Naive T lymphocytes travel to T-cell areas of secondary lymphoid organs in search of antigen presented by dendritic cells. Once activated, they proliferate vigorously, generating effector cells that can migrate to B-cell areas or to inflamed tissues. A fraction of primed T lymphocytes persists as circulating memory cells that can confer protection and give, upon secondary challenge, a qualitatively different and quantitatively enhanced response. The nature of the cells that mediate the different facets of immunological memory remains unresolved. Here we show that expression of CCR7, a chemokine receptor that controls homing to secondary lymphoid organs, divides human memory T cells into two functionally distinct subsets. CCR7- memory cells express receptors for migration to inflamed tissues and display immediate effector function. In contrast, CCR7+ memory cells express lymph-node homing receptors and lack immediate effector function, but efficiently stimulate dendritic cells and differentiate into CCR7- effector cells upon secondary stimulation. The CCR7+ and CCR7- T cells, which we have named central memory (TCM) and effector memory (TEM), differentiate in a step-wise fashion from naive T cells, persist for years after immunization and allow a division of labour in the memory response.

immunology letters to nature 34 asymmetric unit (molecule B). Density for the other two molecules was broken and hard to interpret. The skeleton for molecule B was used to generate a molecular envelope for NCS averaging. Initial rotation matrices describing the relative orientations of molecules A, B and C were calculated from the heavy-atom sites (mercury bound to five sites in each of the three molecules). NCS averaging with DM 25 was used to improve phases at 2.7 Å resolution, and then extend phases to 2.2 Å , the limit of the native data set. The resulting electron density map was readily interpretable (Fig. 2). A model including residues 47-351 of each of the three molecules was built using the graphics program O 26 . No electron density is seen for 24 residues at the amino terminus. The model was refined using simulated annealing and positional refinement in X-PLOR 27 , with tight NCS restraints. The model includes 669 water molecules, which were positioned by using the program ARP (V.Lamzin). An overall thermal B factor and tightly restrained individual B factors were refined, and a bulk-solvent model was incorporated. Crystallographic R values and stereochemical parameters are presented in Table 1.The structure of the Cbl-N/ZAP70 phosphopeptide complex was determined by molecular replacement with the program AmoRe 28 . Use of the complete unliganded Cbl-N structure as a search model yielded clear rotation and translation peaks, and maps phased with the appropriately positioned model revealed strong electron density for the 4H and EF-hand domains, but no interpretable density for the SH2 domain. The model was therefore broken into two fragments (residues 47-265 and residues 266-351), and the rotation and translation searches were repeated with each fragment. The 4H/EF-hand fragment yielded a solution essentially identical to that from the intact model. The SH2 domain was then positioned using translation searches conducted in the context of the appropriately positioned 4H/EF-hand fragment. After rigid-body and positional refinement using X-PLOR 27 , electron density maps calculated with the combined model revealed clear density for all domains, and readily interpretable density for the bound ZAP-70 phosphopeptide. After construction of the peptide, the structure was refined with iterative cycles of manual refitting and simulated annealing and positional refinement in X-PLOR (Table 1). Restrained individual temperature factors were refined. The model includes residues 47-351 of c-Cbl, residues 289-297 of ZAP-70, and 358 water molecules. Figure 1a was prepared with MOLSCRIPT 29 , Fig. 2 with program O 26 , and Figs 1e and 3 with GRASP 30 . Illustrations

The proper function of immune surveillance requires well-coordinated mechanisms in order to guide the patrolling immune cells through peripheral tissues and into secondary lymphoid organs. Analyzing gene-targeted mice, we identified the chemokine receptor CCR7 as an important organizer of the primary immune response. CCR7-deficient mice show severely delayed kinetics regarding the antibody response and lack contact sensitivity and delayed type hypersensitivity reactions. Due to the impaired migration of lymphocytes, these animals reveal profound morphological alterations in all secondary lymphoid organs. Upon activation, mature skin dendritic cells fail to migrate into the draining lymph nodes. Thus, in order to bring together lymphocytes and dendritic cells to form the characteristic microarchitecture of secondary lymphoid organs, CCR7 is required to rapidly initiate an adoptive immune response.

Naive T cells are stimulated by antigen-presenting dendritic cells (DCs) in secondary lymphoid organs, but whether other types of cell participate in T cell priming is unclear. Here we show in mice that natural killer (NK) cells, which are normally excluded from lymph nodes, are rapidly recruited in a CCR7-independent, CXCR3-dependent manner to lymph nodes on stimulation by the injection of mature DCs. Recruitment of NK cells is also induced by some, but not all, adjuvants and correlates with the induction of T helper cell type 1 (T(H)1) responses. NK cell depletion and reconstitution experiments show that NK cells provide an early source of interferon-gamma (IFN-gamma) that is necessary for T(H)1 polarization. Taken together, our results identify an induced pathway of NK cell migration in antigen-stimulated lymph nodes and a mechanism by which some adjuvants may facilitate T(H)1 responses.

Lymphoid follicles are B-cell-rich compartments of lymphoid organs that function as sites of B-cell antigen encounter and differentiation. CXC chemokine receptor-5 (CXCR5) is required for B-cell migration to splenic follicles, but the requirements for homing to B-cell areas in lymph nodes remain to be defined. Here we show that lymph nodes contain two types of B-cell-rich compartment: follicles containing follicular dendritic cells, and areas lacking such cells. Using gene-targeted mice, we establish that B-lymphocyte chemoattractant (BLC/BCA1) and its receptor, CXCR5, are needed for B-cell homing to follicles in lymph nodes as well as in spleen. We also find that BLC is required for the development of most lymph nodes and Peyer's patches. In addition to mediating chemoattraction, BLC induces B cells to up-regulate membrane lymphotoxin alpha1beta2, a cytokine that promotes follicular dendritic cell development and BLC expression, establishing a positive feedback loop that is likely to be important in follicle development and homeostasis. In germinal centres the feedback loop is overridden, with B-cell lymphotoxin alpha1beta2 expression being induced by a mechanism independent of BLC.