The life-span and recirculation of marrow-derived small lymphocytes from the rat thoracic duct

Identification of a glycophorin-like molecule at the cell surface of rat thymocytes

There are three predominant glycoproteins in the rat thymocyte plasma membrane. Two of these have carbohydrate compositions that are characteristic of structures N-glycosidically linked to protein. The other glycoprotein is very different, having about 20 O-glycosidically linked carbohydrate units per 100 amino acids. It has similarities to the major sialoglycoprotein of human erythrocytes.

Ultrastructural localization of acid phosphatase in rosetted T and B lymphocytes of normal human blood

Using electron microscopy and cytochemical techniques, the authors determined the distribution of acid phosphatase (AcPase) within the organelles of lymphocytes from blood rosetted with either neuraminidase-treated sheep erythrocytes (En) or sheep erythrocytes coated with antibody and complement (EACs). Subsequently, the various reactive organelles of the rosetted lymphocytes were counted, affording a comparison of T and B cells. It was found that AcPase was present in approximately 80% of T cells and 45% of B cells and was most frequently observed in secondary lysosomes of varying size and content. Although more T cells than B cells were reactive for AcPase, the extent of reaction in some B cells clearly precludes the use of AcPase for differentiating the two cell lines. It should be recognized that while the En rosetting procedure detects T cells in a nonselective manner, the EAC rosette is a marker of a major subpopulation of B lymphocytes, ie, those bearing complement receptors. We believe that the distribution of lysosomal enzymes in B and T lymphocytes probably reflects the functional state of individual cells rather than being a reliable indicator of cell lineage. A surprising finding (which could be established only by a fine-structural study) was the fact that 20% of circulating "resting" T cells contained reaction product for AcPase within endoplasmic reticulum and the perinuclear cisterna indicating that these cells are actively synthesizing AcPase, probably due to a foregoing inductive event. Such stimulus could be the result of recent endocytosis of surface receptors in combination with antigen, antibody, or immune complexes and/or recent mitosis, or possibly some unrelated autophagic incident.

Mucosal immunology

In this review, we shall highlight some recent advances in mucosal immunology and also those concepts which seem to us to merit more attention than they normally receive. Since we cannot hope to be all inclusive, we recommend the following articles and books to the reader (Tomasi & Bienenstock, 1968; Tomasi & Grey, 1972; Bienenstock, 1974; Heremans, 1974; Mestecky & Lawton, 1974; Lamm, 1976; Tomasi, 1976; Waksman & Ozer, 1976; Porter & Knight, 1977; McGhee, Mestecky & Babb, 1978; Ogra & Dayton, 1979; Befus & Bienenstock, 1980).

H-2 restriction for lymphocyte homing into lymph nodes

Migratory patterns into lymph nodes of labeled spleen lymphocytes, injected intravenously, showed considerable differences according to the genetic identity or disparity of donor and recipient mice. The use of mice carrying different H-2 haplotypes in the same B 10 background, or, conversely, the same H-2b haplotype in different backgrounds, showed that no homing was observed in the case of complete disparity at the H-2 complex, contrasting with optimal homing in the case of H-2 identity. Homing was not influenced by disparity at the genetic background. Irradiation of recipient mice did not change the results and suggested that no immunological host-vs.-graft reaction was involved in the H-2 restriction observed. The use of donor-recipient pairs carrying recombinant H-2 haplotypes showed that a single H-2 K and/or H-2 D identity allowed almost optimal homing. The results indirectly suggested that an immunological graft-vs.-host reaction was not involved in the phenomenon but rather a positive recognition of H-2 K or H-2 D identity between donor and recipient cells.

Migration of human lymphocytes. II. Variation of lymphocyte distribution

Variations in the distribution of 51Cr-labelled human lymphocytes from several donors and from one donor on several occasions have been determined after their injection into carbon-treated mice and reasons for the variation discussed. Natural alteration of the lymphocyte surface (leukaemic lymphocytes) or induced change (trypsin-treatment) reduced lymph node localisation as did treatment with formalin, sodium azide and antilymphocyte globulin. We have concluded that the presence of radioactivity in the lymph nodes is due to the presence of viable, metabolically active lymphocytes with normal surfaces capable of interacting with the endothelium of the post-capillary venules.

