JOURNAL OF PATHOLOGY, VOL. 156: 1-3 (1 988)

EDITORIAL

DO VASCULAR ADDRESSINS MEDIATE TISSUE SPECIFIC LYMPHOCYTE EXTRAVASATION?

In 1964, Gowans and Knight’ first described ‘the route of recirculation of lymphocytes in the rat. By using cells labelled with radioisotopes, they showed that small lymphocytes continuously recirculate between the blood and the lymph; extravasating via high endothelial venules (HEVs) into lymphoid tissues, then draining into lymphatics and re- entering the blood via the thoracic duct. In addition, they demonstrated that many of the large dividing cells in the thoracic duct lymph (which receives its largest contribution from the intestinal lymphatics) extravasated into the lamina propria of the small intestine after being injected intravenously into syn- geneic recipients. Since these observations, the complexity of lymphocyte migration has been demonstrated by an expanding literature on the subject. The migratory pathways of lymphocytes depend on whether they are large dividing or small quiescent cells; whether the cells are from lymph node, spleen, or lymph; whether they originate in the gut-associated lymphoid tissue (GALT) or the peripheral somatic lymphoid tissue (PSLT); and whether the cells being studied are B or T cells.

In a recent publication,’ endothelial molecules believed to be involved in lymphocyte extravasation into the mucosae have been described. These mol- ecules, the putative ‘vascular addressins’, communi- cate the site of the appropriate endothelium to recirculating lymphocytes, resulting in tissue specific extravasation. The existence of such vascu- lar addressins is important not only in the under- standing of the physiology of lymphocyte traffic, but by manipulating such molecules it may be poss- ible to engineer tissue specific immunosuppression or to control the metastasis of lymphoid tumours. Before discussing the evidence for the existence of these molecules, it is worth reviewing briefly the results of some key studies of lymphocyte migration in vivo.

Small lymphocytes from either mesenteric lymph node or peripheral somatic lymph nodes of rats,3 mice,4 and sheep,5 labelled in vitro and returned

intravenously to syngeneic or autochthonous recipi- ent animals, migrate randomly. Small lymphocytes from the lymph of adult sheep, however, show differential migration between GALT and PSLT.6 Technically, these experiments require that both an intestinal lymphatic and an efferent lymphatic from a peripheral somatic lymph node be cannulated. It can then be demonstrated that small lymphocytes obtained from intestinal lymph, when radiolabelled in vitro and returned intravenously to the same animal, preferentially re-enter the intestinal lymph. Similarly, if cells from the efferent lymphatic of a peripheral somatic node are collected, labelled in vitro, and returned intravenously to the same sheep, the cells will tend to reappear in the lymph from the peripheral somatic node. It is not possible to com- pare the migratory characteristics of lymph-borne small lymphocytes from the intestine and peripheral nodes in rats and mice because it is not possible to collect intestinal lymph and lymph from a periph- eral somatic lymph node of the same animal simultaneously.

Small B lymphocytes from mesenteric or periph- eral lymph node or thoracic duct lymph may prefer- entially extravasate into the Peyer’s patches in short-term adoptive transfer experiments (less than 1 h after intravenous injection) in mice7 and rats.’ There are, however, several problems in interpreting these data. Firstly, using thoracic duct lymph of nude rats as a source of B cells for adoptive trans- fer, Fossum et a1.’ showed that the short-term migratory behaviour of B cells is particularly sus- ceptible to changes in behaviour caused by reduc- tion in temperature during handling. Short-term storage at low temperatures reduces B cell extra- vasation into PSLT in particular. This may explain the apparently preferential localization of B cells in the Peyer’s patches, the site least susceptible to tem- perature-induced changes in the B cells. Secondly, after 24 h there is no differential distribution of B and T cells irrespective of their source between the Peyer’s patches and the lymph nodes.’ Thirdly, the

0022-341 7/88/090001-03 $05.00 0 1988 by John Wiley & Sons, Ltd.

2 EDITORIAL

Peyer’s patches, cervical lymph nodes, and the mesenteric lymph nodes of mice each contain approximately 30 per cent kappa chain positive cells (ca. 95 per cent B cells in the rat), giving no grounds to suspect that small B or T cells migrate preferen- tially into any of these tissues.8 Therefore, although there are some in vivo data to suggest that B cells recirculate preferentially through the gut, this is not yet proven because of technical difficulties in the experiments. Differential migration of small lym- phocytes has only been demonstrated conclusively using the true migrating cells from the lymph of sheep, as mentioned above.

