TLR4 on Follicular Dendritic Cells: An Activation Pathway That Promotes Accessory Activity1

Microbial molecular patterns engage TLRs and activate dendritic cells and other accessory cells. Follicular dendritic cells (FDCs) exist in resting and activated states, but are activated in germinal centers, where they provide accessory function. We reasoned that FDCs might express TLRs and that engagement might activate FDCs by up-regulating molecules important for accessory activity. To test this hypothesis, TLR4 expression on FDCs was studied in situ with immunohistochemistry, followed by flow cytometry and RT-PCR analysis. TLR4 was expressed on FDC reticula in situ, and flow cytometry indicated that TLR4 was expressed on surface membranes and TLR4 message was readily apparent in FDCs by RT-PCR. Injecting mice or treating purified FDCs with LPS up-regulated molecules important for accessory activity including, FDC-FcγRIIB, FDC-ICAM-1, and FDC-VCAM-1. Treatment of purified FDCs with LPS also induced intracellular phospho-IκB-α, indicating NF-κB activation, and that correlated with increased FcγRIIB, ICAM-1, and VCAM-1. FDCs in C3H/HeJ mice were not activated with LPS even when mice were reconstituted with C3H/HeN leukocytes, suggesting that engagement of FDC-TLR4 is necessary for activation. Moreover, activated FDCs exhibited increased accessory activity in anti-OVA recall responses in vitro, and the FDC number could be reduced 4-fold if they were activated. In short, we report expression of TLR4 on FDCs for the first time and that engagement of FDC-TLR4 activated NF-κB, up-regulated expression of molecules important in FDC accessory function, including FcγRIIB, ICAM-1, and VCAM-1, as well as FDC accessory activity in promoting recall IgG responses.

A ccessory cells of the immune system tend to remain quiescent until they encounter alarm signals that promote activation. Activation involves up-regulation of molecules, including costimulatory molecules, needed for optimal presentation of Ag to specific lymphocytes and the initiation of adaptive immune responses. Infectious agents are a major source of alarm signals that are recognized by pattern recognition receptors (PRRs) 3 that engage conserved pathogen-associated molecular patterns (PAMPs) (1)(2)(3). The range of molecular patterns recognized by PRRs includes signals from injured or stressed host cells and tissues (4,5). Prominent PRRs include the evolutionarily conserved membrane-bound TLRs that recognize PAMPs and initiate responses in organisms as diverse as flies and mammals (6 -10). TLR4 is an especially clear example of a receptor that is known to engage LPS, an amphiphilic molecule in the outer leaflet of the outer membrane of Gram-negative bacteria, and activate accessory cells (5,11,12).
Follicular dendritic cells (FDCs) are localized to the light zones of germinal centers (GCs), where their dendritic processes interdigitate and form three-dimensional FDC networks or reticula (13,14) that help these noncirculating cells remain fixed while attract-ing specific lymphocytes (15,16). FDC functions include the capture and retention of immune complexes (ICs) (17), promotion of B cell survival (18 -22), and promotion of high-affinity Ab production (23,24). The phenotype of FDCs in primary and secondary follicles is very different. In secondary follicles, FDCs bear high levels of Fc␥RIIB, ICAM-1, and VCAM-1, and these molecules are involved in converting poorly immunogenic ICs into a highly immunogenic form for B cells and in facilitating FDC-B cell interactions (17,(25)(26)(27). In contrast, Fc␥RIIB, ICAM-1, and VCAM-1 levels are low and difficult to detect in FDC reticula of primary follicles (17,26,28). The presence of these resting and activated FDCs prompted the hypothesis that FDCs, like other accessory immune cells, may express PRRs capable of recognizing PAMPs, leading to the acquisition of an activated phenotype required for optimal FDC accessory activity.
To begin hypothesis testing, we sought to determine whether FDCs express TLR4 and whether LPS can activate FDCs via TLR4. Activation was indicated by increased expression of FDC function-associated molecules Fc␥RIIB, ICAM-1, and VCAM-1 by flow cytometry. TLR4 expression on FDCs in situ was studied with immunohistochemistry, followed by flow cytometric and RT-PCR analyses on purified FDCs. The role of FDC-TLR4 engagement with LPS was investigated by adoptive transfer of wild-type LPS-responsive C3H/HeN leukocytes into TLR4-mutated LPS-hyporesponsive C3H/HeJ mice with host FDCs. The TLR4 activation pathway involves NF-B activation, and the involvement of this pathway was examined by flow cytometric analysis of intracellular phospho-IB-␣ in FDCs. Finally, we sought to determine whether activated FDCs have increased accessory activity, as indicated by the enhanced ability to promote specific IgG responses in vitro.
The results indicated for the first time that FDCs express TLR4 on their surfaces; TLR4 engagement mediates activation, as indicated by a marked increase of FDC function-associated molecules; and activation involves the NF-B pathway. Moreover, like other accessory cells, activated FDCs have increased accessory activity, as indicated by increased anti-OVA production in comparison with nonactivated FDCs. An important implication of these studies is that TLR agonists may promote immune responses not only by stimulating dendritic cells (DCs) and enhancing T cell function, but also by stimulating FDCs and promoting B cell function. An understanding of how TLR agonists influence FDC activation may give insight into how adjuvants should be formulated to give optimal humoral immune responses when administering vaccines.

