Follicular dendritic cells produce IL-15 that enhances germinal center B cell proliferation in membrane-bound form

Factors that control the survival and proliferation of Ag-stimulated B cells within the germinal center (GC) are crucial for humoral immune responses with high affinity Abs against infectious agents. The follicular dendritic cell (FDC) is known as a key cellular component of the GC microenvironment for GC-B cell survival and proliferation. In this study, we report that IL-15 is produced by human FDC in vivo and by an FDC cell line, FDC/HK cells, in vitro. IL-15 is captured by IL-15Ralpha on the surface of FDC/HK cells. The surface IL-15 is functionally active and augments GC-B cell proliferation. Because GC-B cells have the signal-transducing components (IL-2/15Rbetagamma), but not a receptor for binding of soluble IL-15 (IL-15Ralpha), IL-15 signaling is possibly transduced by transpresentation from FDCs to GC-B cells via cell-cell contact. Together, these results suggest that IL-15 from FDC, in membrane-bound form, plays an important role in supporting GC-B cell proliferation, proposing a new target for immune modulation as well as treatment of B cell tumors of GC origin.     

Distinct role of follicular dendritic cells and T cells in the proliferation, differentiation, and apoptosis of a centroblast cell line, L3055

Germinal center (GC) B cells undergo complex interactions with follicular dendritic cells (FDC) and T cells in the course of differentiation into memory B and plasma cells. To delineate the individual roles of FDC and T cells at each stage of GC B cell differentiation at the clonal level and to analyze the signals involved, we adopted a unique experimental model using an FDC line, HK, and a lymphoma cell line, L3055, that resembles centroblasts. A detailed phenotypic analysis revealed L3055 cells to be a clonal population originating from the GC. Like freshly isolated centroblasts, L3055 cells underwent spontaneous apoptosis when cultured in the absence of fresh FDC or HK cells. L3055 cells proliferated continuously in the presence of HK cells, while they differentiated into a population with the phenotype of centrocytes after stimulation with CD40 ligand (CD40L) and IL-4. The CD40L-stimulated L3055 cells underwent CD95-mediated apoptosis, which was reminiscent of the feature of CD40L-stimulated tonsillar GC B cells. In contrast to HK cells that did not protect L3055 cells from anti-Ig killing, CD40L plus IL-2, IL-4, and IL-10 prevented anti-Ig-induced apoptosis. These experimental results demonstrate a distinct function of FDC and activated T cells, in that FDC provide signals for rapid proliferation of centroblasts, whereas T cells confer signals for differentiation of centroblasts into centrocytes and resistance to B cell receptor-mediated apoptosis. T cells collaborate with FDC in the protection and expansion of the Ag-specific GC B cells.     

Differential expression of the inhibitory IgG Fc receptor FcgammaRIIB on germinal center cells: implications for selection of high-affinity B cells

The murine low-affinity receptor for IgG, FcgammaRIIB, mediates inhibition of B cell receptor-triggered events in primary B cells. We investigated the expression of FcgammaRIIB on germinal center (GC) cells to better understand its role in memory B cell development. Immunohistological analyses demonstrated differential regulation of FcgammaRIIB on GC cells. Its levels are markedly down-regulated on GC B cells and up-regulated on follicular dendritic cells (FDC) at all times during the GC response. Analyses of surface expression of FcgammaRIIB by flow cytometry and FcgammaRIIB mRNA levels by RT-PCR analysis confirmed that this FcR is down-regulated in GC B cells. In mice lacking FcgammaRIIB, the development of the secondary FDC reticulum in GCs is substantially delayed, although the overall kinetics of the GC response are unaltered. These findings have direct implications for models proposed to account for the selection of high-affinity B cells in the GC and suggest a role for FcgammaRIIB in promoting the maturation of the FDC reticulum.     

