THE PREDOMINANT ROLE OF THE SPLEEN IN LYMPHOCYTE RECIRCULATION

Lymphocyte recirculation through the isolated pig spleen was studied by means of a perfusion system which kept the organ alive for a prolonged period of time. By changing the perfusate to a leucocyte-enriched or cell-free perfusate and taking serial arterial and venous samples, the numbers of lymphocytes which homed to or were released from the spleen were measured. In all experiments more lymphocytes homed than were released per minute. There was no apparent difference when autologous or allogeneic cells were used. The number of lymphocytes released depended on the number of lymphocytes homed previously. During the phase of constant release up to 3-3 × 10⁶ lymphocytes were released per gram spleen per minute. From these values it can be extrapolated that up to 270 × 10⁹ lymphocytes recirculate through the isolated pig spleen per day. Based on kinetic data from other species it is estimated that in the entire pig a total number of 300–400 × 10⁹ lymphocytes recirculate per day. Thus, it can be concluded that the spleen is the most important organ for lymphocyte recirculation in the pig.     

Splenic lymphocytopoiesis and migration pattern of splenic lymphocytes

The spleens of young pigs were selectively labeled with tritiated thymidine ([³H]-TdR) and the relative and absolute numbers of labeled lymphocytes found 24 hr later in different lymphoid and nonlymphoid organs were determined autoradiographically. It was deduced that about 4.6 × 10⁹ lymphocytes (that is, about 15% of all splenic lymphocytes) are produced by the spleen per day and about 17% of the newly formed lymphocytes leave the spleen within the first day of labeling. Spleen-derived lymphocytes could be found in relatively high numbers in the lymph nodes, blood, gut-associated lymphoid tissues, and, surprisingly, in the bone marrow, whereas the concentration in the thymus was very low. In a second series, pigs were labeled with [³H]TdR and only the spleen was excluded from labeling. The labeling index of splenic small lymphocytes was about 10% 1 day later, indicating a high rate of influx of newly formed lymphocytes into the pig spleen. The spleen of the young pig is an important lymphocytopoietic organ and exports and imports newly formed lymphocytes at high rates.     

The predominant role of the spleen in lymphocyte recirculation. II. Pre- and postsplenectomy retransfusion studies in young pigs.

Autologous blood lymphocytes from three normal pigs were labelled with 3H-uridine and retransfused before and after splenectomy. Frequent samples for up to 150 min after retransfusion were evaluated autoradiographically to determine the rate of disappearance of labelled lymphocytes from the blood. In one pig retransfusion was performed before and after sham-splenectomy. In all preoperative experiments the pattern of disappearance of labelled lymphocytes was very similar. After a first rapid decline (halving time on average 8 min) a short rise of the labelling index was observed from 10 to 15 min after retransfusion. Then a second more gradual decrease of labelled lymphocytes followed. The mean halving time during this period was less than 32 min. From 60 min onwards the labelling index remained nearly constant. Retransfusions performed 3 days after splenectomy revealed only one nearly constant decline of the labelling index (halving time on average 129 min). After sham-splenectomy the pattern of disappearance was similar to the preoperative experiment. One hour after the end of retransfusion the labelling index had decreased by three-quarters of the initial value in normal pigs and by only one-third in the splenectomized ones. These results indicate that in the pig the total rate of recirculation is at least 4 times faster with the spleen in situ than without the spleen.     

Role of beta 2-adrenoceptors in the biological effect of autoimmune orchitis spleen cells

Autoimmune orchitis induced an increment in beta 2-adrenoceptor populations in intact mouse spleen lymphocytes. Normal and autoimmune lymphocytes incubated with soterenol increased the mechanical response of isolated atria. Autoimmune cells were more effective than normal cells in inducing this response. Soterenol or spleen cells alone did not modify the contractility at the concentration used. Inhibitors of beta 2-adrenoceptors of spleen cells completely blunted the reaction between soterenol and lymphocytes, while when atria were exposed to butoxamine, mechanical activity induced by soterenol plus lymphocytes was not affected. Cell-free supernatants of lymphocytes exposed to soterenol elicited the reaction in the same way as soterenol-treated lymphocytes. Direct contact of cells with the assay organ was not necessary. Inhibitors of cyclooxygenase on lymphocytes blocked the reaction of soterenol-treated lymphocytes on atria, while inhibitors of lipoxygenase(s) completely blocked the reaction of atria exposed to soterenol-stimulated lymphocytes or supernatants. These results suggest that soterenol reacts with beta 2-adrenoceptors of normal and autoimmune cells. From this reaction, soluble factors are released that in turn trigger stimulation of the atrial contractility as a consequence of the release of oxidative products of the lipoxygenase(s) pathway of arachidonic acid from atria. The high activity of atria in the presence of autoimmune spleen cells is probably related to the increment in number of beta 2-adrenoceptors of these cells.     