Animals
Six-to 8-wk-old BALB/c mice were obtained from the National Cancer Institute. The C3H/HeNTac mice were from Taconic Farms, and the C3H/ HeJ mice were from The Jackson Laboratory. All mice were housed in standard plastic shoebox cages with filter tops and maintained under specific pathogen-free conditions in accordance with guidelines established by Virginia Commonwealth University Institutional Animal Care and Use Committee.

Immunohistochemical labeling of TLR4, FDC-M1, and GL-7 in situ
Popliteal lymph nodes (LNs) were collected from normal BALB/c mice and frozen in CryoForm embedding medium (IEC). Frozen sections of 10 m thickness were cut on a Leica (Jung Frigocut 2800E) cryostat and air dried. Following absolute acetone fixation, the sections were dehydrated and the endogenous peroxidase activity was quenched with the Universal Block. The sections were washed and saturated with 10% BSA. Serial sections were incubated with unlabeled rabbit anti-mouse TLR4 or rat antimouse FDC-M1. Sections were washed and then incubated with biotinconjugated goat anti-rabbit or anti-rat IgG, followed by streptavidin-HRP. The sections were developed using a diaminobenzidine substrate kit. For GC B cell labeling, 10-m cryostat sections of axillary LNs from OVAimmunized BALB/c mice were labeled with IgM rat anti-mouse T and B cell activation Ag (GL-7, Ly-77) (eBioscience 14-5902-85), followed by biotinylated goat anti-rat IgM (Southern Biotechnology Associates 3020-08) and phosphatase-labeled streptavidin (Kirkegaard & Perry Laboratories 15-30-00). The sections were developed with SIGMAFAST 5-bromo-4chloro-3-indolyl phosphate/NBT alkaline phosphatase substrate (Sigma-Aldrich B5655). Rabbit polyclonal IgG (Abcam ab27478), rat IgG2c (BD Biosciences 553982), and rat IgM (eBioscience 14-4341) isotype controls were similarly treated. Images were captured with Optronics digital camera and analyzed with Bioquant Nova software.

DC, FDC, and B cell isolation
DCs were isolated from splenic leukocytes using the CD11c microbead kit from Miltenyi Biotec. FDCs were isolated by positive selection from LNs (axillary, lateral axillary, inguinal, popliteal, and mesenteric) of irradiated adult mice, as previously described (29). One day before isolation, mice were irradiated with 1000 rad to eliminate most lymphocytes, and then mice were killed, and LNs were collected, opened, and treated with 1.5 ml of collagenase D (22 mg/ml, C-1088882; Roche), 0.5 ml of DNase I (5000 U/ml, D-4527; Sigma-Aldrich), and 2 ml of DMEM with 20 mM HEPES. After 45 min at 37°C in a CO 2 incubator, released cells were washed in 5 ml of DMEM with 10% FCS. Cells were then sequentially incubated with FDC-specific Ab (FDC-M1) for 45 min, 1 g of biotinylated anti-rat L chain for 45 min, and 20 l of anti-biotin microbeads (Miltenyi Biotec) for 15-20 min on ice. The cells were layered on a MACS LS column and washed with 10 ml of ice-cold MACS buffer. The column was removed from the VarioMACS, and the bound FDCs were released with 5 ml of MACS buffer. B220 ϩ B cells were isolated from BALB/c LNs using CD45R (B220) MicroBeads from Miltenyi Biotec.

