Ontogeny of ovine lymphocytes. I. An immunohistological study on the development of T lymphocytes in the sheep embryo and fetal thymus

The time of appearance of lymphocytes expressing T-cell markers and the subsequent development of the fetal thymus were studied in ovine embryos using a panel of monoclonal antibodies. Leucocyte common antigen (LCA) and major histocompatibility complex class I (MHCI) antigens were seen on a small number of cells within the ovine embryo at Day 19 of gestation. SBU-T6 (CD1)-positive cells were found at Day 22 of gestation, while major histocompatibility complex class II (MHC II) antigens were first observed at Day 25 of gestation. Large basophilic cells, weakly staining for SBU-T1 (CD5), were present in the mesenchyme of the neck and in the dorsal mediastinum and mesentery of embryonic sheep of 33 days gestational age (g.a.); however, no SBU-T1-positive cells were detected in the thymus at this time. No SBU-T4 (CD4)- or SBU-T8 (CD8)-positive cells were detected in any organs of embryos of this age. SBU-T4- and SBU-T8-positive cells were first seen in fetal thymi, and elsewhere within the fetus, at 35-38 days g.a. SBU-T19-positive cells were first seen within the fetal thymus at 50-58 days g.a.     

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.     

The ontogenetic development of macrophage subpopulations and Ia-positive non-lymphoid cells in gut-associated lymphoid tissue of the rat

The ontogenetic development of macrophage subpopulations and Ia-positive non-lymphoid cells was studied in gut-associated tissue in fetal and neonatal Wistar rats. A two-step immunoperoxidase method was carried out on cryostat sections, a panel of monoclonal antibodies being applied and aimed specifically at rat macrophages (ED1, ED2 and ED3) and at Ia antigen (Ox4). The first ED1-positive macrophages appeared in the liver on Day 15 (gestational age), and they did not express Ia. In developing mesenteric lymph nodes and in the gut wall, macrophages were found for the first time on Day 17 and 18, respectively, of gestation. These early macrophages were also ED1-positive. Until birth, ED2 recognized few cells in the gut-associated tissue; on the day of birth this subpopulation showed a sudden and considerable increase. The distribution pattern of ED3-positive macrophages appeared to be the same in fetal and in adult rats; it was confined to lymph nodes and Peyer's patches. Ia-positive non-lymphoid cells appeared in the abdomen early in ontogeny. The first Ia-positive cells displayed dendritic features and were found on Day 15 of fetal life in the mesenchymal tissue between intestinal loops. A few days later, many Ia-positive cells with a dendritic appearance were demonstrable in the gut wall and in developing mesenteric lymph nodes. Their number increased rapidly during the following days. Based on these results, the existence of two differentiation lines for dendritic cells and classical macrophages is discussed. The function of early Ia-positive cells in the abdomen is suggested as not being an antigen-presenting one.     

The development of gut associated lymphoid tissue in the terminal ileum of fetal human intestine.

Lymphoid tissue in formalin fixed and snap frozen human fetal ileum has been studied using immunohistochemistry. At 11 weeks gestation clusters of cells expressing CD4 (leu-3a positive) are present in fetal ileum but these do not express CD3 (UCHT1 negative) and are probably macrophages. Aggregates of lymphoid tissue are apparent from 14 weeks gestation which contain T cells of helper/inducer and suppressor/cytotoxic phenotype. Both B and T cells are present at 16 weeks but with no cellular zonation. By 19 weeks, distinct follicles of B cells are present surrounded by T cells of helper/inducer and suppressor/cytotoxic phenotype. Follicular dendritic cells are also present within the B cell areas. The B cells at this age express surface IgM and IgD, C3b- and C3d-receptors. They also express the antigen CD5 which has been shown by others to be present on some fetal B cells but which is almost exclusively associated with T cells in the adult. HLA-D region antigens are present on apparently all of the cells within the fetal lymphoid follicles. The antigen on activated B cells, CD23 (recognized by MHM6), was present on some cells scattered within the B cell follicle. This is indicative of antigen independent B cell proliferation.     

