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Therapeutic CD154 antibody for lupus: promise for the future?
Garnett Kelsoe
Department of Immunology, Duke University, Durham, North Carolina, USA
Address correspondence to: Garnett Kelsoe, Department of Immunology, Duke University Medical Center 3010, Duke University, Durham, North Carolina 27710, USA. Phone: (919) 613-7815; Fax: (919) 613-7878; E-mail: ghkelsoe@duke.edu.
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Published November 15, 2003 - More info
J Clin Invest. 2003;112(10):1480–1482. https://doi.org/10.1172/JCI20371.
© 2003 The American Society for Clinical Investigation
The autoimmune disease systemic lupus erythematosus (SLE) is characterized by the production of high-affinity IgG antibodies against double-stranded DNA (dsDNA). Typically, these pathogenic autoantibodies are encoded by mutated Ig genes, and the frequency and pattern of mutations strongly suggest that the source of autoreactive antibodies is B cells that have participated in the germinal center (GC) reaction. Indeed, GCs appear spontaneously in many strains of lupus-prone mice (1), and extraordinary numbers of GC B cells and their progeny are present in the blood of SLE patients (2).
GCs are organized collections of antigen-activated T and B lymphocytes in secondary lymphoid tissues (3) or sites of chronic inflammation (4, 5). Although transient GC-like reactions can be elicited by thymus-independent pathways (6–8), most GC responses are thymus-dependent. GC B cells require ongoing survival and proliferation signals that depend on CD154-CD40 signaling (9). CD154, a member of the TNF family, is inducibly expressed on the surface of CD4 T lymphocytes, whereas its ligand, CD40, is constitutively present on B lymphocytes (reviewed in ref. 10).
T cell–dependent GCs begin with the separate activation of T and B cells by antigen (11). In secondary lymphoid tissues, binding of antigen modifies B cell chemotaxis, resulting in migration toward the T cell zone (12), where cognate T cell–B cell interaction expands both lymphocyte populations. Soon afterward, activated T and B cells emigrate from T cell areas into the reticula of follicular dendritic cells (FDCs) that define the B cell follicle (Figure 1). The immigrant B lymphocytes proliferate in the FDC reticulum to generate nascent GCs and, in the process, acquire distinctive phenotypes, including expression of the CD69 activation antigen and several differentiation markers, including CD38 and CD27. Remarkably, human CD38+ GC B cells can also express variable levels of CD154, especially under conditions of chronic stimulation (13, 14), and appear to have the capacity for continued self-activation.
Figure 1Potential cellular targets for passive CD154 antibody to interrupt thymus-dependent GC and antibody responses. Antigen-specific T and B lymphocytes meet at the interface of T and B cell zones (follicles) in secondary lymphoid tissues (I). CD40-CD154 interaction results in the local proliferation of both lymphocyte types. Following clonal expansion, a fraction of activated T and B cells, prompted by CD40-CD154 signaling, migrate into the follicle to initiate the GC reaction (II). In GCs, B cells proliferate and activate AID-dependent SHM and CSR. Other clonally related B lymphocytes are retained in extra-follicular sites and differentiate into antibody-forming cells (AFCs), or plasmacytes (III). These transient AFCs do not express AID and do not support CSR and SHM. In GCs, centroblasts proliferate in the DZ, migrate to the LZ as centrocytes, present antigen to LZ T cells, and receive signals that direct either their exit from the GC or return to the DZ (IV). Centrocytes that do not receive survival signals die by apoptosis. Active GCs require continuous CD40-CD154 signaling; these signals may represent homotypic interaction between CD40+CD154+ centroblasts or heterotypic signals between CD40+ centrocytes and CD154+CD4 T cells. Selected B cells exit the GC (V) to enter the memory or long-lived plasmacyte compartments. Grammer et al. (17) observe that passive anti-CD154 reduces the numbers of IgD+CD38+ GC precursors (I and II) and CD38bright plasmacytes (III and V) but has little effect on circulating GC (IgD–CD38+) cells (IV). The primary effect(s) of CD154 antibody in SLE patients may be to interrupt the early initiation (I) and/or migrations steps (II) of the GC reaction.
GCs become polarized into histologic dark and light zones (DZ, LZ). The DZ is proximal to the T cell area and contains rapidly dividing B cells called centroblasts that express little or no surface immunoglobulin. The more distal LZ contains the bulk of the activated FDC network, antigen-specific CD4 T cells, and nondividing B cells known as centrocytes. Centrocytes express surface immunoglobulin and are thought to be the progeny of DZ centroblasts. In turn, selected centrocytes likely reenter the DZ and regain the centroblast form.
The enzyme AID (activation-induced cytidine deaminase) drives both somatic hypermutation (SHM) and immunoglobulin class switch recombination (CSR) in GC B cells (15). SHM introduces point mutations and occasional small deletions into the V(D)J regions of transcriptionally active immunoglobulin genes. Mutations are probably introduced in centroblasts and accumulate during repeated rounds of centroblast/centrocyte migration between the DZs and LZs (Figure 1). Intense selection for higher-affinity mutants establishes GCs as foci of rapid somatic evolution and antibody affinity maturation. CSR ensures that mutated, high-affinity antibodies acquire various effector activities and can be delivered throughout the body.
With time, centrocytes leave GCs and enter into at least two phenotypically distinct compartments of memory cells (Figure 1). One compartment, comprising antigen-specific, small, resting B lymphocytes, defines the classical memory B lymphocyte. The other contains long-lived plasmacytes that secrete high-affinity antibody and are capable of maintaining significant levels of serum antibody for years, if not decades. Characteristically, fully differentiated human plasmacytes express high levels of CD38.
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