Biphasic pattern of thymus regeneration after whole-body irradiation

Restoration of thymic T-cell development by bone marrow transplantation in mouse radiation lymphomagenesis

Fractionated total body irradiation (TBI) with X-rays induces thymic lymphoma/leukemia (TL) in C57BL/6 mice. Radiation-induced mouse TL (RITL) can be prevented by bone marrow transplantation (BMT) of unirradiated BM cells. However, the mechanisms underlying the prevention of RITL with BMT remain unclear. Here, we show that BMT restores thymic T-cell differentiation in mice subjected to TBI. TBI (four times of 1.8 Gy X-rays weekly) was conducted with C57BL/6 mice. BMT was performed immediately after the last irradiation of TBI in mice by transplantation of BM cells isolated from enhanced green fluorescence protein (eGFP) transgenic mice. Thymic cell numbers were drastically decreased in TBI and TBI + BMT mice compared to those in non-irradiated mice. Flow cytometry showed a dramatic decrease in double negative (DN, CD4-CD8-) thymocytes, especially DN2 (CD25+CD44+) and DN3 (CD25+CD44-) subpopulations, in the TBI mice on Day 10 after the last irradiation. In contrast, the DN2 and DN3 populations were recovered in TBI + BMT mice. Interestingly, these restored DN2 and DN3 cells mainly differentiated from eGFP-negative recipient cells but not from eGFP-positive donor cells, suggesting that transplanted BM cells may interact with recipient cells to restore thymic T-cell development in the RITL model. Taken together, our findings highlight the significance of restoring thymic T-cell differentiation by BMT in RITL prevention.

Effects of Radioactive 56MnO2 Particle Inhalation on Mouse Lungs: A Comparison between C57BL and BALB/c

The effects of residual radiation from atomic bombs have been considered to be minimal because of its low levels of external radioactivity. However, studies involving atomic bomb survivors exposed to only residual radiation in Hiroshima and Nagasaki have indicated possible adverse health effects. Thus, we investigated the biological effects of radioactive dust of manganese dioxide 56 (56MnO2), a major radioisotope formed in soil by neutron beams from a bomb. Previously, we investigated C57BL mice exposed to 56MnO2 and found pulmonary gene expression changes despite low radiation doses. In this study, we examined the effects in a radiation-sensitive strain of mice, BALB/c, and compared them with those in C57BL mice. The animals were exposed to 56MnO2 particles at two radioactivity levels and examined 3 and 65 days after exposure. The mRNA expression of pulmonary pathophysiology markers, including Aqp1, Aqp5, and Smad7, and radiation-sensitive genes, including Bax, Phlda3, and Faim3, was determined in the lungs. The radiation doses absorbed in the lungs ranged from 110 to 380 mGy; no significant difference was observed between the two strains. No exposure-related pathological changes were observed in the lungs of any group. However, the mRNA expression of Aqp1 was significantly elevated in C57BL mice but not in BALB/c mice 65 days after exposure, whereas no changes were observed in external γ-rays (2 Gy) in either strain. In contrast, Faim3, a radiation-dependently downregulated gene, was reduced by 56MnO2 exposure in BALB/c mice but not in C57BL mice. These data demonstrate that inhalation exposure to 56MnO2 affected the expression of pulmonary genes at doses <380 mGy, which is comparable to 2 Gy of external γ-irradiation, whereas the responses differed between the two mouse strains.

Immune Reconstitution and Thymic Involution in the Acute and Delayed Hematopoietic Radiation Syndromes

Lymphoid lineage recovery and involution after exposure to potentially lethal doses of ionizing radiation have not been well defined, especially the long-term effects in aged survivors and with regard to male/female differences. To examine these questions, male and female C57BL/6 mice were exposed to lethal radiation at 12 wk of age in a model of the Hematopoietic-Acute Radiation Syndrome, and bone marrow, thymus, spleen, and peripheral blood examined up to 24 mo of age for the lymphopoietic delayed effects of acute radiation exposure. Aged mice showed myeloid skewing and incomplete lymphocyte recovery in all lymphoid tissues. Spleen and peripheral blood both exhibited a monophasic recovery pattern, while thymus demonstrated a biphasic pattern. Naïve T cells in blood and spleen and all subsets of thymocytes were decreased in aged irradiated mice compared to age-matched non-irradiated controls. Of interest, irradiated males experienced significantly improved reconstitution of thymocyte subsets and peripheral blood elements compared to females. Bone marrow from aged irradiated survivors was significantly deficient in the primitive lymphoid-primed multipotent progenitors and common lymphoid progenitors, which were only 8-10% of levels in aged-matched non-irradiated controls. Taken together, these analyses define significant age- and sex-related deficiencies at all levels of lymphopoiesis throughout the lifespan of survivors of the Hematopoietic-Acute Radiation Syndrome and may provide a murine model suitable for assessing the efficacy of potential medical countermeasures and therapeutic strategies to alleviate the severe immune suppression that occurs after radiation exposure.

