Tissue-resident dendritic cells (DCs) that migrate from peripheral sites to lymphoid organs are essential in the initiation of adaptive immune responses and for the maintenance of peripheral tolerance, and have been extensively studied. By contrast, blood-borne DCs represent a heterogeneous population, the origin, destination and function of which are still unclear. Recent studies have shown that circulating DCs capture blood-borne antigen and transport it into the extra-vascular space of lymphoid tissues for processing and presentation. Other findings suggest that a fraction of tissue-resident DCs might enter the blood after having acquired antigen in the periphery. Together, these studies imply that circulating DCs might modulate immune responses by translocating antigenic material from its point of origin to remote target tissues.

Normal intestinal mucosa contains abundant immunoglobulin A (IgA)-secreting cells, which are generated from B cells in gut-associated lymphoid tissues (GALT). We show that dendritic cells (DC) from GALT induce T cell-independent expression of IgA and gut-homing receptors on B cells. GALT-DC-derived retinoic acid (RA) alone conferred gut tropism but could not promote IgA secretion. However, RA potently synergized with GALT-DC-derived interleukin-6 (IL-6) or IL-5 to induce IgA secretion. Consequently, mice deficient in the RA precursor vitamin A lacked IgA-secreting cells in the small intestine. Thus, GALT-DC shape mucosal immunity by modulating B cell migration and effector activity through synergistically acting mediators.

Homing of bone marrow (BM)-derived progenitors to the thymus is essential for T cell development. We have previously reported that two subpopulations of common lymphoid progenitors, CLP-1 and CLP-2, coexist in the BM and give rise to lymphocytes. We demonstrate that CLP-2 migrate to the thymus more efficiently than any other BM-derived progenitors. Short-term adoptive transfer experiments revealed that CLP-2 homing involves P-selectin/P-selectin glycoprotein ligand 1 interactions, pertussis toxin-sensitive chemoattractant signaling by CC chemokine ligand 25 through CC chemokine receptor 9, and binding of the integrins alpha4beta1 and alphaLbeta2 to their respective ligands, vascular cell adhesion molecule 1 and intercellular adhesion molecule 1. Preferential thymus-tropism of CLP-2 correlated with higher chemokine receptor 9 expression than on other BM progenitors. Thus, CLP access to the thymus is controlled by a tissue-specific and subset-selective multistep adhesion cascade.

The tumor microenvironment is composed of an intricate mixture of tumor and host-derived cells that engage in a continuous interplay. T cells are particularly important in this context as they may recognize tumor-associated antigens and induce tumor regression. However, the precise identity of cells targeted by tumor-infiltrating T lymphocytes (TILs) as well as the kinetics and anatomy of TIL-target cell interactions within tumors are incompletely understood. Furthermore, the spatiotemporal conditions of TIL locomotion through the tumor stroma, as a prerequisite for establishing contact with target cells, have not been analyzed. These shortcomings limit the rational design of immunotherapeutic strategies that aim to overcome tumor-immune evasion. We have used two-photon microscopy to determine, in a dynamic manner, the requirements leading to tumor regression by TILs. Key observations were that TILs migrated randomly throughout the tumor microenvironment and that, in the absence of cognate antigen, they were incapable of sustaining active migration. Furthermore, TILs in regressing tumors formed long-lasting (>or=30 min), cognate antigen-dependent contacts with tumor cells. Finally, TILs physically interacted with macrophages, suggesting tumor antigen cross-presentation by these cells. Our results demonstrate that recognition of cognate antigen within tumors is a critical determinant of optimal TIL migration and target cell interactions, and argue against TIL guidance by long-range chemokine gradients.