The Use of Stereology Method to Estimate the Volume of Feto‐Maternal Exchange Area of the Bovine Placentome during Gestation

The functional surface areas of the feto‐maternal unit of the bovine placentomes were quantified from Day 100 to 260 of gestation by using stereology. This study was achieved using intact placentomes obtained from an abattoir. There was no change in volume and surface densities of binucleate cells, fetal trophoblast, fetal and maternal tissues, and maternal epithelium with gestation age, although the total volume of these components increased with gestation age from Day 126 to 260. The total surface area of the feto‐maternal interface increased in a similar pattern as the placentomal components without a change in the fetal to maternal tissue ratio when estimated with stereology. This is one of the recent studies in cattle to quantify and describe the functional surface area of intact placentomes at different stages of gestation and it emphasised the differences between the yak and cattle placentomal development. Anat Rec, 299:1571–1577, 2016. © 2016 Wiley Periodicals, Inc.     

Analysis of hemopoietic cells in rat bone marrow and fetal liver with monoclonal antibody

The expression of cell surface antigens recognized by UB-12 monoclonal antibody against rat hemopoietic cells was examined in fetal liver, and adult bone marrow using immunohistochemistry, radioimmunoassay, and flow cytofluorometry. Comparisons were made with the expression of the antigens recognized by OX-7 (anti-Thy-1) or W3/13 (anti-leukocyte sialoglycoprotein) antibodies. UB-12 positive cells appeared at earlier stages of gestation and in higher cell frequency than OX-7 or W3/13 positive cells in fetal liver. The antigen detected with UB-12 antibody was positive in fetal liver at day 12 and increased at day 15, reaching about 90% of the positive cells. In bone marrows, UB-12 positive cells started to appear around birth and were about 30-40% of total nucleated cells during adult periods. Thus, the antigen recognized by UB-12 was found only during early stages of blood cells. UB-12 monoclonal antibody recognized a B-cell lymphoma, Y-3, but did not react with a rat T lymphoma cell line. This monoclonal antibody may be important for distinguishing normal and neoplastic B cells and their precursors.     

A Novel Form of CD4 (L3T4) mRNA in the Murine Fetal Liver Results in Cell-Surface Expression of the L3T4 Antigen

The expression of the L3T4 antigen during ontogeny in the murine fetal liver has been investigated in parallel by northern blot analysis and cytofluorometry. The L3T4 gene is transcribed in the murine fetal liver in two polyadenylated mRNA species with the sin: of 3.5 kb and 3.7 kb. Whereas the 3.5-kb mRNA is expressed from days 13 lo 18 of gestation, expression of the 3.7-kb mRNA is found only from days 16 to 18 of gestation and thus appears to be developmentally regulated. Immunofluorescent staining of fractionated fetal liver cells from days 12 to 18 of gestation with the anti-L3T4 antibody (GK i.5) provides evidence that cell-surface expression of the L3T4 antigen on a subset of lymphohaematopoietic cells in the murine fetal liver is the product of a novel form of L3T4 mRNA with the size of 3.5 kb     

Temporal distribution of retinoic acid and cellular retinoic acid-binding protein (CRABP) in the fetal hamster.

The temporal relationship between the distribution of retinoic acid, a known human and rodent teratogen, and that of cellular retinoic acid-binding protein (CRABP) was investigated from Day 11 to Day 14 of hamster prenatal development. The 11,12-(3)H2 and 15(-14C) forms of all-trans-retinoic acid were used for quantitative distribution studies and autoradiography, respectively, and were evaluated 15 min after a single intravenous injection. Radioactivity was detected in all fetal tissues examined (brain, liver, heart, spinal cord, limb, and skin), and at Day 14, approximately 66% of the total radioactivity was present as parent all-trans-retinoic acid. High concentrations of total radioactivity were observed by autoradiography in the midbrain and hindbrain (mesencephalon, metencephalon, and myelencephalon) and spinal cord, but not in the forebrain. At the earliest time studied, limb buds showed relatively high concentrations of radioactivity. Levels of radioactivity were also high in portions of the developing face, nose, and tongue. Immunohistochemical analyses indicated that the amount of CRABP in Day 14 tissues was the highest in spinal cord followed by limb and skin; heart and liver contained only relatively small amounts of this protein. From Day 11 to Day 14, the amount of CRABP, as measured by high-performance size-exclusion liquid chromatography, in the whole body decreased as gestation progressed. Microscopic immunohistochemical localization of CRABP found the highest concentration in the ventral midbrain and in the ventral and lateral sides of the hindbrain and spinal cord; CRABP was also abundant in tongue, limb, and skin. The distribution of CRABP-positive cells in the central nervous system was similar to the distribution of retinoic acid. The data presented here indicate that fetal CRABP appears to play a role in differential accumulation of retinoic acid in certain structures of the developing hamster. The patterns of tissue retinoid and CRABP distribution observed here are consistent with the patterns of congenital malformations induced by prenatal retinoid exposure.     

