Occurrence of estrogen binding components in mouse thymus: A study using fluorescence technique

The occurrence of estrogen binding components was studied in frozen sections from different organs from adult and neonatal female mice. The sections were incubated with a fluorescein-BSA-estradiol-17β complex and studied in a microscope equipped for epi-illumination. A pronounced fluorescence was seen in the reticuloepithelial cells of the thymus while the thymic lymphocytes were negative. Treatment of ovariectomized females with estradiol and progesterone in combination strongly increased fluorescence. The estrogen thymolytic effect is supposed to be due to an interaction between reticuloepithelial cells and thymocytes.     

Bacterial translocation from the gastrointestinal tract of athymic (nu/nu) mice.

The immune system may be one host defense mechanism preventing viable indigenous bacteria from translocating from the mouse gastrointestinal lumen to the mesenteric lymph nodes, spleen, liver, or kidney. The role of T-cell-dependent immunity in preventing bacteria from translocating from the gastrointestinal tract was tested with congenitally athymic nude (nu/nu) mice, heterozygous (nu/+) mice, and thymus-grafted nude (nu/nu) mice. Viable bacteria were cultured from 50% of the mesenteric lymph nodes, spleens, livers, and kidneys of athymic (nu/nu) mice, whereas heterozygous (nu/+) mice exhibited viable bacteria in only 5.2% of these organs. Both aerobic and strictly anaerobic bacteria were cultured from these organs with Escherichia coli and Lactobacillus predominanting. Grafting thymuses to the athymic (nu/nu) mice restored their immunological responses to sheep erythrocyte antigens. The incidence of bacterial translocation from the gastrointestinal tract was reduced from 50% in the athymic (nu/nu) mice to 7.8% in the thymus-grafted (nu/nu) mice. Thus, T-cell-dependent immunity restored by thymic grafts inhibited the translocation of certain indigenous bacteria from the gastrointestinal tract to the spleen, liver, and kidney in nu/nu mice.     

Effect of interferon-gamma on the abnormality of T cell activation in NZB mice

It is known that New Zealand black (NZB) mice have a defect in the autoantigen recognition mechanism by T cells. The present study was carried out to examine whether the defect could be improved by normalizing the self Ia molecule expression on thymic reticuloepithelial cells (TRC) by using interferon-gamma (IFN-gamma). Syngeneic mixed lymphocyte reaction (SMLR) was used as an indicator of the mechanism of autoantigen recognition. In the reaction, T cells separated from the thymus were used as responders, and machrophages separated from the spleen were used as stimulators. When the responders were cultured with TRC, in which self Ia molecule expressions had been enhanced by IFN-gamma, for activation, the low level of SMLR in NZB mice was elevated close to the normal level. It was suggested, therefore, that IFN-gamma could normalize the defective autoantigen expression on TRC in NZB mice and improve the abnormal T cell activation against autoantigens.     

Restoration of the T lymphoid system of nude mice: different regeneration of neonatal and adult thymuses.

When a neonatal thymus and an adult (reticuloepithelial) thymus from normal mice are simultaneously grafted to individual adult nude mice, the host-derived T cell precursors regenerate the neonatal thymus more efficiently than the adult (reticuloepithelial) thymus. Thus, this is a direct cause for the poor restoration of the T lymphoid system of nude mice by adult and/or reticuloepithelial thymus grafts.     

Inhibition of T-cell differentiation by an antibody to a soluble thymic factor.

An antiserum to chicken soluble thymic factor (STF) was prepared in rabbits and absorbed with chicken thymocytes until it reacted only with thymic reticuloepithelial cells. When injected into 6-day-old embryonated eggs, this antiserum caused a severe depletion of small lymphocytes in the cortical region of the thymus which was filled with large undifferentiated cells. In addition, reticuloepithelial cells displayed a marked disruption of cytoplasmic elements. The percentage of thymic lymphocytes bearing the T antigen was greatly reduced. It is believed that a factor responsible for differentiation of stem cells into T cells has been inhibited.     

Modification of the T cell responsiveness to synthetic peptides by substituting amino acids on agretopes.