Fluid endocytosis by rat liver and spleen. Experiments with 125I-labelled poly(vinylpyrrolidone) in vivo.

1. Rates of fluid endocytosis of rat liver, spleen, hepatocytes and sinusoidal liver cells have been determined, by using 125I-labelled poly(vinylpyrrolidone) as marker. Poly(vinylpyrrolidone) was injected intravenously into rats, and plasma clearance and uptake by liver and spleen were estimated. From these data, rates of fluid endocytosis of 1.2 and 1.8 ml of plasma/g of protein per day were calculated for liver and spleen respectively. Essentially the same results were found in nephrectomized rats. 2. Hepatocytes and sinusoidal cells were separately isolated by the collagenase/Pronase method, and sinusoidal cells were further fractionated by centrifugal elutriation. Hepatocytes, sinusoidal cells, Kupffer cells and endothelial cells showed rates of fluid endocytosis of 0.96, 9.0, 19 and 13 ml of plasma/g of cell protein per day respectively. Total-body X-irradiation did not influence uptake of poly(vinylpyrrolidone) by spleen, indicating that spleen lymphocytes are not significantly involved in fluid endocytosis. 3. For liver a rate constant of exocytosis of 5% per day was found, whereas for spleen no significant loss of accumulated label could be demonstrated during a 21-day period. 4. Distribution of label over a great number of organs and tissues was measured 9 days after the injection. Liver, skin, bone and muscle together contained about 70% of the label present in the carcass; only spleen and lymph nodes contained more label per g fresh weight of tissue than liver.     

The migration of lymphocytes across specialized vascular endothelium. I. The entry of lymphocytes into the isolated mesenteric lymph-node of the rat.

The chain of lymph-nodes in the rat mesentery was isolated and the preparation was perfused via cannulae in the superior mesenteric vessels. The perfusate consisted of serum to which labelled lymphocytes had usually been added. The entry of radioactively labelled lymphocytes from the blood vessels into the lymph-nodes was studied by scintillation counting and autoradiography. It was observed that: (1) In the perfused node labelled lymphocytes localized in and around post-capillary venules in the paracortex as they do early after i.v. injection. (2) The number of lymphocytes which entered the node was directly proportional to the concentration in the perfusate over the range studied. The proportion of cells retained in the node varied considerably around a mean of 11% of lymphocytes reaching it. (3) The isolated lymph-node released few if any lymphocytes into the effluent (venous) perfusate. (4) Large lymphocytes migrated into isolated lymph-nodes slightly more readliy than did small lymphocytes. (5) Unlike intact cells isolated lymphocyte membranes did not adhere to specialized vascular endothelium.     

The metabolism of 3-H-cortisone and 3-H-cortisol by the isolated perfused rat and guinea pig lungs.

Isolated perfused rat lungs removed more than 35% of 3-H-cortisone (1 times 10-9M) from the perfusate during one passage through the pulmonary circulation. The cortisone in the lungs was then rapidly converted to cortisol, which was returned to the perfusate. The tritiated steroid taken up was so rapidly washed from the lung, that only 10% remained after a 12 minute perfusion with steroid-free medium. In recirculating experiments, nearly 60% conversion to cortisol occurred over 32 cycles; in addition, there was a slow increase in the percentage of polar compounds in the medium. Similarly, the perfused hindlimbs preparation from the rat converted cortisone to cortisol and returned the cortisol to the perfusate. In contrast, guinea pig isolated perfused lungs had neglible effect on cortisone. Rat lungs demonstrated only a limited ability to convert 3-H-cortisol to cortisone. The results suggest that the lungs may play an important role in maintaining cortisone/cortisol levels in the plasma.     