The pattern of lymphocytes in the thymus and spleen after labeling with 3H-thymidine and 3H-deoxycytidine

Adult male untreated mice (NMRI) were investigated after radioactive labeling with 3H-thymidine and 3H-deoxycytidine to find out whether the lymphocytes in the cortex and medulla of the thymus as well as in the perifollicular and periarteriolar regions of the spleen show a labeling pattern which allows a classification into T- and B-lymphocytes. The percentages of radioactively labeled small lymphocytes and their mean grain counts were determined. The percentages of radioactively labeled small lymphocytes after 3H-TdR and 3H-CdR showed no significant differences in both splenic zones. The grain counts over the lymphocyte nuclei in the periarteriolar zone showed lower values after 3H-TdR than after 3H-CdR. The lymphocytes in the perifollicular zone were strongly labeled with 3H-TdR and weakly labeled with 3H-CdR. In the thymus medulla, lymphocytes were weakly labeled with 3H-thymidine and strongly labeled with 3H-CdR. In the cortex no significant differences were observed. 75 to 80% of the small lymphocytes in the peripheral blood were weakly and 20-25% strongly labeled after 3H-TdR. Therefore there are similarities in the radioactive labeling pattern of thymic medulla lymphocytes and that of small lymphocytes of the periarteriolar zone of the spleen by both DNA precursors. The small lymphocytes in the peripheral T-dependent tissue zones, for example in the spleen, as well as in the mixed lymphocyte population of the peripheral blood can be differentiated from the B-lymphocytes through the difference in the amount of incorporation of 3H-thymidine and 3H-deoxycytidine.

Non-specific acid esterase activity in rat lymphocytes

In a first night's collection of rat thoracic duct lymphocytes B- and T-cells were distinguished on cytocentrifuge smears by complementary markers: B-cells by rosette-formation with Staphylococcus aureus Strain Cowan 1 armed with rabbit antiserum to rat F(ab')2 and T-cells by high uptake of 3H-uridine demonstrated by autoradiography. On incubation of the cytocentrifuge smears for demonstration of alpha-naphthyl-acetate esterase (ANAE)-activity, most B-cells showed large, intense colour deposits often located in the uropod. T-cell ANAE-activity on the other hand was variable, possibly due to variations in experimental conditions. When present, the T-cell colour deposits were small, dot-like and of low intensity. ANAE-activity may then be used to differentiate between rat T- and B-cells, being a marker for the latter cell type. This is unexpected as others have reported that ANAE-activity is specific for T-cells in mouse and man. Studies with inhibitors indicate that the rat B-cell enzyme belongs to the acetylesterases.

Immunological studies of T-cell receptors. I. Specifically induced resistance to graft-versus-host disease in rats mediated by host T-cell immunity to alloreactive parental T cells

The present studies extend our previous efforts to understand the immunological basis of specifically induced graft-versus-host (GVH) resistance in F1 hybrid rats. Immunization of F1 rats with alloreactive T-cell populations of parental strain origin induces a host-mediated T-cell response which is specific for anti-major hostocompatibility complex receptors on parental T cells. This protective immunity is rapid in onset and once induced, it provides a highly effective, specific resistance to lethal GVH disease which is radioresistant and can be adoptively transferred to syngeneic recipients.

The effects of complement activation by cobra venom factor on the migration of T and B lymphocytes into rat thoracic duct lymph

Experiments were done to see whether C3 or C3-split products are involved in lymphocyte recirculation, with particular reference to B lymphocytes which have C3b receptors. Rats were injected with cobra venom factor (CVF), and the output of subclasses of lymphocytes was measured in thoracic duct lymph in hourly collections during the subsequent 24 h. During the period of acute C3 activation which lasted for 2-8 h, the output of lymphocytes decreased by 47%, but returned to normal at later times, when C3 levels were reduced to less than 20% normal. There was no effect on the output of C3b receptor lymphocytes, and this receptor was not blocked probably because initial C3 levels in lymph were only 13% of blood levels, so that only small amounts of C3b were generated in lymph. When these lymphocytes were labelled and injected i.v. they migrated with the slow rate which is characteristic of normal B lymphocytes. The main effect of CVF was to reduce the output of T lymphocytes by 58% during the phase of acute C3 activation. When normal thoracic duct lymphocytes were labelled and injected, their rate of reappearance in thoracic duct lymph was only reduced during this phase. It was concluded that recirculation of lymphocytes is not C3 dependent, and that insufficient C3b is generated in lymphoid tissues to block C3b receptors on B lymphocytes during periods of rapid C3 activation. However the migratory rate of T lymphocytes through these tissues is reduced during this period, and it is suggested that this may be due to an effect of C3 split products on macrophages which lie along T-lymphocyte traffic routes.

