Follicular Dendritic Cell Makes Environmental Sense

Summary

Follicular dendritic cells (FDCs) are key organizers of B cell follicles and germinal centers. In this issue of Immunity, identify the roles of Toll-like receptors in the responses of FDCs, providing a unique link between innate and adaptive immunity.

Follicular dendritic cells (FDCs) have long been recognized as key organizers of B cell follicles, both in secondary lymphoid tissues such as the spleen, lymph nodes (LNs) and Peyer's patches (PPs) and in reactive sites around localized infections and chronic inflammatory reactions where they are referred to as tertiary lymphoid follicles. Central to the function of B cell follicles is the development of germinal centers (GCs) where cooperation of multiple cell lineages leads to the formation of isotype-switched, high affinity immunoglobulin and the establishment of humoral immune memory. In this issue of Immunity, Suzuki et al. (Suzuki et al., 2010) identify important new activities of FDCs in the establishment of the GC response in PPs and Garin et al. (Garin et al., 2010) identify additional actions of FDCs in the expression of mature GC functions in LNs. These findings provide another link between the innate and adaptive immune systems and raise important questions regarding the nature of the stimuli that activate FDCs for maturation of humoral immune responses.

Toll-like receptors (TLRs) are evolutionarily conserved membrane receptors that were first identified because of their ability to recognize a broad range of microbial products that are not present in the mammalian host (Beutler, 2009). The ability of TLRs to recognize these microbial products, based on their possession of microbe-specific Pathogen-Associated Molecular Patterns (PAMPs), provided a convenient way for the innate immune system to distinguish between dangerous environmental threats and innocuous foreign products. TLRs are expressed by a very broad range of immune and non-immune cell lineages, emphasizing their central role as activators of innate immune responses (Beutler, 2009). Their expression by cells of the myeloid lineage provides a bridge between the innate and the adaptive immune responses with TLRs on dendritic cells and macrophage-monocyte lineage cells enhancing antigen-presenting cell function and the release of cytokines and chemokines that are essential for the activation of both CD8⁺ and CD4⁺ T cells (Manicassamy and Pulendran, 2009). These findings have established TLRs as key activators of processes that lead to fully developed cellular immunity.

There is increasing appreciation that TLRs also augment humoral immune responses. This occurs both directly at the level of the B cell and indirectly through the action of other cells that cooperate with B cells. B cells express TLRs and can be stimulated polyclonally by TLR ligands such as endotoxin to proliferate and differentiate into Ig-secreting cells (Meyer-Bahlburg et al., 2007). TLR signaling also supports B lymphocyte trafficking and clustering within LN follicles and thus promotes sustained B cell proliferation and enhanced generation of memory B and plasma cells (Hwang et al., 2009). In addition, TLR signaling indirectly support the induction of T cell-dependent B cell responses via the enhancement of CD4⁺ T cell help.

Although a role for TLRs in the induction of humoral immune responses is now well established for both T helper cells and intrinsic B cell activation programs, less is known about possible actions of TLR ligands on FDCs. That FDCs might respond importantly to TLR signals is suggested by the observations that FDCs provide central organizing signals for GCs and, at least in the PP, GCs depend on the presence of ongoing stimulation of the gut by commensal microbes, many of which are thought to activate innate host responses through TLRs.

FDCs are a radio-resistant cell population that appears not to be of hematopoietic lineage, but rather to arise from stromal cells located within secondary lymphoid tissues. Their differentiation from stromal cell precursors depends absolutely on signals delivered by several TNF family members, especially by the membrane lymphotoxin (mLT) heterotrimer, LTα₁β₂, expressed by B cells. mLT on B cells induces expression by the FDC of the chemokine CXCL13, leading to the compact clustering of B cells and follicular helper T cells that appears essential for formation of the GC (van Nierop and de Groot, 2002). FDCs are unique in that they present native antigen in the form of Fc receptor-bound immune complexes to responding B cells, support both T cell-dependent and –independent B cell activation and immunoglobulin (Ig) production, and suppress apoptosis in B cells that have successfully completed the somatic hypermutation process to create a high affinity antigen-binding receptor.