Changes in the number of DNA-protein cross links in spleen lymphocytes of mice exposed to low intensity low dose gamma irradiation

Levels of DNA-protein cross-links (DPC) and DNA single-strand breaks (SSB) in spleen lymphocytes were studied in mice exposed to low-intensity gamma-radiation (1.7 mGy/day) for 1, 4, 10, 20, and 30 days. The spleen mass and count of lymphocytes isolated from this organ also has been investigated. The significant increase in the DPC level as compared to the control occurred on the 10-th and 30-th days of irradiation at doses of 1.7 and 5.1 cGy, accordingly. The number of spleen lymphocytes normalized to organ mass significantly decreased on the 4-th and 30-th days of the experiment. No increase was found in levels of alkali-labile sites and SSB. In contrast, the increase in the amount of duplex form DNA was recorded on the 4-th and 30-th days of the experiment. Our indicate that DPC formation after irradiation at low doses represents some form of cellular response to the damaging agent.     

Cellular mechanisms in the protection against infection by Listeria monocytogenes in mice.

Listeria monocytogenes, in doses of 2-0 X 10(3) to 3-0 X 10(3) viable organisms, was injected into athymic nude mice, irradiated mice and mice treated with reticuloendothelial system-blocking agents. Viable counts on liver and spleen homogenates were made at intervals after infection. In both nude mice (nu/nu) and normal littermates (nu/+) of BALB/c background, the bacteria grew rapidly for 24 h but increased only slowly thereafter, to reach a plateau of about 10(5) per organ at 72 h. In nu/+ mice, the number of viable bacteria began to decrease after 6 to 9 days, with complete elimination by day 12. In nude mice, the number of Listeria remained at a stable level of approximately 10(5) per organ during the observation period of 21 days. In lethally irradiated nu/+ mice, bacteria grew progressively and extensively to reach 10(7) per spleen and 10(9) per liver by 72 h. Bacterial growth during the first 72 h was markedly enhanced by treatment with carbon particles, dextran sulphate 500 or silica. These enhancing effects were also observed in nude mice and in AKR, C3H/He and C57BL/6 animals. We conclude that both non-immune phagocytes and T cell-dependent mechanisms contribute to the resistance of mice to Listeria infection.     

Natural killer cell activity in the rat. V. The circulation patterns and tissue localization of peripheral blood large granular lymphocytes (LGL)

Large granular lymphocytes (LGL) and T cells were separated from blood by centrifugation on discontinuous gradients of Percoll, were labeled with [3H]uridine or [111In]oxine, and were injected i.v. into syngeneic euthymic or athymic nude rats. The tissue distribution of these labeled cells was monitored for up to 24 hr after transfer by scintillation counting of tissue homogenates and autoradiography of tissue sections. In normal euthymic rats, the main sites of LGL localization were the alveolar walls of the lungs and spleen red pulp; however, they were not detectable in the major traffic areas of T lymphocyte recirculation, the spleen white pulp, and lymph nodes. Furthermore, the density of labeled LGL was very low in the small intestine, thymus, kidney, and liver, although on a per-organ basis, about 10% of the injected radioactivity was found in the liver by 24 hr post-injection. When 111In-labeled LGL were injected i.v. into rats with an indwelling thoracic duct cannula, they completely failed to enter the thoracic duct lymphocyte (TDL) population over an observation period of 6 days. This finding was markedly different from the results obtained with T cells and was consistent with the lack of natural killer and antibody-dependent cellular cytotoxicity activity observed among TDL, even in rats pretreated with the biological response modifier, poly I:C. LGL in athymic nude rats also failed to recirculate between blood and lymph. However, in contrast to normal euthymic animals, a significant increase in the localization of radiolabeled LGL to lymph nodes was observed in nude rats between 30 min and 24 hr. Taken as a whole, these findings define the areas within the lungs and spleen in which blood LGL normally localize, and clearly demonstrate that LGL do not normally recirculate between blood and lymph.     

Radiosensitivity of defined populations of lymphocytes. VI. Functional, structural, and biochemical consequences of in vitro irradiation.

Single cell suspensions of BALB/c thymocytes (T cells) and nu/nu spleen cells (B cells) were exposed to 0, 50, and 500 rads and examined for topological abnormalities and alterations in surface glycoproteins and capacity to traffic normally upon transfer to syngeneic recipients. The results show that irradiated lymphocytes from both sources undergo extensive topological alterations which become more pronounced with the passage of time and which appear to precede the loss of cell viability. Viable T and B cells irradiated in vitro also fail to recirculate normally and accumulate in abnormal proportions in the spleen at the expense of the lymph nodes and gut-associated lymphoid tissues. Similarly, after a 2-hr incubation in saline at room temperature, irradiation of both T and B cells resulted in alterations in extractable surface glycoproteins. The results are consistent with the hypothesis that alterations in the recirculation of irradiated lymphocytes are associated with alterations in the plasma membrane.     