Mechanisms of dominant tolerance have evolved within the mammalian immune system to prevent inappropriate immune responses. CD4(+)CD25(+) regulatory T (T(reg)) cells have emerged as central constituents of this suppressive activity. By using multiphoton intravital microscopy in lymph nodes (LNs) of anesthetized mice, we have analyzed how cytotoxic T lymphocytes (CTLs) interact with antigen-presenting target cells in the presence or absence of activated T(reg) cells. Nonregulated CTLs killed their targets at a 6.6-fold faster rate than regulated CTLs. In spite of this compromised effector activity, regulated CTLs exhibited no defect in proliferation, induction of cytotoxic effector molecules and secretory granules, in situ motility, or ability to form antigen-dependent conjugates with target cells. Only granule exocytosis by CTLs was markedly impaired in the presence of T(reg) cells. This selective form of regulation did not require prolonged contact between CTLs and T(reg) cells but depended on CTL responsiveness to transforming growth factor-beta. CTLs quickly regained full killing capacity in LNs upon selective removal of T(reg) cells. Thus, T(reg) cells reversibly suppress CTL-mediated immunity by allowing acquisition of full effector potential but withholding the license to kill.

In individuals with chronic myeloid leukemia (CML) treated by autologous hematopoietic stem cell (HSC) transplantation, malignant progenitors in the graft contribute to leukemic relapse, but the mechanisms of homing and engraftment of leukemic CML stem cells are unknown. Here we show that CD44 expression is increased on mouse stem-progenitor cells expressing BCR-ABL and that CD44 contributes functional E-selectin ligands. In a mouse retroviral transplantation model of CML, BCR-ABL1-transduced progenitors from CD44-mutant donors are defective in homing to recipient marrow, resulting in decreased engraftment and impaired induction of CML-like myeloproliferative disease. By contrast, CD44-deficient stem cells transduced with empty retrovirus engraft as efficiently as do wild-type HSCs. CD44 is dispensable for induction of acute B-lymphoblastic leukemia by BCR-ABL, indicating that CD44 is specifically required on leukemic cells that initiate CML. The requirement for donor CD44 is bypassed by direct intrafemoral injection of BCR-ABL1-transduced CD44-deficient stem cells or by coexpression of human CD44. Antibody to CD44 attenuates induction of CML-like leukemia in recipients. These results show that BCR-ABL-expressing leukemic stem cells depend to a greater extent on CD44 for homing and engraftment than do normal HSCs, and argue that CD44 blockade may be beneficial in autologous transplantation in CML.

How is the amoeboid movement of lymphocytes in secondary lymphoid organs orchestrated? In this issue of Immunity, Bajénoff et al. (2006) demonstrate that stromal cell networks serve as guidance structures that direct and limit the migration of B and T cells in lymph nodes.

The migration of B cells into secondary lymphoid organs is required for the generation of an effective immune response. Here we analyzed the involvement of SWAP-70, a Rac-interacting protein involved in actin rearrangement, in B cell entry into lymph nodes. We noted reduced migration of Swap70-/- B cells into lymph nodes in vivo. Swap70-/- B cells rolled and adhered, yet accumulated in lymph node high endothelial venules. This defect was not due to impaired integrin expression or chemotaxis. Instead, Swap70-/- B cells aberrantly regulated integrin-mediated adhesion. During attachment, Swap70-/- B cells showed defective polarization and did not form uropods or stabilize lamellipodia at a defined region. Thus, SWAP-70 selectively regulates processes essential for B cell entry into lymph nodes.

To exert immunological activity, T and B cells must leave the blood and enter different extravascular compartments in the body. An essential step in this process is their adhesion to microvascular endothelium and subsequent diapedesis into a target tissue. Naive and effector/memory T and B cells possess distinct repertoires of traffic molecules that restrict their ability to interact with specialized microvessels in different anatomic compartments and thus exhibit distinct patterns of migration. In addition, antigen-experienced lymphocytes are subdivided into different subsets based on their expression of characteristic sets of adhesion receptors that favor their accumulation in certain target organs, such as the skin and the gut. This article focuses on recent discoveries that have broadened our understanding of the "imprinting" mechanisms responsible for the generation of tissue-specific effector/memory lymphocytes, especially in the intestine. We discuss how gut-specific homing is acquired, maintained, and modulated and how these mechanisms might be harnessed to develop improved vaccine protocols and treatments for intestinal autoimmune diseases.