Memory B cells express a phenotype consistent with migratory competence after secondary but not short-term primary immunization

The cell surface phenotype of dinitrophenol (DNP)-specific memory B cells, defined by their capacity to transfer IgG responses into syngeneic irradiated recipients, was assessed using two markers of relevance to lymphocyte migratory properties: (i) peanut agglutinin, which binds to terminal galactosyl residues expressed at high levels by several nonmigrating lymphocyte subsets and, among lymph node B cells, is highly specific for germinal center cells; and (ii) MEL-14, a monoclonal antibody specific for lymphocyte surface receptors required for migration from the blood into peripheral lymph nodes. At various times after primary or secondary immunization with DNP-keyhole limpet hemocyananin (KLH), lymph node B cells were separated by fluorescence-activated cell sorting on the basis of staining with PNA and/or MEL-14, and the presence of B-memory cells in each fraction was assessed by adoptive transfer with antigen (DNP-KLH) and helper T cells. One week after immunization, most of the memory sorted in the PNAhi population, confirming a previous report by R. F. Coico, B. S. Bhogal, and G. J. Thorbecke (J. Immunol. 131, 2254, 1983) that early memory B cells or their precursors are contained within the germinal center cell population, a population which is known to be MEL-14- and migratory-incompetent. Six weeks after primary stimulation, however, the bulk of memory cells, unlike germinal center cells, were MEL-14hi. After secondary immunization, memory was still predominantly MEL-14+ and PNAlo, although in some experiments adoptive responses were transferred by all sorted fractions. The results are consistent with the hypothesis that antigen-specific B cells initially undergo local (sessile) differentiation and proliferation in germinal centers, where they develop the capacity for adoptive transfer of antigen-specific secondary responses, but that with continued development their long-lived memory-containing progeny express a phenotype permitting their reentry into the recirculating lymphocyte pool.     

The role of antigen-presenting B cells in T cell priming in vivo. Studies of B cell-deficient mice

Mice rendered B cell deficient by treatment with rabbit anti-mouse IgM (anti-mu) antibodies from birth fail to respond when primed with soluble protein antigens in CFA, as measured by T cell proliferation when challenged with antigen in vitro. The role of B cells in T cell priming in vivo was examined by adoptively transferring hapten-specific B cells into anti-mu mice, followed by immunization with haptenated Ag in CFA. The T cell proliferative response to OVA of anti-mu BALB/c mice was partially restored by the administration of TNP or FITC-specific B cells and immunization with TNP-OVA or FITC-OVA, respectively. This reconstitution was Ag-specific, inasmuch as hapten-binding B cells restored the T cell responses to OVA in mice immunized with the same hapten coupled to OVA. The mechanism of B cell reconstitution of T cell priming in anti-mu mice was addressed using parental to F1 B cell transfers. The Ia restriction pattern of the activated T cells from these mice indicated that both direct presentation of Ag by transferred B cells and antibody-mediated enhancement of Ag presentation by non-B, host Ag-presenting cells occurred. Thus, Ag-specific B lymphocytes play a critical role in priming of T cells in vivo.     

Relationship of germinal centers in lymphoid tissue to immunologic memory. VI. Transfer of B cell memory with lymph node cells fractionated according to their receptors for peanut agglutinin

We isolated germinal center B cells by exploiting their high affinity for peanut agglutinin (PNA). The PNA+ and PNA- B cells, fractionated by panning on PNA-coated petri dishes, were examined for their ability to transfer memory responses to irradiated recipients at various times after priming. With such fractionated B cells from lymph nodes taken at the peak of germinal center formation, the largest response was obtained in recipients of the PNA+ B cell population. At 4 to 5 wk after priming, and 10 days after challenge with an unrelated antigen, memory responses were approximately equal in recipients of PNA+ or PNA- B cells. At 14 wk after priming, memory responses were found only in recipients of the PNA- B cell population. Memory B cells from the spleen, taken from mice primed in the footpad 8 wk earlier, were also PNA-. Finally, we show that boosting with a TNP-conjugate in the footpad, 6 mo after priming in the same footpad, induced the reappearance of marked memory responsiveness in the PNA+ B cell fraction of the draining node.     