DN2 Thymocytes Activate a Specific Robust DNA Damage Response to Ionizing Radiation-Induced DNA Double-Strand Breaks

For successful bone marrow transplantation (BMT), a preconditioning regime involving chemo and radiotherapy is used that results in DNA damage to both hematopoietic and stromal elements. Following radiation exposure, it is well recognized that a single wave of host-derived thymocytes reconstitutes the irradiated thymus, with donor-derived thymocytes appearing about 7 days post BMT. Our previous studies have demonstrated that, in the presence of donor hematopoietic cells lacking T lineage potential, these host-derived thymocytes are able to generate a polyclonal cohort of functionally mature peripheral T cells numerically comprising ~25% of the peripheral T cell pool of euthymic mice. Importantly, we demonstrated that radioresistant CD44⁺ CD25⁺ CD117+ DN2 progenitors were responsible for this thymic auto-reconstitution. Until recently, the mechanisms underlying the radioresistance of DN2 progenitors were unknown. Herein, we have used the in vitro “Plastic Thymus” culture system to perform a detailed investigation of the mechanisms responsible for the high radioresistance of DN2 cells compared with radiosensitive hematopoietic stem cells. Our results indicate that several aspects of DN2 biology, such as (i) rapid DNA damage response (DDR) activation in response to ionizing radiation-induced DNA damage, (ii) efficient repair of DNA double-strand breaks, and (iii) induction of a protective G₁/S checkpoint contribute to promoting DN2 cell survival post-irradiation. We have previously shown that hypoxia increases the radioresistance of bone marrow stromal cells in vitro, at least in part by enhancing their DNA double-strand break (DNA DSB) repair capacity. Since the thymus is also a hypoxic environment, we investigated the potential effects of hypoxia on the DDR of DN2 thymocytes. Finally, we demonstrate for the first time that de novo DN2 thymocytes are able to rapidly repair DNA DSBs following thymic irradiation in vivo.

Acute systemic DNA damage in youth does not impair immune defense with aging

Aging‐related decline in immunity is believed to be the main driver behind decreased vaccine efficacy and reduced resistance to infections in older adults. Unrepaired DNA damage is known to precipitate cellular senescence, which was hypothesized to be the underlying cause of certain age‐related phenotypes. Consistent with this, some hallmarks of immune aging were more prevalent in individuals exposed to whole‐body irradiation (WBI), which leaves no anatomical repository of undamaged hematopoietic cells. To decisively test whether and to what extent WBI in youth will leave a mark on the immune system as it ages, we exposed young male C57BL/6 mice to sublethal WBI (0.5–4 Gy), mimicking human survivor exposure during nuclear catastrophe. We followed lymphocyte homeostasis thorough the lifespan, response to vaccination, and ability to resist lethal viral challenge in the old age. None of the irradiated groups showed significant differences compared with mock‐irradiated (0 Gy) animals for the parameters measured. Even the mice that received the highest dose of sublethal WBI in youth (4 Gy) exhibited equilibrated lymphocyte homeostasis, robust T‐ and B‐cell responses to live attenuated West Nile virus (WNV) vaccine and full survival following vaccination upon lethal WNV challenge. Therefore, a single dose of nonlethal WBI in youth, resulting in widespread DNA damage and repopulation stress in hematopoietic cells, leaves no significant trace of increased immune aging in a lethal vaccine challenge model.

IL-23 promotes TCR-mediated negative selection of thymocytes through the upregulation of IL-23 receptor and RORγt

Transient thymic involution is frequently found during inflammation, yet the mode of action of inflammatory cytokines is not well defined. Here we report that interleukin-23 (IL-23) production by the thymic dendritic cells (DCs) promotes apoptosis of the CD4^hiCD8^hi double positive (DP) thymocytes. A deficiency in IL-23 signaling interferes with negative selection in the male D^b/H-Y T-cell receptor (TCR) transgenic mice. IL-23 plus TCR signaling results in significant up-regulation of IL-23 receptor (IL-23R) expressed predominantly on CD4^hiCD8^hiCD3⁺αβTCR⁺ DP thymocytes, and leads to RORγt dependent apoptosis. These results extend the action of IL-23 beyond its peripheral effects to a unique role in TCR mediated negative selection including elimination of natural T regulatory cells in the thymus.