T cell receptors, major histocompatibility complex molecules, and antigens constitute tri-molecular complexes which induce T cell activation. T cells in I-Ab mice generate proliferative responses to a synthetic peptide composed of residues 43-58 of pigeon cytochrome c (p43-58) and its analogs with substitution at position 50 (50A, 50V, 50L, 50N, 50Q, 50K, and 50M). However, none of these peptides stimulate T cells in I-Ak mice. We substituted two residues at positions 46 and 54 of p43-58(50D), 50V, 50L, 50E, and 50K with two amino acids on agretopes of the I-Ak binding HEL52-61 peptide and immunized I-Ak mice with these newly synthesized peptides: 46D50D54R, 46D50V54R, 46D50L54R, 46D50E54R, and 46D50K54R. Apart from 46D50D54R, these peptides elicited T cell responses in I-Ak mice in an immunogen-specific manner, but did not stimulate those in I-Ab mice. Further, 46D50V54R inhibited competitively the responses of I-Ak restricted T cell hybridomas specific for 46D50E54R. These results demonstrate that the residues at positions 46 and 54 on the peptides act as an agretope and the residue at position 50 acts as an epitope in I-Ak mice, as in I-Ab mice, and provide the possibility of opening up a new method to prepare peptide antigens which induce T cell responses in each murine strain by introducing appropriate amino acids on agretopes.     

Distribution of Ia-Antigen-like Molecules on Non-lymphoid Tissues

Ia-antigen-like molecules are expressed on cells within several different non-lymphoid tissues of the guinea-pig. In indirect immunofluorescence analyses anti-Ia-antigen antibodies stained epithelial cells lining the intestinal tract, the bile ducts, the respiratory tract and the urinary tract. The rabbit antibodies against Ia antigens also stained the cells of the parotid and the submandibular glands. Evidence was also obtained suggesting that the reticuloepithelial cells of the thymus, like the Kupffer cells of the liver, express Ia-antigen-like molecules. In several cases indirect immunoprecipitation analyses and SDS-polyacrylamide gel electrophoresis confirmed the immunofluorescence studies inasmuch as Ia-antigen-like subunits with apparent molecular weights of 26,000 and 34,000 could be isolated from the non-lymphoid organs.     

Immunohistology of T cell differentiation in the thymus of H-Y-specific T cell receptor alpha/beta transgenic mice

We examined the immunohistological aspects of the H-Y specific T cell receptor (TcR) alpha/beta transgene expression in the thymus of male and female transgenic (Tg) mice. Virtually all thymocytes expressed the beta transgene in both the male and female thymus. Expression of accessory molecules (co-receptors) in Tg mice deviated from control mice. In the male Tg thymus, CD8 expression was either low or absent on both cortical and medullary thymocytes. In contrast, in the thymus of female mice, CD8+ cells were found both in the cortex and in the medulla. The majority of medullary thymocytes was bright CD8+. This is in clear contrast to the CD8 distribution in control B6 mice, where only a few percent of medullary cells are CD8+. Similarly, the proportion of cells expressing CD4 antigens was reduced in the cortex and medulla of the thymus from male Tg mice, as compared to the thymus of female Tg mice and B6 control mice. Comparative analysis of the stromal cell types of the thymic microenvironments in the three groups of mice revealed that the cortical thymic microenvironment of male Tg mice differed, compared to that of female Tg mice. In particular, the deep cortex showed a closely packed meshwork of epithelial reticular cells. Moreover, H-2Db molecules (which are the restricting elements for the Tg TcR alpha/beta) were abnormally expressed in the thymic cortex of male mice. The cortical microenvironment in female mice, on the other hand, appeared normal. Together, the data indicate that TcR alpha/beta transgene expression in male mice leads to an aberrant co-receptor expression in both cortical and medullary lymphoid cells as well as an abnormal composition of the cortical microenvironment. Both phenomena may be the consequence of "negative selection" of developing H-Y-specific T cells, as it occurs only in the male Tg thymus. The absence of the H-Y antigen, but presence of the restricting element H-2Db in the thymic cortex of female mice, leads to accumulation of CD8+ in the medulla, a phenomenon interpreted as "positive selection".     

The ontogeny of the interleukin 2 receptor expression and of the interleukin 2 responsiveness in the rat thymus

The ontogeny of the interleukin 2 receptor (IL 2 R) expression and of the IL 2 responsiveness has been investigated in the rat thymus. In tissue sections, IL 2 R-bearing cells were first detected at day 16 of gestation using the anti-IL 2 R mAb ART-18. In contrast to mice, IL 2 R-bearing cells of the rat are localized mainly in the thymic medulla from the first day of the corticomedullary compartimentalization, and not in the cortex. They are found in regions with a high expression of MHC class II antigens. The proportion of IL 2 R-bearing cells increases during gestation, reaches a peak at the first day after birth and declines to the adult level in the following weeks. The appearance of medullary localized IL 2 R-bearing cells is paralleled with the capacity of the thymocytes to proliferate in response to IL 2 without any additional stimuli.     