The migration of lymphocytes across specialized vascular endothelium. VI. The migratory behaviour of thoracic duct lymphocytes retransferred from the lymph nodes, spleen, blood, or lymph of a primary recipient

Thoracic duct lymphocytes labelled with 51Cr were injected into a primary recipient and then were transferred for a second time from the lymph nodes (cervical and/or mesenteric), spleen, lymph, or blood into a series of final recipients. Measurement of the organ distribution of labelled lymphocytes in the final recipients enabled three main conclusions to be drawn. (1) Lymphocytes that had localized in the spleen, mesenteric lymph nodes (LN), or cervical LN of the first recipient showed no tendency to return in increased numbers to the same organ in the final recipient. (2) Lymphocytes that had recently entered the spleen or LN were temporarily impaired in their ability to reenter LN. This capacity was recharged when the cells returned to the lymph and the blood. (3) Lymphocytes that had been passaged from blood to lymph and collected for up to 4 hr at room temperature entered the LN of a recipient much faster than did nonpassaged thoracic duct lymphocytes collected overnight at 0 degree C. Supplementary experiments indicated that the different migratory behavior of thoracic duct lymphocytes under these two circumstances was mainly a consequence of their handling in vitro during the collecting and the labelling procedures. This functional impairment was not associated with a diminished ability to enter the spleen and bone marrow or to survive in recipients for up to 24 hr.     

Large quantity of ribosomal RNA exists extracellularly in mouse spleen

When BALB/c mouse spleens were gently homogenized in saline, the resultant supernatant (without cells and tissue debris) contained significant amount of 28S and 18S ribosomal RNA, reaching up to 70% of the total spleen RNA. Haemoglobin assays indicated that less than 15% of the spleen cells were lysed during the homogenization process, indicating that the majority of the spleen `supernatant RNA' was from the extracellular space of the organ rather than released by the splenocytes as a consequence of grinding. Quantitative RNA analysis showed that the ratio of spleen supernatant RNA/total RNA of BALB/c mice was inversely correlated with age (from approximately 70% at 3 weeks to 45% at 6 months), but that of BXSB mice (an animal model for systemic lupus erythematosus) remained at about 70% irrespective of age. Methyl Green–Pyronin Y staining of paraffin sections of mouse spleen revealed that extracellular RNA was distributed mainly in the sinuses of the organ. Culture supernatants of apoptotic splenocytes contained significant amounts of RNA, suggesting that the extracellular RNA in the spleen might have come from apoptotic lymphocytes. This is supported by the fact that `thymus supernatant' also contained significant amount of RNA. A possible correlation between spleen extracellular RNA and autoimmune diseases is discussed.     

THE MIGRATION OF LYMPHOCYTES ACROSS SPECIALIZED VASCULAR ENDOTHELIUM

The chain of lymph-nodes in the rat mesentery was isolated and the preparation was perfused via cannulae in the superior mesenteric vessels. The perfusate consisted of serum to which labelled lymphocytes had usually been added. The entry of radioactively labelled lymphocytes from the blood vessels into the lymph-nodes was studied by scintillation counting and autoradiography. It was observed that: (1) In the perfused node labelled lymphocytes localized in and around post-capillary venules in the paracortex as they do early after i.v. injection. (2) The number of lymphocytes which entered the node was directly proportional to the concentration in the perfusate over the range studied. The proportion of cells retained in the node varied considerably around a mean of 11 % of lymphocytes reaching it. (3) The isolated lymph-node released few if any lymphocytes into the effluent (venous) perfusate. (4) Large lymphocytes migrated into isolated lymph-nodes slightly more readily than did small lymphocytes. (5) Unlike intact cells isolated lymphocyte membranes did not adhere to specialized vascular endothelium.     

The relative importance of the bone marrow and spleen in the production and dissemination of B lymphocytes.