Labeling of purified FDCs and B220 ϩ B cells with anti-TLR4
FDCs and B220 ϩ B cells isolated from BALB/c mice were incubated with mouse Fc-Block (BD Pharmingen) for 15 min on ice, followed by FITCconjugated rat anti-mouse TLR4 or isotype control for 60 min in the dark at 4°C. After washing, the cells were analyzed using an FC500 flow cytometer and Cytomics RXP analysis software. Histograms were gated for FDCs based on their forward and side scatter properties established with FDC phenotypic markers, including FDC-M1, CD21/35, and CD32 (29). The figures were plotted using WinMDI software (Scripps Research Institute).

Detection of TLR4 mRNA in purified FDCs by RT-PCR
Total cellular RNA was extracted from 2 ϫ 10 6 FDCs purified from LNs or spleens as well as control splenic DCs of BALB/c mice using TRIzol reagent (Invitrogen Life Technologies), according to the manufacturer's instructions, and 100 ng was reverse transcribed into cDNA using Gene-Amp Gold RNA PCR Core Kit. One-tenth of cDNA was primed with TLR4-specific primers 5Ј-GCTTTCACCTCTGCCTTCAC-3Ј and 3Ј-CGAGGCTTTTCCATCCAATA-5Ј and amplified by PCR. PCR products were electrophoresed in 1.5% agarose, stained with ethidium bromide, and visualized by UV transilluminator.

Analysis of Fc␥RIIB, VCAM-1, and ICAM-1 on LPS-treated FDCs in vivo and in vitro
FDCs were positively selected with FDC-M1 from the draining LNs of irradiated BALB/c mice 3 days after injecting 25 g of LPS in the four limbs. Control FDCs were isolated from saline-injected mice. In addition, 1 ϫ 10 6 purified FDCs were treated in vitro with 10 g of LPS in 1 ml of medium for 3 days, and the surface expression of Fc␥RIIB, VCAM-1, and ICAM-1 was compared with 1 ϫ 10 6 control FDCs cultured in medium without LPS. Analysis was done by incubating FDCs with mouse Fc-Block (BD Pharmingen) for 15 min on ice, followed by FITC-conjugated rat Abs against Fc␥RIIB, ICAM-1, and VCAM-1 or isotype control for 60 min in the dark at 4°C. After washing, the cells were analyzed by flow cytometry.

Adoptive transfer
C3H/HeJ and C3H/HeN recipients were exposed to 600 rad of irradiation, and 1 day later they were reconstituted with 2 ϫ 10 7 LPS-responsive C3H/ HeN total splenic leukocytes injected behind the neck. The mice were challenged with LPS in the legs, and the FDCs were isolated after 3 days.

Analysis of Fc␥RIIB, VCAM-1, and ICAM-1 on LPS-treated C3H/HeN and C3H/HeJ FDCs
FDCs were positively selected from the LNs of C3H/HeN or C3H/HeJ mice 3 days after injecting 25 g of LPS in each limb, and control FDCs were isolated from saline-injected mice. FDCs were first incubated with mouse Fc-Block for 15 min on ice, followed by FITC-conjugated rat Abs against Fc␥RIIB, ICAM-1, and VCAM-1 or isotype control for 60 min in the dark at 4°C. After washing, the cells were analyzed by flow cytometry.