Studies on lymph humoral factor. Biological characteristics of a lymphocytopoietic factor in rat thoracic duct lymph

The biological characteristics of a lymphocytopoietic factor obtained from rat thoracic duct lymph (Yamashita, Fukumoto & Miyamoto, 1976) were further investigated. The lymph extract from normal rats failed to stimulate both large pyroninophilic cell-proliferation and mitotic response in the spleen and lymph node of the thymectomized, irradiated and marrow reconstituted rat (B rat). This suggests that target or responsive cells for the factor are not marrow-derived (B) cells, but thymus-derived (T) cells. On the other hand, the lymph extract from the lymphopenic lymph-drained B rats showed similar high lymphopoietic activity to those of normal rats, indicating that the existence of a thymus is not essential for the production or secretion of the factor. The fact that lymphocytotic rats produced by syngeneic lymphocyte transfusion are most sensitive to the lymph extract suggests that liver endogenous level of this factor and the responsiveness of target cells are regulated by the number of circulating T lymphocytes.

The importance of serologically detectable histocompatibility antigens in the induction and effector step of cell-mediated lysis

The induction and effector steps of T cell-mediated cytotoxicity (CMC) have been studied using a mouse tumor cell line and its variant, which is deficient in serologically defined (SD) H-2 antigens. In allogeneic mice the SDxcell line induces CMC, while the SD- cell line does not. However, both cell lines can be lysed by xenogeneic rat lymphocytes. Antiserum specific for rat T cells was used to demonstrate that CMC of both targetss is partially due to T cells. In allogeneic or syngeneic mouse systems the SD- cells coupled with the 2,4,6-trinitrophenyl (TNP) residue can neither induce CMC nor serve as targets for CMC, while TNP-coupled SDxcells can serve both as immunogen and as targets. Thus allogeneic or syngeneic mouse T cells do not interact with the TNP group of targets lacking H-2 SD antigens. However, mouse T killer cells sensitized to TNP-coupled cells may lyse TNP-coupled targets carrying different H-2 haplotypes. These experiments show that the induction and effector steps of CMC executed by mouse T cells, using TNP-coupled cells as immunogen or targets, need not necessarily demonstrate restriction with regard to a certain genetically defined H-2 haplotype. The presence of cell surface H-2 SD antigens is however, absolutely necessary for the induction and effector steps of CMC by mouse T cells. Using cold target inhibition assays, it was not possible to demonstrate recognition of the TNP moiety on TNP-coupled SDxcells.

Recirculating, suppressor T cells in transplantation tolerance

An adoptive transfer system was used to examine the capacity of cellular inocula from rats fully tolerant of Ag-B antigens to transfer tolerance to irradiated recipients. Permanent tolerance in these irradiated recipients involved specific suppression of the regenerating immune response. Cells obtained from tissues rich in recirculating lymphocytes were the most effective suppressors. Highly purified inocula of T cells from tolerant donors were potent suppressors in irradiated hosts, but were not capable of direct suppression of peripheral antigen-sensitive T cells.. The role of the thymus in maintaining the complement of recirculating suppressor T cells in tolerant animals was examined after adult thymectomy. Thymectomized tolerant rats did not reject their tolerated grafts, and the longevity of the suppression in tolerant rats was confirmed by showing that adoptive transfer of cells from thymectomized tolerant donors was effective in suppressing irradiated recipients up to 180 days after thymectomy. Cellular inocula from these donors appeared to lose their suppressor function marginally faster than they lost effector function (as measured by their capacity to mediate rejection of third party control grafts). Thymectomy made tolerant rats more vulnerable to the termination of tolerance by challenge with normal cells. Transplantation tolerance is maintained in adult rats by long-lived rapidly recirculating suppressor T cells. The target for the suppressor action of these cells is probably the precursor of alloantigen-sensitive lymphocytes, and the effect of suppression may be deletion or inactivation of the relevant clone of these cells.

The induction of specific resistance in F1 hybrid rats to local graft-vs.-host reactions: nature of the eliciting cell

A specific state of resistance to local graft-vs.-host (GVH) reactions can be induced in F1 hybrid rats, derived from Ag-B incompatible matings, as a consequence of inoculation with low numbers of parental strain lymphocytes. The magnitude of GVH resistance is markedly and directly dependent upon the number of parental strain lymphocytes used in the pretreatment regimen. The relevant constituents of the resistance-inducing parental cell population are thymus-derived (T) lymphocytes possessing immunologic competence for host alloantigens. B cells, themselves incapable of inducing GVH resistance, adversely effect the induction or display of this effect by T cells.