Regulation of Adhesion and Migration in the Germinal Center Microenvironment

T cell dependent humoral immune responses are initiated by the activation of naive B cells in the T cell areas of the secondary lymphoid tissues. This primary B cell activation leads to migration of germinal center (GC) cell precursors into B cell follicles where they engage follicular dendritic cells (FDC) and T cells, and differentiate into memory B cells or plasma cells. Both B cell homing to the GC and interaction with FDC critically depend on integrin-mediated adhesion. We have recently indentified the c-met-encoded receptor tyrosine kinase and its ligand, the growth and motility factor hepatocyte growth factor/scatter factor (HGF/SF), as a novel paracrine signalling pathway regulating B cell adhesion (van der Voort et al., 1997, J. Exp. Med. 185, 2121–2131). The c-Met protein is expressed on B cells localized in the dark zone of the GC (centroblasts) and is induced by CD40 plus BCR ligation. Stimulation of c-Met with HGF/SF. which is produced at high levels by tonsillar stromal cells and FDC, leads to receptor phosphorylation and to enhanced integrin-mediated adhesion of B cells to both VCAM-l and fibronectin. Interestingly, these responses to HGF/SF are promoted by heparan-sulfate proteoglycan forms of CD44 (CD44-HS). Like c-Met, CD44-HS is induced on B cells by CD40 ligation. It efficiently binds HGF/SF and strongly promotes signalling through c-Met. We conclude that integrin regulation during antigen specific B cell differentiation involves cross-talk between the HGF/SF-c-Met pathway and CD44-HS.     

Complementary effects of TNF and lymphotoxin on the formation of germinal center and follicular dendritic cells

The formation of germinal centers (GC) around follicular dendritic cells (FDC) is a critical step in the humoral immune responses that depends on the cooperative effects of B cells and T cells. Mice deficient in either TNF or lymphotoxin (LT) fail to form both GC and FDC network in B cell follicles. To test a potential complementary effect of TNF and LT, a mixture of bone marrow cells from TNF(-/-) mice and LT alpha(-/-) mice was transferred into irradiated LT alpha(-/-) mice or TNF(-/-) mice. Interestingly, the formation of both GC and FDC clusters in B cell follicles was restored in such chimeric mice, suggesting that TNF and LT from different cells could complement one another. To identify the exact contributions of each subset to the complementary effect of TNF and LT, different sources of T and B cells from LT alpha(-/-) mice or TNF(-/-) mice were used for reconstitution. Our study demonstrates that either T or B cell-derived TNF is sufficient to restore FDC/GC in the presence of LT-expressing B cells. However, TNF itself is not required for GC reactions if the FDC network is already intact. Thus, the development and maintenance of these lymphoid structures depend on a delicate interaction between TNF and LT from different subsets of lymphocytes.     

FDC-SP,a novel secreted protein expressed by follicular dendritic cells

To define better the molecular basis for follicular dendritic cell (FDC) function, we used PCR-based cDNA subtraction to identify genes specifically expressed in primary FDC isolated from human tonsils. In this work we report the discovery of a novel gene encoding a small secreted protein, which we term FDC-SP (FDC secreted protein). The FDC-SP gene lies on chromosome 4q13 adjacent to clusters of proline-rich salivary peptides and C-X-C chemokines. Human and mouse FDC-SP proteins are structurally unique and contain a conserved N-terminal charged region adjacent to the leader peptide. FDC-SP has a very restricted tissue distribution and is expressed by activated FDCs from tonsils and TNF-alpha-activated FDC-like cell lines, but not by B cell lines, primary germinal center B cells, or anti-CD40 plus IL-4-activated B cells. Strikingly, FDC-SP is highly expressed in germinal center light zone, a pattern consistent with expression by FDC. In addition, FDC-SP is expressed in leukocyte-infiltrated tonsil crypts and by LPS- or Staphylococcus aureus Cowan strain 1-activated leukocytes, suggesting that FDC-SP can also be produced in response to innate immunity signals. We provide evidence that FDC-SP is posttranslationally modified and secreted and can bind to the surface of B lymphoma cells, but not T lymphoma cells, consistent with a function as a secreted mediator acting upon B cells. Furthermore, we find that binding of FDC-SP to primary human B cells is markedly enhanced upon activation with the T-dependent activation signals such as anti-CD40 plus IL-4. Together our data identify FDC-SP as a unique secreted peptide with a distinctive expression pattern within the immune system and the ability to specifically bind to activated B cells.     

