Lymphopoiesis and lymphocyte recirculation in the sheep fetus

Development and Characterization of Anti-Sheep Podoplanin Monoclonal Antibodies PMab-253 and PMab-260

Anti-podoplanin (PDPN) monoclonal antibodies (mAbs) are needed as markers for lymphatic endothelial cells or type I alveolar cells in immunohistochemical analyses. We have developed anti-PDPN mAbs for many species, including humans, mice, rats, rabbits, dogs, cats, bovines, pigs, Tasmanian devils, alpacas, tigers, whales, goats, horses, and bears. This study develops and characterizes anti-sheep PDPN (sPDPN) mAbs using Cell-Based Immunization and Screening (CBIS) method. A RAP14 tag was added to the N-terminus of sPDPN, and an anti-RAP14 tag mAb (PMab-2) was used to measure the expression level of sPDPN in flow cytometry and Western blots. We immunized mice with sPDPN-overexpressing Chinese hamster ovary (CHO)-K1 (CHO/sPDPN) cells and screened mAbs against sPDPN using flow cytometry. Two of the mAbs, PMab-253 (immunoglobulin M [IgM], kappa) and PMab-260 (IgM, kappa), detected CHO/sPDPN cells specifically using flow cytometry and Western blots. Both PMab-253 and PMab-260 stained the renal glomerulus and Bowman's capsule, lymphatic endothelial cells of the lung and colon, and type I alveolar cells of the lung, suggesting PMab-253 and PMab-260, which were developed by CBIS method, can be applied to functional analyses of sPDPN. We also determined the binding epitope of PMab-253 and PMab-260 using flow cytometry. Analysis of sPDPN deletion mutants revealed that the N-terminus of the PMab-253 and PMab-260 epitope exists between amino acids 110 and 115 of sPDPN. Analysis of sPDPN point mutations revealed that the critical epitope of PMab-253 and PMab-260 includes Thr112 and Ser113 of sPDPN, indicating that the PMab-253 and PMab-260 epitope are independent of the platelet aggregation-stimulating (PLAG) domain or the PLAG-like domain of sPDPN.

The EXTrauterine Environment for Neonatal Development Supports Normal Intestinal Maturation and Development

BACKGROUND AND AIMS

The Extra-Uterine Environment for Neonatal Development (EXTEND) aims to avoid the complications of prematurity, such as NEC. Our goal was to determine if bowel development occurs normally in EXTEND-supported lambs, with specific emphasis on markers of immaturity associated with NEC.

METHODS

We compared terminal ileum from 17 pre-term lambs supported on EXTEND for 2- 4 weeks to bowel from age-matched fetal lambs that developed in utero. We evaluated morphology, markers of epithelial integrity and maturation, enteric nervous system structure, and bowel motility.

RESULTS

EXTEND-supported lamb ileum had normal villus height, crypt depth, density of mucin-containing goblet cells, and enteric neuron density. Expression patterns for I-FABP, activated caspase-3 and EGFR were normal in bowel epithelium. Transmural resistance assessed in Ussing chambers was normal. Bowel motility was also normal as assessed by ex vivo organ bath and video imaging. However, Peyer's patch organization did not occur normally in EXTEND ileum, resulting in fewer circulating B cells in experimental animals.

CONCLUSION

EXTEND supports normal ileal epithelial and enteric nervous system maturation in pre-term lambs. The classic morphologic changes and cellular expression profiles associated with NEC are not seen. However, immune development within the EXTEND supported lamb bowel does not progress normally.

Characterisation of ovine lymphatic vessels in fresh specimens

Background and aim

The development and use of experimental models using lymphatic cannulation techniques have been hampered by the lack of high-quality colour imaging of lymphatic vessels in situ. Most descriptions of lymphatic anatomy in sheep have historically depended on schematic diagrams due to limitations in the ability to publish colour images of the lymphatic vessels with decent resolution. The aim of this work was to encourage more widespread use of the ovine cannulation model by providing clear photographic images identifying the location and anatomical layout of some major lymphatic ducts and their in situ relationship to surrounding tissues.

Methods

The cadavers of the sheep were collected after they had been euthanized at the end of animal trials not associated with this study. The lymphatics were dissected and exposed to show their appearance in the surrounding tissues and their relationship to other organs. Patent Blue was used to locate lymphatic vessels in exploratory preparations. However, in order to present the natural appearance of the vessels, we used minimal dissection and dye was not used for the photographed examples. Instead, we have indicated the course of the vessels with lines where their position is less clear.

Results and conclusion

In this paper, we have used sheep specimens as examples to show characteristic images of lymphatic vessels. The images of in situ lymphatics and lymph nodes combined with schematic summaries provide a concise illustration of the lymphatic drainage scheme in sheep.

