Early block in maturation is associated with thymic involution in mammary tumor-bearing mice

We previously reported that mice implanted with mammary tumors show a progressive thymic involution that parallels the growth of the tumor. The involution is associated with a severe depletion of CD4+8+ thymocytes. We have investigated three possible mechanisms leading to this thymic atrophy: 1) increased apoptosis, 2) decreased proliferation, and 3) disruption of normal thymic maturation. The levels of thymic apoptosis were determined by propidium iodide and annexin V staining. A statistically significant, but minor, increase in thymic apoptosis in tumor-bearing mice was detected with propidium iodide and annexin V staining. The levels of proliferation were assessed by in vivo labeling with 5'-bromo-2'-deoxyuridine (BrdU). The percentages of total thymocytes labeled 1 day following BrdU injection were similar in control and tumor-bearing mice. Moreover, the percentages of CD4-8- thymocytes that incorporated BrdU during a short term pulse (5 h) of BrdU were similar. Lastly, thymic maturation was evaluated by examining CD44 and CD25 expression among CD4-8- thymocytes. The percentage of CD44+ cells increased, while the percentage of CD25+ cells decreased among CD4-8- thymocytes from tumor-bearing vs control animals. Together, these findings suggest that the thymic hypocellularity seen in mammary tumor bearers is not due to a decreased level of proliferation, but, rather, to an arrest at an early stage of thymic differentiation along with a moderate increase in apoptosis.     

Expression of interleukin 2 and the interleukin 2 receptor in aging rats

Lymphocytes of aged animals exhibit a marked decrease in proliferative capacity in response to mitogen stimulation when compared to those of younger animals. In humans and mice the decreased proliferation is due at least in part (i) to the inability of lymphocytes to synthesize sufficient interleukin 2 (IL-2) and (ii) to decreased expression of IL-2 receptors (IL-2R) on the surface of aged lymphocytes. We compared proliferative abilities, IL-2 production, and IL-2R expression in splenocyte cultures of 4- to 5- and 22- to 24-month-old Fischer 344 rats stimulated with either concanavalin A (Con A) or A23187 and phorbol myristate acetate (PMA). Proliferation was significantly decreased in aged lymphocytes (30-50%) with both treatment protocols. However, unlike mice and humans we observed no difference in IL-2 activity, IL-2 mRNA levels, or IL-2R cell surface expression of lymphocytes from young and aged rats stimulated with either Con A or A23187 and PMA. These results indicate that factors other than decreased expression of IL-2 and IL-2R are responsible for the diminished proliferative capacity of aged rat lymphocytes following mitogen stimulation.     

IL-12 inhibits thymic involution by enhancing IL-7- and IL-2-induced thymocyte proliferation

IL-12 has been reported to affect thymic T cell selection, but the role of IL-12 in thymic involution has not been studied. We found that in vivo, IL-12b knockout (IL-12b(-/-)) mice exhibited accelerated thymic involution compared with wild-type (WT) B6 mice. This is characterized by an increase in thymocytes with the early development stage phenotype of CD25(-)CD44(+)CD4(-)CD8(-) in aged IL-12b(-/-) mice. Histologically, there were accelerated degeneration of thymic extracellular matrix and blood vessels, a significantly decreased thymic cortex/medulla ratio, and increased apoptotic cells in aged IL-12b(-/-) mice compared with WT mice. There was, however, no apparent defect in thymic structure and thymocyte development in young IL-12(-/-) mice. These results suggest the importance of IL-12 in maintaining thymic integrity and function during the aging process. Surprisingly, in WT B6 mice, there was no age-related decrease in the levels of IL-12 produced from thymic dendritic cells. Stimulation of thymocytes with IL-12 alone also did not enhance the thymocyte proliferative response in vitro. IL-12, however, provided a strong synergistic effect to augment the IL-7 or IL-2 induced thymocyte proliferative response, especially in aged WT and IL-12b(-/-) mice. Our data strongly support the role of IL-12 as an enhancement cytokine, which acts through its interactions with other cytokines to maintain thymic T cell function and development during aging.     

