Experimentelle Rheumatologie, Medizinische Klinik, Charité, Humboldt-Universität Berlin und Deutsches Rheumaforschungszentrum, 10117 Berlin, Germany
T cells are powerful and flexible regulators of the function of many immune and non-immune cells, besides their direct defence capabilities. The presence or absence of proinflammatory versus regulatory subsets has major effects on the course and outcome of inflammatory reactions, due to the distinct cytokines secreted. It is therefore of significant interest to know how T cells become attracted to sites of inflammation. Findings of the past decade have greatly increased our knowledge about these mechanisms, and strongly support the idea that intervention in these mechanisms might be of therapeutic value.
Homeostasis of the immune system involves the compartmentalization of lymphocyte populations. For example, naive lymphocytes recirculate much more efficiently than effector or memory cells through lymph nodes, Peyer's patches and the spleen. In contrast, activated cells become trapped by adhesion to vascular surface molecules in the liver, where a specific, apparently immunosuppressive environment is encountered, or the lungs. Finally, subsets of T- and B-memory and -effector cells which have been previously activated in gut-associated tissue home specifically to intestinal sites [13].
Most other organs lack significant lymphocyte traffic in the non-inflamed state. It is only upon inflammation that mechanisms for the recruitment of various leucocyte populations are activated locally, leading to infiltration of the diseased tissue. In both homeostatic recirculation and inflammation, the mechanisms that regulate adhesion are similar to those that regulate the transmigration of lymphocytes. A limited number of adhesion molecules and a large variety of chemokines and chemokine receptors synergize in the multi-step process of extravasation of lymphocytes; the presence of a specific combination of them on the endothelium as well as on the circulating cell determines the type of cells recruited in a given location. Accordingly, the expression of receptors is tightly regulated on both endothelium and lymphocytes.
In the above examples of homeostatic trafficking, the endothelia are equipped constitutively with distinct sets of receptors. In the case of recirculation through lymph nodes, Peyer's patches, and presumably also in lymphoid follicles in chronically inflamed tissues, specialized post capillary (high endothelial) venules express as adhesion molecules ICAM-1, ligands for L-selectin, MAdCAM-1 in mucosal sites and some VCAM-1, as well as the chemokines SLC and ELC. The endothelium of mucosal sites is often equipped with MAdCAM-1, which attracts gut-homing subsets of T and B cells. Most other organs (except the lungs and liver) do not express mechanisms for recruitment constitutively.
Inflammatory conditions induce the expression of adhesion molecules on the endothelium. Some of them are largely inflammation-specific, such as E- and P-selectin. Others, such as MAdCAM-1, ICAM-1 and VCAM-1, might be expressed in non-inflamed vessels, but, under inflamed conditions, become upregulated in quantity or at sites previously lacking this receptor [4].
Besides microbial activators lipopoly saccharides, cytokines such as IL-1 and tumour necrosis factor are universal inducers of E- and P-selectin, ICAM-1 and VCAM-1 (and perhaps other, less well characterized molecules) [5]. The nuclear transcription factor NF-B is a ubiquituous regulator of their genes [6, 7]. It is therefore not surprising that some anti-inflammatory drugs reduce adhesion molecule expression by targeting this nuclear factor [8]; however, up- and downregulation of these molecules on the endothelium might also occur via NF-
B-independent regulatory pathways [9, 10]. Other cytokines, such as interferon-
and IL-4, seem to have more selective effects on adhesion molecule expression [5]. It is an interesting finding of recent years that a variety of anti-inflammatory drugs, steroidal and non-steroidal, appear to have effects on the expression of adhesion molecules; to what extent their therapeutic efficacy depends on these effects is unclear.
Early investigations on lymphocyte trafficking nourished the view that lymphocytes might display organ-specific homing patterns in general. Although there is some bias in the use of adhesion molecules, particularly the preferential use of the MAdCAM-1/4ß7-integrin pathway for entry into intestinal sites, this appears less clear for other tissues.
For example, the endothelium of inflamed synovia was found to express rather promiscuously a variety of adhesion molecules, notably VCAM-1, ICAM-1, P- and E-selectin, CD44 ligands and vascular adhesion protein 1 (VAP-1); only MAdCAM-1 appears not to be expressed. No synovia-specific adhesion molecule has been identified so far, and the interaction of lymphocytes or other leucocytes with the synovial endothelium does not appear to rely on a unique combination of these adhesion molecules [1119]. Inflammatory cells residing in the synovia express high levels of the chemokine receptors CCR5 and CXCR3 [20], but this delineates proinflammatory subsets of lymphocytes or macrophages in general [21, 22] rather than being specific for this tissue.
The preponderance of particular molecules affects the type of cells recruited into the inflamed tissue. High levels of E-and P-selectin, as often found in acute inflammation or inflamed skin, might efficiently recruit neutrophils and T-effector cells grown under the influence of IL-12 [23, 24]; high expression of ICAM-1 and VAP-1 would favour attraction of activated CD8+ and NK cells (which express more LFA-1 than CD4+ cells and react preferentially with VAP-1 [25]); intestinal T cells high in 4ß7-integrin are destined to home to sites expressing large amounts of MAdCAM-1 or VCAM-1 [2]. This explains the apparent overlap in the circulation of T-cell subsets between gut mucosa (where MAdCAM-1 and VCAM-1 are involved) and the synovial tissue (rich in VCAM-1).