Lymphocyte subpopulation in neoplastic and non-neoplastic thymus and in blood of patients with Myasthenia gravis

The lymphocyte subpopulations in the thymus and in the blood were investigated in ten myasthenic patients who had been thymectomized. Histologically, the thymuses tested comprised three cases of thymoma including two cases with malignant characteristics, five cases of hyperplastic thymus with lymph follicles and germinal centres, and two cases of persistent thymus without lymph follicles. Virtually all lymphoid cells in the three thymomas formed spontaneous rosettes with sheep red blood cells as did normal thymocytes from non-myasthenic patients. There was no significant proportion of immunoglobulin Ig-bearing lymphocytes. While the majority consisted of cells forming spontaneous rosettes with sheep red blood cells, there was a certain proportion (2-17%) of Ig-bearing lymphocytes in four of five hyperplastic thymuses, in one of two persistent thymuses, and in a residual atrophic thymus of a thymoma. The myasthenic patients tested were for the most part normal, as compared with healthy individuals, in the proportion of rosette-forming lymphocytes and Ig-bearing lymphocytes in the blood collected immediately before and one to three months after thymectomy. The presence of Ig-bearing lymphocytes in the thymus was not necessarily related to the appearance of circulating antibody to striated muscle. The antibody to striated muscle was demonstrated in all myasthenic patients with thymoma.

The mediator of cellular immunity. XII. Inhibition of activated T cells by Newcastle disease virus

Newcastle disease virus (NDV) can interact in at least two ways with rat T cells. By adsorbing to circulating lymphocytes, the virus can transiently deflect the cells from lymph nodes and inflammatory exudates induced in the peritoneal cavity. T cells are affected regardless of age, state of activation, or position in the mitotic cycle. The effect is reversible and is mediated not only by infectious (I)-NDV, but also by UV-NDV which cannot achieve a complete replication cycle in eggs. But I-NDV has another lasting effect on activated T cells. It is revealed in the failure of virus-treated thoracic duct lymphocytes to transfer cellular resistance to Listeria monocytogenes, delayed-type hypersensitivity to soluble antigens of the parasite, and the permanent exclusion of labeled S-phase lymphocytes from inflammatory foci. Activated T cells are inhibited by virus multiplicites which have little if any effect upon the proliferative potential of antigen-sensitive T cells or localization of labeled small lymphocytes in lymph nodes. The underlying mechanism has not been determined; however, there are reasons for thinking that NDV has a lethal effect upon activated T cells, because the latter are permissive for virus replication.

Lymphocyte homing into lymph nodes: in vitro demonstration of the selective affinity of recirculating lymphocytes for high-endothelial venules

An in vitro model is described for studying the interaction between lymphocytes and high-endothelial venules (HEV) of lymph nodes. Rat or mouse lymphocytes which were layered over fixed sections of syngeneic lymph nodes adhered selectively to the endothelium of HEV but did not bind to other vascular endothelia. Evidence is presented that adherence to HEV in vitro is a property of recirculating lymphocytes and not a characteristic of cells which are unable to home into lymph nodes in vivo.

Migration of newly formed small lymphocytes from bone marrow to lymph nodes during primary immune responses

Selective DNA labelling of bone marrow cells in vivo was used to determine the effect of antigenic stimulation on the migration of small lymphocytes from bone marrow to popliteal lymph nodes. Following footpad injection of keyhole limpet haemocyanin (KLH) in guinea-pigs the regional nodes showed an early increase in weight and cellularity together with a progressive increase in cell proliferation. When [³H]thymidine was injected into tibial and femoral marrow 2 days before KLH administration the DNA radioactivity of the KLH-stimulated nodes increased rapidly and always exceeded that of contralateral nodes. Simultaneously, in radioautographic sections of lymph nodes labelled small lymphocytes, indicative of an origin from marrow precursors, appeared throughout the cortex, post-capillary venules, subcapsular sinus, medullary cords and sinuses. In KLH-stimulated nodes the number of labelled small lymphocytes per section was higher than in contralateral nodes, especially in the cortex, and some of these cells appeared in germinal centres. Labelled large blast cells and macrophages were also increased in numbers. Similar changes were observed in lymph nodes of parental strain rats following intramyeloid [³H]thymidine administration and footpad injection of lymphoid cells from F1 hybrid rats. The results demonstrate that, during the early response of lymph nodes to various antigens, local changes in cell traffic include an enhanced accumulation of newly formed small lymphocytes, putative virgin B lymphocytes, generated in the bone marrow prior to the antigenic stimulation.