Differentiation of murine B cell precursors in agar culture. Frequency, surface marker analysis and requirements for growth of clonable pre-B cells

A semi-solid agar assay is described in which B cell progenitors, already present in day 12 fetal liver, generate colonies which contain antibody-secreting cells. Panning experiments, in which cells which initiate colony formation are depleted on plates coated with monoclonal antibodies, suggest that by the 13th day of gestation they express the antigen recognized by the monoclonal antibody AA4.1 and by day 14 they also express the B220 form of Ly-5 recognized by the monoclonal antibody 14.8. By similar criteria the precursor cells do not express mu, I-A, I-E or Lyb-2. Growth of cells in this assay is dependent upon soluble products provided by either fetal liver adherent cells, bone marrow adherent cells or colony-stimulating factor-containing conditioned media derived from placenta cells, L929 cells, WEHI-3 B(D-) cells, T helper cells or mouse lung cells. These experiments define two sets of growth conditions. In the first, when support is provided by fetal liver adherent cells, the limiting component appears to be the B cell precursor, allowing us to estimate the frequency of these cells during ontogeny. We find approximately 1 clonable pre-B cell in 300 000 fetal liver cells on day 12 of gestation and 1 in 6000 by day 16. Under the second set of growth conditions, when support is provided by bone marrow adherent cells or colony-stimulating factor-containing conditioned media, more than one cell, colony or cell product is limiting. Highly purified samples of granulocyte/macrophage colony-stimulating factor, colony-stimulating factor 1 and multilineage-hematopoietic growth factor are effective in this assay suggesting that the colony-stimulating factors are the active components under these conditions.     

Heterogeneity of non-lymphoid cells expressing HLA-D region antigens in human fetal gut.

Human fetal ileum contains an abundance of cells expressing HLA-D region (HLA-DR) antigens. In this study we characterized the HLA-DR positive cellular infiltrate in fetal ileum using a panel of monoclonal antibodies against cells of the macrophage/monocyte lineage. As well as anti-HLA-DR, the whole infiltrate was recognized by three of the antibodies in the panel studied: RFD1, reported to be an antibody to 'dendritic cells', leu3a which recognizes CD4 (an antigen expressed on helper/inducer T cells and macrophages), and PD7/26 which recognizes the leukocyte common antigen. The macrophage specific antibodies RFD7 and 3.9 stained fewer cells than the anti-HLA-DR. By sequential staining it was clear that most of the RFD7 positive macrophages in the fetal gut lamina propria were not recognized by 3.9 which is a broad specificity antimacrophage antibody in adult tissue. In contrast to this, more of the macrophages within the Peyer's patches of the fetal gut were recognized by 3.9 than by RFD7. In fetal lamina propria not all of the HLA-D region positive cells expressed macrophage markers and some expressed both markers associated with macrophages and 'dendritic' cells. Macrophages with different surface phenotypes were differentially distributed between the lamina propria and the primitive Peyer's patches; RFD7+, 3.9- macrophages were concentrated in the lamina propria whereas RFD7-, 3.9+ macrophages were abundant in Peyer's patches. Within the Peyer's patch lymphoid tissue RFD7+, 3.9- cells were present in the T cell zone whereas RFD7-, 3.9+ cells were concentrated in the dome region as they are in the adult. This suggests that functional heterogeneity of organized macrophage populations may occur in fetal ileum which is free of dietary and bacterial antigens.     

