCL-6 expression. Moreover, the transcriptional co-activator drives MEF2B expression. On the other hand, IRF4 seems to modulate the expression of BOB.1/OBF.1. In the dark zone, PAX5, E2A and IRF8 induce AID and therefore SHM. BOB.1/OBF.1 was also shown to be involved in AID, BCL-6 and FOXO1 induction. BCL-6 and FOXO1 inhibit BLIMP-1 and BOB.1/OBF.1 blocks IRF4 expression to prevent initiation of plasma cell differentiation at this stage. In the light zone, IRF4 is upregulated and prevents BCL-6 expression to terminate the dark zone program. IRF4, PAX5 and E2A promote AID expression and thereby affinity maturation and CSR. BOB.1/OBF.1 (POU2AF1) is again involved in regulation of AID and FOXO1 expression. The TF MYC probably regulates re-entry into the dark zone. IRF4 and BOB.1/OBF.1 are involved in driving BLIMP-1 expression. BLIMP-1 in turn represses BCL-6, AID, PAX5 and MYC and on the other hand facilitates expression of XBP1 together with IRF4 resulting in plasma cell differentiation. In contrast, ABF-1 induces memory B cell differentiation. This figure was created with BioRender.com.

Figure 2

Transcriptional control of Tfh cell differentiation. Tfh cell differentiation occurs in three main steps. The first step of Tfh cell differentiation is the interaction of naïve CD4⁺ T cells with DCs in the T cell zone. The expression of BCL-6, master TF of Tfh cells, is induced on multiple levels. Cytokines including IL-21, IL-6 and type I IFNs (IFN-a/b) drive Bcl-6 expression via the activation of STAT1/3. Besides, TFs LEF-1 and TCF-1 as well as the transcriptional co-activator BOB.1/OBF.1 together with TFs Oct-1/Oct-2 induce Bcl- 6 expression. BCL-6 in turn represses Blimp-1 and TFs favoring development into TH subpopulations. BCL-6 and the TF ASCL2 induce the expression of CXCR5 and suppresses CCR7 expression inducing migration of pre-Tfh cells to the B/T cell border. The second step of Tfh cell differentiation then occurs upon interaction of pre-Tfh cells with Ag-specific B cells in the B/T cell border. ICOS/ICOSL signaling represses the TF KLF2 thereby inhibiting migration back to the T cell zones. ICOS signaling also induces the TF c-MAF, which facilitates IL21 expression. The third step of differentiation occurs within GCs, where SLAM/SAP binding is required to stabilize B/T cell interactions. Here, pre-Tfh cells differentiate into Tfh cells, a further polarized state characterized by highest expression levels of BCL-6, CXCR5, PD-1, ICOS and BTLA. This figure was created with BioRender.com.

Figure 3

Transcriptional control of Tfr cell differentiation. Initial Tfr cell differentiation depends on CD28 and ICOS signaling, whereas interaction with B cells via SLAM/SAP is required for their full differentiation. Tfr cells express both BCL-6 and BLIMP-1 and possibly their balanced regulation is essential for Tfr cell differentiation. BCL-6 is regulated by ICOS and mTORC1. ICOS induces PI3K and its subunit p85α interacts with osteopontin (OPN-i). OPN-i translocates to the nucleus preventing BCL-6 from ubiquitin-dependent proteasome degradation. mTORC1 induces phosphorylation of STAT3, activating TCF-1 expression, which upregulates BCL-6. Stromal interacting molecules (STIM) and ORAI proteins mediate Ca2+ entry inducing NFAT2, which upregulates CXCR5. Id2/Id3, c-Maf and BOB.1/OBF.1 are also involved in Tfr cell differentiation, but the exact mechanisms are incompletely understood so far. This figure was created with BioRender.com.