Catecholaminergic signalling through thymic nerve fibres, thymocytes and stromal cells is dependent on both circulating and locally synthesized glucocorticoids

Glucocorticoids have been shown to modulate the expression of noradrenaline metabolizing enzymes and β(2)- and α(1B)-adrenoceptors in a tissue- and cell- specific manner. In the thymus, apart from extensive sympathetic innervation, a regulatory network has been identified that encompasses catecholamine-containing non-lymphoid and lymphoid cells. We examined a putative role of adrenal- and thymus-derived glucocorticoids in modulation of rat thymic noradrenaline levels and adrenoceptor expression. Seven days postadrenalectomy, the thymic levels of mRNAs encoding tyrosine hydroxylase, dopamine β-hydroxylase, monoamine oxidase-A and, consequently, noradrenaline were decreased. Catecholamine content was diminished in autofluorescent nerve fibres (judging by the intensity of fluorescence) and thymocytes (considering HPLC measurements of noradrenaline and the frequency of tyrosine hydroxylase-positive cells), while it remained unaltered in non-lymphoid autofluorescent cells. In addition, adrenalectomy diminished the thymocyte expression of β(2)- and α(1B)-adrenoceptors at both mRNA and protein levels. Administration of ketoconazole (an inhibitor of glucocorticoid synthesis/action; 25 mg kg(-1) day(-1), s.c.) to glucocorticoid-deprived rats increased the thymic levels of tyrosine hydroxylase, dopamine β-hydroxylase and, consequently, noradrenaline. The increased intensity of the autofluorescent cell fluorescence in ketoconazole-treated rats indicated an increase in their catecholamine content, and suggested differential glucocorticoid-mediated regulation of catecholamines in thymic lymphoid and non-lymphoid cells. In addition, ketoconazole increased the thymocyte expression of α(1B)-adrenoceptors. Thus, this study indicates that in the thymus, as in some other tissues, glucocorticoids not only act in concert with cateholamines, but they may modulate catecholamine action by tuning thymic catecholamine metabolism and adrenoceptor expression in a cell-specific manner. Additionally, the study indicates a role of thymus-derived glucocorticoids in this modulation.

Glucocorticoids, master modulators of the thymic catecholaminergic system?

There is evidence that the major mediators of stress, i.e., catecholamines and glucocorticoids, play an important role in modulating thymopoiesis and consequently immune responses. Furthermore, there are data suggesting that glucocorticoids influence catecholamine action. Therefore, to assess the putative relevance of glucocorticoid-catecholamine interplay in the modulation of thymopoiesis we analyzed thymocyte differentiation/maturation in non-adrenalectomized and andrenalectomized rats subjected to treatment with propranolol (0.4 mg.100 g body weight-1.day-1) for 4 days. The effects of beta-adrenoceptor blockade on thymopoiesis in non-adrenalectomized rats differed not only quantitatively but also qualitatively from those in adrenalectomized rats. In adrenalectomized rats, besides a more efficient thymopoiesis [judged by a more pronounced increase in the relative proportion of the most mature single-positive TCRalphabetahigh thymocytes as revealed by two-way ANOVA; for CD4+CD8- F (1,20) = 10.92, P < 0.01; for CD4-CD8+ F (1,20) = 7.47, P < 0.05], a skewed thymocyte maturation towards the CD4-CD8+ phenotype, and consequently a diminished CD4+CD8-/CD4-CD8+ mature TCRalphabetahigh thymocyte ratio (3.41 +/- 0.21 in non-adrenalectomized rats vs 2.90 +/- 0.31 in adrenalectomized rats, P < 0.05) were found. Therefore, we assumed that catecholaminergic modulation of thymopoiesis exhibits a substantial degree of glucocorticoid-dependent plasticity. Given that glucocorticoids, apart from catecholamine synthesis, influence adrenoceptor expression, we also hypothesized that the lack of adrenal glucocorticoids affected not only beta-adrenoceptor- but also alpha-adrenoceptor-mediated modulation of thymopoiesis.