Immunophenotyping of Sheep Paraffin-Embedded Peripheral Lymph Nodes

Sheep are not only a major livestock species globally, they are also an important large animal model for biomedical research and have contributed to our understanding of the ontogeny and architecture of the mammalian immune system. In this study, we applied immunohistochemistry and multicolor immunofluorescence in fixed and paraffin-embedded lymph nodes to phenotype the key populations of antigen presenting cells, lymphocytes, and stromal cells that orchestrate the host adaptive immune response. We used an extensive panel of antibodies directed against markers associated with dendritic cells (MHC class II, CD83, and CD208), macrophages (CD11b, CD163, and CD169), stromal cells (CNA.42, S100, and CD83), and lymphocytes (CD3, Pax5, CD4, CD8). Using different methods of tissue fixation and antigen retrieval, we provide a detailed immunophenotyping of sheep lymph nodes including the identification of potential subpopulations of antigen presenting cells and stromal cells. By characterizing cells expressing combinations of these markers in the context of their morphology and location within the lymph node architecture, we provide valuable new tools to investigate the structure, activation, and regulation of the sheep immune system in health and disease.

Lymphatic cannulation models in sheep: Recent advances for immunological and biomedical research

Lymphatic cannulation models are useful tools for studying the immunobiology of the lymphatic system and the immunopathology of specific tissues in diseases. Sheep cannulations have been used extensively, as models for human physiology, fetal and neonatal development, human diseases, and for studies of ruminant pathobiology. The development of new and improved cannulation techniques in recent years has meant that difficult to access sites, such as mucosal associated tissues, are now more readily available to researchers. This review highlights the new approaches to cannulation and how these, in combination with advanced omics technologies, will direct future research using the sheep model.

Reassortment of cell populations within the lymphoid apparatus of the sheep

As lymphocytes recirculate through the blood tissues and lymph they are sorted into populations which have varying morphological and functional characteristics. Lymphocytes are added, deleted and transformed within the lymphoid apparatus as a consequence of non-random migration and antigenic stimulation. There is evidence that the physiological characteristics of peripheral and central lymph nodes vary as a result of differences in the origins of the cells entering the nodes. Lymphocytes enter the lymph nodes from the blood and lymph in varying numbers; consequently the cell population in the efferent lymph of central and peripheral lymyph nodes contains different proportions of blood-borne and lymph-borne cells. Cells arriving in lymph nodes by way of the blood or the lymph migrate differently within the node. Those entering from the blood go principally to the paracortex and the follicular areas. Lymphocytes entering in the lymph are distributed through both the cortex and the medulla. In humoral antibody responses and in the response that occurs during the rejection of a renal allograft, lymph-borne cells populate the medullary cords, cortex and germinal centres of the nodes they enter. Within these nodes, new populations of cells are generated which have different functional attributes from the cells which provoked their formation.

Lymphocyte recirculation in the sheep fetus

The numbers of circulating thymus-derived and surface Ig-bearing lymphocytes in the fetal lamb increase exponentially over the last third of gestation. Experiments in which [3H]thymidine was continuously infused into fetal lambs have established that these cells are long-lived in the fetus. The migration of 51Cr-labelled autologous lymphocytes from intestinal or prescapular lymph was compared in fetal lambs and adult sheep. A subpopulation of thymus-derived lymphocytes present in intestinal lymph of adults which migrated preferentially to the small intestine was not found in fetal intestinal lymph. There were marked differences in the migration of fetal and adult lymphocytes to the lungs and liver. In spite of the absence of circulating antibodies or immunoglobulins and of extrinsic antigen in the immunologically virgin sheep fetus, the circulation of lymphocytes through the spleen and lymph nodes of fetal lambs was more intense than in the adult, indicating that the pathways of recirculation and the capacity of cells to recirculate arise as a physiological process independently of antigenic stimulation.

Expression of mucosal chemokines TECK/CCL25 and MEC/CCL28 during fetal development of the ovine mucosal immune system

CCL25/TECK and CCL28/MEC are CC chemokines primarily expressed in thymic dendritic cells and mucosal epithelial cells. The cognate receptors of CCL25 and CCL28, CCR9 and CCR10, respectively, are mainly expressed on T and B lymphocytes. In human, mouse and pig, CCL25 and CCL28 play a key role in the segregation and the compartmentalization of the mucosal immune system through recruitment of immune cells to specific locations. However, little is known about their role in the ontogeny of the mucosal immune system during fetal development. In the present paper, we report the cloning and the sequencing of ovine CCL25, CCL28, CCR9 and CCR10 and the subsequent assessment of their mRNA expression by q-polymerase chain reaction in several tissues, including thymus, gut-associated lymphoid tissue and mammary gland, from young and adult sheep and in the fetal lamb during the development of the immune system. CCL25 mRNA was highly expressed in thymus and gut while CCL28 mRNA was more expressed in large intestine, trachea, tonsils and mammary gland, especially at the end of gestation. These results are consistent with observations in other species suggesting similar roles for these chemokines in sheep. In fetuses, mRNA of CCL25, CCL28 and their receptors are expressed early in the thymus and mucosal tissues, including the small intestine and the nasal mucosa. Furthermore, their expression increased towards the end of gestation. Consequently, we hypothesize that CCL25 and CCL28 play an important role in the lymphocyte colonization of fetal tissues, enabling the development of a functional immune system.

