Figure 4. Memory response in CD41CD45RO1CXCR52 cells. (A) PBLs were sorted as indicated and cultured with autologous serum and ir- radiated autologous macrophages in the presence or absence of TT for 7 d. For the last 24 h of culture, [3H]thymidine was added to the medium and the amount of incorporated 3H was determined (every point repre- sents the mean of 12 replicas). Similar results were obtained in a second experiment. In a third experiment, we also observed a weak proliferative response in the TT-treated CXCR51 fraction at the highest T cell count tested. (B) The same as described for A, with the exception that TT was not added and allogenic (allo) macrophages were substituted for autolo- gous macrophages. Data shown are representative of three experiments. (C) CXCR52 and CXCR51 tonsillar CD4 cells were analyzed for the expression of CD95 by flow cytometry. (Fig. 4 B), demonstrating that CXCR51 T cells have the capability to respond to foreign antigens. As these data sug- gest that CXCR51 T cells might represent finally differen- tiated effector T cells prone to be removed by apoptosis, we stained tonsillar T cells with anti-CD95 mAb. Interestingly, 94% of the CD41CXCR51 cells expressed CD95, whereas ,30% of the CD41CXCR52 cells were associated with this molecule (Fig. 4 C). This observation indicates that CXCR51 T cells might be particularly prone to die by ap- optosis, which might help to explain why we found long- lived memory cells within the CXCR52 subset. Discussion Molecular mechanisms underlying the process of lym- phocyte homing have been extensively studied during the last few years. Data derived from plt mice (paucity of LN T cells [18]), a mouse carrying a mutation in one of the SLC 1549 Breitfeld et al. gene loci (19), and observations made on CCR7 gene-tar- geted mice (3) clearly demonstrate that trans-HEV migra- tion of T cells, and to a lower extent migration of B cells, requires the interaction of HEV-expressed SLC/CCL21 and ELC/CCL19 with its cognate receptor CCR7. Once inside the lymphoid organ, T cells migrate to the DCs to screen these cells for foreign peptides presented to them. The molecular mechanisms underlying this migratory pro- cess are poorly understood. The data shown here provide evidence that this might also be mediated by the interaction of T cell–expressed CCR7 with CCL19, produced by in- terdigitating DCs. More important, compared with other chemokine receptors involved in T cell trafficking such as CXCR4, CCR7 is rather resistant towards ligand-induced receptor internalization. After exposure to its ligand, 50% of surface CXCR4 is removed within 10 min (20), whereas a 2-h period was required to observe comparable effects on CCR7 (Fig. 1 C). This slow kinetic of CCR7 downregula- tion would enable T cells to follow a new ELC/CCL19 gradient build-up by DCs after having exploited the same chemokine/chemokine receptors for transendothelial mi- gration. DC-activated T cells then migrate to the outer T zone, which represents the primary site for Th cell cognate help to B cells specific for the same antigen (9). Although it is currently unknown what cues direct activated T cells to the outer T zone, we recently demonstrated that B cells use CCR7 to stay in this area for some hours to increase the probability of meeting and interacting with T cells (3). Some of the activated B cells migrate to extrafollicular foci, again by unknown mechanisms, to rapidly produce Abs of relatively low affinity. In the second/parallel phase of the immune response, CD41 T and B cells migrate to B cell follicles to form GCs. Data derived from mice deficient for CXCR5 or deficient for its ligand, BLC/CXCL13, show a profound lack in attracting B cells to follicles containing FDCs, demonstrating that this chemokine/chemokine re- ceptor controls B cell migration to follicles (4, 5). In addi- tion, data derived from various mouse models support the idea that Th cell migration to follicles is controlled by the same mechanisms that direct B cell migration to these areas. Walker et al. recently demonstrated that CD28-dependent OX40 ligation of Th cells is closely linked to upregulation of CXCR5 and the appearance of these cells in B cell folli- cles (21). Once they start to express CXCR5, T cells be- come increasingly unresponsive to ELC/CCL19 and SLC/ CCL21 (22). These mechanisms, together with ligand- induced receptor internalization, might release T cells from the ELC/SLC gradients of the T cell–rich area. Interest- ingly, transgenic mice that constitutively express OX40 ligand on DCs show a strong increase in the numbers of CXCR51 T cells and have a high frequency of CD4 T cells locating to B cell areas (23). Although there is strong evi- dence that T cells need CXCR5 to migrate to the follicles, expression of this receptor on T cells does not inevitably re- sult in follicular localization of CXCR51 CD4 cells, as demonstrated by adoptive transfer experiments (22). How- ever, as in vivo activation of T cells with antigen plus adju- vant initiates T cell homing to follicles (22), it seems likely