Chemorepulsion and thymocyte emigration Commentary See related article, pages 1101–1110. Jason G. Cyster Howard Hughes Medical Institute, and Department of Microbiology and Immunology, University of California, San Francisco, 513 Parnassus Ave., San Francisco, California 94143-0414, USA. Phone: (415) 502-6427; Fax: (415) 502-8424; E-mail: [email protected]. J. Clin. Invest. 109:1011–1012 (2002). DOI:10.1172/JCI200215511. The possibility that chemorepulsion, cellular motility increases without any help in developing an understanding or cell migration away from a stimu- directional specificity. of how a chemokine-mediated repul- lus, plays a role in the immune system Like SDF1/CXCL12–mediated che- sive response might occur. is exciting from both clinical and cell- motaxis, the putative chemorepul- biological viewpoints. For the clini- sion/fugetaxis behavior is sensitive to Chemorepulsion in vivo? cian, what could be a more satisfying pertussis toxin (PTX), implicating Gi- Having found suggestive in vitro evi- way to treat a cell-mediated immune mediated signaling in this process. dence for lymphocyte chemorepul- disease than local application of a These observations suggested not sion/fugetaxis, the Scadden group (2) chemorepellent? For the cell biologist, only that a cell can sense differences sought to identify an in vivo example eukaryotic cell chemorepulsion repre- in chemokine receptor occupancy of the process. Revisiting a decade-old sents a novel behavior to be explored between its front and back, but also observation — that cell emigration and explained at the molecular level. that it can respond differently from the thymus is inhibited by the Although the repulsion of neurons depending on the extent of receptor ADP-ribosylating subunit of PTX (6) — and their axons has emerged as an occupancy. Indeed, Zlatopolskiy and Poznansky and coworkers (2) exam- important process in patterning the Laurence (4), noting that the ined a variety of thymic culture systems nervous system (1), a physiological SDF1/CXCL12 receptor CXCR4 and cell types and showed that mature role for chemorepulsion in directing binds its ligand with high and low cells exit thymic fragments or movement in the immune system has affinity sites, have proposed that organoids in a PTX-inhibitable man- yet to be revealed. Therefore, any evi- binding to the lower affinity site is ner. Similarly, Lee et al. (7) recently dence that chemorepulsion regulates required to induce a chemorepulsive reported that the PTX-treated cells immunocyte migration demands signal. Such a mechanism would dis- accumulate inside thymic vascular close attention. tinguish this process from the best- channels, structures that may be asso- In this issue of the JCI, Poznansky characterized examples of chemore- ciated with T cell exit. In support of a and coworkers suggest that chemore- pulsion, found in the nervous system role for chemorepulsion in thymocyte pulsion, which they term “fugetaxis,” (1). There, the type of response is emigration, Poznansky et al. found plays a role in T cell emigration from determined by the type of receptor that if thymic stroma–conditioned the thymus (2). This group first report- expressed by the cell, rather than by medium was placed in the lower Tran- ed evidence for immune cell chemore- ligand concentration (5). It is impor- swell chamber, cells no longer migrat- pulsion in a study of human peripher- tant to recall, however, that only a ed as efficiently away from the thymic al blood T cells. In both Transwell fraction of the T cells undergo either organoids. The thymus is an estab- chemotaxis chamber assays and a attraction or repulsion in the in vitro lished source of SDF1/CXCL12, and methylcellulose migration assay, they assays. For this reason, it cannot be antibodies to the SDF1/CXCL12 recep- tor, CXCR4, could partially inhibit cell emigration from the thymic cultures. Switching to CD4+ T cells as the indi- Poznansky and coworkers suggest that chemorepulsion, cator population, Poznansky et al. which they term “fugetaxis,” plays a role showed that thymic stroma–condi- tioned medium contains another, pos- in T cell emigration from the thymus. sibly chemorepulsive, activity. In this case, however, it was not excluded that this activity was purely chemokinetic. Moreover, since several of the systems showed that T cells could migrate away excluded that different subsets of that have been used to assess this effect from a high concentration source of cells, with intrinsic differences that are quite complex, alternatives to true the chemokine SDF1/CXCL12 (3). favor one response or the other, chemorepulsion have not always been Experiments in the Transwell system, exhibit chemorepulsion or chemoat- ruled out. In particular, if thymocyte where cells were incubated in the same traction. The increasing effort that is motility increases as these cells mature, high concentrations of SDF1/CXCL12 