Andras K. Szakal

Follicular Dendritic Cells as Accessory Cells

Follicular dendritic cells (FDC) represent a novel cell type found in the B-cell regions of all secondary lymphoid tissues. The name follicular dendritic cell and the abbreviation FDC have been recommended by a committee on nomenclature (Tew et al. 1982) but a number of names have been used in the literature including: dendritic reticulum cells (DRC), antigen retaining reticular cells, follicular ii6mune complex retaining cells, follicular antigen-binding dendritic cells, follicular reticular cells, follicular dendritic reticulum cells, Nossals follicular dendritic reticulum cells and dendritic macrophages (Tew et al. 1982). However, in recent years the name follicular dendritic cell has been generally adopted. Two features differentiate FDC from other accessory cells and are considered diagnostic or cardinal: 1) these dendritic cells are restricted to lymphoid follicles and 2) they trap and retain immune complexes on their surfaces for long periods of time (Tew et al. 1982). It was these two features that caught the attention of Szakal & Hanna as well as Nossal et al. in 1968, when both groups identified FDC as the cells responsible for retaining antigen. Important features which help to further characterize FDC are listed in Table I. This is a selected series of markers chosen to emphasize relationships between FDC and other leukocytes as well as discrepancies in the literature. References at the bottom of Table I contain some further phenotypic information. In addition to the cardinal features relating to anatomical location and retention of surface immune complexes, a number of other features appear to be highly consistent among FDC in different species. These include: lack of phagocytie activity, highly convoluted dendritic processes, an irregularly shaped euchromatic nucleus, presence of receptors for complement fragments, and the expression of the adhesion molecule ICAM-1. As indicated in Table I, there are features where the

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.

Germinal Centers and Antibody Production in Bone Marrow

The bone marrow is the major site of immunoglobulin production during anamnestic antibody respotises (Benner et al. 1974, I98Ib, 1981a. Haaijman et al. 1979). However, antigen trapping, processing and presentation occur in secondary lymphoid tissues tiear the site of immunological challenge (Mandel et ah 1980, Szakal et al. 1989). It appears that B cells stimulated by antigen in the secondary lymphoid tissues migrate from these tissues to the bone marrow where they produce the large amounts of antibody associated with secondary immune responses (Koch et al. 1981, Betiner et al. 1977). Thus the site where antigen presentation takes place may be separated by a considerable distance from the site where most immunogiobuiin production occurs. The emigration of these stimulated B cells appears to occur within a few days after secotidary challenge (Koch et al. 1981, Benner et al. 1977). For example, splenectomy of mice 4 or more days after a booster immunization does not influence the bone marrow plaque-forming cell response, whereas splenectomy on the 2nd d prevents antibody formation in bone marrow (Benner et al. 1977). Furthermore, it is known that large numbers of antigen-specific B-cell blasts are present in the efferent lymph of the draining lymph nodes of sheep by 60 hours following secondary immunization (Hay et al. 1972). About 40% of these cells in the efferent lymph of sheep are classified as blast cells and 50-66% of these blast cells were specific immature antibody-forming cells (AFC) (Hay et al. 1972). In other studies, an intlux of blast cells in the bone marrow was detected usitig autoradiography and this inllux also occurred a few days after antigen challenge (Koch et al. 1981), Germinal centers are characterized as loci of rapidly proliferating B cells

Follicular dendritic cells (FDC) in retroviral infection : host/pathogen perspectives