Peanut agglutinin. I. A new tool for studying T lymphocyte subpopulations.

Fluorescein-coupled peanut agglutinin (PNA) has been used at the single-cell level to study mouse lymphocyte subpopulations. PNA not only binds to most thymocytes, as has already been shown by other authors, but also binds to a small fraction of peripheral lymphocytes that are all T cells (theta+Ig-) or null cells (theta-Ig-). Most PNA-positive thymocytes are sensitive to in vivo corticosteroids and irradiation (450 rads) treatments. Conversely, the positive spleen cells (5% of total spleen lymphocytes) are essentially resistant to corticosteroids and irradiation. Study of PNA binding during ontogenesis shows the occurrence of PNA-positive cells in the fetal liver before thymus constitution and in the very beginning of embryonic thymus and spleen development. These data indicate that PNA is a marker of early T cell subpopulations but that there are probably several distinct subsets of PNA-positive T cells.     

A novel in vivo follicular dendritic cell-dependent iccosome-mediated mechanism for delivery of antigen to antigen-processing cells

Recent scanning electron microscopic studies on isolated follicular dendritic cells (FDC) showed that dendrites of certain FDC were "beaded," i.e., consisting of a series of interconnected immune complex coated bodies (termed "iccosomes," measuring 0.3 to 0.7 micron diameter). In vitro these iccosomes detach from one another with ease. The major objectives herein were to establish whether these structures can be detected in sections and whether iccosomes serve to disseminate antigen in vivo. Beginning at day 1, the time point used for isolating beaded FDC, the popliteal lymph nodes of immune C3H mice were studied with light and transmission electron microscopy for 2 wk (i.e., at days 1, 3, 5, 8, and 14) after hind footpad injection of the histochemically detectable antigen, horseradish peroxidase (HRP). Iccosomes (0.25 to 0.38 micron diameter), contoured by a peroxidase (PO)-positive coat of HRP-anti-HRP complexes, were first detected by transmission electron microscopy at day 1 adjacent to cell bodies of certain FDC. Within their limiting membrane they contained flocculent material that was PO positive. At day 3 by light microscopy, germinal centers were seen enlarged and the antigen-retaining reticulum, composed of antigen-bearing FDC, appeared diffuse. This coincided with the transmission electron microscopic visualization of a dispersed state of iccosomes among the follicular lymphocytes. At that time iccosomes were seen attached to the surface of lymphocytes via PO-positive immune complexes and were surrounded by microvillous processes of these cells. Germinal center lymphocytes and tingible body macrophages both responded to contact with iccosomes by endocytosis. Antigen-containing tingible body macrophage were most conspicuous by light microscopy at day 5, when transmission electron microscopy showed that the majority of germinal center lymphocytes contained endocytosed HRP in secondary lysosome-like granules associated with the Golgi apparatus. The number of dispersed iccosomes was markedly reduced by day 5. In controls injected with HSA, a PO-negative antigen, lymphocytes and tingible body macrophages were PO-negative. The presence of antigen in both cell types was confirmed through the use of a gold-conjugated antigen (goat IgG). Simultaneous immunoperoxidase labeling of the same tissues with anti-Ia showed the gold conjugate containing B cells to be Ia+. Antigen-positive B cells and tingible body macrophages were greatly reduced in numbers by day 14, suggesting the intracellular fragmentation of the antigen.(ABSTRACT TRUNCATED AT 400 WORDS)     

Germinal center cells are a major IL-5-responsive B cell population in peripheral lymph nodes engaged in the immune response