FT-2 antigen for distinguishing lymphocyte subpopulations in the developing thymus

The surface phenotypes of lymphoid cells in the developing embryonic thymus were characterized by using monoclonal antibodies. FT-2 antigen thus defined was predominantly expressed on thymocytes in the earlier embryonic stages in all the inbred mouse strains tested. The immunofluorescence and immunoperoxidase tests indicated that, like FT-1 antigen, the proportion of FT-2+ fetal thymocytes rapidly decreased with increase in gestation time, and these cells disappeared by day 19 of gestation. The treatment of fetal thymocytes with anti-FT-1 plus complement eliminated not only FT-1+, but also FT-2+ cells, whereas the treatment with anti-FT-2 failed to eliminate approximately 40% of FT-1+ cells, suggesting that embryonic thymocytes can be provisionally divided into at least three subpopulations, FT-1+2+, FT-1+2- and FT-1-2-.     

A unique human B lymphocyte antigen defined by a monoclonal antibody

We produced a hybridoma designated 4G7 from a mouse immunized with chronic lymphocytic leukemia cells. The 4G7 hybridoma secretes an IgG1 antibody that is specific for normal and malignant B lymphocytes. Using dual color immunofluorescence staining, this antibody reacted with all immunoglobulin-positive cells but no T cells in normal peripheral blood. There was no detectable 4G7 antigen on monocytes, platelets, red cells, granulocytes, or phytohemagglutinin-activated T cells. When PBL were depleted of 4G7 positive cells and stimulated with pokeweed mitogen, secreted immunoglobulin levels fell to less than 10% of control values on Day 5 and less than 1% of control on Day 7. This antibody was reactive with 155 of 176 B lineage neoplasms on which it was screened. Thirty-five cases of myeloid or T-lymphoid malignancy were negative. Our studies show that the 4G7 antigen modulates in the presence of excess antibody. Free 4G7 antigen was not found circulating in human serum. The cell surface antigen identified by 4G7 was sensitive to pronase proteolysis but resistant to trypsin and chymotrypsin digestion. A comparison of 4G7 with other known B-cell antibodies indicates that the 4G7 antigen has not been previously identified. This antibody is of use for the identification of normal B lymphocytes, the study of B-cell differentiation, and the characterization of lymphoid malignancies.     

Costimulatory molecules in the developing human gastrointestinal tract: a pathway for fetal allergen priming

BACKGROUND: Antigen-specific responses can be detected in umbilical cord blood mononuclear cells. The fetal immune system must therefore attain a level of maturity compatible with the initiation of such responses as well as be exposed to antigen. OBJECTIVE: We sought to assess the expression of costimulatory molecules in fetal gut and the presence of cytokines in amniotic fluid at this time as a preliminary analysis of the suitability of the fetal gut as a site of antigen priming during intrauterine life. METHODS: Human fetal gut was analyzed for cells expressing costimulatory molecules through use of immunohistochemistry. Amniotic fluid was studied by ELISA, for cytokines regulating the nature of the response, and as a source of the common dietary antigen ovalbumin. RESULTS: MHC class II--positive cells were abundant over the period examined (11-24 weeks of gestation), other surface antigens showing spatial and temporal variation in expression. From 11 to 14 weeks of gestation, CD68-positive and CD40-positive cells, like MHC class II--positive cells, were present throughout the lamina propria; few CD3-positive cells (T cells) were observed. With the emergence of lymphoid aggregates (14-16 weeks), CD83-positive cells (dendritic cells) and CD20-positive cells (B cells) could be detected in fetal gut; however, expression was restricted to the lymphoid aggregates. In contrast, MHC class II, CD40, and CD68 continued to be expressed in the lamina propria. CD28-positive cells were also evident from 14 weeks of gestation, occurring throughout the lamina propria and lymphoid aggregates; this corresponded to the increasing numbers of CD3-positive cells. The occasional CD86-positive, CD40L-positive, or CTLA4-positive cell could be seen in or around lymphoid aggregates after 14 weeks of gestation. Lymphoid follicles forming after 16 weeks of gestation contained MHC class II--positive, CD83-positive, CD20-positive, CD40-positive, CD86-positive, CD3-positive, CD28-positive, CD40L-positive, and CTLA4-positive cells. MHC class II--positive, CD40-positive, CD68-positive, CD3-positive, and CD28-positive cells continued to be present in the lamina propria at this time. At all times studied, CD14 was not expressed in the lamina propria or lymphoid follicles. Prostaglandin E(2), TGF beta(1), and IL-10 dominated the amniotic fluid cytokine milieu, and ovalbumin was also detectable in amniotic fluid from 3 of 26 women who had detectable circulating levels. CONCLUSION: Of the costimulatory molecules studied, CD40 was the most abundant. However, both of the ligand families studied (CD40-CD40L and CD86-CD28/CD152) could provide the costimulatory signals required for the initiation of antigenspecific reactivity in the gastrointestinal tract of the human fetus as early as 16 weeks of gestation. The cytokine milieu would favor the development of T(H)2-type reactivity to antigens, such as ovalbumin, that are present at this time.     