The relative importance of the bone marrow and spleen in the production of B lymphocytes was investigated in guinea pigs by the combined use of [³H]TdR radio-autography and fluorescent microscopy after the staining of B cells by FITC-F(ab′)₂-goat-anti-guinea pig Ig. Large and small lymphoid cells possess sIg in the marrow and spleen but B cell turnover in the marrow exceeds that in the spleen. That newly generated bone marrow B cells are not derived from an extramyeloid bursa equivalent was demonstrated by the absence of [³H]TdR labeled B cells in tibial marrow 72 hr after [³H]TdR was administered systemically, while the circulation to the hind limbs was occluded. Pulse and chase studies with [³H]TdR showed that large marrow B cells are derived from sIg-negative, proliferating precursors resident in the bone marrow and not from the enlargement of activated small B lymphocytes. The acquisition of [³H]TdR by splenic B cells lagged behind that observed in the marrow. Three days after topical labeling of tibial and femoral bone marrow with [³H]TdR, a substantial proportion of splenic B cells were replaced by cells that had seeded there from the labeled marrow. The studies unequivocally identify the bone marrow as the organ of primary importance in B cell generation and indicate that in the guinea pig rapidly renewed B lymphocytes of the spleen are replaced by lymphocytes recently generated in bone marrow. The rate of replacement of B lymphocytes in the lymph node by cells newly generated in the bone marrow takes place at a slower tempo than in the spleen.     

Innervation of the guinea pig spleen studied by electron microscopy

The innervation of the guinea pig spleen was investigated by electron microscopy. Unmyelinated nerve fibers in the capsulotrabecular and arterial systems were found to contain large and small granular and small agranular synaptic vesicles in their terminals and are thought to be sympathetic adrenergic in nature. They influence the contraction of the smooth muscle cells by diffusion innervation in these systems. These nerve terminals were also scattered in both the red and the white pulp. Pulp nerves wrapped by Schwann cells were further enclosed by myofibroblastic reticular cells. This condition revealed that the pulp nerves pass through the connective-tissue spaces of the reticular fibers, which contain elastic fibers, collagenous fibrils, and lamina densa-like materials of the usual basement laminae. One of the target cells for the pulp nerves is considered to be the myofibroblastic reticular cell in the reticular meshwork. Neurotransmitter substances released from the naked adrenergic nerve terminals travel through the reticular fibers and may play a role, by both close association innervation and diffusion innervation, in the contraction of reticular cells to expose the reticular fibers. At the exposed sides, connective-tissue elements of the reticular fibers are bathed with blood plasma, and the included naked nerve terminals, devoid of Schwann cells but with basement laminae of these cells, face free cells at some distance or are in close association with free cells, especially lymphocytes, macrophages, and plasma cells. The close ultrastructural relationship between the naked adrenergic nerve terminals and immunocytes strongly suggests that there is an intimate relationship between the immune system and the sympathetic nervous system through both close association innervation and diffusion innervation. Thus splenic adrenergic nerves of the guinea pig may play a triple role in 1) contraction of smooth muscle cells to regulate blood flow in the organ, 2) induction of the exposure of reticular fibers by contraction of the reticular cells in order to form a close relationship of the nerve terminals with the immunocytes, and 3) subsequent neuroimmunomodulation of the immunocytes.     

Distribution of substance P receptors on murine spleen and Peyer's patch T and B cells

Previous studies have demonstrated that the sensory neuropeptide substance P (SP) can modulate immune responses in vitro. Work from this laboratory has shown that SP enhances immunoglobulin synthesis by murine splenic and Peyer's patch lymphocytes stimulated with concanavalin A. One mechanism underlying these effects is the binding of SP to specific receptors on lymphocytes. We examined the distribution of SP receptors on murine T and B lymphocytes and their subsets by one and two color fluorescence-activated cell sorter analysis. The specificity and nature of binding was examined using radiolabeled SP, and competitive inhibition experiments were performed with cold SP. In cytofluorimetry experiments, both T and B lymphocytes from Peyer's patches and spleen were bound to SP, with those from Peyer's patches having a higher proportion than lymphocytes from the spleen. The majority of T cells from both organs bound SP with binding being evenly distributed between Lyt-1+ and Lyt-2+ cells. Similarly, the majority of B lymphocytes from spleen and Peyer's patches showed SP binding. There were no significant isotype-specific differences within any organ. Studies using 125I-labeled SP showed specific binding to all lymphocyte subpopulations examined. SP receptors were fewer in number on cells isolated from spleen than on cells from Peyer's patches although the dissociation constants were similar for all populations examined. These studies demonstrated that SP receptors are present both on murine T and B lymphocytes from Peyer's patches and spleen. There is no evidence for differential SP receptor expression on distinct lymphocyte subsets in spleen or Peyer's patches.     