Quantitative studies of lymphocytes and other cell populations in the bone marrow of neonatally thymectomized C3H mice

The effects of neonatal thymectomy on the development of the lymphoid, erythroid and granulocytic cell populations in mouse bone marrow have been assessed by quantitative techniques. The numbers per unit volume of bone marrow of 17 cell types were determined in neonatally thymectomized and sham thymectomized C3H mice at two, four and eight weeks of age, and compared with those of normal C3H mice. After neonatal thymectomy the numbers of small lymphocytes, large and medium-sized lymphoid cells, and erythroid cells reached normal levels at two weeks but fell progressively to 18%, 22% and 42% of normal, respectively, by eight weeks. In sham thymectomized mice these cell populations did not differ significantly from normal. Immature and mature granulocytes were elevated in numbers two weeks after either neonatal thymectomy or sham thymectomy, suggesting a transient non-specific stimulation of granulocytopoiesis. During continuous infusion of 3H-thymidine for ten days in neonatally thymectomized mice aged four weeks and eight weeks many bone marrow small lymphocytes remained unlabeled. The results demonstrate that early postnatal development of bone marrow lymphoid and erythroid cells proceeds normally in the absence of the thymus, in accord with the concept of the bone marrow as a primary site of lymphocyte production and differentiation. In addition, some slowly-renewing small lymphocytes in bone marrow appear to be thymus-independent cells.

Sheep red blood cell-specific helper activity in rat thoracic duct lymphocyte populations positively selected for reactivity to specific strong histocompatibility alloantigens

These studies show that positively selected T-cell populations, having enriched reactivity in the mixed lymphocyte interaction and the graft-versus-host reaction to strong alloantigens of a chosen major histocompatibility complex haplotype, also possess helper activity which is quantitatively normal in the generation of primary antibody responses to sheep red blood cells in vitro. Such positively selected populations give a linear dose plaque-forming cells response curve indistinguishable from that seen with normal unselected T-cell populations. These findings imply that T cells reactive to histocompatibility antigens also react to conventional antigens, and the possibility is raised that they may do so by some recognition mechanism involving multiple specificities.

75SeL-selenomethionine: a new isotopic marker for lymphocyte localisation studies

L-selenomethionine (SeM), an analogue of methionine containing Se, is incorporated by mouse and rat lymphocytes in vitro: labelled with 75Se (75SeM) it emits gamma-rays distinguishable in energy from those of 125I and 51Cr. The usefulness of 75SeM for lymphocyte localisation studies was investigated. Both B and T cells incorporated 75SeM. Reutilization of the isotope in the mouse after injection of killed cells or allogeneic cells into immunized recipients was 1-2% of the injected dose. The immunocompetence of labelled mouse T and B cells was assessed in irradiated recipients, no effect of 75SeM incorporation being found. The distribution of labelled lymphocytes from various tissues in syngeneic recipients was found to be comparable to that previously obtained with 51Cr.

Lymphopoiesis and lymphocyte recirculation in the sheep fetus

The production and the circulation of lymphocytes has been examined in the sheep fetus where neither foreign antigen nor immunoglobulins occur. It was found that as the lymphoid organs increased in size during fetal life, the numbers and the output of lymphocytes in the thoracic duct lymph increased. The recirculating pool of lymphocytes was estimated to be 5.5 +/- 1.5 X 10(8) cells in fetal lambs 95-100 days of age, 5.7 +/- 1.2 X 10(9) cells in fetuses 130-135 days of age, and 1.2 +/0 9.3 X 10(10) cells in fetuses near to term. The rate of addition of lymphocytes to the recirculating pool was 3.2 +/- 1.9 X 10(6) cells/h in fetuses of 100 days and 3.4 +/- 0.9 X 10(7) cells/h in fetuses of 130 days of age. Lymphocytes recirculated from blood to lymph in fetuses; labeled cells injected into the blood stream reappeared in the thoracic duct lymph promptly and reached maximum levels around 12-18 h after they were injected. Labeled lymphocytes were detected subsequently in greatest numbers in the lymph nodes, particularly in the mesenteric lymph nodes and in the interfollicular areas of the Peyer's patches. Chronic drainage of thoracic duct lymph from fetuses in utero for periods of up to 36 days had no obvious effects on the growth or development of the fetus and only minimal effects on the content of lymphocytes in the various lymphoid tissues even though the number of cells in the blood and lymph were reduced to between 20-30% of normal levels. Thymectomy done in fetuses about 2 mo befor cannulation of the thoracic duct reduced the output of cells in the thoracic duct to about 25% of normal levels and caused a significant reduction in the content of lymphocytes in the various lymphoid tissues. Thymectomized fetal lambs subjected to thoracic duct drainage for periods up to 2 wk in utero had a similar complement of lymphocytes in their lymphoid tissues to intact thymectomized fetal lambs. Lymphocytes obtained from the thoracic duct lymph of lambs thymectomized 2 mo previously recirculated from blood to lymph when they were injected intravenously, although they did this at a significantly slower rate than did lymphocytes from normal lambs.