Quantitation and localization of ENaC subunit expression in fetal, newborn, and adult mouse lung

The newborn lung is cleared of fetal liquid by active Na+ transport. The heterotrimeric (alpha, beta, gamma) epithelial Na+ channel, ENaC, mediates this process. To understand the role of individual ENaC subunits in Na+ transport during development, we quantified murine ENaC (mENaC) subunit messenger RNA (mRNA) expression levels of fetal, neonatal, and adult mouse lung by Northern blot analysis and studied regional expression by in situ hybridization. alphamENaC and gammamENaC mRNA expression increased sharply in late fetal gestation and reached near-adult levels by Day 1 of postnatal life. betamENaC expression increased more gradually through late fetal and early postnatal life and increased progressively until adulthood. In situ hybridization studies showed similar localization patterns of alphamENaC and gammamENaC subunit expression in fetal and postnatal lung. gammamENaC and alphamENaC subunits were initially localized to fetal lung bud tubules and by late gestation both subunits were expressed in all regions (acinar and bronchiolar) of the distal lung epithelium. betamENaC was detected from 16 d gestation onward and was expressed most intensely in small airways. There was little expression of betamENaC in the alveolar region. In postnatal lung all three subunits were expressed intensely in small airways. In adult lung, alphamENaC and gammamENaC were expressed in a pattern consistent with an alveolar type II (ATII) cell distribution. The timing of quantitative changes in mENaC subunit expression is consistent with a role of Na+ transport in liquid clearance of the perinatal lung. Intense expression of mENaC subunits in medium and small airway epithelium and in ATII cells suggests that these regions are a primary location for liquid absorption in the perinatal and postnatal murine lung.     

Indole-3-acetic acid induces microencephaly in mouse fetuses

To determine the effect of indole-3-acetic acid (IAA), known as natural auxin, on developing fetus, pregnant mice were injected with 500 or 1000 mg/kg on various gestation days (Days). With the repeated treatment during Days 7-15, the fetal brains exhibited a reduction in size and weight in a dose-dependent manner on Day 18. Histopathologically, hypoplasia of the cortical plate, piriform cortex, hippocampus and thalamus were observed. From the single treatment on 1 day during Days 9-14, the sensitive period of IAA-induced microencephaly was found to be during Days 10-13 and the most significant response in the fetuses was seen on Day 11 or 12. With the repeated treatment during Days 11-13, apoptotic cells mainly increased in the medial and dorsal layer of the neuroepithelium and prepalate with a reduction in cell density in the telencephalon, diencephalon, mesencephalon and metencephalon on Day 12.5. p53-positve cells were detected associated with apoptotic cells in neuroepithelium. Therefore, IAA administration to pregnant mice induces apoptosis mediated by p53 in the embryo's neuroepithelium, decreases formation of neurons and leads to microencephaly in the fetuses.     

Discontinuous expression of a membrane antigen (HB-7) during B lymphocyte differentiation

We have examined the expression of a cell surface antigen by B lineage cells in human fetuses, newborns and adults using a newly produced monoclonal antibody, HB-7. The HB-7 antigen was found to be a protease sensitive 45,000 MW molecule that appeared to be the same molecule recognized by the OKT-10 antibody. The HB-7 reactive molecule was expressed by all fetal pre-B and B cells, and 50% of newborn blood and adult bone marrow B cells. In contrast, only a small minority of B cells (2–12%) from blood, spleen and tonsil of adults were weakly HB-7⁺. The pokeweed mitogen-responsive B cell precursors of plasma cells were also HB-7⁻, but the HB-7 antigen was re-expressed during the plasma cell stage. We conclude that this antigen is unique among known B cell differentiation antigens in its intermittent pattern of expression during B cell development. The reactivity of the HB-7 antibody with immature, but not mature, B cells makes it well suited for studies of B cell ontogeny.     