Expression of alpha1-adrenoceptors on thymic cells and their role in fine tuning of thymopoiesis

The study was undertaken to explore: i) the presence of alpha(1)-adrenoceptors (AR) on thymic lymphoid and non-lymphoid cells and ii) their putative role in T-cell development. The expression of alpha(1)-AR on thymic cells was assessed using both immunohistochemistry and flow cytometric analyses, while their putative role in thymopoiesis was estimated by analyses of thymocyte proliferation and apoptosis, and major thymocyte subset distribution in adult rats subjected to 14-day-long treatment with the alpha(1)-AR blocker urapidil. The presence of alpha(1)-AR was demonstrated on both thymocytes (mainly less mature CD3(-) and CD3(low) cells) and thymic non-lymphoid cells (thymic epithelial cells and CD68-positive cells). Chronic treatment with urapidil increased the thymic weight and thymocyte number. The increase in thymocyte number might, at least partly, be related to an enhanced thymocyte proliferation. In addition, an altered thymocyte subset distribution was observed in these rats. The increase in the percentage of CD4+CD8+ double positive (DP) TCRalphabeta(-) thymocytes was accompanied by the reduction in that of CD4+CD8+ (DP) TCRalphabeta(low) cells, and divergent changes in the percentage of the most mature single positive (SP) TCRalphabeta(high) thymocytes. In urapidil-administered rats the percentage of CD4+CD8- SP TCRalphabeta(high) thymocytes was increased, while that of the CD4-CD8+ TCRalphabeta(high) was reduced, compared with controls. In addition, proportions of CD4+CD25+RT6.1- and CD161+TCRalphabeta+ regulatory cells were increased. Collectively, the results indicate that alpha(1)-AR are involved in complex network of neuro-thymic and intrathymic communications that provide fine tuning of both conventional effector and regulatory T-cell development.

ALTERATIONS IN THE THYMOPOIESIS IN EXPERIMENTAL AUTOIMMUNE MYASTHENIA GRAVIS

Experimental autoimmune myasthenia gravis (EAGM) was induced in female AO rats, by a single immunization with Torpedo acetylcholine receptor (AChR). Animals injected with saline served as controls. The study showed substantial changes in EAMG rats in the thymopoiesis, causing an increase in the relative proportion of mature CD8+ and, particularly, CD4+ (possibly autoreactive) single positive (SP) cells expressing TCRalphabeta at high level (TCRalphabeta(high)), as well as in that of mature double negative (DN) TCRalphabeta(high) cells, which are believed to be the immunoregulatory cells that augment autoantibody (autoAb) production. These results indicate that an augmented production of autoreactive CD4+ cells, on one side, and an increase of the immunoregulatory T cells that augment autoAb production, on the other side (reflecting, most likely, an increased entry of activated autoreactive CD4+ T cells from the periphery into the thymus), may have a significant role in the sustention of immune response in EAMG, and may suggest a putative mechanism underlying the sustention of autoimmune response in acquired MG.

EFFECTS OF BETA-ADRENOCEPTOR BLOCKADE ON THE PHENOTYPIC CHARACTERISTICS OF THYMOCYTES AND PERIPHERAL BLOOD LYMPHOCYTES

The study revealed that beta-adrenoceptor blockade with propranolol (0.40 mg/100 g/day, s.c.) in adult male DA rats: (i) increased the thymocyte proliferation and apoptosis, (ii) caused disturbances in kinetics of T cell differentiation leading to distinguishable changes in relative proportion of thymocytes at distinct maturational steps and to an expansion of the most mature single positive (CD4+, CD8+) thymocyte pool, (iii) affected the relative proportion of neither CD4+ nor CD8+ peripheral blood lymphocytes (PBL), and (iv) augmented the relative number of CD8+CD25+ cells. Thus, the results suggest the role of beta-adrenoceptors in fine-tuning of T cell maturation, and, possibly, distribution and activation of distinct PBL subsets.