Defective thymic T cell activation by concanavalin A and anti-CD3 in autoimmune nonobese diabetic mice. Evidence for thymic T cell anergy that correlates with the onset of insulitis.

In nonobese diabetic (NOD) mice, T cells play a major role in mediating autoimmunity against pancreatic islet beta-cells. We and others previously reported that age-related alterations in the thymic and peripheral T cell repertoire and function occur in prediabetic NOD mice. To study the mechanism responsible for these T cell alterations, we examined whether a defect exists in the thymus of NOD mice at the level of TCR-mediated signaling after activation by Con A and anti-CD3. We found that thymocytes from NOD mice respond weakly to Con A- and anti-CD3-induced proliferation, compared with thymocytes from control BALB/c, BALB.B, (BALB.B x BALB.K)F1, C57BL/6, and nonobese non-diabetic mice. This defect correlates with the onset of insulitis, because it can be detected at 7 to 8 weeks of age, whereas younger mice displayed a normal T cell responsiveness. Thymic T cells from (NOD x BALB/c)F1 mice, which are insulitis- and diabetes-free, exhibit an intermediate stage of unresponsiveness. This T cell defect is not due to a difference in the level of CD3 and IL-2R expression by NOD and BALB/c thymocytes, and both NOD CD4+ CD8- and CD4- CD8+ mature thymic T cells respond poorly to Con A. BALB/c but not NOD thymic T cells respond to Con A in the presence of either BALB/c or NOD thymic APC, suggesting that the thymic T cell defect in NOD mice is intrinsic to NOD thymic T cells and is not due to an inability of NOD APC to provide a costimulatory signal. The defect can be partially reversed by the addition of rIL-2 to NOD thymocytes. To determine whether a defect in signal transduction mediates this NOD thymic T cell unresponsiveness, we tested whether these cells elevate their intracellular free Ca2+ ion concentration in response to Con A. An equivalent Con A-induced increase in Ca2+ ion concentration in both NOD and BALB/c thymocytes was observed, suggesting a normal coupling between the CD3 complex and phospholipase C in NOD thymocytes. In contrast to their low proliferative response to Con A or anti-CD3, NOD thymocytes respond normally (i.e., as do BALB/c thymocytes) to the combinations of PMA plus the Ca2+ ionophore ionomycin and PMA plus Con A but weakly to Con A plus ionomycin. Our data suggest that the age-related NOD thymocyte unresponsiveness to Con A and anti-CD3 results from a defect in the signaling pathway of T cell activation that occurs upstream of protein kinase C activation.     

The effect of a peat-based preparation on mitogen-induced proliferation of thymocytes in non-treated and hydrocortisone-suppressed mice.

The studies were carried out on Balb/c mice (5-6 weeks of age) treated with a peat-based preparation (PBP), administered i.p. once or four times at 24 h intervals at doses of 0.01; 0.1 or 1 mg/kg. Additionally, hydrocortisone was injected i.p. to selected mice at a single dose of 125 mg/kg. The results show that PBP temporarily enhances the proliferative capability of murine thymocytes stimulated in vitro with concanavalin A (Con A) and phytohemagglutinin (PHA). The effect of PBP depends on the number of subsequent doses, but does not depend on the dose applied. A single PBP administration does not affect the proliferative response of thymocytes to Con A and PHA. A single injection of PBP (doses from 0.01 to 1 mg/kg) does not change the number of thymic cells and weight ratio of this organ. Increased doses of subsequent PBP injections (0.01 and 0.1 mg/kg) do not affect the number of thymocytes, but temporarily increase the weight ratio of the thymus two days after the last injection. Administration of PBP prior to hydrocortisone prevents the suppressive effect of the drug on proliferative response of thymocytes stimulated in vitro with Con A and PHA, at the same time increasing the proliferative response of thymic cells to the two mitogens in relation to the control group (hydrocortisone-free). The effect of a single dose of PBP depends on the dose applied--the weakest preventive effect was observed at a dose of 0.01 mg/kg. An increase in the number of subsequent PBP doses, irrespective of a dose applied, prolongs the protective action of the drug on proliferative activity of thymocytes stimulated in vitro with these mitogenes. Moreover, the results obtained in the studies show that PBP partially prevents the suppressive effect of hydrocortisone, as the number of thymic cells and weight ratio of this organ drastically decreased. PBP accelerates regeneration of the thymus, but this depends on a dose applied and the number of subsequent doses. The result was the strongest and the fastest when PBP was injected four times at a dose of 1 mg/kg. It seems quite likely that the thymic regeneration due to PBP is connected with the effect of this drug on maturation and differentiation of thymic cells.     