Intracellular labeling of phospho-IB-␣ in purified FDCs treated with LPS in vitro
Purified FDCs were serum starved for 24 h, and then treated with 10 g/ml LPS overnight in DMEM with 10% FCS. Control groups of unstimulated FDCs as well as macrophage cell line J774 were similarly treated. Cells were washed and then fixed and permeabilized using Fix & Perm cell permeabilization kit (Caltag Laboratories). Intracellular phospho-IB-␣ was labeled with rabbit anti-phospho-IB-␣ (Cell Signaling Technology), followed by PE-conjugated goat anti-rabbit IgG, and cells were analyzed with flow cytometry. Purified FDC preparations were labeled with anti-TLR4-FITC and compared with an isotype control. The MFI of TLR4-FITC-labeled FDCs was double the isotype control. D, TLR4 labeling of purified B220 ϩ B cells. Purified B220 ϩ B cell preparations were labeled with anti-TLR4-FITC and compared with an isotype control. B220 ϩ B cells were hardly labeled with TLR4-FITC, and the MFI of TLR4-FITC-labeled B cells was 0.2 above the MFI of the isotype control. These data are representative of three separate experiments of this type. labeling in the area between the arrows. The characteristic labeled FDC network or reticulum made up by interactive dendrites of FDCs can be seen. GC B cells intermingle with FDCs, and we sought to determine whether they label with anti-TLR4 and contribute to the labeling in Fig. 1Ac. GC B cells are activated and labeled with anti-mouse T and B cell activation Ag GL-7 (Fig.  1Ba), but in contrast with FDCs, they did not label with TLR4 in adjacent LN sections (Fig. 1Bc). Thus, TLR4 labeling in GCs appeared to be attributable to FDCs and not B cells. Isotype controls (Fig. 1Ab, Ad, Bb, and Bd) were included and shown to the right of the corresponding labeled sections.

TLR4 labeling of purified FDCs
To confirm expression of TLR4, FDCs were purified by positive selection using the mAb FDC-M1, and labeled with anti-TLR4 FITC. As illustrated in Fig. 1C, flow cytometric analysis revealed that virtually the entire FDC population shifted to the right when labeled with anti-TLR4 with more than a 2-fold increase in mean fluorescent intensity (MFI) over the background control. Analysis was restricted to viable cells, indicating that TLR4 is expressed on the surface of FDC membranes. B cells positively selected with B220, an Ag expressed on B lineage cells throughout their development, but not on plasma cells, also appeared to shift to the right as a consequence of anti-TLR4 labeling (Fig. 1D). However, in contrast with FDCs, in which the MFI more than doubled upon labeling with anti-TLR4, the increased MFI for the B cells was only 0.2 above the isotype control. In short, murine B cells may express some TLR4, but it is unlikely that they are making a significant contribution to the strong TLR4 labeling in GC areas where the FDC-reticulum identified by FDC-M1 is present.

TLR4 mRNA in purified FDCs
RNA extracted from purified FDCs from murine LNs ( Fig. 2A) and spleen (Fig. 2B) was reverse transcribed into cDNA. TLR4-specific primers were used to amplify the relevant DNA, and the results were compared with similarly treated purified splenic DCs as a positive control (Fig. 2C). Gel electrophoresis of amplified PCR products revealed a single 361-bp band shared between DCs and FDCs, indicating TLR4 expression by FDCs.

LPS-mediated up-regulation of FDC-Fc␥RIIB, FDC-ICAM-1, and FDC-VCAM-1
Injecting LPS increased the levels of Fc␥RIIB, ICAM-1, and VCAM-1 on FDCs from the draining LNs (Fig. 3A), where virtually the entire FDC population shifted to the right when labeled with Abs reactive with these markers and the MFI approximately doubled in each case. Similarly, treating purified FDCs in vitro ( Fig. 3B) with LPS for 3 days increased the levels of Fc␥RIIB, ICAM-1, and VCAM-1 expression on FDCs, although the intensity of VCAM-1 up-regulation was comparatively less.

FDCs purified from TLR4-mutated C3H/HeJ mice did not respond to LPS even when reconstituted with leukocytes from TLR4 wild-type mice
FDCs from wild-type LPS-responsive C3H/HeN mice up-regulated FDC-Fc␥RIIB, FDC-ICAM-1, and FDC-VCAM-1 when injected with LPS (Fig. 4A) much like the BALB/c mice shown in Fig. 3. Moreover, this response persisted after irradiation and reconstitution with LPS-responsive C3H/HeN total splenic leukocytes (Fig. 4B). In marked contrast, FDCs from TLR4-mutated C3H/HeJ mice failed to up-regulate FDC-Fc␥RIIB, FDC-ICAM-1, and FDC-VCAM-1 after LPS injection (Fig. 4C) even after reconstitution with wild-type TLR4 LPS-responsive C3H/HeN leukocytes (Fig. 4D). This suggests that LPS engagement of TLR4 on the FDC is important, and that FDC activation is not simply a response to cytokines being produced by other TLR4-responsive leukocytes.