It is commonly believed that only T lymphocytes and B lymphocytes expressing recombination-dependent antigen-specific receptors mediate contact hypersensitivity responses to haptens. Here we found that mice devoid of T cells and B cells demonstrated substantial contact hypersensitivity responses to 2,4-dinitrofluorobenzene and oxazolone. Those responses were adaptive in nature, as they persisted for at least 4 weeks and were elicited only by haptens to which mice were previously sensitized. No contact hypersensitivity was induced in mice lacking all lymphocytes, including natural killer cells. Contact hypersensitivity responses were acquired by such mice after adoptive transfer of natural killer cells from sensitized donors. Transferable hapten-specific memory resided in a Ly49C-I(+) natural killer subpopulation localized specifically in donor livers. These observations indicate that natural killer cells can mediate long-lived, antigen-specific adaptive recall responses independent of B cells and T cells.

Naive and effector/memory T cells have distinct repertoires of trafficking ligands and receptors that restrict their ability to interact with specialized microvessels in different anatomical compartments and, consequently, have distinct patterns of migration. Antigen-experienced lymphocytes can be further subdivided into different subsets based on their expression of characteristic sets of trafficking receptors that favor their accumulation in certain target organs, including the skin and gut. Here, we summarize recent advances that have broadened our understanding of the cellular and molecular events that induce the generation of tissue-specific effector/memory T cells and discuss how these mechanisms could be harnessed for the therapeutic manipulation of T-cell-dependent pathologies.

The transcription factor T-bet (Tbx21) plays a major role in adaptive immunity and is required for optimal IFN-gamma production by DCs. Here we demonstrate an essential function for T-bet in DCs in controlling inflammatory arthritis. We show that collagen antibody-induced arthritis (CAIA), a model of human RA, is a bipartite disease characterized by an early innate immune system component intact in RAG2 mice and a later adaptive immune system phase. Mice lacking T-bet had markedly reduced joint inflammation at both early and late time points and RAG2T-bet double-deficient mice were essentially resistant to disease. Remarkably, adoptive transfer of T-bet-expressing DCs reconstituted inflammation in a T-bet deficient and T-bet/RAG2-deficient milieu. T-bet regulates the production of proinflammatory cytokine IL-1alpha and chemokines macrophage inflammatory protein-1alpha (MIP-1alpha) and thymus- and activation-related chemokine (TARC) by DCs. Further, T-bet expression in DCs is required for T helper cell activation. We conclude that T-bet plays a vital function in DCs that links innate and adaptive immunity to regulate inflammatory responses. T-bet provides an attractive new target for the development of novel therapeutics for inflammatory arthritis.

Dendritic cells (DCs) carry antigen from peripheral tissues via lymphatics to lymph nodes. We report here that differentiated DCs can also travel from the periphery into the blood. Circulating DCs migrated to the spleen, liver and lung but not lymph nodes. They also homed to the bone marrow, where they were retained better than in most other tissues. Homing of DCs to the bone marrow depended on constitutively expressed vascular cell adhesion molecule 1 and endothelial selectins in bone marrow microvessels. Two-photon intravital microscopy in bone marrow cavities showed that DCs formed stable antigen-dependent contacts with bone marrow-resident central memory T cells. Moreover, using this previously unknown migratory pathway, antigen-pulsed DCs were able to trigger central memory T cell-mediated recall responses in the bone marrow.

Normal bone marrow (BM) contains T cells whose function and origin are poorly understood. We observed that CD8+ T cells in BM consist chiefly of CCR7+ L-selectin+ central memory cells (TCMs). Adoptively transferred TCMs accumulated more efficiently in the BM than naive and effector T cells. Intravital microscopy (IVM) showed that TCMs roll efficiently in BM microvessels via L-, P-, and E-selectin, whereas firm arrest required the VCAM-1/alpha4beta1 pathway. alpha4beta1 integrin activation did not depend on pertussis toxin (PTX)-sensitive Galphai proteins but was reduced by anti-CXCL12. In contrast, TCM diapedesis did not require CXCL12 but was blocked by PTX. After extravasation, TCMs displayed agile movement within BM cavities, remained viable, and mounted potent antigen-specific recall responses for at least two months. Thus, the BM functions as a major reservoir for TCMs by providing specific recruitment signals that act in sequence to mediate the constitutive recruitment of TCMs from the blood.