Semiquantitative Detection of Cytokine Messages in X-Irradiated and Regenerating Rat Thymus

We investigated the expression of cytokine mRNA derived from thymocytes or thymic epithelial cells in X-irradiated (8 Gy) and recovering rat thymuses, according to our previous observation (Mizutani et al., Radiat. Res. 157, 281-289, 2002). The changes in mRNA expression level of interleukin 2 (Il2), Il4, tumor necrosis factor alpha (Tnf), interferon gamma (Ifng), and transforming growth factor beta (Tgfb) were examined. The mRNA expression of Il2 and Il4 decreased from day 5 to day 14 after irradiation. Thereafter, the expression level of Il2 mRNA recovered to normal control levels; however, the expression of Il4 mRNA tended toward significantly low levels. Tnf mRNA expression decreased on day 5 after irradiation and then showed a gradual increase back to normal control levels. Tgfb mRNA expression did not change significantly. Ifng mRNA expression was transiently enhanced from day 11 to day 14. The mRNA expression levels of Il10 increased significantly from day 3 to day 7 after irradiation. In addition, the mRNA expression of thymic epithelial cell-derived Il7 showed a transient decrease on day 3; however, then it showed a continuous increase from day 5 to day 21, finally reaching twice the normal control levels after X irradiation. These observations suggest that the expression of cytokine messages in the irradiated thymus changed significantly and did not return to normal for a long time after 8 Gy irradiation.

Vanin-1-/- mice exhibit a glutathione-mediated tissue resistance to oxidative stress

Vanin-1 is an epithelial ectoenzyme with pantetheinase activity and generating the amino-thiol cysteamine through the metabolism of pantothenic acid (vitamin B(5)). Here we show that Vanin-1(-/-) mice, which lack cysteamine in tissues, exhibit resistance to oxidative injury induced by whole-body gamma-irradiation or paraquat. This protection is correlated with reduced apoptosis and inflammation and is reversed by treating mutant animals with cystamine. The better tolerance of the Vanin-1(-/-) mice is associated with an enhanced gamma-glutamylcysteine synthetase activity in liver, probably due to the absence of cysteamine and leading to elevated stores of glutathione (GSH), the most potent cellular antioxidant. Consequently, Vanin-1(-/-) mice maintain a more reducing environment in tissue after exposure to irradiation. In normal mice, we found a stress-induced biphasic expression of Vanin-1 regulated via antioxidant response elements in its promoter region. This process should finely tune the redox environment and thus change an early inflammatory process into a late tissue repair process. We propose Vanin-1 as a key molecule to regulate the GSH-dependent response to oxidative injury in tissue at the epithelial level. Therefore, Vanin/pantetheinase inhibitors could be useful for treatment of damage due to irradiation and pro-oxidant inducers.

Inflammatory and anti-inflammatory cytokines regulate the recovery from sublethal X irradiation in rat thymus

We investigated the regeneration of rat thymus after sublethal X irradiation (6 Gy). The number of thymocytes was much lower on day 3 after irradiation, and many apoptotic cells were observed. However, by day 5, there had been a rapid proliferation of thymocytes. Since cytokines are considered to be important regulatory factors in postirradiation recovery, we performed in vivo cytokine assays using semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and found serial changes in the cytokine message. The messenger RNA (mRNA) expression of the pro-inflammatory cytokines interleukin 1 beta (Il1b), Il6 and tumor necrosis factor alpha (Tnf) was higher than normal on day 3, lower on day 5, and higher again on day 7. In particular, Tnf was completely absent on day 5 and was expressed again on day 7. Of the anti-inflammatory cytokines Il4, transforming growth factor beta (Tgfb) and Il10, only the Il10 message changed substantially. Il10 expression was very high on day 5 but was completely absent on day 7. Thus the Tnf and Il10 messages were expressed alternately. The changes in the distribution of macrophages detected by the immunohistochemical analysis may be related to the changes in the cytokines. Analysis of cytokine messages in the regenerating thymus in vivo may provide new insights into potential therapies for radiation-induced damage.

Semiquantitative morphological analysis of stromal cells in the irradiated and recovering rat thymus

To understand the roles of thymic stromal cells in T-lymphocyte development, we semiquantitatively analysed rat thymi recovering from irradiation (6 Gy), using a transmission electron microscope. The most striking findings were that the percentage of subcapsular epithelial cells significantly increased in the cortex on day 3 after irradiation compared with the control; the percentage of intermediate epithelial cells significantly increased in the cortex on days 3 and 5 after irradiation and in the medulla on days 5 and 7 compared with the control; the interdigitating cells disappeared from the medulla by day 7 after irradiation and reappeared on day 9. The present data thus reveal that during recovery after irradiation (6 Gy), marked changes occur in the relative proportions of different epithelial cell subtypes in the cortex and medulla of the rat thymus. In addition, the percentages of macrophages and interdigitating cells also changed during the recovery. These changes, which may be associated with the abrupt proliferation of thymocytes after irradiation, should shed light on the significance of stromal cells in the T cell development.