Cell death and thymic export during fetal life

In the fetus the peripheral T cell pool expands as the fetus grows, but the mechanisms that regulate T cell homeostasis during fetal life are unknown. Here, we show that the peripheral T cell pool in the sheep fetus is established by the export from the fetal thymus of twice as many CD8+ as CD4+ thymic emigrants every day. Clonal deletion of CD4+ thymocytes in the fetal thymus appeared to be more stringent than was the case for CD8+ thymocytes because only 1 in 35 single-positive CD4 (SPCD4) thymocytes was exported from the thymus whereas the majority (2/3) of the single-positive CD8 (SPCD8) thymocytes were exported from the fetal thymus each day. Furthermore, within the thymus, the number of apoptotic SPCD4 thymocytes was 40 times greater than the number of apoptotic SPCD8 thymocytes. A tissue-specific migration of CD8+ emigrants localizing in the spleen was also established in the fetus in contrast to CD4+ emigrants, which migrated randomly to spleen and LN.

Cell adhesion molecule expression in the sheep thymus

Cell adhesion molecules are potential regulating factors in both prethymic and intrathymic T cell development. An experimental challenge has been the development of a large animal model that facilitates in vivo studies of both intrathymic development and lymphocyte migration. To extend earlier studies of thymic development, we have developed a panel of monoclonal antibodies (mAb) to a variety of sheep cell adhesion molecules. Immunohistochemistry was used to define mAb reactivity and flow cytometry was used to quantify expression of cell adhesion molecules within the thymus. To facilitate flow cytometry definition of cortical thymocytes, mAbs were developed to the sheep CD1 antigen. Dual parameter flow cytometry provided a phenotypic characterization of cell adhesion molecule expression on both CD1(+) and CD1(-) sheep thymocyte populations. These studies demonstrated significantly enhanced cortical thymocyte expression of three cell adhesion molecules: beta1 integrin (CD29), ICAM-2 and LFA-3. The beta1 integrin cell adhesion molecule was also expressed at higher levels on CD1(+) thymocytes in post-natal lambs as compared to adult sheep. These studies of thymocyte membrane molecule expression should facilitate future investigations of sheep intrathymic development and T lymphocyte immigration.

An in vivo model to investigate lymphocyte-mediated immunity during acute hemoparasitic infections. Use of a monoclonal antibody to selectively deplete CD4+ T lymphocytes from thymectomized calves

Thymectomized calves were selectively depleted of CD4+ T lymphocytes with a monoclonal antibody (mAb) specific for the bovine CD4 monomer (ILA-11). Calves were treated with high loading doses of ILA-11 during the first week of the study then treated with subsequent lower maintenance doses. Depletion of CD4+ T lymphocytes was assessed weekly by flow cytometric analysis of PBMC and mononuclear cells from lymph node and spleen biopsies. Treatment with high doses of ILA-11 resulted in rapid and marked depletion of CD4+ T lymphocytes from the peripheral blood, peripheral lymph nodes, and spleen. Although CD4+ T lymphocytes slowly returned to the peripheral blood, peripheral lymph nodes, and spleen by day 21 post-treatment, the numbers of CD4+ T lymphocytes in depleted calves remained below pre-depletion levels for the duration of the study. CD4+ T lymphocytes failed to be effectively depleted from a non-thymectomized calf treated with the mAb ILA-11. Development of a T lymphocyte depletion model in thymectomized calves will permit testing of the hypothesis that CD4+ T lymphocytes and IFN-gamma are required in cattle for control of acute anaplasmosis. In subsequent planned studies, thymectomized calves depleted of CD4+ T lymphocytes will be experimentally infected with A. marginale and parameters of disease compared between depleted and non-depleted calves.

