CELLULAR INTERNET IN THE GERMINAL CENTER 115 FIGURE 4 FDC-B-cell cluster consisting of 2 to 4 FDC and 15 to 20 B cells in a 6-hr culture is heavily labeled for Cx43 gap junctions. Note that gap junctions in some places outline B-cell borders (star is in the middle of a B cell). Immunofluorescence reaction. Bar: 10/m. morphogens (Loewenstein and Rose, 1992; Warner, 1992). Several lines of evidence suggest that cell growth and proliferation are inversely related to the extent of direct intercellular communication. Growth factors, oncogene activation, and tumor-promoting phorbol-esters downregulate gap-junction expression and function in cells (Trosko et al., 1993). Retinoids have the opposite effect (Hossain et al., 1989). In addition there is the observation that communication- deficient transformed cells can normalize their growth pattern after their cell-cell communication had been restored by transfecting them with connexin cDNA (Metha et al., 1991). It seems, therefore, that two of the important functions for gap-junctional communi- cation are (1) signal propagation across coupled functional compartments in tissues and (2) serving to inhibit cell proliferation and growth that directs the cells toward differentiation. Communication Pathways in the Germinal Center FDC meshworks, produced in response to antigen challenge with T-cell help, capture activated highly proliferating B cells and form round compartments called germinal centers in lymphoid tissues (MacLen- nan, 1994). This germinal center reaction gives rise to memory and plasma B cells specific for the antigen. Our results indicate a positive correlation between Cx43 expression and the size of the FDC network on immunization, where FDC are functionally coupled through gap junctions (Krenacs et al., 1997). From the growth regulatory function of gap junctions (Loewen- stein and Rose, 1992), we hypothesize that upregula- tion of their expression in mature FDC may be involved in controlling the size of the follicles. Reaching a plateau in gap junction expression prob- ably prevents follicles from further enlargement through the control of FDC growth. Germinal centers of secondary lymphoid follicles are structurally divided into a dark zone filled with highly proliferating B centroblasts (see Figure 2) and a light zone with dense FDC meshwork enveloping maturing B centrocytes, a few germinal center T cells and macrophages (MacLennan, 1994; Thorbecke et al., 1994). B cells entering the light zone are rescued from apoptosis on the basis of their binding to FDC and the presented antigen (MacLennan and Gray, 1986; Lindhout et al., 1993). The nature of the final rescue signal blocking endonucleases in B cells is still unknown (Lindhout et al., 1995). Since FDC can communicate with B lymphocytes through gap junc- tions and this is the region where gap-junction expression is the highest (Krenacs and Rosendaal, 1995; Krenacs et al., 1997), one can imagine that this signal may be delivered through gap junctions. If this is so, the specific binding to the antigen presented by FDC may signal B cells to form gap junctions. The fast kinetics of gap-junction formation reflected by the short half-life (1-2 hr) of connexins (Laird et al., 1991) means that gap junctions could form in B cells within 4 hr, the predicted time needed for B-cell rescue (Ernst Lindhout, personal communication). In the light zone of germinal centers, FDC envelope both B and T lymphocytes while forming a meshwork of interconnected cells. We showed that the members of this meshwork are functionally coupled with Cx43 gap junctions (Krenacs et al., 1997), hence they may provide a cellular internet in the germinal center