LPS-mediated up-regulation of intracellular phospho-IB-␣ in FDCs indicative of NF-B activation
Signaling through TLR4 typically involves the NF-B pathway, prompting the hypothesis that stimulation of FDCs with LPS would up-regulate intracellular phospho-IB-␣ in FDCs. Purified FDCs were incubated with 10 g/ml LPS overnight. Control groups included unstimulated FDCs and murine macrophage cell line J774 that were similarly treated. Flow cytometric analysis revealed that the MFI of intracellular phospho-IB-␣ in the control macrophage cell line about doubled after exposure to LPS (Fig.  5A). The FDC response to LPS was very similar, and the increase in MFI was even higher (Fig. 5B). Similar results were obtained with 0.1 and 1 g of LPS, but the results were most apparent with the 10 g dose illustrated in Fig. 5.

LPS-activated FDCs exhibited enhanced accessory activity in promoting recall responses in vitro
FDCs from normal mice are known to enhance recall responses optimally at ratios between 1 FDC and 4 -16 lymphocytes, although detectable accessory activity may be apparent with more lymphocytes (31). We reasoned that LPS-activated FDCs would have enhanced accessory activity, and that might be most apparent in IgG recall responses at low FDC to lymphocyte ratios. When cultured at a ratio of 1 FDC to 64 lymphocytes, the IgG anti-OVA produced in cultures containing activated FDCs was ϳ4 times higher than the production with FDCs from normal mice, which in turn produced anti-OVA IgG comparable to 1 activated FDC to 256 lymphocytes (Fig. 6). In marked contrast, FDCs from normal mice were without detectable activity at 1 FDC to 256 or even 128 lymphocytes (Fig. 6).