Effects of environmental temperature on thymus and spleen weights and lymphocytes in mice

Thymus and spleen weights and lymphocytes in the blood were examined in mice transferred from 22 degrees C to 12 degrees C or 32 degrees C environments. After the exposure to either environment, organ weights tended to decrease. In mice exposed to 12 degrees C, the number of WBCs and mononuclear cells and the ratio and the number of T cells decreased on day 1 and/or 3 after the exposure. The number of B cells also declined on day 3, but the ratio of B cells increased on days 3 and 7. In mice exposed to 32 degrees C, the number of WBCs and mononuclear cells and the number of T cells decreased on day 1 and increased on day 7 and/or 14. The ratio of T cells declined on days 1, 3 and 7, while that of B cells increased on day 3, and the number of B cells increased on day 7. These results show that wide variations in environmental temperature affect the weights of organs and the number of cells which act on immune responses in mice.     

Migration patterns of dendritic cells in the rat: comparison of the effects of gamma and UV-B irradiation on the migration of dendritic cells and Lymphocytes

To further define the underlying mechanisms of immune suppression induced by UV-B irradiation, we have examined the kinetics of homing patterns of in vitro UV-B-irradiated and gamma-irradiated-thoracic duct lymphocytes (TDL) compared to dendritic cells (DC). Our findings show that 111In-oxine-labeled TDL specifically home to the spleen, liver, lymph nodes, and bone marrow with subsequent recirculation of a large number of cells from the spleen to lymph nodes. In contrast, DC preferentially migrate to the spleen and liver with a relatively insignificant distribution to lymph nodes and an absence of subsequent recirculation. Splenectomy prior to cell injection significantly diverts the spleen-seeking DC to the liver but not to the lymph nodes, while the homing of TDL to lymph nodes is significantly increased. In vitro exposure of 111In-oxine labeled TDL to gamma irradiation does not significantly impair immediate homing to lymphoid tissues but inhibits cell recirculation between 3 and 24 hr. In contrast, gamma irradiation does not affect the tissue distribution of labeled DC, suggesting that DC are more radioresistant to gamma irradiation than TDL. Unlike the findings in animals injected with gamma-irradiated cells, UV-B irradiation virtually abolished the homing of TDL to lymph nodes and significantly reduced the homing of the spleen-seeking DC to the splenic compartment while a large number of cells were sequestered in the liver. The results of in vitro cell binding assay show that TDL, unlike DC, have the capacity to bind to high endothelial venules (HEV) within lymph node frozen sections while gamma and UV-B irradiation significantly inhibit the binding of TDL to lymph node HEV. These findings suggest that: (i) DC, unlike TDL, are unable to recirculate from blood to lymph nodes through HEV; (ii) although gamma irradiation impairs TDL recirculation, it does not affect DC tissue distribution; and (iii) UV-B irradiation impairs both TDL and DC migration patterns. We conclude that the lack of capacity of irradiated TDL to home to lymph nodes is due to damage to cell surface homing receptors and that the failure of DC to home to the lymph node microenvironment is related to the absence of HEV homing receptors on their cell surface.     

Physiology of B cells in mice with X-linked immunodeficiency. I. Size, migratory properties, and turnover of the B cell pool

In terms of certain immune functions and density of surface IgM, B cells from xid mice are often viewed as the equivalent of the immature (Lyb-5-) B cell subset of normal adult mice. In this paper we examine xid B cells with regard to certain physiologic functions, including homing to the lymphoid tissues, recirculation, and turnover. Xid mice were found to possess about one-third of the total number of B cells found in normal mice. This applied irrespective of whether one examined the spleen, lymph nodes, or outputs of B cells in thoracic duct lymph. In terms of migration to spleen, lymph nodes, and Peyer's patches, capacity to recirculate from blood to thoracic duct lymph, and turnover, xid B cells proved to be indistinguishable from normal spleen or thoracic duct B cells. Within these parameters, most xid B cells closely resemble the normal mature long-lived population of B cells residing in the recirculating pool of normal mice. Because xid B cells are functionally quite different from normal mature B cells, it seems reasonable to view xid B cells as an abnormal population not represented in normal mice.