Major histocompatibility complex antigens in v-Ki-ras transformed cells: the different antigens are expressed and induced by interferons independently of one another and of the anti-viral state.

A series of cell lines, which differed in their expression of class I (H-2K) or II (I-A) major histocompatibility complex (MHC) antigens, was derived from a line of C3H/He (H-2k) mouse embryo fibroblasts transformed by the v-Ki-ras oncogene. These were prepared by fluorescence-activated cell sorting of cells stained for these antigens either without interferon (IFN) treatment or after induction of antigen expression by either IFN-alpha beta or IFN-gamma, selecting for low or high staining. Cells selected for low (undetectable) constitutive H-2Kk expression were still strongly inducible by either IFN; cells selected for high constitutive expression were induced by IFN to express still higher levels. In all cell lines induction of H-2Kk with one IFN type was paralleled by induction with the other. Expression of H-2Kk appeared largely independent of H-2Dk; in lines which were selected for low H-2Kk expression (constitutive or induced), H-2Dk expression was not much reduced, and lines selected for high H-2Kk expression showed only modest augmentation of H-2Dk. Varying inducibility of I-Ak by IFN-gamma was not closely paralleled by H-2K inducibility by either IFN-alpha beta or -gamma, with again only weak correlation of high and low expression of H2-Kk and I-Ak. On the other hand, expression of I-Ak seemed to be correlated with I-Ek. None of these variable effects could be attributed to differing sensitivity to IFN-alpha beta or -gamma since all the lines showed about the same sensitivity to the anti-viral effects of IFN.     

Early steps of a thymic tumor in SV40 transgenic mice: hyperplasia of medullary epithelial cells and increased mature thymocyte numbers disturb thymic export

Bone marrow progenitors migrate to the thymus, where they proliferate and differentiate into immunologically competent T cells. In this report we show that mice transgenic for SV40 T and t antigens under the control of the L-pyruvate kinase promoter develop, in a first step, thymic hyperplasia of both thymocytes and epithelial cells. Morphological studies (histology, immunohistolabeling and electron microscopy) revealed modifications of the thymic microenvironment and gradual expansion of medullary epithelial cells in 1 month-old mice, taking over the cortical region. Then, a thymic carcinoma develops. Two-color labeling of frozen sections identified the transgene in medullary epithelial cells. Flow cytometry analysis demonstrated a marked increase in mature CD4+ and CD8+ thymocytes in adult mice (39 +/- 10 x 10(6) in transgenic mice and 12 +/- 5 x 10(6) in age-matched controls). Furthermore, thymocyte export was disturbed.     

Development of immune competence.

Present evidence indicates that the precursors of B- and T-cells can be found in the extra-embryonic tissues four days after implantation (day 5) and that by the following day (day 10) certain of the B-cell precursors have differentiated to the stage of the antigen-binding cell. Cells able to secrete antibody are not detected, however, until the later stages of pregnancy. B-cell differentiation has been shown to advance in a stepwise manner through several compartments, and the early stages of maturation are independent of thymic or T-cell regulation. The thymic rudiment appears by the 12th day of pregnancy and the reticuloepithelial tissue is quickly colonized by T-stem cells which migrate from the fetal liver. Within 4 days these cells respond to PHA, recognize and respond to allogeneic antigens, and begin to seed to the peripheral lymphoid tissues. During the latter stages of pregnancy and until about the sixth week after birth T-cell mediated suppressor activity predominates. T-cell killer function can be detected in the neonatal thymus shortly after birth, but this activity increases slowly in the peripheral lymphoid tissues. T-cell helper activity increases slowly after birth as suppressor activity declines. The adult levels of helper-suppressor function are approached about six to eight weeks after birth.     

The effect of peripheral immunization with Mls-1a on the emigration of antigen-specific cells from the thymus.