Although FDCs play a central role in GC development and function in secondary lymphoid tissues in general, their role in the specialized function of PP to produce high amounts of secretory IgA has not been resolved. In this issue, Suzuki et al. (Suzuki et al., 2010) provide elegant data supporting a primary role for PP FDCs in the production of IgA. They demonstrate that FDCs express high cell surface amounts of several TLRs, including TLR2 and TLR4, allowing them to respond to stimulation by LPS and lipopeptides derived from gut microbes. PP FDCs also constitutively express high amounts of the intracellular retinoic acid receptor RAR-β, preparing these cells to respond to the high amount of retinoic acid that are produced by gut epithelial cells. Coordinated signaling through TLR2 and TLR4 and RAR-β synergistically up-regulates the expression of CXCL13, BAFF, MAdCAM-1, clusterin (all of which support recruitment and survival of B cells) and a group of molecules associated with the secretion and activation of the important IgA switch factor TGF-β, including CD36, latent TGF-β binding proteins, matrix metalloproteinases 2 and 9, and bone morphogenic protein 2. Thus, signaling through RAR and TLRs on FDCs up-regulates not only the secretion of TGF-β1, but also its conversion from the latent to the active form. This production of TGF-β1 was dependent on the critical TLR signaling adapter protein MyD88 in the FDC and was also dependent both on BAFF and expression of the RAR. Thus, signaling in FDC through TLRs in the presence the high amounts of retinoic acid that are typical of the gut leads to preferential class switching to the production of IgA, identifying a central role for FDCs and TLRs in the production of this key mucosal immunoglobulin (Figure 1).

Figure 1.

FDCs integrate signals from TLR and other receptors to support an effective germinal center response.

FDCs serve a critical organizing function in B cell follicles within secondary lymphoid tissues. Under stimulation by antigen, FDCs provide key signals for the development of the germinal center. In the Peyer's patch, shown here, signals delivered through cell surface Toll-like receptors (TLR) and through intracellular retinoic acid receptors (RAR) activate the FDC to support TGF-β-dependent B cell isotype switching to the production of IgA. In lymph nodes, TLR stimulation induces enhanced Ig somatic hypermutation and selection of high affinity variants.

Garin et al., in contrast, explore the role of TLR4 signaling in FDCs in shaping the GC affinity maturation response (Garin et al., 2010). In addition to addressing the role of TLRs in regulating the GC response, they have begun to address the nature of the environmental signals that activate signaling through the TLRs. Studies over the several years have questioned whether TLRs act solely as sensors of contact with microbial pathogens, or rather serve more broadly as sensors of inflammation and tissue injury. In this regard, expanding numbers of endogenously produced TLR ligands are being identified. Classified under the general term “alarmins”, these endogenous molecules are thought to serve as indicators of tissue damage and have also termed damage-associated molecular patterns (DAMPS) (Bianchi, 2007; Erridge, 2010).

Garin et al. show that immunization up-regulates expression of TLR4 on FDCs. They also demonstrated that the endogenous TLR ligand oxidized phosopholipid (OxPl), thought to be generated by tingible body macrophages when they catabolize apoptotic B cells, is present in easily detectable quantities in close association with FDCs in the GC. Using TLR4 antibody-based strategies, as well as bone marrow chimeric mice, Garin et al. emphasize the TLR4 dependency of FDC maturation and GC development. They propose a two-phase model for the GC reaction, in which activated ICAM-1 expressing FDCs assist in the early phase expansion of antigen-driven GC B cells and in a late phase of affinity-dependent selection of B cells that carry somatically mutated antigen receptors. Using a combination of in silico mathematical modeling and in vivo experimentation, they provide compelling evidence that signals through TLR4 govern critical functions of FDCs as they regulate the quantity and quality of the GC response. Further, they highlight an important role for endogenous TLR ligands in improving the overall efficiency of the GC reaction. Garin et al. propose that endogenous ligands, generated from necrotic or apoptotic cells that are present in large quantities in GCs, trigger FDC activation via TLR4, thereby enhancing isotype switching, somatic hypermutation and the production of high affinity Ig.

Together these studies emphasize that optimal humoral immunity is dependent on TLRs expressed by FDCs. Not yet clear is whether similar signaling pathways are active in the ectopic lymphoid follicles and GCs that form at sites of localized microbial infection or chronic inflammation. Also the relative role of microbial and endogenous TLR ligands in the activation of FDCs is unclear. Although PAMPs are generally thought to be absent in the healthy setting, there is evidence that PAMPs derived from the intestinal microflora, such as bacterial DNA and bacterial peptidoglycans, may circulate and accumulate at sites of inflammation or tissue damage, such as arthritic joints (van der Heijden et al., 2000). This accumulation could be enhanced by the presence of endogenous TLR ligands that have been noted to have both PAMP-binding and sensitizing properties (Hreggvidsdottir et al., 2009). This process provides a mechanism by which exogenous and endogenous TLR ligands can contribute to the link between innate and adaptive immune responses in the settings of both normal immune responses and destructive chronic inflammatory processes.