Summary: Follicular dendritic cells (FDC) are found in the follicles of virtually all secondary lymphoid tissues. In health, these cells trap and retain antigens (Ag) in the form of immune complexes and preserve them for months in their native conformation. FDC thus serve as a long‐term repository of extracellular Ag important for induction and maintenance of memory responses. In retroviral infection. FDC trap and retain large numbers of retroviral particles with profound effects on FDC. FDC‐trapped retrovirus induces follicular hyperplasia, and conventional Ag trapped prior to infection are lost and new Ag cannot be trapped. Concomitantly, antibody‐forming cells (AFC) specific for Ag lost from FDC decrease follow I by loss of specific serum antibody (Ab). Eventually, FDC die and follicular lysis occurs. From the pathogen perspective, binding to FDC is remarkably beneficial, bringing together virus and activated target cells that are highly susceptible to infection. Furthermore, FDC permit HIV to infect surrounding cells even in the presence of a vast excess of neutralizing Ab. Preliminary data suggest that FDC maintain virus infectivity ‐ even when the virus cannot replicate. Thus retrovirus infection monopolizes FDC networks, thereby transforming the FDC Ag repository into a highly infectious retroviral reservoir.

Follicular Dendritic Cells and the Persistence of HIV Infectivity: The Role of Antibodies and Fcγ Receptors

Large quantities of HIV are found trapped on the surface of follicular dendritic cells (FDCs), and virus persists on these cells until they ultimately die. We recently found that FDCs maintain HIV infectivity for long periods in vivo and in vitro. Because FDCs trap Ags (and virus) in the form of immune complexes and are rich in FcγRs, we reasoned that Ab and FcγRs may be required for FDC-mediated maintenance of HIV infectivity. To investigate this hypothesis, HIV immune complexes were formed in vitro and incubated for increasing times with or without FDCs, after which the remaining infectious virus was determined by HIV-p24 production in rescue cultures. FDCs maintained HIV infectivity in vitro in a dose-dependent manner but required the presence of specific Ab for this activity regardless of whether laboratory-adapted or primary X4 and R5 isolates were tested. In addition, Abs against either virally or host-encoded proteins on the virion permitted FDC-mediated maintenance of HIV infectivity. We found that the addition of FDCs to HIV immune complexes at the onset of culture gave optimal maintenance of infectivity. Moreover, blocking FDC-FcγRs or killing the FDCs dramatically reduced their ability to preserve virus infectivity. Finally, FDCs appeared to decrease the spontaneous release of HIV-1 gp120, suggesting that FDC-virus interactions stabilize the virus particle, thus contributing to the maintenance of infectivity. Therefore, optimal maintenance of HIV infectivity requires both Ab against particle-associated determinants and FDC-FcγRs.

Age-related depression of FDC accessory functions and CD21 ligand-mediated repair of co-stimulation.

Morphological and kinetic studies of immune complex (IC) trapping by follicular dendritic cells (FDC) show marked age‐related deficits. We postulated that a reduction in trapped IC, which generate CD21 ligands (L) on FDC, would lead to inadequate FDC‐Ag‐B cell interactions resulting in depressed Ab responses. To determine whether the age‐related defect was the result of the aging of FDC or changes in the in vivo microenvironment of FDC (i.e. aging B and T cells), FDC‐B cell‐T cell‐Ag interactions were studied in in vitro germinal centers where various combinations of old and young cells could be compared. Since we reasoned that reduced IC on FDC would generate less CD21L needed to stimulate the B cell co‐receptor via CD21, we also examined the role of complement (C′). The hypothesis that aging reduces the accessory activity of FDC was tested with increasing numbers of FDC from young (12 weeks) or old (20 months) mice in the presence of young (12 weeks) B and T lymphocytes. The Ag‐specific stimulatory activity of FDC was studied using the OVA‐specific Ab response which was reduced by 40–50% in the presence of old FDC. Antigen‐independentFDC‐mediated co‐stimulation was studied by using LPS to stimulate B‐lymphocytes to produce immunoglobulin (Ig). In the presence of old FDC, co‐stimulation was decreased by 70–80% in the LPS system.Incubation of aged FDC with IC and C′ to provide FDC with CD21L restored co‐stimulatory activity to near normal levels. In marked contrast, no defects in old B and T cells were apparent. The data suggest that the Ag handling capacity and co‐stimulatory activity of old FDC become defective with aging and this appears to be a consequence of reduced trapping and presentation FDC‐Ag and CD21L to B cells.