Mature T cells found in the lymph nodes and spleen have the capacity to become activated and to proliferate in response to foreign antigens. The response of the thymus to such immunization is less well understood. We have examined one aspect of the thymic response by determining the effect of peripheral immunization upon cell emigration from the thymus. BALB/c (Mls-1b) mice were injected with spleen cells from DBA/2 (Mls-1a) mice, and V beta 6+ (Mls-1a-reactive) thymic emigrants were identified 3-30 days after immunization. Neither the rate of total cell migration from the thymus nor the proportion of V beta 6+ cells was altered, even though the immunizing spleen cells elicited an immune response in the draining (parathymic) lymph nodes. The same immunogen caused deletion of V beta 6+ cells in both the thymus and lymph nodes after intraperitoneal injection into the neonate. The inability of DBA/2 splenocytes to modify the development of adult thymocytes after intrathymic injection of the cells precluded the lack of entry into the thymus as the reason for the lack of any observed effect in the adult. Our results, therefore, indicate that the development of adult thymocytes is not modified by immunization, and suggest that the differing thymic response of mice injected as adults or neonates is related to changes in the intrathymic antigen presentation capacity associated with age.     

Vitamin D receptor is required to control gastrointestinal immunity in IL-10 knockout mice

The vitamin D receptor (VDR) is a nuclear receptor expressed in a number of different cells of the immune system. This study was performed to determine the effect of VDR deficiency on immune function and inflammation of the gastrointestinal tract in a model of inflammatory bowel disease, namely interleukin-10 (IL-10) knockout mice. IL-10 knockout mice were generated which either could or could not respond to vitamin D (double IL-10/VDR knockout; DKO). The distribution and function of lymphocytes in both the primary and secondary lymphoid organs were compared and determined as a function of the severity of intestinal inflammation. DKO mice had normal thymic development and peripheral T-cell numbers at 3 weeks of age, but a week after intestinal disease was detected the thymus was dysplastic with a reduction in cellularity. The atrophy was coupled with increased apoptosis. The spleen weight of DKO mice increased as a result of the accumulation of red blood cells; however, there was a 50% reduction in the numbers of T and B cells. Conversely, the mesenteric lymph nodes were enlarged and contained increased numbers of lymphocytes. The T cells from DKO mice were of a memory phenotype and were hyporesponsive to T-cell receptor stimulation. Colitis in the DKO mice was associated with local and high expression of IL-2, interferon-gamma, IL-1beta, tumour necrosis factor-alpha and IL-12. The primary and secondary lymphoid organs in DKO mice are profoundly altered as a consequence of the fulminating inflammation in the gastrointestinal tract. VDR expression is required for the T cells and other immune cells to control inflammation in the IL-10 KO mice.     

Differentiation and maturation of macrophages into interdigitating cells and their multicellular complex formation in the fetal and postnatal rat thymus

Three mouse anti-rat macrophage monoclonal antibodies, TRPM-1, TRPM-2, and TRPM-3, as well as anti-rat Ia monoclonal antibody, were used to study the emergence, differentiation, and maturation of macrophages in the fetal and postnatal rat thymus immunohistochemically and immunoelectron microscopically. At 14 days of gestation, primitive/fetal macrophages entered the thymic primordium and showed Ia expression, where afterwards the epithelial cells also expressed Ia antigens prominently at 15 days of gestation. After 16 days of gestation, differentiation of a subpopulation of primitive/fetal macrophages into interdigitating cells (IDCs) is suggested. From 19 days of gestation, TRPM-1-positive dendritic cells including IDCs started forming multicellular complexes with thymocytes and the epithelial cells also formed similar complexes with thymocytes. One day after birth, TRPM-1 positive IDC-thymocyte complexes distributed throughout the thymic medulla. The number of TRPM-1- and Ia-positive IDCs increased by day, and Langerhans cells (LCs) appeared in the thymic medulla within a few days after birth. By two weeks after birth, the distribution pattern of Ia- and TRPM-1-positive cells became similar to that of adult rats. In ontogeny, intimate cell membrane appositions were frequently observed between thymocytes and Ia-positive epithelial cells or IDCs in the thymic multicellular complexes. These complexes were discriminated into two types; epithelial cell-thymocyte complexes and IDC- or LC-thymocyte ones. In vitro, two types of the thymic nurse cells (TNCs) were identified: epithelial cells and IDCs or LCs. Besides epithelial cells, IDCs or macrophages formed rosettes with thymocytes. These TNCs and rosettes in vitro seem to correspond to the thymic multicellular complexes in vivo.     