Dysregulation of T-cell development in adrenal glucocorticoid-deprived rats

A number of different experimental approaches have been used to elucidate the impact of basal levels of adrenal gland-derived glucocorticoids (GCs) on T cell development, and thereby T cell-mediated immune responses. However, the relevance of the adrenal GCs to T cell development is still far from clear. This study was undertaken to explore the relevance of basal levels of GCs to T cell differentiation/maturation. Eight days post-adrenalectomy in adult male rats the thymocyte yield, apoptotic and proliferative rate and the relationship amongst major thymocyte subsets, as defined by TCRalphabeta/CD4/CD8 expression, were examined using flow cytometry. Adrenal GC deprivation decreased thymocyte apoptosis and altered the kinetics of T cell differentiation/maturation. In the adrenalectomized rats there was increased thymic hypercellularity and an over-representation of the CD4+CD8+ double positive (DP) TCRalphabeta(low) cells entering selection, as well as increased numbers of their DP TCRalphabeta(-) immediate precursors. These changes were accompanied with under-representation of the postselected DP TCRalphabeta(high) and the most mature CD4-CD8+ and, particularly, CD4+CD8- single positive (SP) TCRalphabeta(high) cells. This data suggests that withdrawal of adrenal GCs produces alterations in the thymocyte selection processes, possibly affecting the diversity of functional T cell repertoire and generation of potentially self-reactive cells as indicated by the reduced proportion and number of CD4-CD8- double negative TCRalphabeta(high) cells. In addition, it indicates that GCs influence the post-selection maturation of thymocytes and plays a regulatory role in controlling the ratio of mature CD4+CD8-/CD4-CD8+ SP TCRalphabeta(high) cells.

Murine monoclonal antibody 26 raised against tetanus toxoid cross-reacts with beta2-glycoprotein I: its characteristics and role in molecular mimicry

PROBLEM

Studies on experimental antiphospholipid syndrome (APS) models proved that molecular mimicry between plasma protein beta(2)-glycoprotein I (beta(2)GPI) and structure within micro-organisms or their products, might be a cause for experimental APS. Considering the heterogeneity of polyclonal antiphospholipid antibodies (aPLs), it is important to define the precise characteristics of pathogenic aPLs. To avoid the influence of polyclonality and to further analyse the connection between molecular mimicry and APS, we produced monoclonal antibodies (MAbs) against tetanus toxoid (TTd) and tested their reactivity against beta(2)GPI.

METHOD OF STUDY

In this report, we analysed the characteristics of MAb26 raised against TTd and cross-reactive with beta(2)GPI: its binding properties in various in vitro immunoassays, its specific interactions with surface epitopes expressed on apoptotic cells and its role in vivo.

RESULTS

We have demonstrated that MAb26: (i) binds beta(2)GPI being immobilized on an appropriate surface: irradiated polystyrene plates, non-irradiated plates pre-coated with anionic phospholipids and polyvinylidene fluoride membrane; (ii) binds specifically to apoptotic but not to viable cells and the binding is beta(2)GPI-dependent; and (iii) induces a pathologic pregnancy outcome when passively injected into BALB/c mice.

CONCLUSION

This study concluded that certain subpopulations of antibodies raised against TTd and cross-reactive with beta(2)GPI, because of the molecular mimicry mechanism, could have pathologic potential.

Neonatal testosterone imprinting affects thymus development and leads to phenotypic rejuvenation and masculinization of the peripheral blood T-cell compartment in adult female rats

Exposure of female rodents to testosterone in the critical neonatal period produces defeminization/masculinization of the hypothalamo-pituitary-gonadal (HPG) axis, i.e. neonatal androgenization and postpones axis maturation. To address the hypothesis that HPG axis signaling is involved in the programming of thymic maturation/involution and sexual differentiation we studied the impact of neonatal androgenization on thymic cellularity, development of effector and regulatory T cells, and phenotypic characteristics of peripheral blood T lymphocytes in adult rats. A single injection of testosterone on postnatal day 2 postponed thymic maturation/involution as revealed by organ hypercellularity, increased cellularity of the most mature (CD4+CD8- and CD4-CD8+) TCRalphabeta(high) thymocyte and both recent thymic emigrant (RTE) subsets and caused phenotypic defeminization/masculinization of thymic (decreased CD4+CD8-TCRalphabeta(high)/CD4-CD8+TCRalphabeta(high) cell ratio) and peripheral blood T-cell compartments (decreased CD4+RTE/CD8+RTE and CD4+/CD8+ cell ratio). In addition, neonatal androgenization increased the relative and absolute numbers of both CD4+CD25+Foxp3+ and natural killer (NK) regulatory T cells in peripheral blood. These findings, in conjunction with thymocyte overexpression of Thy-1 that is assumed to reduce negative selection affecting self-reactive cell generation, suggest a new relationship between self-reactive and regulatory T cells. In conclusion, our study provides additional evidence for a role of HPG signals (i.e. sex steroids and gonadotropins) in programming the kinetics of thymic maturation/involution and in establishing immunological sexual dimorphism.

