116 T. KRENACS and M. ROSENDAAL between the different lymphocytes. We did not find gap junctions between FDC and T cells or between FDC and macrophages. However, there is in vitro evidence that T cells, probably via cytokine action or the CD40-CD40L pathway modify FDC growth and phenotype (Clark et al., 1992; Kim et al., 1994), which should have consequences on FDC-B-cell interactions. It has to be noted that antigen-specific T cells play a pivotal role in FDC and germinal center formation, B-cell activation, and the final maturation steps of B lymphocytes (Clark and Ledbetter, 1994; Thorbecke et al., 1994). T cells act through receptor-ligand pathways represented by interactions between cyto- kines/growth factors, adhesion molecules, and their receptors in the target cell membrane. In recent work, however, we have confirmed the existence of another direct pathway for information exchange in the lymphoid germinal center that occurs through gap junctions (Krenacs and Rosendaal, 1995; Krenacs et al., 1997). This direct cell-cell communication may explain some unclear features of B-cell maturation and show how germinal center events are integrated. There is a lot more to do in revealing exactly how this cellular internet works and what role(s) it serves in germinal center reactions. FIGURE 5 Cytospin samples of isolated tonsillar mononuclear cells simultaneously immunolabeled either for (A) IgM and (B) Cx43, or for (C) IgD and (D) Cx43. Note that some of the (A) IgM positive and (C) less IgD positive B cells express Cx43 gap-junction protein. Colocalization of pairs of antigens are indicated with arrowheads on identical cells, that is, in (A) and (B), or in (C) and (D). Bar: 10 /m.