Studies on the thymus in Chagas' disease. I. Changes in the thymic microenvironment in mice acutely infected with Trypanosoma cruzi

Previous observations demonstrated severe thymocyte depletion in mice undergoing acute Chagas' disease. These data led us to investigate the status of the thymic microenvironment in these animals. Young adult C57BL/6 and C3H/HeJ mice were infected i.p. with 10(5) blood-derived trypomastigote forms of Trypanosoma cruzi (CL strain) and killed 7-14 days after infection. Sera were then analyzed for thymic hormone (thymulin) levels, and frozen thymus sections were studied by immunohistochemistry for the expression of functional antigens (thymulin and Ia), the distribution of distinct thymic epithelial cell subsets and extracellular matrix components. Infected mice exhibited a transient decrease in thymulin production and those with severe thymic atrophy showed a denser Ia-bearing cellular network. In addition, an abnormal localization of the TR5 and CK18 antigens restricted to the medullary and cortical TEC subsets, respectively, was observed. Furthermore, an increase in the basement membrane proteins was detected within thymic lobules. We suggest that the thymic microenvironment is also affected during T. cruzi infection, extending the concept that the thymus should be regarded as a target in Chagas' disease.     

Rat thymic epithelium positively selects mouse T cells with specificity for rat MHC class II antigens but fails to induce detectable tolerance in the mouse T cells to the rat MHC antigens.

BALB/c (H-2d) nude mice were grafted with allogeneic AKR/J (H-2k) or xenogeneic (ACI-N rat, RT1av1) fetal thymuses which were depleted of hemopoietic cells by incubating with 2'-deoxyguanosine (2'dGuo) in vitro prior to grafting. The nylon-wool-passed LN T cells from nude mice grafted with 2'dGuo-treated AKR/J thymus showed a poor proliferative response to B10BR (H-2k) stimulator cells, confirming that mouse thymic epithelium has the capacity to induce tolerance against the mouse MHC antigens on the thymic epithelium. On the other hand, the nylon-wool-passed LN T cells from nude mice grafted with untreated or 2'dGuo-treated ACI/N rat thymus showed significant proliferative responses to ACI/N, which can be blocked by anti-rat MHC class II mAb, whereas the nylon-wool-passed LN T cells from nude mice grafted with syngeneic thymus hardly responded to the xenogeneic stimulator cells. These results suggest that rat thymic stromal cells including thymic epithelium can not induce detectable tolerance in mouse T cells to rat MHC antigens; but rat thymic epithelium may positively select mouse T cells with specificity for rat MHC class II antigens, resulting in a mouse T cell repertoire with strong xeno-reactivity.     

T cell clones are killed by a thymic stromal cell monolayer following stimulation of T cell receptor with antigen and/or H-2 molecules on the monolayer

A thymic stromal cell clone, MRL104.8a, expressed class I and class II H-2k antigens after exposure to gamma-interferon (gamma-IFN) and produced thymic stroma-derived T cell growth factor (TSTGF) irrespective of gamma-IFN exposure. Culturing the keyhole limpet hemocyanin (KLH)-specific, I-Ek-restricted 9-16 helper T cell (Th) clone on an Ia (I-Ak and I-Ek)-expressing MRL 104.8a monolayer induced potent proliferation of the 9-16 cells by virtue of the TSTGF produced by the monolayer. In contrast, the addition of KLH to cultures resulted in lethal growth inhibition of the 9-16 Th clone. Such a phenomenon was also observed for various Th as well as cytotoxic T lymphocyte (CTL) clones, and the following were revealed: (i) the growth of the ovalbumin (OVA)-or bovine thyroglobulin (BTg)-specific Th clone on the la-expressing MRL 104.8a monolayer was also inhibited by addition of the relevant antigen. The fact that these Th clones required antigen-presenting cells (APC) capable of processing antigen for the recognition of the respective target antigen suggested the potential of MRL 104.8a cells for antigen-processing; (ii) the lethal growth inhibition of KLH-specific, I-Ak (23-1-8)- or I-Ek (9-16)-restricted Th clone was prevented selectively by anti-I-Ak or anti-I-Ek antibody respectively; (iii) the I-Ek-alloreactive Th clone (2-13) was supported for its growth on a gamma-IFN-unexposed MRL 104.8a monolayer, whereas this clone was killed on an I-Ek-expressing monolayer; and (iv) when I-Ak-reactive CTL clones were cultured on an Ia- or Ia+ monolayer, CTL clones failed to exhibit cytotoxic effect on either the Ia- or the Ia+ monolayer, but were conversely killed by the Ia+ monolayer. Its killing was also prevented by an antibody which inhibits the recognition of Ia antigen on the monolayer by CTL clones.(ABSTRACT TRUNCATED AT 250 WORDS)     