Upregulated Expression of Fibronectin Receptors Underlines the Adhesive Capability of Thymocytes to Thymic Epithelial Cells During the Early Stages of Differentiation: Lessons From Sublethally Irradiated Mice

A 250-cGy whole-body γ-radiation dose was used to induce thymus regression in mice, and to study the expression and function of extracellular matrix (ECM) receptors in distinct thymocyte subsets emerging during repopulation of the organ. The onset of regeneration was detected from day 2 to 3 postirradiation (P-Ir), when a remarkable increase in the absolute counts of CD3−CD25hiCD44+ and CD3−CD25in/hiCD44−cells occurred. Enhanced expression of L-selectin, 4, and 5 integrin chains (L-selhi 4hi5hi) was also exhibited by these cells. This pattern of expression was maintained until the CD4+CD8+ (DP) young stage was achieved. Afterward, there was a general downregulation of these ECM receptors in DP as well as in CD4+ or CD8+ single positive (SP) thymocytes (L-selin 4in5in). In some recently generated SP cells, 4 expression was downregulated before the 5 chain, and L-selectin was upregulated in half of more mature cells. The expression of the 6 integrin chain was downregulated only in maturing CD4+cells. Importantly, the increased expression of L-selectin and 4 and 5 chains in thymocytes was strongly correlated with their adhesiveness to thymic epithelial cells (TEC) in vitro. Blocking experiments with monoclonal antibody or peptides showed the following: (1) that the LDV rather than the REDV cell attachment motif in the IIIC segment of fibronectin is targeted by the 4 integrin during thymocyte/TEC adhesion; (2) that the RGD motif of the 120-kD fragment of fibronectin, a target for 5 integrin, has a secondary role in this adhesion; and (3) that the YIGSR cell attachment motif of the β1 chain of laminin/merosin recognized by a nonintegrin receptor is not used for thymocyte adherence. In conclusion, our results show that an upregulated set of receptors endows CD25+ precursors and cells up to the young DP stage with a high capability of interacting with thymic ECM components.

Upregulated Expression of Fibronectin Receptors Underlines the Adhesive Capability of Thymocytes to Thymic Epithelial Cells During the Early Stages of Differentiation: Lessons From Sublethally Irradiated Mice

A 250-cGy whole-body γ-radiation dose was used to induce thymus regression in mice, and to study the expression and function of extracellular matrix (ECM) receptors in distinct thymocyte subsets emerging during repopulation of the organ. The onset of regeneration was detected from day 2 to 3 postirradiation (P-Ir), when a remarkable increase in the absolute counts of CD3−CD25hiCD44+ and CD3−CD25in/hiCD44−cells occurred. Enhanced expression of L-selectin, 4, and 5 integrin chains (L-selhi 4hi5hi) was also exhibited by these cells. This pattern of expression was maintained until the CD4+CD8+ (DP) young stage was achieved. Afterward, there was a general downregulation of these ECM receptors in DP as well as in CD4+ or CD8+ single positive (SP) thymocytes (L-selin 4in5in). In some recently generated SP cells, 4 expression was downregulated before the 5 chain, and L-selectin was upregulated in half of more mature cells. The expression of the 6 integrin chain was downregulated only in maturing CD4+cells. Importantly, the increased expression of L-selectin and 4 and 5 chains in thymocytes was strongly correlated with their adhesiveness to thymic epithelial cells (TEC) in vitro. Blocking experiments with monoclonal antibody or peptides showed the following: (1) that the LDV rather than the REDV cell attachment motif in the IIIC segment of fibronectin is targeted by the 4 integrin during thymocyte/TEC adhesion; (2) that the RGD motif of the 120-kD fragment of fibronectin, a target for 5 integrin, has a secondary role in this adhesion; and (3) that the YIGSR cell attachment motif of the β1 chain of laminin/merosin recognized by a nonintegrin receptor is not used for thymocyte adherence. In conclusion, our results show that an upregulated set of receptors endows CD25+ precursors and cells up to the young DP stage with a high capability of interacting with thymic ECM components.

Early steps in T cell development are affected by aging

Involution of the thymus accompanies aging, a process in which the organ diminishes in size and cellularity and becomes disorganized. The rate of T cell emigration from the thymus is markedly reduced with age, and phenotypic analyses have identified alterations in the relative proportions of the major thymocyte subpopulations. The present studies made use of the capacity of the thymus to regenerate following irradiation from an intrathymic radio-resistant precursor population. By analysis of the differentiation of this "wave" of thymocytes, it was determined that aging most severely affects the earliest developmental transitions. While the overall rate of differentiation does not appear to be affected in older mice, fewer thymic progenitors initiate differentiation. The reduced expansion of late pre-T cells in the middle-aged is due to the smaller pool size of these cells.