Discussion
Engagement of TLRs on DCs leads to physiological and phenotypic changes that dramatically alter their ability to present Ag to T cells (1,32,33). DC alterations include changes in the following: survival, chemokine receptor expression, chemokine secretion, migration, cell shape, and endocytic activity (34). LPS and other TLR agonists promote DC maturation and accessory activity in priming naive T cells, promoting clonal expansion, and stimulating differentiation of T cells into effectors (35). We reasoned that for the immune system to respond to microbial invasion as an integrated unit, microbial patterns encountered early in immune responses should activate not only the DC-T cell axis, but also the FDC-B cell axis. The data reported in this study indicated that FDCs express surface TLR4 and that engagement of this receptor in vivo and in vitro altered their physiology and phenotype, leading to FDC activation. Moreover, activated FDCs were far more effective in enhancing the production of specific IgG than FDCs from normal mice. IC-bearing FDCs reside in GCs where B cells do the following: proliferate, isotype switch, somatically hypermutate, become memory cells, and transition from B cells into Ab-forming cells (reviewed in Refs. 13 and 36). Considerable data indicate that FDCs influence these B cell functions (reviewed in Refs. 13 and 14). An understanding of FDC activation could facilitate our ability to manipulate the immune system such that the appropriate adjuvants could be selected to activate FDCs and optimize humoral immune responses when vaccines are administered.
Analysis of mRNA by RT-PCR in the present study suggested that TLR4 expression in DCs and FDCs is comparable. The fact that FDCs are barely endocytic and retain Ag in ICs on their surfaces (37) allows the surface-expressed FDC-TLR4, as shown by flow cytometry, to engage LPS in an environment spatially associated with other FDC accessory functions, including IC trapping and receptor-mediated B cell-FDC clustering and interactions. The presence of TLR4 molecules inside the FDCs has not been excluded, but clearly, anti-TLR4 labels FDC membranes intensely. Cytokines help activate many cell types, and we reasoned that cytokines induced by LPS may play an important role in FDC activation. However, no support for this idea was found in the present study. When normal splenic leukocytes that are fully capable of secreting cytokines upon TLR4 engagement were injected into irradiated TLR4-mutated C3H/HeJ mice and challenged with LPS, the FDCs with mutated TLR4 still failed to activate (Fig.  4D). These results argue that functional TLR4 on FDCs is important. Moreover, when purified wild-type FDCs were treated with LPS in vitro, they responded well, suggesting that FDC-TLR4 is sufficient (Fig. 3B).
FDC-associated GC B cells failed to label immunohistochemically in situ with anti-TLR4, and purified B cell preparations positively selected with the pan-B cell marker CD45R (B220) were barely labeled with anti-TLR4 by flow cytometry. These data are consistent with similar results reported by Akashi et al. (38), in which cell surface TLR4-MD-2 was hardly detectable on CD19positive B cells and intracellular staining by the saponin detergent did not make a difference.
Fc␥RIIB plays an important role in FDC-B cell interactions. Both FDC and B cells express Fc␥RIIB (25,39), but B cell activation may be inhibited by ITIM signaling mediated by IC coligating the BCR for Ag and B cell-Fc␥RIIB (40,41). Engagement of BCR in the absence of such coligation induces rapid activation of tyrosine kinases, generation of inositol phosphates, elevation of cytoplasmic Ca 2ϩ concentrations, and activation of MAPKs (reviewed in Refs. 40 and 41). These events promote B cell activation and lead to proliferation, differentiation, and Ab secretion (42). In marked contrast, coligation of BCR and Fc␥RIIB by ICs leads to inhibition of the extracellular Ca 2ϩ influx, reduction of cell proliferation, blockage of blastogenesis, and inhibition of Ig synthesis (40 -42). Nevertheless, ICs on FDCs provide potent activation signals for B cells that result in induction of GC dark zones, where B cells rapidly proliferate and differentiate (reviewed in Ref. 36). However, FDCs lacking Fc␥RIIB or expressing only low levels of Fc␥RIIB are unable to convert poorly immunogenic IC into a highly immunogenic form for B cells, although they can trap IC using complement receptors (17). In active GCs, FDCs express high levels of FcR relative to B cells, and these FDC-FcRs bind Fc portions of Ab in ICs and appear to minimize their binding to FcR on B cells (14,17,25). Accordingly, cross-linking of BCR and Fc␥RIIB via IC is minimized, ITIM signaling is reduced, and B cell proliferation and differentiation are promoted. Thus, activating FDCs and increasing FDC-Fc␥RIIB levels would be expected to enhance accessory activity.
FDCs and B cells form large clusters in culture, and B cells proliferate adjacent to FDCs much like they do in the dark zone of GCs in vivo (27). FDC-ICAM-1 and FDC-VCAM-1 are important adhesion molecules, and blocking these molecules inhibits clustering of FDCs and B cells and the ability of B cells to proliferate in GC reactions in vitro (27). Up-regulating these adhesion molecules should promote FDC-B cell interactions, facilitate clustering and presentation of FDC-Ag to B cells, and enhance the GC reaction. The signaling pathway known to up-regulate FDC-ICAM-1 and FDC-VCAM-1 is mediated via the NF-B (43), and up-regulation of FDC-ICAM-1 and FDC-VCAM-1 may be initiated by ICs engaging FDC-Fc␥RIIB (44). Similarly, the NF-B pathway appeared to be activated when FDC-ICAM-1 and FDC-VCAM-1 were up-regulated by LPS, engaging FDC-TLR4 in the present study.
The initial exposure to appropriate immunogen primes T cells and stimulates specific B cells to begin making Abs. Specific Abs bind the immunogen and form ICs, ICs are trapped by FDCs, B cells are stimulated by the Ag in FDC-ICs, and GC reactions are initiated. The data presented in this study suggest that FDCs can be activated by TLR4 agonists and such agonists can be delivered by the pathogen or with the immunogen. Thus, FDCs could be activated and ready to trap ICs and optimally stimulate B cells as soon as the first ICs are made. The ICs themselves can activate FDCs, but that requires 24 -48 h after encountering adequate amounts of ICs (44). The present study indicated that FDCs were activated via TLR4 engagement in 3 days, and that would precede Ab production and IC formation. Thus, ICs could be trapped by activated FDCs, and that would speed up and enhance GC events, including B cell proliferation, isotype switching, somatic hypermutation, memory cell formation, transition from B cells to Ab-forming cells, selection, and affinity maturation. In addition to TLR4, preliminary studies using RT-PCR indicated that FDCs express TLR-2, TLR-3, and TLR-9. By use of flow cytometry with purified FDCs, we were able to confirm expression of TLR-2 and TLR-3. Stimulation of FDCs with the TLR-3 ligand poly(I:C) resulted in up-regulation of complement receptor 1/2, Fc␥RIIB, and VCAM-1. However, the data suggest that TLRs may differ in their impact on FDC phenotype and function, and further studies in this area are in progress. Nevertheless, an understanding of how FDCs can be optimally activated via TLR engagment could provide information needed to control and enhance the development of protective humoral immune responses.