Intrathymic Tolerance Induction: Determination of Tolerance to Class II Major Histocompatibility Complex Antigens in Maturing T Lymphocytes by a Bone Marrow-Derived Non-Lymphoid Thymus Cell

Two new experimental approaches were established to analyse the influence of the thymus on tolerance induction to major histocompatibility complex (MHC) antigens: The aim of the first experiment was to perform successful transplantation of adult allogeneic thymus tissue into nude mice, an attempt that has been unsuccessful in the past. Tolerance for the MHC genotype of a prospective thymus graft recipient (A) was induced in mice of strain B by injection of (A X B) splenocytes during the neonatal period. Adult thymic tissue obtained from these allogeneic donors (B) were grafted into the nude mice of strain A. The allogeneic thymus was accepted by the nude mice and immunoreconstitution was achieved. Subsequently the recipients developed tolerance to the MHC antigens of the allogeneic thymus donor as proved by mixed lymphocyte cultures and the acceptance of skin grafts. The second experiment was designed to determine which Ia-positive thymic compartment participates in conferring tolerance to MHC antigens in maturing T lymphocytes. Chimaeric thymus grafts were created by transplantation of neonatal thymus (A) into allogeneic nude mice (B) for a period of 8 weeks. The graft was populated with host bone marrow-derived Ia antigen-positive cells. The chimaeric thymuses consisting of type A epithelium but populated with both type A and B lymphocytes and non-lymphoid cells (i.e. Ia-positive macrophages and dendritic cells), were newly transplanted into nude mice of strain A. The engraftment lead to immunological reconstitution and the nude mice acquired tolerance to the MHC antigens expressed by the allogeneic Ia-positive cells populating the chimaeric graft. Irradiation of the chimaeric thymus prior to transplantation allowed transplantation of chimaeric thymus devoid of living thymocytes but still populated with functionally intact Ia-positive non-lymphoid cells. Transplantation of irradiated chimaeric thymuses resulted in immunoreconstitution and induced exactly the same allotolerance pattern as described above. The results demonstrate that not thymus epithelial cells but a bone-marrow-derived non-lymphoid thymus cell, most likely the Ia-antigen-positive thymic macrophage of dendritic cell, is responsible for the induction of tolerance to MHC antigens in developing T lymphocytes.     

Cytogenetics of murine thymic lymphomas induced by N-methyl-N-nitrosourea: Difference in incidence of trisomy 15 in thymic lymphomas induced in neonatal and adult mice

Chromosome complements of murine thymic lymphomas induced by an alkylating agent N-methyl-N-nitrosourea (MNUA) were analyzed microscopically and karyotypically using the Q-banding technique. The chemical carcinogen was injected intraperitoneally into either neonatal or 7-week-old CFW/D mice. In addition, thymic lymphomas induced in 7-week-old AKR mice and thymic lymphomas developed spontaneously in this strain were also examined. All six lymphomas induced in neonatal CFW/D had hyperdiploid cell lines that accounted for 90% of the cells analyzed. Chromosome analysis of lymphomas induced in adult CFW/D mice showed that only four out of nine lymphomas had predominantly hyperdiploid cell lines. The remaining five lymphomas had diploid modal chromosome number although they also carried a variant line characterized by 41 chromosomes. All eight lymphomas induced in adult AKR mice and six out of seven spontaneous AKR lymphomas showed predominantly diploid modal line. The remaining spontaneous lymphoma had a hyperdiploid stem line of 41 chromosomes. Microscopic and karyotypic analysis further identified trisomy 15 as the regular chromosome abnormality in the hyperdiploid cells in lymphomas of each group, whereas cells with diploid chromosome number had no detectable chromosome abnormality. Additional trisomies were also found, but their appearance was restricted to individual tumors. Thus, the incidence of trisomy 15 in lymphomas induced by MNUA in adult CFW/D and AKR mice, as well as in the spontaneous AKR lymphomas, is significantly lower than that in lymphomas induced in neonatal mice by the same carcinogen.