Jiuhua Wu

Follicular dendritic cells and presentation of antigen and costimulatory signals to B cells

Summary: This review focuses on how immunogens trapped by FDC In the form of Ag‐Ab complexes productively signal B cells. In vitro, Ag‐Ab complexes are poorly immunogenic but m vivo immune complexes elk ii potent recall responses. FDC trap Ag‐Ab complexes and make immune complex mated bodies or “iccosomes”. B cells endocytose iccosomes, the Ag is processed, and T‐cell help is elicited. In vitro, addition of FDC bearing appropriate Ag‐Ab complex to memory T and B cells provoke potent recall responses (IgG and IgE). FDC also provide nonspecific costimulatory signals which augment B‐cell proliferation and Ab production. B cell‐FDC contact is important and interference with ICAM‐1‐LFA‐1 interactions reduces FDC‐mediated costimulation. Preliminary data suggest that a costimulatory signal may be delivered via CRZL on FDC binding CR2 on B cells. FDC can also stimulate B cells to become chemotactically active and can protect lymphocytes from apoptosis. FDC also appear to be rich in that groups and may replace reducing compounds such as 2 mercaptoethanol in cultures. In short, FDC‐Ag specifically signals B cells through BCR, and FDC provide B cells with iccosomal‐Ag necessary for processing to elicit T i ell help. In addition, FDC provide nonspecific signals that are important to promote B‐cell proliferation, maintain viability, and induce chemotactic responsiveness.

Fcγ Receptor IIB on Follicular Dendritic Cells Regulates the B Cell Recall Response

Generation of the B cell recall response appears to involve interaction of Ag, in the form of an immune complex (IC) trapped on follicular dendritic cells (FDCs), with germinal center (GC) B cells. Thus, the expression of receptors on FDC and B cells that interact with ICs could be critical to the induction of an optimal recall response. FDCs in GCs, but not in primary follicles, express high levels of the IgG Fc receptor FcγRIIB. This regulated expression of FcγRIIB on FDC and its relation to recall Ab responses were examined both in vitro and in vivo. Trapping of IC in spleen and lymph nodes of FcγRII−/− mice was significantly reduced compared with that in wild-type controls. Addition of ICs to cultures of Ag-specific T and B cells elicited pronounced Ab responses only in the presence of FDCs. However, FDCs derived from FcγRIIB−/− mice supported only low level Ab production in this situation. Similarly, when FcγRIIB−/− mice were transplanted with wild-type Ag-specific T and B cells and challenged with specific Ag, the recall responses were significantly depressed compared with those of controls with wild-type FDC. These results substantiate the hypothesis that FcγRIIB expression on FDCs in GCs is important for FDCs to retain ICs and to mediate the conversion of ICs to a highly immunogenic form and for the generation of strong recall responses.

Follicular dendritic cells: beyond the necessity of T-cell help

Follicular dendritic cells (FDCs) are potent accessory cells for B cells, but the molecular basis of their activity is not understood. Several important molecules involved in FDC-B-cell interactions are indicated by blocking the ligands and receptors on FDCs and/or B cells. The engagement of CD21 in the B-cell coreceptor complex by complement-derived CD21 ligand on FDCs delivers a crucial signal that dramatically augments the stimulation delivered by the binding of antigen to the B-cell receptor (BCR). The engagement of Fc gamma receptor IIB (FcgammaRIIB) by the Ig crystallizable fragment (Fc) in antigen-antibody complexes held on FDCs decreases the activation of immunoreceptor tyrosine-based inhibition motifs (ITIMs), mediated by the crosslinking of BCR and FcgammaRIIB. Thus, FDCs minimize a negative B-cell signal. In short, these ligand-receptor interactions help to signal to B cells and meet a requirement for B-cell stimulation that goes beyond the necessity of T-cell help.

FcγRII expression on follicular dendritic cells and immunoreceptor tyrosine-based inhibition motif signaling in B cells

Immune complexes (IC) initiate immunoreceptor tyrosine‐based inhibition motif (ITIM) signaling and inhibit B cell activation by coligating B cell receptor for antigen (BCR) and FcγRII. Nevertheless, IC on follicular dendritic cells (FDC) stimulate rapid germinal center (GC) B cell proliferation suggesting that interactions between IC and FDC render IC capable of B cell activation. Tounderstand this, we studied the kinetics of FDC FcγRII and complement receptors 1 and 2 (CR1&2) expressions during the GC reaction and determined whether FDC FcγRII could bind Fc in IC and block ITIM signaling. Mice were immunized with sheep red blood cells (SRBC), and CR1&2 and FcγRII levels in FDC reticula were monitored. The role of FDC FcγRII was studied using anti‐BCR‐stimulated A20 cells. Levels of FDC FcγRII in spleens of SRBC‐injected mice increased within 24 h and were dramatically increased (∼50‐fold) on days 3 and 5. In contrast, CR1&2 levels increased less than twofold. Addition of normal FDC, but not FDC lacking FcγRII, reduced and reversed anti‐BCR‐induced SH2 domain‐containing inositol phosphatase (SHIP)‐1 phosphorylation in A20 cells. FDC wereable to induce normal recall responses even after overnight incubation of the lymphocytes with IC to stimulate ITIM signaling. Engagement of Ig Fc with numerous FcγRII on FDC appears to minimize IC‐induced ITIM signaling. Thus, rapid up‐regulation of FDC FcγRII may explain why poorly immunogenic IC are rendered highly immunogenic when presented by FDC in GC.