Influence of a radioprotector WR-638 on the lymphoid compartment of the irradiated rat thymus: a flow cytometric analysis

The T cell composition of the thymus of X-ray irradiated (3.5 Gy) Wistar rat protected with WR-638 was analyzed by flow cytometry using monoclonal antibodies directed to the Thy 1.1, CD43, CD2, CD5, CD4, CD8 and class I and II MHC antigens. It was shown that this dose of X-rays caused cyclic changes in thymic cellularity manifested as: primary involution (until day 2), primary regeneration (from days 2 to 14), secondary involution (from days 14 to 21) and secondary regeneration (from days 21 to 30). WR-638 reduced the magnitude of thymocyte depletion in the primary involutive phase of the irradiated thymi, primarily as a result of protection of Thy 1.1high+ CD2low+ CD5high+ CD4+ CD8+ class I antigen high+ subpopulations of thymocytes. In the early regenerative phase, WR-638 accelerated the regeneration of CD4-CD8- and CD4-CD8+ thymocyte subsets, followed by subsequent increase of CD4+CD8+ and CD4+CD8- thymocyte subsets. Secondary involutive and regenerative phases in protected animals were characterized by higher absolute cell number of almost all thymocyte subpopulations in comparison with those in irradiated, non-protected animals.

Early thymic regeneration after irradiation

Whole body irradiation produces profound thymic atrophy. After sublethal irradiation, regeneration begins promptly and the earliest regeneration is from radioresistant intrathymic precursors. The progeny of these precursors expand rapidly and restore thymic cellularity to near normal within 2 weeks. We have used monoclonal antibodies specific for a variety of differentiation markers of the T lineage to analyze the early events in thymic regeneration. A three-color flow microfluorometric analysis revealed that the majority of the cells found early in the regenerative process have the phenotype of mature T cells. These include CD4-/CD8-; CD3hi as well as CD4+/CD8-; CD3hi and Cd4-/CD8+; CD3hi. The proportion of cells with mature phenotypes declines rapidly between day 6 and day 12. Not all of the early appearing cells have mature phenotypes. Among the early cells that do not express CD3 are both CD4 and CD8 single positive cells that express HSA and resemble the intrathymic precursors found in other systems. In these mice CD4 single positive predominate. There are other cells that are HSA positive but express low levels of CD4 and very low levels of Thy-1. These appear to include the earliest members of the T-lineage. In addition to relatively mature conventional T cells and early progenitors, the early developing population includes cells that express markers of the T-cell lineage including the T-cell receptor but do not express Thy-1. These Thy-1 negative T cells comprise a significant number of the earliest cells found after regeneration.

Differential sensitivity of DBA/2 and C57BL/6 mice to cyclophosphamide

Early and delayed toxicity of a single dose of 300 mg kg-1 cyclophosphamide (CP) was investigated in male DBA/2 and C57BL/6 mice. Early toxicity (up to 30 days after CP administration) resulted in 36.6% lethality in DBA/2 and no mortality in C57BL/6 mice. Delayed toxicity (after 30 days) occurred primarily in DBA/2 mice, resulting in a survival of 3% in DBA/2 and 93% in C57BL/6 mice on day 125 after CP administration. Early modifications brought about by CP in erythrocytes and leucocytes, and spleen and liver indexes (mg organ g-1 body wt.) were rather similar in DBA/2 and C57BL/6 strains. However, CP treatment caused a profound cell depletion in DBA/2 bone marrow owing, in part, to the fact that the number of cells in bone marrow of normal DBA/2 mice was much lower than that in normal C57BL/6 mice. Furthermore, the thymus index (mg organ g-1 body wt.) decreased sooner in DBA/2 than in C57BL/6 animals and no sign of recovery was evident in the former even after 10 days, whereas recovery in the latter started on day 5 after injection. Differential sensitivity of DBA/2 and C57BL/6 strains to CP could be due to differences in activation and/or inactivation of the drug, or to the increased effect of CP on DBA/2 bone marrow resulting in damage to pre-T cells that normally participate in thymus recovery.

Sequential appearance of T-cell receptor gamma delta- and alpha beta-bearing intestinal intra-epithelial lymphocytes in mice after irradiation

We have previously reported that T-cell receptor (TcR) gamma delta-bearing T cells precede TcR alpha beta-bearing T cells in appearance in the thymus after whole-body irradiation. In the present study, the kinetics of appearance of intestinal intra-epithelial lymphocytes (IEL) was examined in mice after whole-body irradiation with a lethal dose of 9.5 Gy or with a sublethal dose of 6 Gy. The number of CD3+ IEL decreased to the lowest value 4 days after irradiation with 9.5 Gy, and thereafter increased to half as many as the normal level by day 7. Thy-1+TcR alpha beta- IEL and Thy-TcR alpha beta- IEL recovered considerably by day 7 after the irradiation, whereas Thy-1+TcR alpha beta+ IEL and Thy-1+TcR alpha beta+ IEL hardly recovered at this stage. All mice died within 12 days after irradiation with a lethal dose of 9.5 Gy. On the other hand, when irradiation dose was decreased to 6 Gy, all mice survived beyond 40 days after irradiation. The number of CD3+ IEL recovered to the normal level by 10 days after irradiation with 6 Gy. Consistently with the results in mice irradiated with a lethal dose, the first cells to increase in IEL of mice irradiated with a sublethal dose were TcR gamma delta+ IEL expressing Thy-1 antigen. The number of Thy-1+TcR gamma delta+ IEL increased to approximately two-fold as many as that in normal mice by day 10, while TcR alpha beta+ IEL began to increase in number from day 20 after irradiation and recovered to the normal level by day 40 after irradiation. Thus, sequential appearance of TcR gamma delta+ and TcR alpha beta+ IEL was evident after irradiation, similar to that seen in the thymus after irradiation. The IEL on day 10 after a sublethal irradiation, which is composed mainly of Thy-1+TcR gamma delta+ IEL, exhibited a strong cytolytic activity against P815 in the presence of anti-CD3 mAb, suggesting that the early appearing Thy-1+TcR gamma delta+ IEL may play important roles in epithelial immunity at an early stage after irradiation.