Beta-adrenoceptor blockade ameliorates the clinical course of experimental allergic encephalomyelitis and diminishes its aggravation in adrenalectomized rats

As glucocorticoids influence both catecholamine synthesis and adrenoceptor expression by immune cells, the current study was undertaken to distinguish their direct effects on the development of experimental allergic encephalomyelitis from those induced by alteration of catecholamine signaling. We examined the influence of 16-day-long beta-adrenoceptor blockade with propranolol (0.40 mg/100 g body weight/day, s.c.) beginning 3 days before immunization on the development of experimental allergic encephalomyelitis in adrenalectomized (7 days before immunization) and in non-operated male Dark Agouti rats. Adrenalectomy aggravated the clinical course of experimental allergic encephalomyelitis. In contrast, propranolol attenuated both the clinical signs of the disease and decreased the number of lesions in the spinal cord. Furthermore, propranolol prevented adrenalectomy-induced aggravation of the disease course without affecting mortality. We also found that the percentage of CD4(+)CD25(+) T lymphocytes (recently activated or regulatory cells) was increased in peripheral blood of experimental allergic encephalomyelitis rats over that in the corresponding non-immunized and bovine serum albumin immunized rats. However, the percentage of these cells was reduced in adrenalectomized and/or propranolol-treated experimental allergic encephalomyelitis rats compared to control experimental allergic encephalomyelitis rats. Our findings, coupled with the clinical course of the disease and the underlying pathomorphological changes, clearly suggest that differential mechanisms were responsible for the changes in the percentage of CD4(+)CD25(+) T lymphocytes in propranolol-treated adrenalectomized rats and only propranolol-treated rats with experimental allergic encephalomyelitis. Our results, when viewed globally, indicate that: i) beta-adrenoceptor-dependent mechanisms are involved in the immunopathogenesis of experimental allergic encephalomyelitis, ii) experimental allergic encephalomyelitis has a more severe course in adrenalectomized rats and iii) beta-adrenoceptor-mediated mechanisms operate in adrenalectomy-induced aggravation of the disease.

Neonatal castration affects intrathymic kinetics of T-cell differentiation and the spleen T-cell level

To test putative interdependence in the ontogenesis of the hypothalamic-pituitary-gonadal and thymic-lymphatic axes, thymocyte differentiation and maturation was examined in neonatally castrated (Cx) adult rats. In the hypercellular thymi of Cx rats, the proportion of the least mature CD4(-)CD8(-)TCRalphabeta(-) triple negative (TN) thymocytes was reduced, while the proportions of all downstream double positive (DP) subsets (TCRalphabeta(-), TCRalphabeta(low) and TCRalphabeta(high)) were increased when compared with neonatally sham-castrated (Sx) adult rats. This suggested an accelerated thymocyte transition from the TN to DP TCRalphabeta(low) developmental stage accompanied by an increased positive/ reduced negative thymocyte selection. The increased thymocyte surface density of Thy-1, which is implicated in thymocyte hyposensitivity to negative selection, in Cx rats further supports the previous assumption. The finding that the proportions of both single positive (SP) TCRalphabeta(high) thymocyte subsets were reduced, while their numbers were increased (CD4(+)CD8(-)) or unaltered (CD4(-)CD8(+)), coupled with results demonstrating an increased level of CD4(-)CD8(+) cells without changes in that of CD4(+)8(-) cells in the spleen indicate: (i) accelerated differentiation and maturation of the positively selected DP TCRalphabeta(high) thymocytes towards CD4(-)8(+) TCRalphabeta(high) cells followed by increased emigration of the mature cells and (ii) decelerated differentiation and maturation towards CD4(+)8(-)TCRalphabeta(high) cells in Cx rats. Furthermore, the unaltered proportion of intrathymically developing CD4(+)CD25(+)Foxp3(+) regulatory cells in Cx rats, in light of putative hyposensitivity of thymocytes to negative selection suggesting reduced elimination of autoreactive cells, may provide a firm basis for understanding the reasons behind increased susceptibility of Cx rats to autoimmune disease induction.