Thymic lymphocytes. III. Cooperative phenomenon in the proliferation of thymocytes under Con A stimulation

In the present paper, the response of thymocytes to Con A is analyzed in terms of a cooperative phenomenon between medullary thymocytes, cortical thymocytes, thymic accessory cells, and interleukin 2. Medullary thymocytes respond spontaneously to Con A and produce IL-2. The addition of exogenously produced IL-2 enhances their proliferation. Small numbers of cortical (PNA+) thymocytes do not respond to Con A, even in the presence of IL-2-containing supernatant. By increasing the number of PNA+ cells per well, sensitivity to Con A and IL-2 appears. This response may be linked either to the increase in a minor PNA+-responding population and/or to the enhanced contamination by medullary thymocytes and macrophages in non-responding PNA+ thymocyte population. In this hypothesis, either the contaminating cells respond by themselves and/or cooperate with PNA+ cells to induce their proliferation. Coculture of non-responding low numbers of PNA+ thymocytes with Con A- and IL-2-containing supernatant in the presence of PNA- cells containing thymic medullary thymocytes and macrophages always produces a higher response than that of each individual population. These results show that a cooperative phenomenon occurs in the cocultures of PNA+ and PNA- thymic cells. We can show using PNA+ and PNA- thymocytes with different Thy 1 alleles, that indeed both PNA+ and populations participate PNA-thymocytes with different Thy 1 alleles, that indeed both PNA+ and PNA- populations participate in the generation of proliferating cells. We can demonstrate, by lysis experiments with monoclonal antibodies and complement that at the end of coculture, most of the proliferating cells are Lyt 1+, and part are Lyt 2+ or L3T4+. We discuss the fact that the phenotype of the cells after activation does not allow us to deduce the phenotype of their precursors. Lysis of Ia+ cells prior to coculture, reduces the level of the proliferative response but does not modify the percentage of cooperation produced by the coculture. Cooperation with medullary mature thymocytes or the presence of active Ia- accessory cells possibly able to convert to Ia expression during coculture experiments may account for these results.     

The role of thymus gland in the regulation of the activity of peripheral T-lymphocytes in the process of spontaneous blastogenesis in mice

The total count, spontaneous proliferation and proliferative response of thymic and spleen T-cells in mixed lymphocyte culture and Con A-induced suppressor activity do not reveal significant disorders in 10-14-month A/Sn and C3H/He mice with spontaneous mammary tumors (weight under 2-3 g). However, these indices are quite different in varying age (2, 6, 12 month) A/Sn mice and C57Bl/6 mice with low rates of spontaneous tumors. The analysis of thymus-dependent immunity changes observed with age shows that relatively intensive migration of nonmature thymocytes, T-suppressor precursors is noted in mice with high cancer incidence. This phenomenon is considered to be one of major mechanisms regulating immune response in spontaneous-carcinogenesis.     

