Heterogeneity and cellular origin of follicular dendritic cells

How do follicular dendritic cells interact intimately with B cells in the germinal centre?

The germinal centre is a dynamic microenvironment where antigen-activated B cells rapidly expand and differentiate, generating plasma cells and memory B cells. These cellular events are accompanied by dramatic changes in the antibody molecules that undergo somatic hypermutation and isotype switching. Follicular dendritic cells (FDCs) are the stromal cells located in the germinal centre. Although the capacity of FDCs to present antigen to B cells through antigen-antibody complexes has been recognized for many years, additional critical functions of FDCs have only recently been recognized. FDCs prevent apoptosis of germinal centre B cells and stimulate cellular interaction and proliferation. Here, we review the FDC signalling molecules that have recently been identified, some of which offer potential therapeutic targets for autoimmune diseases and B-cell lymphomas.

Immunohistochemical recognition of human follicular dendritic cells (FDCs) in routinely processed paraffin sections

A number of monoclonal antibodies (MAbs) that recognize human follicular dendritic cells (FDCs) have been identified. Although some of them have already been applied individually in routine immunolabeling using formalin-fixed paraffin sections for diagnostic and experimental purposes, many antibodies are still employed only for immunolabeling using cryostat sections or particularly processed sections because they have been thought unsuitable for routine sections. A comprehensive examination re-evaluating their suitability in paraffin sections has not been reported. Accordingly, there is limited ability to examine the immunopathological contribution or diagnostic value of FDCs using routinely processed specimens or archived materials. In this study a broad panel of antibodies was systematically applied to the immunolabeling of paraffin sections of reactive tonsils or lymph nodes, in combination with advanced antigen retrieval (AR) techniques. Several antibodies, including Ki-M4p, X-11, 12B1, CNA.42, 1F8/BU32 (anti-CD21), BU38/1B12 (anti-CD23), Ber-MAC-DRC/To5 (anti-CD35), 1.4C3 (anti-CD106), NGFR5 (anti-nerve growth factor receptor p75), IIH6 (anti-CD55), 55K-2 (anti-fascin), and anti-S100 protein alpha-chain, were found to label FDCs in routine sections when combined with suitable AR techniques. Our results are easily adaptable for routine practice and provided useful suggestions concerning the immunopathological behavior and diversity of the particular cells.

Dismantling the germinal center: comparing the processes of transformation, regression, and fragmentation of the lymphoid follicle

The pathologic and immunologic features of the formation of the germinal center have been extensively studied. The process of dissolution of the germinal center is not as well understood. Different patterns of germinal-center breakdown are commonly encountered in diagnostic lymph node biopsy specimens and frequently present difficulties in diagnosis. The current immunologic understanding of germinal-center dynamics is reviewed and correlated with the histologic and immunophenotypic features of three broad classes of germinal center dissolution, namely progressive transformation, regression, and follicle fragmentation. The author suggests that these different patterns represent alternate responses to antigenic stimulation. The relationships of progressive transformation to Hodgkin's disease and of follicular regression to Castleman's disease are discussed.

Reticulum cell sarcoma of lymph node with mixed dendritic and fibroblastic features

We report a case of clinically aggressive reticulum cell sarcoma with mixed follicular dendritic cell (FDC) and fibroblastic reticular cell (FRC) features. Histologically, the tumor was confined to lymph nodes occurring as a multifocal epithelioid and spindle cell proliferation with appreciable mitotic rate and numerous admixed non-neoplastic B-cells. Ultrastructural examination revealed elongated cells with prominent nucleoli, interdigitating cell processes and frequent desmosomes. These features are typical of FDC sarcoma. However, immunohistochemical stains showed no expression of antigens characteristic of FDCs, including CD21, CD23 and CD35. Cytogenetic characterization of this tumor, by conventional G-banding and multicolor spectral karyotyping, revealed multiple clonal chromosomal aberrations, including del(X)(p11.4) and add (21)(p11.2). Gene expression analysis by cDNA microarray of RNA obtained from short-term tumor cultures revealed high-level expression of a set of genes (including PDGF receptor-alpha and -beta, certain metalloproteinases, and CD105) that were also highly expressed in cultures of nodal FRC cultured from non-neoplastic lymph nodes. We propose that this tumor represents a nodal sarcoma with intermediate differentiation between FDCs and FRCs. This case adds to the diversity of tumors that may arise from lymph node stroma and supports a possible relationship between the FDC and FRC lineages.

The Warthin-Finkeldey-type giant cell in HIV infection, what is it?

A study was conducted to ascertain the origin of the Warthin-Finkeldey-type giant cell that is common to lymphoid tissues of HIV-infected individuals. Light microscopy (LM) and transmission electron microscopy (TEM), in situ hybridization (ISH) (HIV-specific RNA), and immunohistochemistry (HIV p24, OPD4, CD3, CD45 UCHL, CD21, CD35, S-100, p55 (actin-bundling protein), CD68, HAM56, alpha-1-antitrypsin, alpha-1-antichymotrypsin, and lysozyme) studies were performed on hyperplastic tonsil, adenoid, lymph node, and intestinal MALT specimens from HIV+ patients. Warthin-Finkeldey-type giant cells (WFTGC) and follicular dendritic cells (FDC) shared characteristic morphologic (high N: C ratio; crowded, irregular nuclei; thin filaments with dense bodies; desmosomes; and cilia) and immunophenotypic (CD21+, CD35+, S-100+, p55, and vimentin+) features. Also, transitional forms between binucleated FDC and WFTGC were identified by TEM. TEM and ISH revealed evidence of HIV expression by FDC, but not WFTGC. WFTGC in HIV- lymphoid specimens displayed identical LM and IHC characteristics. The WFTGC in HIV infection appears to represent a multinucleated form of FDC.

Morphology, function and pathology of follicular dendritic cells

The precise ultrastructural morphology and functions in reactive conditions of lymphoid follicles (LF) and dendritic cells, including follicular dendritic cells (FDC) are reviewed; as well as the pathognomonic role of FDC in some disease conditions and finally, the cellular origin of FDC. In reactive conditions, FDC in each of the five follicular zones have distinct ultrastructural features, reflecting the different three-dimensional structures and functions of these zones. The FDC framework may be supported by some characteristic factors, including desmosome-like junctions between FDC and the expression of fibronectin and laminin receptors and caldesmon on FDC. FDC, especially in the light zone, express various cytokine receptors, but produce only one cytokine, TGF-beta. The outer zone may not only be a cellular pathway in the LF, but may also provide a site for germinal center B cell proliferation, and the FDC-lymphocyte cluster is not the site of germinal center B cell division. In patients with auto-immune diseases, such as Hashimoto's thyroiditis and rheumatoid arthritis, FDC may be in a hyperfunctional state, whereas those in patients with immunosuppressive disorders, such as Kimura's disease and AIDS, may be in a dysfunctional state. FDC may be derived from fibroblastic reticulum cells in lymphatic tissues rather than in bone marrow cells. The data discussed in this review provide fascinating insight into the roles of FDC, which are intimately related to the migration, proliferation, cell selection and differentiation of B cells in secondary LF.