Long-term beta-adrenergic receptor blockade increases levels of the most mature thymocyte subsets in aged rats

Age-related increase in the density of thymic noradrenergic fibres and noradrenaline (NA) concentration is proposed to be associated with thymic involution and altered thymopoiesis. To test this hypothesis thymocyte differentiation/maturation and thymic structure were studied in 18-month-old male Wistar rats subjected to 14-day-long propranolol (P) blockade of beta-adrenoceptors (beta-ARs). The treatment primarily resulted in changes in the T-cell receptor (TCR)-dependent stages of thymopoiesis, which led to an increase in both the relative and absolute numbers of the most mature single positive (SP) CD4(+)CD8(-) (including cells with the CD4(+)CD25(+) regulatory phenotype) and CD4(-)CD8(+) TCRalphabeta(high) thymocytes. Accordingly, in the thymi of these rats an increase in both numerical density and absolute number of medullary thymocytes encompassing mainly the most mature SP cells was found. These findings, together with an increase in the thymocyte surface expression of the regulatory molecule Thy-1 (CD90) (implicated in negative regulation of TCRalphabeta-dependent thymocyte selection thresholds) in the same rats, may suggest increased positive/reduced negative thymocyte selection. Collectively, the results indicate that a decline in thymic efficiency in generating both conventional and regulatory T cells, and consequently in immune function, in aged rats may be, at least partly, attenuated by long-term blockade of beta-ARs with P.

Strain differences and the role for HSP47 and HSP70 in adjuvant arthritis in rats

Because of high sequence homology between microbial and endogenous heat shock proteins (HSP), immunological cross-reactivity to microbial HSP has been suggested as a possible cause of the development of autoimmune diseases, such as rheumatoid arthritis. The present study aimed to determine a potential role of HSP47, a molecular chaperone involved in the synthesis and assembly of collagen molecules, and microbial HSP71 (mHSP71) in adjuvant arthritis (AA) in two rat strains: Dark Agouti (DA), susceptible to AA induction and Albino Oxford (AO), which is resistant to AA induction. Immunization with complete Freund's adjuvant (CFA) induced an increased expression of HSP47 in joints of DA rats, which exhibited severe clinical signs of AA at the time of disease peak, while this protein was not detectable in joints of AO rats. In contrast, no strain differences in HSP72 (rat analogue of mHSP71) expressions in joints were observed. The increased levels of anti-HSP47 antibodies were detected in sera of DA rats during the AA peak, while the immunization with CFA increased levels of anti-mHSP71 antibodies in sera of AO rats. HSP47 and mHSP71 reduced proliferation of draining inguinal lymph node cells (LNC) in resistant AO rat strain, leading to a hypothesis that both HSP participated in AA control. Finally, mHSP71 potentiated the apoptotic response of LNC in susceptible DA rat strain. In conclusion, our findings indicate involvement of HSP47 in the development of AA in the rat, and point out to the regulatory role for both HSP47 and mHSP71.

Neuropeptide Y (NPY) modulates oxidative burst and nitric oxide production in carrageenan-elicited granulocytes from rat air pouch

We studied the effects of neuropeptide Y (NPY) and NPY-related receptor specific peptides on functions of carrageenan-elicited granulocytes in vitro and ability of NPY to modulate carrageenan-induced air pouch inflammation in rats in vivo. Anti-inflammatory effect of NPY comprises reduced granulocyte accumulation into the air pouch, to some extent attenuation of phagocytosis, attained via Y1 receptor, and considerable decrease in peroxide production, albeit mediated via Y2 and Y5 receptors activation. Conversely, NPY increases nitric oxide production and this potentiation is mediated via Y1 receptor. It is concluded that NPY Y1 and Y2/Y5 receptors' interaction participates in NPY-induced modulation of granulocyte functions related to inflammation.