Regulation of T cell homeostasis during fetal and early postnatal life

Before parturition the fetal lamb develops a large pool of long-lived recirculating T cells which provides a large population of naive T cells with a diverse TcR repertoire. After birth and concomitant with exposure to environment antigens, fetal T cells are rapidly replaced by short-lived cells formed postnatally. The majority of thymic emigrants homing to spleen in postnatal lambs are short-lived, in contrast to emigrants targeting lymph nodes where a population appears to be long-lived. The lifespan of thymic emigrants in the fetus is unknown as in the relative importance of antigen-driven processes versus developmental programming in regulating T cell homeostasis in early postnatal life.

The ontogeny of T cell recirculation during foetal life

Studies on the ontogeny of the immune system in the sheep foetus, which remains immunologically naive until after birth, indicate that a large scale recirculation of T cells is just as much a feature of the foetal immune system as it is in animals after birth. An extensive recirculation of T cells and dendritic cells through peripheral tissues-including the gastrointestinal tract and skin-develops early in foetal life, although a population of gut-homing memory T cells does not develop until postnatal life. Current evidence suggests that two populations of thymic emigrants with distinct tissue-homing specificities to spleen and lymph node contribute to the development of the foetal peripheral T cell pool. CD8(+) thymic emigrants mostly target spleen while CD4(+) thymic emigrants home predominantly to lymph nodes. The lifespan of thymic emigrants is uncertain, although cells entering lymph are long-lived and form the basis of a diverse pre-immune repertoire of recirculating T cells. The life history and growth rates of non-recirculating T cells in spleen and lymph nodes during foetal life are at present unknown.

The genesis, tutelage and exodus of B cells in the ileal Peyer's patch of sheep

The ileal Peyer's patch (PP) is a prominent lymphoid organ that extends 1-2 meters along the terminal small intestine of sheep. It is comprised of rapidly proliferating B cells that make major contributions to the animals total B cell system. The characteristics of this tissue in sheep have enabled a variety of novel approaches to studying both the B cell system and the contribution of PP to the mucosal immunity. The sheep ileal PP has characteristics that place it in a category similar to that of the thymus, bone marrow and the avian bursa of Fabricius. The ileal PP develops before birth and involutes while the sheep is still young. It produces B cells that populate the immune system but most of the large numbers of newly-formed B cells are rapidly destroyed by apoptosis. It has been concluded that this death is related to a selection event that examines each newly-formed cell. Antibody diversity in sheep is a post-rearrangement event, generated by the process of somatic hypermutation. A comparison of the lambda light chain gene from surviving and dying B cells indicates that when the PP is at its greatest size the dying cells have the characteristics of cells with high affinity receptors for a selecting ligand/antigen. It is proposed that if B cells proliferate in the continual presence of selecting ligand/antigen that the B cell receptor might develop a sufficient affinity to trigger apoptosis. This process might contribute to elimination of clones with high affinity for either self antigens or continually present environmental antigens.

Fetal lambs are depleted of IgM+ cells following a single injection of an anti-IgM antibody early in gestation

B-cell depleted fetal sheep were created following a single injection of an anti-IgM monoclonal antibody early in gestation. Six sheep fetuses were given a single intraperitoneal injection of a monoclonal antibody directed against IgM at 63 days of gestation (gestation in sheep = 150 days). The fetuses were killed at 138-142 days of gestation and lymphoid tissues were collected for subsequent light microscopy and immunohistochemical examination. The ileal and jejunal Peyer's patch (PP) follicles in four of the six injected fetuses were markedly reduced in size. Cells in the rudimentary follicles of the ileal PP of these animals showed no reactivity for IgM and most were negative for CD45. The dome regions contained many T cells, which were predominantly CD8+ cells and included gamma delta T cells. The interfollicular areas of the PP of the markedly affected fetuses contained large populations of T cells. The spleen and lymph nodes were also markedly depleted of IgM+ cells and these tissues contained only a small, scattered population of weakly IgM+ cells. Follicular accumulations of IgM+ cells were absent. Large populations of T cells were present in the white pulp of the spleen and cortex of the lymph nodes. The liver did not contain IgM+ cells and the medulla of the thymus was depleted of IgM+ cells. The results of this study suggest that a surface IgM+ B-cell population is present in the sheep fetus at 63 days of gestation, which is essential for the colonization of the ileal PP and subsequent B-cell development.

Late-term fetal thymectomy does not prevent the development of gut-homing T cells after birth

Tissue-specific circulation of T cells is a critical element in the integration of systemic immune responses. Current models of T-cell migration suggest that homing specificities of T cells for tissues such as gut and skin are generated outside the thymus as a result of activation of virgin T cells by antigen in lymph nodes. We have used the sheep fetus (which is immunologically virgin and contains no memory or effector T-cell subsets) to examine the migration of 51Cr-labelled T cells in vivo. We report that gut-homing T cells are not present in the fetus and that gut-homing T cells from postnatal lambs home normally to fetal gut. Fetal thymectomy performed immediately prior to birth failed to prevent the development of gut-homing T cells in postnatal life. Gut-homing specificities on T cells are thus acquired extrathymically.