"Radioresistant" intrathymic T cell precursors express T cell receptor C gamma 4- and C delta-specific gene messages

We have studied the expression and sequences of T cell receptor gamma and delta chain gene messages in intrathymic T cell precursors of mice irradiated with 600 rads. On day 7 after irradiation a high level of expression of gamma and delta chain messages was detected in thymocytes which were composed of a relatively high proportion of CD3+CD4-CD8- thymocytes. During further development of the precursors from day 7 to day 14 after irradiation, gamma and delta chain messages fell to low levels and alpha and beta mRNA levels increased. Nucleotide sequence analysis of 14 gamma and 10 delta chain complementary DNA (cDNA) in the thymocytes on day 7 revealed that there were 7 functional gamma chain transcripts composed of V gamma 2-J gamma 2-C gamma 2 or V gamma 1-J gamma 4-C gamma 4 gene segments, and only 1 functional delta chain transcript composed of the V delta M23-D delta 1-D delta 2-J delta 1-C delta gene segments. The repertoire of gamma chain and delta chain genes used in "radioresistant" intrathymic T cell precursors of adult mice appears to be limited.

Seeding of thymic microenvironments defined by distinct thymocyte-stromal cell interactions is developmentally controlled

Seeding of distinct intrathymic microenvironments defined by direct thymocyte-stromal cell interactions was correlated with T cell development in situ using radiation and nonradiation chimeras of Thy-1.1/1.2 congenic mice. The results identify associations of thymocytes with I-A- macrophages in the cortex as the earliest discernible cell-cell interactions during thymopoiesis. After a significant delay, this recognition stage is followed by concomitant interactions of T cells with I-A+ epithelial cells in the cortex and bone marrow-derived I-A+ dendritic cells in the medulla. All three types of T cell-stromal cell interactions occur after seeding of the intrathymic precursor cell subset and before development of mature medullary-type T cells. The seeding kinetics imply that recognition of cortical epithelial cells by thymocytes in situ represents a relatively late stage of cortical T cell development, whereas thymocyte-dendritic cell interactions denote a very early stage of T cell development in the medulla. The relative positioning of these cell-cell recognition stages during the course of T cell maturation pertains to a putative role of these microenvironments in selection and tolerization of the T cell repertoire.

Dynamics of early T cells: prothymocyte migration and proliferation in the adult mouse thymus

The object of this review has been to consider precursor cell migration into the normal adult thymus, using the mouse model. We have presented a series of experiments and discussed them in the context of other relevant experiments in the literature. The conclusions, qualified in the text, can be summarized as follows: There is a continual input of precursor cells into the normal undepleted adult thymus. The daily input of precursors under normal circumstances is very low (e.g. several per day). Once a precursor enters the pool of proliferating cells inside the thymus, its proliferation is limited to only several weeks. There is no permanent endogenous stem cell. There are a number of different precursor microenvironments in the thymus with different controls, since the kinetics of early (bone marrow-derived) and late (thymus-derived) precursors is quite different. All of these points require further analysis, and we have presented a minimal model as a basis for further experiment.

Enhancement of hemopoietic recovery by indomethacin after sublethal whole-body gamma irradiation

The effect of the non-steroid anti-inflammatory drug, indomethacin, a potent prostaglandin synthesis inhibitor, on the recovery of hemopoiesis was investigated in sublethally gamma irradiated mice. Treatment with indomethacin after irradiation was found to increase the granulocyte and lymphocyte counts in peripheral blood. Furthermore, an increased rate of the restitution of bone marrow cellularity and of the spleen weight was observed. Using the method of 125iodo-deoxyuridine uptake in the spleen, the ability of indomethacin to potentiate cell proliferation was demonstrated.