Most chemokines involved in the recruitment of leucocyte populations are inducible and are secreted only under inflammatory conditions. Differential expression of the members of this large family of mediators by tissue cells or the endothelium is believed to have a strong effect on the type of cells recruited. However, the circumstances which induce expression of distinct chemokines and how this depends on the tissue, the type of inflammatory signals and the progress of an immune reaction are poorly defined so far.
It might be assumed that the regulation of the above trafficking signals influences the type of effector mechanisms dominating within the tissue; the character and time course of disease symptoms would depend on this.
Naive T cells are equipped with a basal level of the adhesion molecules L-selectin, LFA-1 and 4-integrins; these, together with the chemokine receptor CCR7, enable them to recirculate through lymphoid tissues. This is not sufficient, however, to enable them to enter inflamed tissues with good efficiency. Indeed, activation and differentiation into T-effector/memory cells is required for the expression of inflammation-specific receptors, such as the carbohydrate epitopes recognized by E-and P-selectin [23, 26, 27] or receptors for the inducible chemokines, such as CCR1-5 and CXCR3 [28].
In addition, the integrins LFA-1 and 4ß7 or
4ß1, CD44 and other adhesion molecules become upregulated on all or subsets of T cells on activation [29], corresponding to the increased levels of their counterparts ICAM-1, MAdCAM-1 or VCAM-1, and hyaluronic acid, respectively, on the endothelium of inflamed areas.
It is important to realize that not every T-effector cell is able to enter a given inflamed site. It seems that trafficking properties are tightly regulated during the activation, differentiation and functional specialization of T cells, processes in which the cytokine milieu is crucial.
One example is the induction of the fucosyltransferases required for the synthesis of selectin-binding epitopes. It has been shown previously that these epitopes appear only on some T-effector cells and that they are required for the entry of T cells into some inflamed sites, including the inflamed synovia. Initial in vitro studies suggested that P-selectin binding is restricted to Th1 cells, but among effector cells generated in vivo some of the Th2 cells were found to express selectin ligands [23, 26]. IL-12 has a crucial (although not exclusive) role in the induction of selectin ligands during T-effector cell maturation [27, 30, 31]; its release from activated antigen-presenting cells might prime the T cells to migrate into acutely inflamed sites. The same applies to natural killer cells, which up-regulate their selectin-binding epitopes under the influence of IL-12 [32]. In the human, skin-associated sites seem to be especially prone to induce selectin ligands on developing T-effector cells, as the E-selectin binding epitope CLA (cutaneous lymphocyte antigen) is found on a large number of cutaneous T cells [33].
The factors that regulate the differential expression of integrins, especially 4ß7-integrin, are unknown. The various cytokines tested so far have shown no effect on its expression (Lehmann and Hamann, unpublished data).
Chemokine receptor expression is regulated differently from T-cell activation and differentiation; the state of activation and cytokines seem to be of importance. Again, different expression patterns were found to be associated with activation stage and with functional T-cell subsets. Expression of the receptors CCR5 and CXCR3 on in vitro-generated T-effector cells, preferentially within the Th1 subset, was found by several (but not all) groups, whereas CCR3, CCR4, CCR8 and CXCR4 expression was biased towards Th2 cells [34, 35]. Whether this holds true when effector cells generated in vivo are analysed remains to be shown. It is clear that the conditions during the activation and differentiation of T-effector cells dictate their trafficking behaviour, as for selectin-binding epitopes.
It is still not known whether the expression of distinct sets of chemokine receptors and adhesion molecules depends only on the actual milieu or whether it involves long-term memory, leading to subsets with a polarized, stable homing phenotype. The finding that increased expression of 4ß7-integrin is associated with memory cells specific for intestinal antigens, and of CLA when subcutaneous immunization is used [3638], seems to favour (but does not prove) the latter possibility. If this holds true, analysis of the homing phenotype of T cells might help to identify the origin of T cells of pathophysiological importance. This is of special interest with regard to the suggested link between intestinal immune responses and reactive arthritis.
With regard to therapeutics, fundamental questions are still unresolved. Is the main reason for the specialized trafficking properties of T-effector cells to target them into compartments presenting recall antigens, as present thinking assumes? Or does the development of the specialized trafficking properties of T cells proceed in parallel with distinct functional differentiation? This would imply a major effect, for example on the local balance of proinflammatory and regulatory subsets. An understanding of the mechanisms regulating homing specificity could provide new targets for immunomodulatory strategies in inflammatory diseases. Indeed, a variety of experimental and a few clinical studies have provided evidence that blocking either adhesion molecules [15, 3946] or chemokines [4749] could result in the control of autoimmune diseases such as rheumatoid arthritis. These studies have indicated that the reduction of proinflammatory effector cells within the tissue is a possible explanation for the therapeutic efficacy that has been observed. The present development of small molecule-based antagonists for adhesion receptors and chemokines by a variety of companies will overcome several constraints inherent in experimental therapies based on monoclonal antibodies. Even more challenging will be to direct regulatory T cells to sites of inflammation, or, vice versa, to direct inflammatory cells into compartments such as the gut or liver, where the default reaction is the induction of anergy or tolerance. With the emerging knowledge on homing mechanisms, these developments might soon become less speculative.
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