Downregulation of interleukin-7 and hepatocyte growth factor in the thymic microenvironment is associated with thymus involution in tumor-bearing mice

During mammary tumorigenesis, there is a profound thymic involution associated with severe depletion of the most abundant subset of thymocytes, CD4⁺CD8⁺ immature cells, and an early arrest in at least two steps of T cell differentiation. Thymic atrophy that is normally related with aging has been observed in other model systems, including graft-vs-host disease (GVHD) and tumor development. However, the mechanisms involved in this phenomenon remain to be elucidated. Vascular endothelial growth factor (VEGF) has been associated with thymic involution, when expressed at high levels systemically. In thymuses of D1-DMBA-3 tumor-bearing mice, this growth factor is diminished relative to the level of normal thymuses. Interestingly, the expression of hepatocyte growth factor (HGF), which has been associated with proliferation, cell survival, angiogenesis and B-cell differentiation, is profoundly down-regulated in thymuses of tumor bearers. In parallel, IL-7 and IL-15 mRNA, crucial cytokines involved in thymocytes development and cellular homeostasis, respectively, are also down-regulated in the thymuses of tumor hosts as compared to those of normal mice. Injection of HGF into mice implanted with mammary tumors resulted in normalization of thymic volume and levels of VEGF, IL-7 and IL-15. While, injections of IL-7 partially restored the thymic involution observed in the thymuses of tumor-bearing mice, injection of IL-15 did not have any significant effects. Our data suggest that the downregulation of HGF and IL-7 may play an important role in the thymic involution observed in tumor-bearing hosts.     

Indomethacin inhibits thymic involution in mice with streptozotocin-induced diabetes

Diabetes is chronic disease that is accompanied by a rapid thymus involution. To investigate the factors responsible for thymic involution in a model of STZ-induced diabetes, mice were injected with STZ alone or in combination with the cyclooxygenase 2 inhibitor indomethacin (INDO). Thymus weight, glycemia and serum corticosterone were measured, and apoptosis in thymus and thymocyte cultures was analyzed by flow cytometry. Although earlier studies report that streptozotocin (STZ) is toxic to lymphoid tissues, in our experiments even massive doses of STZ did not negatively affect thymocyte cultures. Cultured thymocytes also seemed unaffected by high glucose concentrations, even after 24 h of exposure. Administration of INDO concomitantly with STZ reduced thymic involution but did not prevent the onset of hyperglycemia or reduce established hyperglycemia. When INDO was given before STZ, the same degree of thymic involution occurred; however, hyperglycemia was reduced, although normoglycemia was not restored. INDO also reduced serum corticosterone. Because thymocytes are known to be sensitive to glucocorticoids, this finding suggests that cyclooxygenase 2 inhibition may retard thymic involution by reducing serum glucocorticoids. In conclusion, our results show that STZ and hyperglycemia are not toxic to thymocytes and that cyclooxygenase 2-mediated mechanisms are involved in thymic involution during diabetes.     

Altered T cell differentiation and Notch signaling induced by the ectopic expression of keratin K10 in the epithelial cells of the thymus

Transgenic mice expressing hK10 under the keratin K5 promoter display several alterations in the epidermis including decreased cell proliferation, and reduced susceptibility to tumor development. Given that K5 promoter is also active in the epithelial cells of the thymus, we explored the possible alterations of the thymus because of K10 transgene expression. We found severe thymic alterations, which affect not only the thymic epithelial cells (TEC), but also thymocytes. We observed altered architecture and premature thymus involution in the transgenic mice associated with increased apoptosis and reduced proliferation of the thymocytes. Interestingly, prior to the development of this detrimental phenotype, thymocytes of the transgenic mice also displayed altered differentiation, which is aggravated later on. Molecular characterization of this phenotype indicated that Akt activity is reduced in TEC, but not in thymocytes. In addition, we also observed altered expression of Notch family members and some of their ligands both in TEC and T cells. This produces reduced Notch activity in TEC but increased Notch activity in thymocytes, which is detectable prior to the disruption of the thymic architecture. In addition, we also detect altered Notch expression in the epidermis of bK5hK10 transgenic mice. Collectively the present data indicate that keratin K10 may induce severe alterations not only in a cell autonomous manner, but also in neighboring cells by the modulation of signals involved in cell-cell interactions.     