Follicular dendritic cell accessory activity crosses MHC and species barriers.

Productive follicular dendritic cell (FDC)‐B cell interactions appear to involve critical ligand‐receptor interactions. Immune complexes (IC) on FDC activate complement and provide FDC with a complement‐derived CD21 ligand (CD21L), which bind CD21, while antigen in the IC binds on the B cell‐BCR. Further, FDC‐FcγRIIB binds Fc regions of antibodies in IC and reduces coligation of BCR and FcγRIIB minimizing an inhibitor of B cell activation. Given that Fc receptors and complement receptors bind immunoglobulins and complement fragments of other species, we reasoned that FDC accessory activity should cross MHC and species barriers. This prediction was tested using memory lymphocytes from OVA‐immune mice and TT‐immune humans in combination with FDC from murine lymph nodes and human tonsils. Human and murine FDC converted IC into potent immunogens (specific antibody increased from background to thousands of ng / ml). MHC barriers did not restrict this activity and human FDC worked with murine lymphocytes and murine FDC worked with human lymphocytes. Furthermore, stimulation via MHC‐dependent allogeneic or zenogeneic mechanisms did not promote antibody production by FDC. Polyclonal responses stimulated by lipopolysaccharide and pokeweed mitogen were also promoted (10 – 100‐fold) and anti‐CD21 blocked FDC activity. These results substantiate the hypothesis that FDC are necessary for strong recall responses and that FDC‐CD21L is critical.

Follicular dendritic cells mediated maintenance of primary lymphocyte cultures for long-term analysis of a functional in vitro immune system.

Primary lymphocyte cultures are important for analysis of cellular and molecular events occurring during immune responses. However, the lymphoid cells (especially B cells) typically only survive for a few days in vitro which limits studies. Establishment of long-term primary lymphocyte cultures where a functioning humoral immune responses can be maintained and regulated is still a challenge. Follicular dendritic cells (FDC) are immune accessory cells that reside in the follicles of secondary lymphoid organs and are known to protect lymphocytes from apoptosis. We hypothesized that addition of FDC to primary lymphocyte cultures may help maintain humoral immune responses in vitro as they do in vivo. To test the hypothesis, freshly isolated lymphocytes were cultured with or without FDC. The B cells in cultures were labeled using B220 and apoptotic cells were labeled using the TUNEL assay. Antibody production was monitored in supernatant fluids using ELISA. The results showed that FDC reduced apoptosis and helped sustain primary lymphocyte cultures and antibody production was maintained throughout the entire period (e.g., 8 weeks). This FDC dependent system should be useful for analysis of cellular and molecular events over extended periods in vitro.

A Role for CR2 in FDC-B Cell Interactions

If animals lack C3 or if C3 is destroyed by cobra venom factor, antibody responses are dramatically depressed1–5. Treatment of animals with mAb against CR2 or CD19, which are part of the CD19, CR2 TAPA-1 complex on B cells, also result in dramatically depressed antibody responses6–8. Furthermore, if animals are treated with a soluble construct of CR2, which will bind C3 fragments, the ability to mount a humoral response is markedly suppressed’. In addition, a recent study of CR2 knockout mice revealed that B cell expression of CR2 is required for immune responses to T-dependent antigens9. Furthermore, complement markedly lowers the threshold at which B cells respond to antigen and this effect may be attributable to the co-ligation of CR2 to BCR via the association with C3b-associated antigen10,11. It is also known that cross-linking of CR2 on the B cell by multiple C3b fragments on a carrier renders B cells more easily stimulated by mitogens including anti-μ12,13. These results suggest that CR2 is associated with an important signaling mechanism which is involved when B cells proliferate and differentiate into antibody forming cells (AFC).

Iccosomes and Induction of Specific Antibody Production in Vitro

Follicular dendritic cells (FDCs) are immune accessory cells, located in lymphoid follicles1. FDCs can trap and retain antigen antibody complexes on their long slender dendrites for a long periods of time2 and it appears that antigen retained on FDCs can induce primed B cells to make specific antibody3. FDCs can form liposome-like immune complexes coating bodies (iccosomes)4. These iccosomes may be released and then engulfed by adjacent B lymphocytes. The engulfed iccosomes appeared to disintegrate in the B cells close to Golgi complex. We believed the iccosomal Ag is being processed and Ag fragments are being delivered to the B cell surface for presentation to T cells5. Antibody forming cells (AFC) emerge in germinal centers 3 to 4 days after antigen challenge6. The coincidence of iccosome release and AFC emergence led to the hypothesis that antigen bearing iccosomes might be able to induce B cells to differentiate into AFC7. In this study, we sought to test the hypothesis that iccosomal Ag is involved in the induction of specific Ab production. Iccosomes were prepared from FDCs 3 days after OVA challenge by sonicating FDCs or shaking them in the upper chamber of a transwell apparatus with a 3.0 um pore diameter filter and collecting icccosomes from the lower chamber of the apparatus. Both iccosome preparations were able to induce OVA primed lymphocytes to produce anti-OVA. However intact FDCs were critical for optimal Ab production.