Lymphocyte traffic and positioning in vivo: an expanded role for the ECM, the VLA proteins and the cytokines

The continuous circulation of lymphocytes between blood and lymph, constitutes, with the specific recognition of foreign antigens and with memory, one of the hallmarks of the immunological system. In the routine process of recirculating between blood and lymph in adult life, lymphocytes display the unique capacity of recognizing "self within self". The physiological recognition of "self within self" is expressed at three levels: (1) an overall ability to distinguish between peripheral lymphoid and non-lymphoid organs, (2) the ability of lymphocyte subpopulations to discriminate between peripheral lymphoid organs, (3) a fine recognition of distinct microenvironments within the peripheral lymphoid organs. Historically, interest in regulation of entry into the lymph dominated this field, focusing primarily on the interaction of lymphocytes with post-capillary venules with high endothelium found in some higher vertebrate lymph nodes. Lymphocytes, however, enter the lymph in species without lymph nodes, and lymphocytes recirculation is well established in fetal life. Regardless of route of entry, lymphocytes in birds, fish, rodents, and humans enter the peripheral lymphoid organs and display the capacity to segregate and arrange themselves in distinct territories, a phenomenon called "ecotaxis". This paper reviews the evidence for the relative contribution of so called specialized lymphocyte endothelium interactions and other interactions, to the physiological regulation of lymphocyte traffic and positioning. Of the latter, interactions of lymphocytes with the ECM appear of some significance for two reasons: there is a selective distribution of some ECM components in peripheral lymphoid organ areas, as recently shown for tenascin in the thymus-dependent zones (Chilosi et al., Am J Pathol 143: 1348-1355, 1993). This selective distribution may serve as a basis for preferential T cell migration through those zones. The abnormal expression of tenascin in non-lymphoid organs with autoimmune lesions may serve as an identical basis for abnormal T lymphocyte migration in autoimmunity (Chilosi, personal communication). Additional evidence for abnormal cell-ECM interactions playing a role in autoimmune-like lesions comes from the recent observations in TGF-beta 1 knockout mice, in which mononuclear cell infiltration of the heart and lungs has been corrected by the systemic administration of synthetic FN peptides (Hines at al., PNAS 91:5187-5191, 1994). Changes in expression of ECM components have aso being described in rat heart allografts preceding lymphocyte accumulation in the process of allograft rejection (Coito et atl. Transplantation 57:599-605, 1994).(ABSTRACT TRUNCATED AT 400 WORDS)

Development and cell phenotypes in primary follicles of foetal sheep lymph nodes

Lymph nodes from sheep foetuses and postnatal lambs were examined to determine the participation of different leucocyte populations in primary follicle formation, with special emphasis on the emergence and subsequent development of follicular dendritic cells during late gestation and early postnatal life. A series of immune and enzyme histochemical markers was used. The first 5'-nucleotidase-positive primary follicles were found at 80 days gestational age (gestation in sheep is 150 days) in superficial cervical lymph nodes. In the last month of gestation the primary follicles possessed follicular dendritic cells, macrophages, dendritic cells, and CD5-positive lymphocytes, in addition to IgM-positive cells. Follicular dendritic cells in primary follicles were found to be ultrastructurally immature. These follicular dendritic cells were characterised by a few, course surface projections and many ribosomes attached to the endoplasmic reticulum. A final differentiation to mature follicular dendritic cells was coincident with the postnatal germinal centre reaction. Computer-assisted morphometric analysis demonstrated that the size of 5'-nucleotidase-positive primary follicles in the distal jejunal lymph node, but not in the superficial cervical lymph node, increased significantly during late gestation. It was concluded that stromal cells in primary follicles of foetal sheep lymph nodes were a continuously developing population but that ultrastructural maturity was only achieved in the germinal centres of postnatal lambs.

Homing of naive, memory and effector lymphocytes

Lymphocyte subsets home in a particular manner depending on their class, state of activation and 'allegiance' to certain tissues. Naive lymphocytes home preferentially through lymphoid tissues, whereas effector and memory cells home preferentially through non-lymphoid tissues and exist as phenotypically distinct subsets with preferential localization to gut, skin or other tissues. These migration pathways, which correlate with different functional properties of lymphocyte subsets, increase the efficiency of the immune system.

