Recirculating lymphocytes (RL) 1 are long-lived small lymphocytes which migrate continuously from blood to lymph, predominantly at specialized traffic sites in lymph nodes (LN) (1, 2). In individual LN, recirculation of lymphocytes accounts for over 95% of the cell output (3). In LN undergoing an immune response, where the cell output may increase by as much as 10-fold over the first 48 h after antigen, the migration of RL from blood to lymph still accounts for over 95% of the cell output (3-5).The observation that increased numbers of intravenous (i.v.)-injected ~lCr-labeled lymphocytes are present in LN after antigen stimulation has been attributed to a failure of RL to leave LN, as a direct consequence of antigen priming (6, 7). The transient fall in cell output in the efferent lymph of an antigen-stimulated LN was originally considered by Hall and Morris as possibly reflecting a temporary reduction in the entry of RL into the LN (8). Hall (9) has since reported that lymphocytes are entering a LN from the bloodstream during a period when the output of cells from an antigen-stimulated LN is decreased.Thus, at present, the effect of antigen on the migration of RL through LN is interpreted in two contradictory ways. According to one view, antigen provokes an increase in the input of lymphocytes from the blood into LN but is assumed to prevent those same lymphocytes from leaving the LN (6, 7). According to the other view, antigen provokes both an increased input of lymphocytes from blood into LN and an increased output of the same lymphocytes into the efferent lymph (3-5).Recently, by infusing i.v. ~Cr-labeled autologous RL obtained from efferent lymph draining single lymph nodes, we have demonstrated (10) that the majority of RL required about 30 h to pass from the bloodstream through a single resting LN and reach the efferent lymph. The present experiments were designed to decide between the conflicting opinions on immune response traffic changes and to examine the effects of antigenic stimulation on the migration of RL through single LN. In particular, to determine how the presence of antigen ~Abbreviations used in this paper: BCG, Bacille Calmette-Gu~rin; HRBC, horse red blood cells; i.v

A pronounced asymmetry in the recirculation from blood to lymph of resting small recirculating T lymphocytes is described. When 51Cr-labeled small T-recirculating lymphocytes (TRL) from intestinal lymph were infused intravenously their relative recovery in intestinal lymph was about twice that in nodal lymph. In contrast, the relative recovery in nodal lymph of 51Cr-labeled nodal TRL was twice that in intestinal lymph. Intestinal TRL migrated in large numbers through the small intestine. Nodal TRL did not. It is proposed that the pool of recirculating small T lymphocytes consists of two major subdivisions, an intestinal pool and a nodal pool. The nodal circulation comprises small TRL which traverse PCV in all lymph nodes (LN) but not the small intestine. The intestinal circulation comprises small TRL which do not traverse PCV in LN, but which do recirculate through the small intestine from which they pass via afferent lymphatics to the mesenteric LN and subsequently via the thoracic duct into the blood. It is suggested that the intestinal circulation is present in the fetus and that its initial development is independent of extrinsic antigen.

Using cannulated single lymph nodes in sheep the migration of 51 Cr‐labeled autologous and allogeneic recirculating lymphocytes (RL) into efferent lymph was studied after i. v. infusion and infusioninto an afferent lymphatic. The majority of autologous and allogeneic RL appeared in efferent lymph 27–36 h after their infusion i. v. and 6–12 h after intralymphatic infusion. The recovery of i. v. infused allogeneic RL in efferent lymph was only 5–10% that of autologous RL. In contrast, there was no difference in the recovery of autologous and allogeneic RL after infusion into an afferent lymphatic. In primed animals the migration of allogeneic RL through a lymph node was completely inhibited.

The size and docility of the sheep permit various surgical interventions and repeated collections of biological samples. Development of lymphatic cannulation techniques in this species enabled the investigation of the kinetics of lymphocyte migration in single lymph nodes of not only postnatal animals but also of fetuses at various stages of gestation. It was first demonstrated in the sheep that lymphocyte recirculation commences in the fetus without any exogenous antigenic stimulus. Using these cannulation techniques, it is also possible to investigate humoral events such as the secretion of lymphokines taking place in single lymph nodes with regard to the regulation of lymphopoiesis and the immune response. An extracorporeal perfusion system has been used successfully to investigate the emigration of cells from various lymphoid organs in the sheep. This apparatus enables cells to be labelled in their normal microenvironment with radioisotopes and/or fluorescent probes without destroying the normal tissue architecture. In studies with outbred animals such as the sheep, an investigation in which an individual animal is studied as a case history over a long time often provides much more information than studies based on single-point examinations of many animals and is much closer to the clinical study of immunological problems in individual humans. The recent development of an array of monoclonal antibodies against lymphocyte surface antigens in sheep will help to further dissect the complexity of immunological phenomena. Therefore, the sheep is a useful animal model to study physiological events taking place in the lymphoid system, and in vivo studies in this species will continue to offer a great potential for research of biological relevance and supplement the research done on the in vitro manipulation of cells and biological products related to the immune system.

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