The role of macrophages in thymocyte mitogenesis.

The thymic lymphocytes of CBA/J mice respond to the mitogen Concanavalin A (Con A) only in the presence of adherent cells of the monocyte-macrophage series. Depletion of adherent cells abrogates the response, and macrophage-rich population of cells restore it. The need for macrophages and mitogen is completely provided by irradiated splenic macrophages which have been exposed to Con A and washed free of the soluble mitogen. The mitogenmacrophage effect in this case is apparently not due to soluble factors. Even more striking than the effect of macrophages on fresh cultures of thymic lymphocytes is their ability to restimulate quiescent cells 72 hr after their first stimulation with Con A. The quiescent cells respond immediately and quantitatively to Con A in the presence of fresh macrophages. This stimulation, like that of fresh thymocytes, is also controlled by a lymphokine ("costimulator") produced by mixing macrophages, mitogen, ant T lymphocytes. Our data suggest a model in which two signals are required for mitogenesis. First, the interaction of macrophage, T cell, and mitogen elicits a soluble costimulator, which is itself not mitogenic. Secondly, in the presence of costimulator, the mitogen (either soluble, or, more efficiently, bound to macrophages) induces a proliferative response in the T cell.     

Selective support of a mouse thymus epithelial cell line （MTEC1） to the viability and proliferation of CD4＋CD8—,CD4^—CD8^—,and CD4^＋CD8^＋thymocytes in vitro

The MTEC1 cell line,established in our laboratory,is a normal epithelial cell line derived from thymus medulla of Balb/c mice and these cells constituteively produce multiple cytokines.The selection of thymic microenvironment on developing T cells was investigated in an in vitro system.Unseparated fresh thymocytes from Balb/c mice were cocultured with MTEC1 cells or/and MTEC1-SN,then,the viability,proliferation and phenotypes of cultured thymocytes were assessed.Without any exogenous stimulus,both MTEC1 cells and MTEC1-SN were able to maintain the viability of thymocytes,while only the MTEC1 cells,not the MTEC1-SN,could directly activate thymocytes to exhibit moderate proliferation,indicating that the proliferative signal is delivered through cell surface interatcions of MTEC1 cells and thymocytes.Phenotype analysis on FACS of viable thymocytes after coculture revealed that MTEC1 cells preferentially activate the subsets of CD4^ CD8^-,CD4^ CD^8 and CD^4- CD^8- thymocytes;whereas MTEC1-SN preferentially maintained the viability of CD4^ CD^8- and CD4^-CD8^ thymocyte subsets.For the Con A-activated thymocytes.both MTEC1 cells and MTEC1-SN provided accessory signal(s) to significantly increase the number of viable cells and to markedly enhance the proliferation of thymocytes with virtually equal potency,phenotyped as CD4^ CD8^-,CD4^-CD8^ ,and CD^4-CD8^-subests,In summary,MTEC1 cells displayed Selection of thymic epithelial cells on thymocyte subsets. selective support to the different thymocyte subsets,and the selectivity is dependent on the status of thymocytes.     

The GTPase Rho has a critical regulatory role in thymus development.

The present study employs a genetic approach to explore the role of Rho GTPases in murine thymic development. Inactivation of Rho function in the thymus was achieved by thymic targeting of a transgene encoding C3 transferase from Clostridium botulinum which selectively ADP-ribosylates Rho within its effector domain and thereby abolishes its biological function. Thymi lacking functional Rho isolated from C3 transgenic mice were strikingly smaller and showed a marked (90%) decrease in cellularity compared with their normal litter mates. We also observed a similar decrease in levels of peripheral T cells in C3 transgenic mice. Analysis of the maturation status of thymocytes indicated that differentiation of progenitor cells to mature T cells can occur in the absence of Rho function, and both positive and negative selection of T cells appear to be intact. However, transgenic mice that lack Rho function in the thymus show maturational, proliferative and cell survival defects during T-cell development that severely impair the generation of normal numbers of thymocytes and mature peripheral T cells. The present study thus identifies a role for Rho-dependent signalling pathways in thymocyte development. The data show that the function of Rho GTPases is critical for the proliferative expansion of thymocytes. This defines a selective role for the GTPase Rho in early thymic development as a critical integrator of proliferation and cell survival signals.     