Lymphocyte recirculation and life span in vivo

The physiological process of lymphocyte migration is a complex and dynamic process. The differential migration and life span of lymphocyte subsets is inherent to the normal function of the mammalian immune system. Adequate assessment of the involved processes requires the presence of an intact blood and lymphatic circulatory system and the ability to isolate individual tissues. The sheep provides an invaluable experimental model for studying these processes. Recent data suggest that direct quantitation of the life span of individual subsets of recirculating memory and naive lymphocytes is now possible, and that the long-term characterization of the behaviour of recirculating cells can be undertaken. Finally, it appears that previous qualitative data on tissue-specific homing pools can now begin to be understood in the context of phenotypic analysis for T cell markers and adhesion molecules, combined with long-term tracking techniques.

T-cell memory: the connection between function, phenotype and migration pathways

Immunological memory is a fundamental feature of vertebrate immune systems, providing enhanced protection against previously encountered antigens. The established view has been that immunological memory results from clonal expansion and long-term survival of specialized memory cells. Recently, the nature of memory T cells has come under closer scrutiny because of the ability to distinguish naive and memory T cells phenotypically, particularly in humans. In this article, Charles Mackay discusses three features of memory T cells that help to explain the nature and function of these cells: the increased expression of adhesion and activation molecules on memory T cells, their potent functional status and their specific pathways of recirculation.

Naive and memory T cells show distinct pathways of lymphocyte recirculation

In this report, we have addressed two questions concerning immunological memory: the way in which naive and memory T cells recirculate through the body, and the intrinsic rate of division within the naive and memory populations. We identified naive and memory T cells in sheep by their cell surface phenotype and their ability to respond to recall antigen. Memory T cells were CD2hi, CD58hi, CD44hi, CD11ahi, and CD45R-, as pertains in man. T cells that crossed from blood to the tissues of the hind leg and accumulated in the popliteal afferent lymph were all of memory phenotype. Conversely, T cells in efferent lymph, 90% of which entered the lymph node (LN) via high endothelial venules (HEV), were mostly of the naive phenotype (CD2lo, CD58lo, CD44lo, CD11alo, and CD45R+). The marked enrichment of these two phenotypes in different recirculatory compartments indicated that memory T cells selectively traffic from blood to peripheral tissues to LN (via afferent lymph), whereas naive T cells selectively traffic from blood to LN (via HEV). We argue that the differential use of these two recirculation pathways probably optimizes lymphocyte interactions with antigen. The nonrandom distribution of T cell subsets in various recirculatory compartments may be related to the relative proportion of memory cells in each subset. In particular, gamma/delta T cells in blood were almost exclusively of memory phenotype, and accumulated preferentially in afferent, but not in efferent, lymph. Finally, using the bromo-deoxyuridine labeling technique, we found that at least a sizeable proportion of memory T cells, whether in blood or afferent lymph, were a dividing population of cells, whereas naive T cells were a nondividing population. This result supports an alternative model of lymphocyte memory that assumes that maintenance of memory requires persistent antigenic stimulation.

Lymphocyte homing

The phenomenon of "homing" is discussed with respect to patterns of lymphocyte circulation and the molecules on the surface of both endothelium and lymphocytes that mediate this process. In addition, the data are analysed in the context of a model for lymphocyte homing.

Traffic and proliferative responses of recirculating lymphocytes in fetal calves

The thoracic duct or efferent prescapular duct was cannulated in four fetal calves aged 121-259 days post-conception. The duration of lymph flow ranged from 2 to 20 days and the mean flow rates sustained over these collection periods varied from 5.4 to 48.8 ml/hr. Lymphocyte output ranged from 4.4 x 10(6) cells/hr in thoracic duct lymph from a 121-day fetus to 3.9 x 10(8) cells/hr in efferent prescapular lymph from a 259-day fetus. The circulating lymphocyte pool in fetal calves of about 120 and 190 days gestational age was calculated to contain, respectively, 4 x 10(8) cells and 2 x 10(10) cells. The proportion of lymphocytes bearing surface immunoglobulin detected in fetal lymph ranged from 2.1% to 8.7%. Recirculating lymphocytes from fetal calves produced strong proliferative responses when stimulated by T-cell mitogens but responded poorly to B-cell mitogens. Fetal lymphocytes also responded to stimulation by allogeneic cells and stimulated other cells to proliferate during mixed lymphocyte culture. When stimulated with Con A, fetal lymphocytes secreted IL-2 to a degree that was indistinguishable from the secretory behaviour of lymphocytes from adult animals. The results presented in this paper show that chronic lymphatic fistulae can be established successfully in fetal calves to give access to recirculating lymphocytes. This provides a new experimental approach for studying the development of the bovine immune system.

