Follicular dendritic cells

Follicular dendritic cells (FDCs) are cells of the immune system found in primary and secondary lymph follicles of the B cell areas of the lymphoid tissue. These cells were first described in 1965 and, although they have a very dendritic morphology, are not dendritic cells (DCs). Unlike DCs, FDCs are not derived from the bone-marrow hematopoietic stem cell, but are of mesenchymal origin.

Follicular DCs are a non-migratory population found in primary and secondary follicles of the B cell areas of lymph nodes, spleen, and mucosa-associated lymphoid tissue (MALT). They form a stable network due to intercellular connections between FDCs processes and intimate interaction with follicular B cells. Follicular DCs network typically forms the center of the follicle and does not extend from the follicle to the interfollicular regions or T- cell zone. Supposedly, this separation from the sites of earliest antigen processing and capture provide a protected environment in which opsonized antigens can be displayed for a long time without being proteolyzed or removed by phagocytic cells. Follicular DCs have high expression of complement receptors CR1 and CR2 (CD 35 and CD 21 respectively) and Fc-receptor FcγRIIb (CD32). Further FDCs specific molecular markers are FDC-M1, FDC-M2 and C4. Unlike other DCs and macrophages, FDCs lack MHC class II antigen molecules and express few pattern-recognition receptors, so they have little ability to capture non-opsonized antigens.

Follicular DCs develop from putative mesenchymal precursors. Severe combined immunodeficiency (SCID) mice models demonstrate that these precursors may be transmitted to recipients with bone marrow allotransplants, in which case both donors' and recipients' FDCs networks may later be found in recipients' lymphoid compartments. Interaction between FDCs precursors and lymphoid cells mediated by TNF-a and lymphotoxin (LT) is crucial for normal FDC development and maintenance. TNF-a binds on the TNFRI receptor, while LT interacts with LTβ-receptor expressed on FDC precursors. In mice lacking B cells, or with blocked TNF-a and lymphotoxin (LT) production, cells with FDC phenotype are missing.

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