Germinal center formation and the development of B cell memory in lymphoid tissue is a T cell-dependent process. The specific B cell-T cell interactions, and/or cytokines, resulting in germinal center cell growth have not yet been identified. Germinal center B cells were separated from other lymph node (LN) B cells by panning on peanut agglutinin (PNA)-coated dishes. Resulting fractions enriched for PNA+ (germinal center) B cells, and the PNA- (other) LN B cells from immune SJL mice were assayed for proliferation in the presence of cytokines. PNA+ and PNA- B cells responded equally to IL-4 in the anti-mu co-stimulator assay. In contrast, PNA+ B cells responded to murine (r)IL-5 or human B cell growth factor in the dextran sulfate (DxS) co-stimulator assay, to a much greater degree than did PNA- B cells. The same results were obtained with PNA+ and PNA- cells from LAF1 mice. Unfractionated LN B cells from nonimmunized SJL or BALB/c mice did not respond to IL-5 with or without DxS. B cell populations from BALB/c mice such as from spleen and peritoneal cavity, which are known to be high in Ly-1+B cells, responded to IL-5 alone, and more dramatically, to IL-5 as a co-stimulator with DxS. Such populations of cells from SJL mice, which are known to contain low numbers of Ly-1+B cells, responded markedly less. These results are consistent with those of others which show that in nonimmunized mice, Ly-1+B cells are a major IL-5 responsive subpopulation. IL-1 enhanced the proliferation of PNA+ cells in response to rIL-5 and had no effect on PNA- cells. IL-4 and IL-5 did not enhance each other's effects as co-stimulators of proliferation. In contrast to PNA+ B cells from immune LN, B cells activated by Escherichia coli endotoxin exhibited no responses to rIL-5. The present results indicate that in immune LN, PNA+, germinal center B cells constitute a prominent IL-5-responsive population.     

Follicular dendritic cell-dependent adhesion and proliferation of B cells in vitro.

In response to an antigenic challenge, B cells proliferate in germinal centers within secondary lymphoid tissue. Specialized accessory cells, follicular dendritic cells (FDC), and T cells are necessary to drive this reaction. Indirect evidence suggests that FDC provide signals which not only induce B cell proliferation but can rescue B cells programmed to die by apoptosis. An in vitro system was developed to: 1) define the role of FDC and 2) identify molecules involved in this response. Activated, low density B cells and T cells were coisolated with FDC from immune mouse lymph nodes. Upon culturing, large cellular aggregates formed, composed of 1 to 3 FDC interdigitating between 30 to 90 B cells and 1 to 5 T cells. Many of these B cells were undergoing DNA synthesis. Depleting FDC or T cells from the cultures immediately stopped cluster formation and proliferation. Separating clustered vs nonclustered cells revealed that the FDC-associated population remained viable, whereas cells in suspension became apoptotic. The adhesion/activation molecules ICAM-1, LFA-1, and CD44 supported both cluster formation and proliferation. In addition, anti-class II and anti-kappa L chain mAb interfered dramatically with DNA synthesis. This model mimics many of the features of a germinal center and can be used to further study B cell activation, proliferation, and differentiation in vitro.     

Unresponsiveness to lymphoid-mediated signals at the neonatal follicular dendritic cell precursor level contributes to delayed germinal center induction and limitations of neonatal antibody responses to T-dependent antigens

The factors limiting neonatal and infant IgG Ab responses to T-dependent Ags are only partly known. In this study, we assess how these B cell responses are influenced by the postnatal development of the spleen and lymph node microarchitecture. When BALB/c mice were immunized with alum-adsorbed tetanus toxoid at various stages of their immune development, a major functional maturation step for induction of serum IgG, Ab-secreting cells, and germinal center (GC) responses was identified between the second and the third week of life. This correlated with the development of the follicular dendritic cell (FDC) network, as mature FDC clusters only appeared at 2 wk of age. Adoptive transfer of neonatal splenocytes into adult SCID mice rapidly induced B cell follicles and FDC precursor differentiation into mature FDC, indicating effective recruitment and signaling capacity of neonatal B cells. In contrast, adoptive transfer of adult splenocytes into neonatal SCID mice induced primary B cell follicles without any differentiation of mature FDC and failed to correct limitations of tetanus toxoid-induced GC. Thus, unresponsiveness to lymphoid-mediated signals at the level of neonatal FDC precursors delays FDC maturation and GC induction, thus limiting primary Ab-secreting cell responses to T-dependent Ags in early postnatal life.     