The studies discussed above relate to the migration of quiescent small lymphocytes. Acti- vated lymphocytes, i.e., immunoblasts, however, have quite different migratory characterisitics. Dividing cells (B or T immunoblasts) derived from GALT migrate via the regional lymphatics to the blood and then ‘home’ back to the gut. Dividing cells from either mesenteric lymph nodes or intesti- nal or thoracic duct lymph will, after radioactive labelling in vitro, extravasate in the small intestine of syngeneic or autochthonous recipient animals.’-” This differential distribution is independent of anti- gen, although antigen may cause local proliferation resulting in accumulation of antigen reactive cells.” A study of the migration of immunoblasts from the mesenteric lymph nodes in relation to the blood flow and the burden of parasites in the gut has shown that retention of immunoblasts occurs within the infected regions of the small inte~t ine.’~ Czinn and Lamm14 have recently described a chemo- tactic factor from epithelialcells which may influence the retention of IgA producing cells in the gut, and Tsaacson15 has observed that IgA plasma cell locali- zation adjacent to carcinomas in the colon occurs only when the tumours are well differentiated and synthesize secretory component.

In summary, experiments in vivo have shown that small lymphocytes teased from lymph nodes migrate randomly. In sheep, small lymph-borne lympho- cytes show a tendency to recirculate either through GALT or PSLT depending on their lymph of origin. Small B cells may extravasate preferentially into Peyer’s patches but these experiments may be influenced by the handling of B cells in vitro. Prefer- ential ‘homing’ to the tissue of origin, either to GALT or to PSLT, is a fundamental property of dividing B and T cells.

In contrast, experiments in vitro suggest that Peyer’s patches contain small lymphocytes which migrate preferentially through the Peyer’s patches.I6

Most of this work is based on an in vitro model of lymphocyte extravasation involving the incubation of lymphocytes at 7°C on glutaraldehyde fixedI7 or ~ n f i x e d ~ , ~ ’ ’ ~ sections of tissue. This technique has shown preferential adherence of lymphocytes derived from Peyer’s patches to Peyer’s patch HEV and peripheral node lymphocytes to peripheral node HEV. These experiments were reported together with substantiating evidence from in vivo adoptive transfer of Peyer’s patch lymphocytes. Peyer’s patch lymphocytes showed a greater tend- ency to extravasate into Peyer’s patches than lym- phocytes from peripheral lymph nodes. Lymphomas which show distinct adherence to either Peyer’s patch HEV or lymph node HEV have also been demon- strated.I6 The endothelial cell/lymphocyte recog- nition systems are postulated to extend beyond GALT/non-GALT; for example, a separate system in the synovium of joints has been proposed.I8

Another approach to the study of the interaction between small lymphocytes and endothelium has been to use monoclonal antibodies to the proposed receptor-ligand systems that mediate the initial attachment of lymphoid cells to the endothelium. MEL-14 is an antibody recognizing the receptor on lymphocytes which adhere to the HEVs in periph- eral lymphoid tissues in vitro.” This antibody is able to block the in vitro adherence of cells to HEVs in peripheral nodes. Recently, two monoclonal anti- bodies have been described which bind to HEVs. These antibodies recognize all HEVs in the organ- ized lymphoid tissue in the gut (the Peyer’s patches), most HEVs in the mesenteric lymph nodes, and only occasional HEVs in the peripheral somatic lymph nodes2 One of these antibodies was able to prevent lymphocyte extravasation into the Peyer’s patches almost completely and also impair lympho- cyte extravasation into mesenteric lymph nodes. Lymphocyte extravasation into peripheral somatic lymph nodes was unaffected, correlating with the activity of the antibody on the HEVs in tissue sec- tions. The endothelial molecules have been termed ‘vascular addressins’. It has been proposed that they ‘communicate’ the site of the endothelium express- ing them to blood-borne lymphocytes. Proposed counterparts to the vascular addressin described in mucosae which confer positional information in the peripheral lymphoid tissue and synovium of joints have not yet been identified. It is important, how- ever, in the excitement of interpreting such novel data and extrapolating to the potential use of vascu- lar addressins in the clinic to remember, firstly, that most of the evidence for their role in the trafficking

EDITORIAL 3

of lymphocytes has been obtained using as assay at 7°C on a tissue section. Secondly, painstakingly careful experiments using cells from mesenteric lymph nodes (through which the cells from the Peyer’s patches must migrate) and secondary transfer of thoracic duct lymphocytes which extra- vasate into mesenteric nodes have failed to demon- strate differential migration of small lymphocytes in rats or mice. Differential recirculation of small lymphocytes between GALT and PSLT has only been demonstrated using small lymphocytes from the lymph of sheep. Sheep, however, do not have well-developed high endothelial venules in the para- cortices of their lymph nodes as do rats and man (J . G. Hall, personal communication). The results of further studies that may resolve these paradoxes are eagerly awaited.

ACKNOWLEDGEMENTS

We thank Professor J. G. Hall and Professor

J. SPENCER and P. G. ISAACSON University College and Middlesex

School of Medicine London WCl E 6JJ

D. M. V. Parrott for valuable discussions.

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