being directed at determining how a the effect would mimic that of in the absence of a gradient, provided cell can sense different concentra- chemorepulsion in some experiments. evidence that the migration was not tions of ligand across its length to Interestingly, Lee and colleagues simply due to chemokinesis, in which make a chemotactic response should recently reported that positive selec- The Journal of Clinical Investigation | April 2002 | Volume 109 | Number 8 1011 tion in fetal thymic organ culture is numbers of T cells within the spleen, families (see Perspective by Goshima et indeed associated with increased motil- although, surprisingly, they do not have al. [ref. 1]), participate in chemorepul- ity of the selected cells (7). increased proportions of mature single- sion of lymphoid cells and whether, Analysis of the SDF1/CXCL12– positive CD4 or CD8 T cells in the thy- like neurons, lymphoid cells can CXCR4 contribution to thymocyte mus, in contrast to what is observed in undergo intrinsic changes that switch emigration in vivo has been difficult, PTX transgenic mice (6). In addition, their response to a ligand from attrac- as deficiency for either gene causes adult CCR7-deficient mice accumulate tive to repulsive. perinatal lethality. However, studies of approximately normal numbers of 1. Goshima, Y., Ito, T., Sasaki, Y., and Nakamura, F. lethally irradiated mice reconstituted peripheral T cells, making it clear that 2002. Semaphorins as signals for cell repulsion with CXCR4-deficient fetal liver or the CCR7 pathway cannot provide the and invasion. J. Clin. Invest. 109:993–998. lymphocyte-deficient mice grafted whole explanation for thymocyte emi- DOI:10.1172/JCI200215467. 2. Poznansky, M.C., et al. 2002. Thymocyte emigra- with CXCR4-deficient fetal thymi gration. Interestingly, ELC/CCL19 is tion is mediated by active movement away from appear to indicate that CXCR4 is not expressed at higher levels in medullary stroma-derived factors. J. Clin. Invest. limiting in thymocyte emigration (8, epithelial cells than in the adjacent 109:1101–1110. DOI:10.1172/JCI200213853. 3. Poznansky, M.C., et al. 2000. Active movement of 9). Further studies are needed, for endothelial structures that are believed T cells away from a chemokine. Nat. Med. example, in mice selectively lacking to be associated with exit (10), consis- 6:543–548. CXCR4 in T cells, before we can fully tent with the idea that this factor helps 4. Zlatopolskiy, A., and Laurence, J. 2001. ‘Reverse understand the role of this receptor in drive thymocyte emigration by a gear’ cellular movement mediated by chemokines. Immunol. Cell Biol. 79:340–344. thymocyte development. chemorepulsive mechanism. 5. Giger, R.J., and Kolodkin, A.L. 2001. Silencing the In general, it is difficult to judge from Overall, while there is still much to siren: guidance cue hierarchies at the CNS mid- these in vivo studies whether direct be learned, the new studies are signifi- line. Cell. 105:1–4. 6. Chaffin, K.E., and Perlmutter, R.M. 1991. A per- chemorepulsion is involved in thymo- cant in that they develop systems for tussis toxin sensitive process controls thymocyte cyte motility. In this regard, it is inter- studying thymocyte exit and they pro- emigration. Eur. J. Immunol. 21:2565–2573. esting to consider another recent vide hints as to the pathways that are 7. Lee, C.K., et al. 2001. Thymic emigrants isolated by a new method possess unique phenotypic and report, indicating that ELC/CCL19 and likely to be involved. The findings on functional properties. Blood. 97:1360–1369. its receptor CCR7 participate in thy- chemorepulsion are tantalizing and 8. Zou, Y.R., Kottmann, A.H., Kuroda, M., Taniuchi, mocyte exit, again possibly through a should certainly prompt further inves- I., and Littman, D.R. 1998. Function of the chemokine receptor CXCR4 in haematopoiesis chemorepulsive mechanism. Addition tigation of this process, particularly in and in cerebellar development. Nature. of ELC/CCL19 to the medium of fetal systems where the chemokine concen- 393:595–599. thymic organ cultures strongly aug- tration gradient can be precisely quan- 9. Ma, Q., Jones, D., and Springer, T.A. 1999. The chemokine receptor CXCR4 is required for the ments thymocyte emigration (10). Mice tified and controlled. It will also be retention of B lineage and granulocytic precursors lacking CCR7 or mice treated with an interesting to see whether molecules within the bone marrow microenvironment. ELC/CCL19–neutralizing polyclonal established to have repellent functions Immunity. 10:463–471. 10. Ueno, T., et al. 2002. Role for CCR7 ligands in the antibody have slightly increased total in the nervous system, such as mem- emigration of newly generated T lymphocytes numbers of thymocytes and reduced bers of the ephrin and semaphorin from the neonatal thymus. Immunity. 16:205–218. 1012 The Journal of Clinical Investigation | April 2002 | Volume 109 | Number 8.