T and B cells have similar recirculation kinetics in sheep

T and B cells continually migrate around the body by recirculating between blood and lymph via lymphoid tissues. This is an important way of fostering the cell-cell interactions required for the initiation of an immune response. Data from previous studies of T and B cell recirculation, based on relatively complex approaches, have been interpreted as evidence that the recirculation of B cells is more sluggish than for T cells. We have shown from a study in sheep that this is not the case. Lymphocyte migration through both the intestine and the prescapular lymph node was analyzed by cannulating efferent lymphatics. The lymph cells were labeled in vitro with Hoechst 33342, or other fluorochromes, then injected i.v., and the rate of their recirculation into the lymph was determined by fluorescence microscopy. A multicolor immunofluorescence assay was used to classify each recirculated cell as either a T or B cell. This established that the T and B cells from intestinal lymph and from prescapular lymph recirculate with similar kinetics. The recovery of i.v. injected T and B cells was the same in prescapular lymph, but fewer injected B cells than T cells were recovered in intestinal lymph. Thus, although the recirculation kinetics are the same for T and B cells, the two populations are handled differently in intestinal tissues and a lymph node remote from the gut.

Ontogeny of ovine lymphocytes. III. An immunohistological study on the development of T lymphocytes in sheep fetal lymph nodes

The development of T lymphocytes in ovine fetal lymph nodes was studied immunohistologically using a panel of monoclonal antibodies. T lymphocyte subsets appeared within the ovine fetal lymph node in a specific sequence. SBU-T1- and SBU-T8-positive lymphocytes were seen at Day 47 of gestation. LCA, sIg, MHC I, MHC II, SBU-T6 and 46.66 antigens were also seen within the lymph node at this time. The first SBU-T4-positive cells were seen within the fetal lymph node at Day 50 of gestation, along with the first 20.96-, 25.69- and 38.38-positive cells. SBU-T19 lymphocytes appeared later in gestation, being present in fetal lymph nodes from Day 69 of gestation. The appearance and distribution of T and B cells in the developing lymph nodes of the ovine fetus is described and compared with the ontogeny of lymphocytes in the fetal thymus and spleen.

Lymphocyte migration and differentiation in a large-animal model: the sheep

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.

Lymphocyte migration into collagen gels: role of lymph

In the present investigation we found that the presence of lymph within a 3-D collagen gel potentiates the invasion of mature recirculating lymphocytes into the gel. Preferential accumulation of lymphocytes in lymph-containing gels follows the same rules that apply for both in vitro lymphocyte adhesion to high endothelial venules of frozen sections of the lymph nodes and in vivo lymph node entry of lymphocytes. Furthermore, the results obtained suggest that lymph is chemotactic for lymphocytes. This chemoattractant activity of lymph could be assigned mainly to a protein fraction precipitating in the range of 40-60% concentration of ammonium sulphate. The biological significance of these findings for the selective process of lymphocyte emigration from blood to lymph, across the parenchyma of the nodes, is discussed.

Studies on the differentiation of T lymphocytes in sheep. I. Recognition of a sheep T-lymphocyte differentiation antigen by a monoclonal antibody T-80

The results presented in this paper demonstrate that a mouse IgM monoclonal antibody (T-80) recognizes an antigen on cells of the T-lymphocyte lineage of sheep. However, this antibody does not identify all T cells, as 10-20% of thymocytes and some peripheral-blood T cells are negative. T-80- thymocytes reside in the medulla. The majority of cortical thymocytes are T-80+ and classified as dull cells on the basis of antigen density per cell as measured by flow microfluorometry. In contrast, T-80+ cells in the periphery can be categorized into two populations, i.e., dull cells and bright cells. Suggestive evidence was obtained that bright T-80+ cells are fast recirculating T cells, whereas dull cells are sessile or less easily mobilizable T cells in the periphery. In foetal environment, over 90% of thymocytes and approximately 5% of spleen cells are T-80+ at 54 days of gestation (gestation period = 150 days), which may indicate that T-cell emigration from the thymus commences well before mid-gestation in sheep.

The response of the pregnant ewe to challenge with foetal and paternal lymphocytes

The efferent lymph from the popliteal lymph nodes of pregnant ewes challenged between 93 and 127 days with specific paternal or foetal lymphocytes was examined. There was an increased efflux of lymphocytes and blast cells from the challenged node, similar to that observed during the response of normal, non-pregnant ewes to allogeneic cells. Additionally, there was a decrease in the mixed lymphocyte and mitogen responsiveness of the efferent lymphatic cells that was comparable with that evoked by challenge of non-pregnant sheep. While cytotoxic cells could not be detected in the lymph after challenge of normal or pregnant animals, specific cytotoxic antibody was invariably produced in both instances. The present observations that the immune responsiveness of the pregnant ewe to foetal lymphocytes remains normal are consistent with an earlier report on the antifoetal reactivity of maternal cells in vitro. It is inferred that the in vitro reactivity of maternal lymphocytes was a valid reflection of the capacity of these cells in the intact ewe.