Thymic hormones in radiation-induced immunodeficiency. I. Induction of mature interleukin 1 responsive cell in the thymus by thymosin fraction 5

The restorative effect of thymosin fraction 5 (TF5) on the thymus of gamma-irradiated mice was examined. Four different mouse strains were used in this study since earlier work determined that the degree of response to TF5 is strain dependent. The responsiveness to comitogenic effect of interleukin 1 (IL-1) was used to measure the rate of recovery of immunocompetent cells in the thymus, since only more mature PNA-, Lyt-1+-2- medullary cells respond to this monokine. Contrary to several earlier reports that radioresistant cells repopulating the thymus within the first 10 days after irradiation are mature, corticosteroid resistant, immunocompetent cells, the thymic cells from irradiated mice in all strains used had greatly reduced responses to IL-1. Daily intraperitoneal injections of TF5 increased significantly the responses of thymic cells to IL-1 in 10- to 13-weeks-old C57Bl/KsJ, C57Bl/6, C3H/HeJ, and DBA/1 mice. Older mice, 5 months or more in age, of DBA/1 strain did not respond to treatment with TF5. However, C3H/HeJ mice of the same age were highly responsive. In conclusion, (1) cells repopulating the thymus within 12 days after irradiation contain lower than normal fraction of mature IL-1 responsive cells, (2) thymic hormones increase the rate of recovery of immunocompetent cells in the thymus, and (3) the effect of thymic hormones is strain and age dependent.

Restriction of regenerative capacity of the rat thymus after the application of cyclophosphamide

Restriction of thymic regenerative capacity, in contrast to the data in the literature, was found 30 days after a single (300 mg per kg) or fractionated doses (5 x 60 mg per kg) of Cyclophosphamide. Detailed cellular analysis revealed myelometaplastic changes in the thymic parenchyma, especially in the outer cortex and cortico-medullary zone. Plasmoid elements also appeared in the outer cortex. The capsular and septal connective tissue was markedly thickened. The thymic mass was decreased. The changes were more marked after the fractionated than after the single dose of Cyclophosphamide.

Influence of nandrolone decanoate on the repopulation of the thymus after total body irradiation of mice

It has been reported that nandrolone decanoate is helpful in overcoming the neutropenic phase following irradiation. In the present study the influence of nandrolone decanoate on the thymus' cellularity after total body irradiation was investigated. In comparison with a placebo-treated group, mice receiving nandrolone decanoate showed a similar pattern of thymus repopulation, but a significantly lower number of thymocytes over the whole period of treatment was found. Nonirradiated mice also had a significantly lower number of thymocytes when treated with nandrolone decanoate. In addition, the number of circulating leukocytes was also evaluated over a period of 1 month after total body irradiation. On 11 of the 21 days investigated, a significantly higher number of leukocytes was found in the nandrolone decanoate-treated group. We conclude that the action of nandrolone decanoate was not clearly distinct from that of testosterone regarding either granulopoiesis or thymic involution.

Studies on the termination of immunological tolerance in the mouse thymus cell population after irradiation

Immunological tolerance in the mouse thymus cell population induced by the intravenous injection of deaggregated bovine gamma globulin was terminated by whole body irradiation. After irradiation, the weight of the thymus recovered biphasically, and the termination of tolerance occurred as early as in the first phase. Both Thy-1 antigen expression and helper activity of the thymus cell population in irradiated mice recovered in parallel with the recovery of the thymus weight. Sensitivity of the regenerating thymus cell to the tolerogen was not different from that of the normal thymus cell. The first phase of thymus regeneration may be caused by the proliferation and differentiation of relatively radioresistant and tolerogen insensitive precursors residing in the thymus. Tolerogen and/or immunogen reactive thymus cells may originate from the precursor.

Modification of radiation-induced spleen weight changes in mice by 2-mercaptopropionylglycine

It was found that the MPG partially protects the spleen against weight loss due to radiation, and exaggerates the compensatory reaction in the tissue during recovery. It is also concluded that MPG protects the stem cells in the spleen, which helps to restore the peripheral blood by extramedullary erythropoiesis.

Differentiation of mouse T-lymphocytes. 19 Characterization of thymocyte precursors in bone marrow by one G velocity sedimentation

Bone marrow cells were transferred into 700 R X-irradiated syngeneic mice, and regeneration of the thymus was demonstrated by determining the total amount of DNA in the thymus. The amount of DNA in the regenerated thymus 9 days after X-irradiation paralleled the number of cells injected, the range being 0 to 10(6) cells. These results indicate that regeneration during this period reflects the proliferation of thymocyte precursors existing in bone marrow cells, therefore such can be used as an indicator to determine the number of thymocyte precursors. Thymocyte precursors were separated from spleen colony forming cells and soft agar colony forming cells by one G velocity sedimentation. Thymocyte precursors (more than 4.3 mm/hour sedimentation rate) sedimented faster than did spleen colony forming cells (peak at about 3.0 mm/hour) and Thy-1 antigen was already evidenced on the cell surface as demonstrated by complement dependent cytotoxicity. Soft agar colony forming cells also sedimented faster, suggesting that cells destined to become one type of cell are larger than multi-potential stem cells.