Impairment of T-cell functions with the progressive ascitic growth of a transplantable T-cell lymphoma of spontaneous origin

It has been observed that the progressive ascitic growth of a transplantable T-cell lymphoma of spontaneous origin, designated Dalton's lymphoma (DL), in a murine host induces inhibition of various immune responses and is associated with an involution of thymus accompanied by a massive depletion of the cortical region and alteration in the distribution of thymocytes caused by tumour serum-dependent induction of apoptosis with a decrease of CD4(+)CD8(+), CD4(+)CD8(-) and CD4(-)CD8(+) thymocytes. Here, we report that thymocytes of DL-bearing mice are defective in their proliferative ability and in their response to non-specific mitogenic stimulus in vitro. Also, antigen-specific T-cell proliferative ability representing the fundamental T(H) function declines under DL-bearing conditions and upon treatment with serum of DL-bearing mice. Moreover, a significant inhibition of T-cell cytolytic activity with a decreased ability to produce interferon gamma is shown by the T cells of DL-bearing mice and by the T cells treated with DL-ascitic fluid, DL-conditioned medium or serum of DL-bearing mice. Further, addition of interleukin-2 and anti-interleukin-10 to the cultures of thymocytes treated with serum of DL-bearing mice is found to inhibit the induction of apoptosis in thymocytes, a phenomenon associated with the progression of DL growth. Analysis of the results indicates an immune deviation with the predominance of a T(H2)-type response with the progression of tumour. We further discuss the possible mechanisms that may explain the observed tumour-induced diminution of T-cell immunity.     

Plasminogen activator and protease inhibitor activities in isolated rat thymocytes

Summary Plasminogen Activator (PA) and its response to glucocorticoids and androgens was studied in viable rat thymocytes in suspension. PA was measured by its ability to convert plasminogen to plasmin, and the formed plasmin determined by cleavage of ¹⁴C-labeled globin. Using this functional assay, PA was found to be associated with the outer surface of thymic cells, and only negligible activity recovered from the incubation medium. Rat thymocytes also contain cytoplasmic and nuclear inhibitor(s) of the serine proteases plasmin, trypsin, chymotrypsin and thymic PA. Release of these inhibitors prevented determination of thymic PA activity in presence of lysed cells.The specific activity of PA in thymocytes isolated from adrenalectomized-castrated rats did not differ significantly from the specific activity associated with cells from intact animals. Furthermore, treatment of adrenalectomized-castrated rats with 0.1 mg of dexamethasone/ kg for 2 days induced thymic involution without affecting thymic PA activity. These observations suggest that PA activity of thymocytes is not involved in glucocorticoid-mediated thymic involution.     

Ontogeny of interleukin 2 responsive cells in the fetal thymus

The ontogeny of IL 2-responsive cells in the thymus of CBA/J mice was examined in neonatal animals and in fetuses at 14, 16, and 18 to 20 days gestation. The thymocytes were tested for responsiveness to 2 micrograms/ml Con A, TCGF, IL 2, and co-stimulation by Con A plus TCGF or IL 2. These responses were compared with those of thymocytes of 6- to 8-wk-old CBA/J. Thymocytes (1 X 10(5)) were cultured, and the reaction was measured at maximum response (96 hr). Neonatal animals gave an unusually high response to TCGF or partially purified IL 2 alone, approximately five times greater than the adult. A low but significantly enhanced proliferation, stimulated by partially purified IL 2 alone, was observed with 14-day fetal thymocytes, even though cultures of these cells in medium alone had higher background proliferation than any other age tested. In the co-stimulator reaction, proliferation significantly above background was measured at 16 days of gestation with Con A plus TCGF. The magnitude of the co-stimulator reaction increased with age, especially between the 16th and 18th day of gestation and immediately after birth.     

