F. Bouzahzah

Follicular Dendritic Cells: Origin and Function

The human immune system is made up of one trillion lymphocytes. Although dispersed throughout the organism, these cells behave as if they belonged to a single organ. To attain this homeostasis, the lymphoid cells home to and are controlled in lymphoid tissues where, in specific microenvironments, they communicate with each other or with accessory cells and proliferate, mature, or die under stringently controlled conditions. The germinal center microenvironments are among those controlling the B cells which, during the T-dependent humoral immune responses, undergo important phases of their life cycles: activation, proliferation, the isotype switch, affinity maturation, deactivation, apoptosis, etc. The follicular dendritic cells (FDC) are major components of the germinal center microenvironments. Here, we examine their origin and their influence on B cells in the light of recent experimental data.

Human germinal center CD4^+CD57^+ T cells act differently on B cells than do classical T-helper cells

We have isolated two subtypes of helper T cells from human tonsils: CD4+ CD57+ cells, mostly located in the germinal center (GC), and CD4+ CD57- cells, distributed through the interfollicular areas but also present in the GC. In a functional study, we have compared the capacities of these T-cell subtypes to stimulate B cells in cocultures. In order to block T-cell proliferation while maintaining their activation level, we pretreated isolated T cells with mitomycin C prior to culture in the presence of B cells and added polyclonal activators such as PHA and Con A, combined or not with IL-2. Contrary to CD4+ CD57- cells, CD4+ CD57+ cells did not markedly enhance B-cell proliferation. Even when sIgD-B cells typical of germinal center cells were tested, the CD4 CD57 cells had no significant effect. This is in accordance with the location of these cells: They mainly occupy the light zones of the GC where few B cells divide. Even when added to preactivated, actively proliferating cells, CD4+ CD57+ cells failed to modulate B-cell multiplication. On the supernatants of B-cell-T-cell cocultures, we examined by the ELISA technique the effect of T cells on Ig synthesis. Contrary to CD57- T cells, whose effect was strong, CD57+ T cells weakly stimulated Ig synthesis. More IgM than IgG was generally found. Because CD57 antigen is a typical marker of natural killer cells, we tested the cytolytic activity of tonsillar CD4+ CD57+ cells on K562 target cells. Unlike NK cells, neither CD4+CD57+ nor CD4+ CD57- cells exhibit any cytotoxicity. Thus, germinal center CD4+ CD57+ cells are not cytolytic and do not strongly stimulate either B-cell proliferation or Ig secretion. CD4+ CD57- cells, however, enhance B-cell proliferation and differentiation, thus acting like the classical helper cells of the T-dependent areas.

THE INFLUENCE OF FOLLICULAR DENDRITIC CELLS ON B-CELL PROLIFERATION DEPENDS ON THE ACTIVATION OF B CELLS AND THE MITOGEN USED

Follicular dendritic cells (FDC) are unique non‐lymphoid cells found only in lymph follicles. They play a part in the survival, proliferation and differentiation of B cells. To analyse the influence of FDC on B‐lymphocyte proliferation, we isolatedthem from human tonsils on albumin gradients and treated them with mitomycin C to prevent the multiplication of lymphoid cells harboured in their cytoplasmic evaginations. FDC cultured for 12–16 h remained attached to the substrate;non‐adherent cells were carefully eliminated by washing. Purified B cells cultured alone or with contaminant‐cleared FDC were maintained for 2 days in the presence or absence of various stimulants, after which tritiated thymidine uptake by these cells was measured. In the absence of activators, FDC did not induce B‐cell multiplication. B cells cultured in the presence of FDC exhibited increased 3H‐TdR uptake when activated with anti‐CD40 MoAb, anti‐immunoglobulin MoAb or transferrin, but notwhen stimulated with Staphylococcus aureus strain Cowan I (SAC) at a given concentration. In the latter case, B‐cell proliferation clearly decreased. In control cocultures where mitomycin‐C‐treated non‐adherent cells were used instead of FDC inthe presence of the different stimulants, no increase in B‐cell proliferation was observed. The results suggest that, inside the germinal centres, FDC modulation of B‐cell proliferation depends on the activation state of the B cells and on the stimulantencountered.

Chemotaxis-Promoting and Adhesion Properties of Human Tonsillar Follicular Dendritic Cell Clusters

Lymph follicles are globular and compact due to aggregation of lymphoid cells on follicular dendritic cells (FDC). To probe the mechanisms underlying this accumulation of cells, we analyse here the role played by FDCs in attracting and binding cells. FDCs prepared from human tonsils by mild separation techniques appeared in the form of clusters (FDC clusters), where, via cytoplasmic extensions, they enveloped lymphoid cells. Using Boydens chambers, we demonstrated that these FDC clusters produced one or more chemoattractants capable of inducing chemotaxis of lymphoid cells. Supernatants of FDC cluster cultures also exerted a chemotaxis-promoting effect. FDC clusters induced true chemotaxis, not merely chemokinesis due to cell activation. They secreted a substance or substances that stuck to the substrate (a cellulose filter) and thus induced haptotaxis. B as well as T cells were attracted, but B cells apparently required the presence of T cells to respond fully to the chemoattractant(s). Subtypes of B cells (IgD+ and IgD-) and T cells (CD4+, CD8+, CD57+ AND CD57-) were tested and all were attracted. Since purified lymphoid cells did not induce these phenomena, FDCs were suspected to do so. FDCs have been shown to establish contact with lymphoid cells. Here we have determined that CD4+ T cells adhere in greater number to FDC clusters than do CD8+ T cells. We thus propose that FDCs specifically contribute to construction of lymph follicles by attracting and determining their cell composition.

Germinal Center T Cells: Analysis of Their Proliferative Capacity

Several immunohistochemical studies have revealed the existence of T cells expressing the CD57 antigen in the germinal center1. A few are also found in the interfollicular zones and the mantle zone2. Phenotypically they are CD4+, CD8- cells. They do not express Leu8, CD16 or CDllb3,4,5. These cells are not fully activated being CD25-,CD71- and HLA-DR- cells6.

Modulation of B lymphocyte proliferation inside the germinal center.

Germinal centers of stimulated lymphoid follicules comprise different microenvironments where B cells proliferate or differentiate into memory B cells or precursors of Ig-producing cells [1]. Follicular dendritic cells (FDC), unique non lymphoid cells, mainly compose these microenvironments. FDC are closely associated with B cells and can be distinguished from other accessory cells in secondary lymphoid tissues by a number of features, as lack of phagocytic activity and a characteristic set of cell surface markers.