Cellular repopulation and recovery of the phytogemagglutinine (PHA) responding cell pool in the thymus of sublethally irradiated mice

Cellular repopulation and recovery of PHA reactivity was investigated in the thymus of mice exposed to 400 R either on the whole body or with one leg protected. In the whole body irradiated cases both the cellular repopulation and the restoration of PHA reactivity in the thymus showed a cyclic pattern: an initial depression and a subsequent first recovery being followed by a secondary cellular depletion and decrease of reactivity before another recovery phase started. The phase of the secondary decrease lasted fo a similar period for both the cell number and reactivity, but was shifted in time and the reduction in reactivity began some 12 days later than that in the cell number. In the leg-shielded animals, the cellular repopulation progressed without cyclic changes, but the recovery of PHA reactivity maintained a cyclic pattern. However, due to a smaller secondary decrease, it was less pronounced than in the whole body irradiated cases. During the aging of mice from 50 to 90 day old, the cell number in the involuting thymus decreased to half of the original. The decrease in the cell population was associated with an about four-fold increase of PHA reactivity. The results were discussed in regard to the origin of the immunoreactive thymic cells. Arguments were presented in favour of the hypothesis that both the reactive and nonreactive cells originate from precursors in common, but require different lengths of time for maturation.

The role of bone marrow in different phases of the cellular repopulation of irradiated mouse thymus

In one series of experiments, the cellular repopulation of the thymus was investigated in mice exposed first to 200 R on the whole body and, after various intervals, to 700 R with one leg protected or unprotected during the exposure. When no protection was made, the mice were transplanted with syngeneic bone marrow cells in a defined number immediately after irradiation. Repopulation was fastest when the interval between exposures was 5 days, and most delayed when it was 14 days; with a 30 day interval the speed of repopulation was intermediate, and resembled that of a control group exposed to only the second dose. In another experimental series, thymus repopulation was studied after exposure of mice first to 200 R with one leg protected or unprotected and, after an interval of 5, 14 or 30 days, to a second dose of 700 R on the whole body. In all cases, syngeneic bone marrow cells were grafted intravenously after the second irradiation. The thymus repopulation was enhanced by protection of the leg when 14 day interval separated the exposures. In the other cases, no enhancement was noted. The findings were interpreted to indicate that the cellular composition of the thymus and, in particular, the frequency of the proliferating stem cells at the time of the exposure determines thymic repopulation for about two weeks after irradiation. After this period, repopulation is due to new precursors from the bone marrow which seeded the thymus.

Experimental canine distemper virus-induced lymphoid depletion

Thirty-four gnotobiotic dogs were used to study the effects of R252 canine distemper virus (CDV) infection on the lymphoid system. Twenty-one dogs were inoculated parenterally, and 6 dogs were infected by contact exposure to inoculated littermates. Seven littermate dogs were maintained as uninfected controls. The means of the absolute lymphocyte counts of dogs infected with R252 CDV were significantly decreased for 7 weeks after infection. The lymphocyte counts failed to return to normal in 7 dogs which became moribund with neurologic signs and were sacrificed 27 to 47 days after infection (group I). The 19 dogs which survived the 12-week observation period had normal lymphocyte counts by 8 weeks after infection. Five of these dogs (group II) were subsequently found to have demyelination of the central nervous system (CNS), while no CNS lesions were demonstrated in the remaining 14 infected dogs (group III). The thymuses of dogs of group I were atrophic and their lymph nodes were depleted in lymphoid cells. In addition, the number of Hassall's corpuscles per thymic lobule were decreased in dogs of group I. Viral nucleoprotein was demonstrated in thymus and lymph node cells by electron microscopy. No lymphoid lesions were found in the dogs of group II. Some of the thymuses of dogs of group III were significantly larger than those of uninfected controls, and they contained increased numbers of Hassall's corpuscles per thymic lobule. Further, germinal centers were found in two thymuses of dogs of group III.

Proliferation of donor marrow and thymus cells in the myeloid and lymphoid organs of irradiated syngeneic host mice

CBA/HT(6)T(6) bone marrow cells (1 x 10(7)) or CBA/H bone marrow cells (1 x 10(7)) plus CBA/HT(6)T(6) thymus cells (5 x 10(7)) were injected intravenously into lethally (800 R) irradiated CBA/H mice. Chromosome analyses of dividing cells in the host lymphoid and myeloid organs were performed at intervals after irradiation. Donor marrow cells settled and proliferated in the host bone marrow, spleen, and lymph nodes soon after injection, but donor marrow cells did not proliferate in the host thymus until day 10; then host-type cells were quickly replaced by donor-type cells in the thymus by day 20. On the other hand, donor thymus cells settled and proliferated in the host thymus and lymph nodes soon after injection but they gradually disappeared from these organs. On day 20, a few donor-type dividing cells (of thymus origin) were found in the host lymphoid and myeloid organs.