The role of B cells in the development of CD4 effector T cells during a polarized Th2 immune response

Previous studies have suggested that B cells promote Th2 cell development by inhibiting Th1 cell differentiation. To examine whether B cells are directly required for the development of IL-4-producing T cells in the lymph node during a highly polarized Th2 response, B cell-deficient and wild-type mice were inoculated with the nematode parasite, Nippostrongylus brasiliensis. On day 7, in the absence of increased IFN-gamma, IL-4 protein and gene expression from CD4 T cells in the draining lymph nodes were markedly reduced in B cell-deficient mice and could not be restored by multiple immunizations. Using a DO11.10 T cell adoptive transfer system, OVA-specific T cell IL-4 production and cell cycle progression, but not cell surface expression of early activation markers, were impaired in B cell-deficient recipient mice following immunization with N. brasiliensis plus OVA. Laser capture microdissection and immunofluorescent staining showed that pronounced IL-4 mRNA and protein secretion by donor DO11.10 T cells first occurred in the T cell:B cell zone of the lymph node shortly after inoculation of IL-4-/- recipients, suggesting that this microenvironment is critical for initial Th2 cell development. Reconstitution of B cell-deficient mice with wild-type naive B cells, or IL-4-/- B cells, substantially restored Ag-specific T cell IL-4 production. However, reconstitution with B7-1/B7-2-deficient B cells failed to rescue the IL-4-producing DO11.10 T cells. These results suggest that B cells, expressing B7 costimulatory molecules, are required in the absence of an underlying IFN-gamma-mediated response for the development of a polarized primary Ag-specific Th2 response in vivo.     

Ultrastructural study of highly enriched follicular dendritic cells reveals their morphology and the periodicity of immune complex binding

Follicular dendritic cells (FDCs) are immune accessory cells found in the follicles of secondary lymphoid organs where they promote B cell maturation in germinal centers (GCs) that develop following antigen exposure. Recently, we published a method for isolating functional murine FDCs in high purity. We reasoned that disruption of FDC reticula in vivo would alter FDC morphology. The present study was undertaken to determine the morphological features of isolated FDCs. FDC-M1 and immune complex (IC) labeling were used to identify FDCs in isolated preparations. Results at the light-microscopic level revealed that isolated FDCs trapped ICs, expressed FDC-M1 and cadherins, but generally appeared non-dendritic. However, at the ultrastructural level, the majority of FDCs exhibited dendrites and typical euchromatic nuclei that appeared as single, bilobed, or double nuclei. Based on morphology, four varieties of FDCs were distinguishable, possibly indicative of differences in maturity. Remarkably, ICs trapped by FDCs showed a distinctive periodic arrangement consistent with that known to induce immune responses by thymus independent-2 (TI-2) antigens that engage and cross-link multiple B cell receptors. The ability of FDCs to trap ICs and then display these T-cell-dependent antigens with repeating periodicity suggests that multiple B cell receptors are cross-linked by antigen on FDCs, thus promoting B cell stimulation and proliferation. Rapid proliferation is characteristic of the GC reaction, and the arrangement of T-dependent antigens in this periodic fashion may help to explain the profuse B cell proliferation in the GC microenvironment. This work was supported by National Institutes of Health Grant AI-17142. Electron microscopy and confocal microscopy were performed at the VCU Department of Neurobiology and Anatomy Microscopy Facility supported, in part, by funding from an NIH-NINDS Center core grant (5P30NS047463).     

Differential induction of cytokine genes and activation of mitogen-activated protein kinase family by soluble CD40 ligand and TNF in a human follicular dendritic cell line.