Small lymphocytes in peripheral lymphoid tissues of nude mice. Life-span and distribution

The distribution of small lymphocytes according to life-span in the peripheral lymphoid tissues of the mouse mutant "nude" has been studied by means of auto-radiography and scintillation counting to evaluate the localization of B lymphocytes with varying life-span. The vast majority of the lymphocytes in this congenitally athymic mouse are relatively long-lived, although few cells live for 6 weeks or more. Differences in labelling percentages of blood, spleen and lymph node lymphocytes indicated a production of lymphocytes with a short residence time in the spleen. A similar production was not seen in the lymph nodes. While the lymphocytes in the spleen were evenly distributed according to life-span, the paracortical lymphocytes in lymph nodes were found to have a generally shorter life-span than those of the cortex, in opposition to findings in normal mice. The cortical cells which were by far the most numerous in the lymph nodes seemed to be more sessile than para-cortical lymphocytes. The life-span of these latter cells are comparable to those of thoracic duct lymphocytes, and the scarcity of cells in the paracortex reflects the small number of recirculating lymphocytes in nude mice.

Collaboration of allogeneic T and B lymphocytes in the primary antibody response to sheep erythrocytes in vitro

This study provides a direct quantitative comparison of the helper effects of allogeneic and syngeneic rat T cells in the production of direct SRBC plaque-forming cell (PFC) responses by B cells in culture. In syngeneic T-B combinations, log-log plots of the number of PFC generated after 5.5 days in culture vs. the number of T cells employed as helpers showed a linear response between 10(4) and 2.5 times 10(5) T cells added. Allogeneic T-B combinations, in which the T cells possess the capacity for reactivity to major alloantigens of the B-cell donor, showed a different dose/response relationship in which PFC responses were decreased at high T/B ratios and augmented at low T/B rations. In this system responses were detected with as few as 10(3) allogeneic T cells. Use of negatively selected allogeneic T populations, specifically depleted of mixed lymphocyte interaction (MLI) and graft-vs-host reactivity for B-cell alloantigens, as helpers gave dose/response curves quantitatively identical to responses with syngeneic T-B combinations and also with F1 T-cell parental B-cell combinations. These data indicate that rat T and B cells need not share a major histocompatibility complex haplotype in order to collaborate effectively in a primary direct PFC response to SRBC in culture. In addition, the PFC response required the combinaed presence of T and B cells as well as antigen in the cultures, a finding consistent with the two signal model of B-cell activation. Finally, the dose/response data obtained suggest the possibility that although SRBC antigen is required in the cultures helper activity with low numbers of normal allogeneic T cells may not depend on T cells having specificity for this antigen.

Life span of small lymphocytes in the thymolymphatic tissues of normal and thymus-deprived BALB/C mice

Autoradiography and scintilation counting have been used after various schedules of 3H-thymidine injections to evaluate lymphocyte kinetics in normal and thymus-deprived BALB/C mice. The thymus was found to be an active production site of small lymphocytes, the majority of these cells having thymic residence times of three to five days. Peripheral lymphoid tissues--including spleen, lymph nodes and Peyer's patches--were dominated by long-lived lymphocytes and produced very few small lymphocytes. Nearly identical percentages of long-lived lymphocytes with comparable grain counts were found in the peripheral tissues. In adult thymectomized animals, and in mice thymectomized and reconstituted with bone marrow cells following total body irradiation, percentages and lifespan of long-lived lymphocytes were found to be of the same order as in normal or sham-operated controls. It is concluded that T cells in the peripheral long-lived pool can be formed outside the thymus and that the bone marrow probably produces long-lived B lymphocytes.