Failure of rearranged TCR transgenes to prevent age-associated thymic involution.

After puberty, the thymus undergoes a dramatic loss in volume, in weight and in the number of thymocytes, a phenomenon termed age-associated thymic involution. Recently, it was reported that age-associated thymic involution did not occur in mice expressing a rearranged transgenic (Tg) TCRalphabeta receptor. This finding implied that an age-associated defect in TCR rearrangement was the major, if not the only, cause for thymic involution. Here, we examined thymic involution in three other widely used MHC class I-restricted TCRalphabeta Tg mouse strains and compared it with that in non-Tg mice. In all three TCRalphabeta Tg strains, as in control mice, thymocyte numbers were reduced by approximately 90% between 2 and 24 mo of age. The presence or absence of the selecting MHC molecules did not alter this age-associated cell loss. Our results indicate that the expression of a rearranged TCR alone cannot, by itself, prevent thymic involution. Consequently, other presently unknown factors must also contribute to this phenomenon.     

Effects of costimulator on immune responses in vitro.

We recently described a factor, costimulator, that is required for the concanavalin A-induced proliferation of CBA mouse thymocytes in vitro (see Reference 1). Using the costimulator dependence of mouse thymocytes as an assay, we have now determined that spleen cells from congenitally athymic (nude) BALB/c mice do not produce costimulator in response to Con A, and spleen cells depleted of Thy 1-positive cells do not respond to it in the presence of Con A. Thus, costimulator both requires thymus-derived (Thy 1+ lymphocytes for its production and has an effect on this type of cell. (However, the costimulator-producing and responsive cells may be different.) Purified costimulator preparations are a source of the required second component for the stimulation of adult, CBA/J thymic lymphocytes by PHA, normally a poor mitogen for these cells. They also enhance the level of DNA synthesis in a mixed leukocyte reaction, and the specific generation of cytotoxic lymphocytes to allogeneic tumor cells in vitro. Costimulator is not H-2 restricted in its effects, and it is produced in mixed leukocyte reactions. Finally, it has been possible to grow normal, primary thymic lymphocytes in culture for about 20 days by adding partially purified costimulator to the cultures.     

Exogenous interleukin-2 abrogates differences in the proliferative responses to T cell mitogens among inbred strains of mice.

The proliferative response of spleen cells from BALB/c mice to stimulation with a T cell mitogen, concanavalin A (Con A), was two or more times stronger than that of cells from C57BL/10SnSc (B10) mice. In contrast, the cells from B10 mice responded better to B cell mitogen bacterial lipopolysaccharide (LPS). The differences in the proliferative response to Con A stimulation were not associated with the function of macrophages nor did they depend on IL-1. Spleen cells from BALB/c and B10 mice synthesized comparable amounts of mRNA for IL-1 alpha, and the production of biologically active IL-1 was even higher in the B10 strain. Indomethacin, an inhibitor of prostaglandin synthesis, had no effect on the differences in reactivity between the cells from BALB/c and B10 mice. In addition, no differences in the synthesis of mRNA for the inducible 55-kDa interleukin-2 (IL-2) receptors were found between the spleen cells from BALB/c and B10 mice. However, Con A-stimulated spleen cells from B10 mice produced a significantly lower amount of biologically active IL-2 than similarly stimulated cells from BALB/c mice. In the presence of exogenous IL-2, these low responder spleen cells from the B10 mice responded by proliferation to Con A stimulation to the same extent as cells from the BALB/c mice. These results thus show that a low proliferative response to Con A stimulation in B10 mice was a consequence of a lower production of IL-2 and possibly abrogated the proliferative hyporeactivity produced by exogenous IL-2. We suggest that the differences in the ability to produce IL-2 could be a reason for the discrepancies observed in the immunological responsiveness between BALB/c and B10 mice.