Follicular dendritic cells (FDC)3 play crucial roles in germinal center (GC) formation and differentiation of GC B cells. Many aspects of FDC function are influenced by contact with B or T cells, and by cytokines produced in the GC, which involve stimulation of CD40 and TNF-alpha receptors on FDC. In this study, using an established FDC line, HK cells, we compared the effects of CD40 and TNF receptor triggering on cytokine induction and activation of mitogen-activated protein kinase family. We show that HK cells spontaneously produced IL-6, M-CSF, and G-CSF mRNA. Both the soluble form of CD40 ligand (sCD40L) and TNF increased the level of M-CSF and G-CSF mRNA. While TNF strongly induced IL-6 mRNA, its expression was not affected by sCD40L treatment, differing from the strong IL-6 induction in other cell types upon CD40 stimulation. In addition, sCD40L treatment resulted in activation of extracellular signal-related kinase 1 and 2 (ERK1/2) and p38 without significant increase in c-Jun N-terminal kinase (JNK) activity. Lack of JNK activation differs in that most B cells respond to CD40 stimulation by inducing JNK activity strongly, suggesting distinct characteristics of CD40 signaling in FDC. Compared with the effects of sCD40L, TNF was capable of inducing JNK activity in addition to the activation of ERK1/2 and p38. Furthermore, the proximal signaling elements activated by TNF differed from those activated by sCD40L, in that TNF did not require PMA-sensitive protein kinase C isoforms in the activation of ERK and p38, whereas sCD40L did. However, signals activated by these stimuli converged on cytokine gene expression in a synergistic manner, which may have implication in augmenting FDC function during GC reaction.     

Cell cycle block in anergic T cells during tolerance induction

The induction of anergy, or T cell unresponsiveness to antigen, is preceded by T cell activation and cell division in response to fed antigens. These events parallel the activation observed in T cells following sensitization with antigen and adjuvant. The events that distinguish eventual sensitization versus tolerance remain unclear. Using a T lymphocyte transfer model specific to OVA, we demonstrated previously that oral encounter with antigen leads to functional anergy. Antigen-specific CD4+ T cells nevertheless become activated and cycle briefly after encounter with antigen. In this study, we measured the extent of cell cycling of antigen-specific T cells after oral encounter with their antigen. Whereas T cells cycle on the average of eight times in 4 days after conventional immunization, an abortive proliferation was observed in the draining LN T cells after oral encounter with antigen; OVA-specific T cells divided fewer times after exposure to fed OVA, compared to T cells in mice immunized with OVA. This abortive proliferation is antigen specific and not due to bystander suppression, as coadministration of an unrelated antigen that was previously used as a tolerogen does not alter the degree of abortive proliferation. Measurement of BrdU incorporation in mice that were previously fed ovalbumin indicates that up to 3 days following feeding, OVA-specific cells are actively cycling in vivo. However, by day 4, they have stopped cycling while identical T cells in OVA-sensitized mice continue to cycle. Our results indicate either that tolerance is a default pathway and a secondary stimulus is required at day 3 to progress to sensitization, or that elements that limit cell cycle progression are provided for tolerance induction.     

Tumor immunotherapy by epicutaneous immunization requires langerhans cells

A role for Langerhans cells (LC) in the induction of immune responses in the skin has yet to be conclusively demonstrated. We used skin immunization with OVA protein to induce immune responses against OVA-expressing melanoma cells. Mice injected with OVA-specific CD8(+) T cells and immunized with OVA onto barrier-disrupted skin had increased numbers of CD8(+) T cells in the blood that produced IFN-gamma and killed target cells. These mice generated accelerated cytotoxic responses after secondary immunization with OVA. Prophylactic or therapeutic immunization with OVA onto barrier-disrupted skin inhibited the growth of B16.OVA tumors. LC played a critical role in the immunization process because depletion of LC at the time of skin immunization dramatically reduced the tumor-protective effect. The topically applied Ag was presented by skin-derived LC in draining lymph nodes to CD8(+) T cells. Thus, targeting of tumor Ags to LC in vivo is an effective strategy for tumor immunotherapy.     

Fc gamma 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 Fc gamma RIIB. This regulated expression of Fc gamma RIIB 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 Fc gamma RII-/- 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 Fc gamma RIIB-/- mice supported only low level Ab production in this situation. Similarly, when Fc gamma RIIB-/- 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 FcgammaRIIB 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.