Development, differentiation, and proliferation of epidermal Langerhans cells in rat ontogeny studied by a novel monoclonal antibody against epidermal Langerhans cells, RED-1.

An antirat monoclonal antibody (mAb) against nonlymphoid dendritic cells, RED-1, was produced using epidermal Langerhans cells (LCs) as the immunogen. This mAb reacted mainly with the LCs and indeterminate dendritic cells (ICs), interdigitating cells in the T cell areas of lymphoid tissues, and monocyte/macrophages in various organs and tissues of adult rats. In the epidermal sheets prepared from adult rats, it specifically recognized the cell surface antigen(s) present on LCs and ICs. In the fetal rat skin, primitive or fetal macrophages migrated into the epidermis and expressed RED-1 at fetal day 17. With advance of gestation, RED-1-positive cells increased, started expressing Ia antigens at fetal day 18, and subsequently differentiated into dendritic cells. Most of them showed Ia expression by fetal day 20 and differentiated into LCs within a few days after birth. The labeling index of 5-bromo-2'-deoxyuridine in RED-1-positive cells was 18% at fetal day 17 and decreased to 5 to 6% in the postnatal period. These results imply that proliferative capacity of RED-1-positive cells is important for the formation and expansion of the IC population in the fetal stage and for the survival of LCs in the postnatal period.     

Fetal antigen 1 (FA1) in the human pancreas: cell type expression, topological and quantitative variations during development

Monospecific rabbit anti-human fetal antigen 1 (FA1), was used to examine the distribution of FA1 during the development of the human fetal pancreas and liver using an indirect immunoperoxidase technique. FA1 was expressed by 94% of the glandular epithelial cells of the branching ducts in the pancreatic anlage at week 7 of gestation. This pattern changed during the development of the human pancreas, 64% of the glandular cells being FA1 positive at week 17 of gestation, decreasing to 11% in the infant (4 months after birth). In the infant and adults the FA1 expression was restricted to a subpopulation of -cells within the islets of Langerhans. Insulin immunoreactive cells were scattered throughout the epithelium of primitive branching pancreatic ducts at week 7 of gestation, well before the formation of islets. From the 7th through to the 17th week of gestation, FA1 was found in the cytoplasm of fetal hepatocytes, whereas no staining was observed in the liver from a 4-month-old infant. No FA1 expression was found in the epithelium of the developing gut. The present findings indicate that the glandular epithelial cells in the developing pancreas may serve as stem cells, which, if appropriately induced, may differentiate into endocrine cells. Fetal antigen 1 (FA1) may take part in or be a result of this differentiation.     

Characterization of a 95,000 molecule on sheep leucocytes homologous to murine Pgp-1 and human CD44.

The phagocyte glycoprotein-1 (Pgp-1) antigen of mice is a 94,000 MW molecule with a wide tissue distribution, but no attributed function. We produced a monoclonal antibody (mAb) termed 25-32 which recognizes the Pgp-1 molecule of numerous mammalian species, including humans and sheep. Preclearing experiments with I42/5, a rat anti-mouse Pgp-1 mAb that cross-reacts with human Pgp-1, established the specificity of 25-32 for human and sheep Pgp-1. Moreover, an antibody recognizing human CD44, termed F10-44-2, also reacted with the same molecule as that recognized by 25-32 and I42/5, so establishing the co-identity of CD44 and Pgp-1. Within the sheep thymus, Pgp-1 was expressed most strongly by medullary thymocytes and stromal cells, and by small numbers of cells at the subcapsular cortex. Pgp-1 was expressed early in thymic ontogeny; all 35-40-day gestation fetal sheep thymocytes were intensely Pgp-1+, but by 80 days the number of reactive thymocytes had decreased to adult levels. The expression of Pgp-1 on lymphocytes was markedly increased after stimulation with mitogens, or with phorbol esters and ionomycin. The highly conserved nature of Pgp-1 through evolution, its expression on virtually all cell types within the body, and its increased expression on rapidly dividing cells indicate that this molecule mediates an important function, possibly serving as a hormone or metabolite receptor.