The response of the foetal lamb to maternal lymphocytes

Foetal lambs were inoculated with either maternal or third-party lymphocytes. Of foetuses transfused in the first half of pregnancy (from 49 to 73 days), one quarter survived until the fifth month. Examination of the immunological reactivity of these survivors revealed that all rejected skin grafts from the lymphocyte donors and manifested normal mixed lymphocyte reactivity. In two instances, responsiveness of the transfused lambs to normal lymphocyte transfer was reduced. Foetal lambs transfused with large numbers of maternal lymphocytes in the last third of pregnancy could survive provided the donor ewe had not been sensitized against foetal or paternal determinants. Following intravenous challenge with maternal lymphocytes, cells collected over a prolonged period from the thoracic duct of the foetal recipient exhibited depression of anti-maternal reactivity in mixed lymphocyte culture.

Immunological responsiveness of maternal and foetal lymphocytes during normal pregnancy in the ewe

Peripheral blood lymphocytes collected from ewes before and during pregnancy manifested constant reactivity to concanavalin A and to paternal and third party peripheral blood lymphocytes. However, in some instances, the reactivity of these maternal cells against lipopolysaccharide and lymphatic lymphocytes from paternal, foetal and third-party donors increased markedly during pregnancy. Apart from an indication that plasma from some pregnant ewes acquired the capacity to depress lymphocyte reactivity non-specifically, no evidence was obtained to suggest that maternal lymphocyte reactivity observed in vitro did not accurately reflect the capacity of these cells in the donor ewe. In particular, there was no indication that populations of maternal peripheral blood lymphocytes returning from the gravid uterus had undergone any modification of reactivity against foetal determinants.

The migration of lymphocytes in the fetal lamb

The migration of 51Cr-labeled autologous lymphocytes from intestinal or prescapular lymph was compared in fetal lambs and adult sheep. A subpopulation of lymphocytes present in intestinal lymph of adults which migrated to the small intestine was not found in fetal intestinal lymph. There were marked differences in the migration of fetal and adult lymphocytes to the lungs and liver. In spite of the absence of circulating antibodies or immunoglobulins and of extrinsic antigen in the immunologically virgin sheep fetus, the circulation of lymphocytes through the spleen and lymph nodes of fetal lambs was more intense than in the adult.

Two distinct pools of recirculating T lymphocytes: migratory characteristics of nodal and intestinal T lymphocytes

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.

Studies on the lymphocytes of sheep. III. Destination of lymph-borne immunoblasts in relation to their tissue of origin

Lymph-borne immunoblasts were labeled in vitro with 125I[]iodo-deoxy-uridine, washed and returned by intravenous injection to the sheep from which they had been collected. Twenty h later the sheep were killed and the distribution of the immunoblasts was determined by assaying the radioactivity in various organs. Immunoblasts from the efferent lymph of peripheral somatic lymph nodes (PSLN) went mainly to the spleen, lungs and other PSLN, while immunoblasts from intestinal lymph went mainly to the small gut. This ability of intestinal immunoblasts to home to the gut was demonstrated also in the sterile environment of fetuses in utero; apparently the migratory behavior of immunoblasts, like that of small lymphocytes, is not primarily "antigen-driven". A technique was devised for the collection of peripheral (i.e. afferent to the mesenteric node) intestinal lymph which was found to contain 10-20 times the numbers of small lymphocytes that occur in the peripheral lymph from other tissues. Immunoblasts from peripheral intestinal lymph also homed to the gut. The immunoglobulin content of immunoblasts was studied by making detergent extracts of lymph cells, by applying immuno-peroxidase techniques to cell films and by investigating the incorporation of 14C-labeled amino acids into immunoglobulins by immunoblasts in vitro. Immunoblasts from both somatic and intestinal lymph contained and made IgG and IgM, but many intestinal immunoblasts contained and made IgA. It is not known whether this immunoglobulin mediates the extravasation of immunoblasts into the gut. Nonetheless, there is compelling evidence that there are two major migratory pathways for lymphoid cells; one through the gut-associated lymphoid tissue and the other through the somatic-splenic lymphoid tissues.

Development of sheep thymus in relation to in utero thymectomy experiments

The developing thymus of sheep embryos has been examined from 20 days of gestation onwards. The initial relationships of the thymus to the parathyroid III and the pericardium are maintained so that the thymus develops as an extensive and complex structure consisting of paired cords of tissues on either side of the neck continuous with a single, fused structure in the anterior mediastinum. Histological examination of the developing thymus revealed that thymic lymphopoiesis commences at 36 days of gestation (term = 150 days). The results indicate that conclusions drawn from sheep thymectomy, usually performed after 60 days of gestation, must be interpreted with caution.