Early postnatal castration affects thymic and thymocyte noradrenaline levels and beta-adrenoceptor-mediated influence on the thymopoiesis in adult rats

The interactions among the nervous, endocrine and immune system were studied by examining: i) thymic and thymocyte catecholamine levels in adult rats castrated (Cx) at postnatal day 3 and ii) effects of 14-day-long propranolol (P) treatment on main thymocyte differentiational molecule expression in adult non-Cx and Cx rat. The results demonstrated that castration in early postnatal period lowers levels of both neurally- and thymocyte-derived noradrenaline in adult rats, and thereby diminishes beta-adrenoceptor-mediated fine tuning of the T-cell differentiation/maturation. In non-Cx rats P affected TCRalphabeta-dependent stages of thymocyte differentiation/maturation decreasing frequency of CD4+8+ double positive (DP) TCRalphabeta(low) cells entering selection processes and increasing relative number of positively selected DP TCRalphabeta(high) (most likely due to an increased thymocyte surface density of Thy-1 that is involved in negative control of TCRalphabeta-mediated signaling/selection thresholds) and the most mature CD4+8- TCRalphabeta(high) cells (including CD4+25+ regulatory cells). However, in Cx rats P failed to produce any significant changes in thymocyte subset composition.

Age-related effect of peptide YY (PYY) on paw edema in the rat: the function of Y1 receptors on inflammatory cells

It is well documented that neuropeptides participate in local inflammatory reaction and modulate functions of inflammatory cells. The aim of the study was to determine a link between in vivo and in vitro effects of NPY-related peptides on inflammatory response with respect to ageing. Peptide YY (PYY) intraplantarly applied decreases concanavalin A-induced paw edema in 3 and 8 months, but not in 24 months old male rats of Albino Oxford strain. The use of NPY-related receptor-specific peptides and Y1 receptor antagonist revealed that anti-inflammatory effect of PYY is mediated via NPY Y1 receptors. PYY in vitro decreases adherence of macrophages from 8 months, but not from 3 and 24 months old rats and this effect is also mediated via NPY Y1 receptor. Additionally, PYY (10(-6)M) decreases NBT reduction in macrophages from 3 and 8 months old rats, and suppresses NO production in cells from 24 months old rats, albeit regardless of absence of in vivo effect of PYY on inflammation in aged rats. It is concluded that aged rats are less responsive to anti-inflammatory action of PYY compared to adult and young rats, and that ageing is associated with altered NPY Y1 receptor functioning.

Age-associated changes in CD90 expression on thymocytes and in TCR-dependent stages of thymocyte maturation in male rats

To elucidate the effects of ageing on T-cell-maturation, in 3- and 18-month-old rats, we analysed the expression of: (i) CD4/CD8/TCRalphabeta and (ii) Thy-1, which is supposed to be a regulator of TCRalphabeta signalling, and thereby the thymocyte selection thresholds. Since an essential role for TCRalphabeta signalling in the development of CD4+25+T(reg)-cells was suggested, the frequency of these cells was also quantified. We demonstrated that, as for mice, early thymocyte differentiational steps within the CD4-8- double negative (DN) developmental stage are age-sensitive. Furthermore, we revealed that TCRalphabeta-dependent stages of T-cell development are affected by ageing, most likely due to an impaired expression of Thy-1 on TCRalphabeta(low) thymocytes entering selection processes. The diminished frequency of the post-selection CD4+8+ double positive (DP) cells in aged rats, together with an overrepresentation of mature single positive (SP) cells, most probably suggests more efficient differentiational transition from the DP TCRalphabeta(high) to the SP TCRalphabeta(high) developmental stage, which is followed by an increase in pre-migration proliferation of the mature SP cells. Moreover, the study indicated impaired intrathymic generation of CD4+25+T(reg)-cells in aged rats, thus providing a possible explanation for the increased frequency of autoimmune diseases in ageing.