Appearance and phenotype of murine follicular dendritic cells expressing VCAM‐1

The architecture of lymphoid follicles is determined by a series of interactions between lymphoid and follicular stromal cells. A cardinal population in the non‐lymphoid compartment is the follicular dendritic cell (FDC), whose communication with resting and activated B cells involves various adhesive interactions. The FDC phenotype variably includes the display of vascular cell adhesion molecule (VCAM‐1). In this report we investigated the appearance and follicular tissue distribution of VCAM‐1 in murine peripheral lymphoid tissues, and compared VCAM‐1 with other FDC markers using immunohistochemistry. Correlating the appearance of VCAM‐1 with other murine FDC‐associated markers (CR1.2 [complement receptor 1.2 or CD35/21] and FDC‐M1) revealed that the display of VCAM‐1 is restricted to a subset of CR1.2‐positive FDCs. We found that the expression of VCAM‐1 antigen in the spleen or peripheral lymph nodes on FDCs requires antigenic stimulus, and that it coincides with germinal center formation. The VCAM‐1 expression is associated with the appearance of mucosal addressin cell adhesion molecule (MAdCAM‐1), with some slight differences in occurrence. The appearance of VCAM‐1 and MAdCAM‐1 antigens on FDCs may serve as indicators of FDC activation. Anat Rec 268:160–168, 2002.

Molecular interactions of FDCs with B cells in aging.

Follicular dendritic cells (FDCs), as accessory cells to B cells, promote germinal center (GC) development. Age-related defects in the role of FDCs are well documented in vivo. In old mice, FDCs bind fewer immune complexes (ICs) and produce few iccosomes for endocytosis by B cells, antigen processing, and presentation to T cells. We recently studied whether these defects are due to changes in the FDC microenvironment or to changes in FDCs and their surface molecules. In vitro evidence suggests that age-related defects in both B cell stimulation via the BCR and co-stimulation via CD21/CD21L are related to IC-trapping by FDCs in vivo-a defect which is repairable, at least, in vitro.

Targeted Deletion of CD44v7 Exon Leads to Decreased Endothelial Cell Injury but Not Tumor Cell Killing Mediated by Interleukin-2- activated Cytolytic Lymphocytes*

In the current study, we investigated the nature and role of CD44 variant isoforms involved in endothelial cell (EC) injury and tumor cell cytotoxicity mediated by IL-2-activated killer (LAK) cells. Treatment of CD44 wild-type lymphocytes with IL-2 led to increased gene expression of CD44 v6 and v7 variant isoforms and to significant induction of vascular leak syndrome (VLS). CD44v6-v7 knockout (KO) and CD44v7 KO mice showed markedly reduced levels of IL-2-induced VLS. The decreased VLS in CD44v6-v7 KO and CD44v7 KO mice did not result from differential activation and expansion of CD8+ T cells, NK, and NK-T cells or from altered degree of perivascular lymphocytic infiltration in the lungs. LAK cells from CD44v7 KO mice showed a significant decrease in their ability to adhere to and mediate lysis of EC but not lysis of P815 tumor cells in vitro. CD44v7-mediated lysis of EC by LAK cells was dependent on the activity of phosphatidylinositol 3-kinase and tyrosine kinases. Interestingly, IL-2-activated LAK cells expressing CD44hi but not CD44lo were responsible for EC lysis. Furthermore, lysis of EC targets could be blocked by addition of soluble or enzymatic cleavage of CD44v6-v7-binding glycosaminoglycans. Finally, anti-CD44v7 mAbs caused a significant reduction in the adherence to and killing of EC and led to suppression of IL-2-induced VLS. Together, this study suggests that the expression of CD44v7 on LAK cells plays a specific role in EC injury and that it may be possible to reduce EC injury but not tumor cell killing by specifically targeting CD44v7.

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.