The burgeoning field of leukocyte trafficking has created new and exciting opportunities in the clinic. Trafficking signals are being defined that finely control the movement of distinct subsets of immune cells into and out of specific tissues. Because the accumulation of leukocytes in tissues contributes to a wide variety of diseases, these 'molecular codes' have provided new targets for inhibiting tissue-specific inflammation, which have been confirmed in the clinic. However, immune cell migration is also critically important for the delivery of protective immune responses to tissues. Thus, the challenge for the future will be to identify the trafficking molecules that will most specifically inhibit the key subsets of cells that drive disease processes without affecting the migration and function of leukocytes required for protective immunity.

Over the past decades, intravital microscopy (IVM), the imaging of cells in living organisms, has become a valuable tool for studying the molecular determinants of lymphocyte trafficking. Recent advances in microscopy now make it possible to image cell migration and cell-cell interactions in vivo deep within intact tissues. Here, we summarize the principal techniques that are currently used in IVM, discuss options and tools for fluorescence-based visualization of lymphocytes in microvessels and tissues, and describe IVM models used to explore lymphoid and non-lymphoid organs. The latter will be introduced according to the physiologic itinerary of developing and differentiating T and B lymphocytes as they traffic through the body, beginning with their development in bone marrow and thymus and continuing with their migration to secondary lymphoid organs and peripheral tissues.

It is widely believed that rolling lymphocytes require successive chemokine-induced signaling for lymphocyte function-associated antigen 1 (LFA-1) to achieve a threshold avidity that will mediate lymphocyte arrest. Using an in vivo model of lymphocyte arrest, we show here that LFA-1-mediated arrest of lymphocytes rolling on high endothelial venules bearing LFA-1 ligands and chemokines was abrupt. In vitro flow chamber models showed that endothelium-presented but not soluble chemokines triggered instantaneous extension of bent LFA-1 in the absence of LFA-1 ligand engagement. To support lymphocyte adhesion, this extended LFA-1 conformation required immediate activation by its ligand, intercellular adhesion molecule 1. These data show that chemokine-triggered lymphocyte adhesiveness involves a previously unrecognized extension step that primes LFA-1 for ligand binding and firm adhesion.

The interaction of L-selectin on lymphocytes with sulfated ligands on high endothelial venules leads to rolling and is critical for recruitment of lymphocytes into peripheral lymph nodes. Peripheral node addressin represents a class of L-selectin ligands recognized by the function-blocking monoclonal antibody MECA-79. Its epitope overlaps with sialyl 6-sulfo Lewis X, an L-selectin recognition determinant. Here, mice lacking two N-acetylglucosamine-6-O-sulfotransferases (GlcNAc6ST-1 and GlcNAc6ST-2) demonstrated elimination of both peripheral node addressin and sialyl 6-sulfo Lewis X in high endothelial venules, considerably reduced lymphocyte homing to peripheral lymph nodes and reduced sticking of lymphocytes along high endothelial venules. Our results establish an essential function for the sulfotransferases in L-selectin ligand synthesis and may have relevance for therapy of inflammatory diseases.

Mature recirculating B cells are generally assumed to exist in follicular niches in secondary lymphoid organs, and these cells mediate T-dependent humoral immune responses. We show here that a large proportion of mature B lymphocytes occupy an anatomically and functionally distinct perisinusoidal niche in the bone marrow. Perisinusoidal B cells circulate freely, as revealed by parabiosis studies. However, unlike their counterparts in the follicular niche, these cells are capable of being activated in situ by blood-borne microbes in a T-independent manner to generate specific IgM antibodies. The bone marrow represents a unique type of secondary lymphoid organ in which mature B cells are strategically positioned in the path of circulating microbes.