Characterization of thymocyte phenotypic alterations induced by long-lasting beta-adrenoceptor blockade in vivo and its effects on thymocyte proliferation and apoptosis

Adult male Wistar rats were subjected to propranolol (P, 0.40 mg/100 g/day) or saline (S) administration (controls) over 14 days. The expression of major differentiation molecules on thymocytes and Thy-1 (CD90) molecules, which are shown to adjust thymocyte sensitivity to TCRalphabeta signaling, was studied. In addition, the sensitivity of thymocytes to induction of apoptosis and concanavalin A (Con A) signaling was estimated. The thymocytes from P-treated (PT) rats exhibited an increased sensitivity to induction of apoptosis, as well as to Con A stimulation. Furthermore, P treatment produced changes in the distribution of thymocyte subsets suggesting that more cells passed positive selection and further differentiated into mature CD4+ or CD8+ single positive (SP) TCRalphabeta(high) cells. These changes may, at least partly, be related to the markedly increased density of Thy-1 surface expression on TCRalphabeta(low) thymocytes from these rats. The increased frequency of cells expressing the CD4+25+ phenotype, which has been shown to be characteristic for regulatory cells in the thymus, may also indicate alterations in thymocyte selection following P treatment. Inasmuch as positive and negative selections play an important role in continuously reshaping the T-cell repertoire and maintaining tolerance, the hereby presented study suggests that pharmacological manipulations with beta-AR signaling, or chemically evoked alterations in catecholamine release, may interfere with the regulation of thymocyte selection, and consequently with the immune response.

Chronic alpha1-adrenoreceptor blockade produces age-dependent changes in rat thymus structure and thymocyte differentiation

In order to examine the influence of chronic alpha1-adrenergic receptor (alpha1-AR) blockade on the thymus structure and T-cell maturation, peripubertal and adult male rats were treated with urapidil (0.20 mg/kg BW/d; s.c.) over 15 consecutive days. Thymic structure and phenotypic characteristics of the thymocytes were assessed by stereological and flow cytometry analysis, respectively. In immature rats, treatment with urapidil reduced the body weight gain and, affecting the volume of cortical compartment and its cellularity decreased the organ size and the total number of thymocytes compared to age-matched saline-injected controls. The percentage of CD4+8- single positive (SP) thymocytes was decreased, while that of CD4-8+ was increased suggesting, most likely, a disregulation in final steps of the positively selected cells maturation. However, alpha1-AR blockade in adult rats increased the thymus weight as a consequence of increase in the cortical size and cellularity. The increased percentage of most immature CD4-8- double negative (DN) cells associated with decreased percentage of immature CD4+8+ double positive (DP) thymocytes suggests a decelerated transition from DN to DP stage of T-cell development. As in immature rats, the treatment in adult rats evoked changes in the relative numbers of SP cells, but contrary to immature animals, favoring the maturation of CD4+8- over CD4-8+ thymocytes. These results demonstrate that: i) chronic blockade of alpha1-ARs affects both the thymus structure and thymocyte differentiation, ii) these effects are age-dependent, pointing out to pharmacological manipulation of alpha1-AR-mediated signaling as potential means for modulation of the intrathymic T-cell maturation.

Gonadotropin-releasing hormone agonist administration affects the thymopoiesis in adult female rats independently on gonadal hormone production

PROBLEM

In addition to having an indirect effect on the T-cell development by controlling the production of ovarian steroids, an accumulating body of evidence suggest that GnRH analogue (GnRH-A) administration may exert a thymopoietic regulatory effect that is not mediated by ovarian hormones.

METHOD OF STUDY

In non-ovariectomized (non-OVX) and OVX adult female AO rats treated s.c. with GnRH-A or saline (controls), over 14 days, were estimated the thymic cellularity and thymocyte expression of CD4/CD8/TCRalphabeta by stereological analysis and three-color flow cytometry, respectively.

RESULTS

GnRH-A in both groups of rats diminished the thymic cellularity. In non-OVX rats GnRH-A increased the relative numbers of immature cells (CD4-8-TCRalphabeta(-), CD4-8-TCRalphabeta(low) and CD4+8-TCRalphabeta(low)), and reduced those of positively selected CD4+8+TCRalphabeta(high) and mature (CD4-8+TCRalphabeta(high), CD4(+8)-TCRalphabeta(high)) cells, suggesting decelerated expression of TCRalphabeta followed by less efficient positive selection and further maturation of the selected cells. Differently, in OVX rats GnRH-A decreased the percentage of immature (CD4-8-TCRalphabeta(-), CD4+8+ TCRalphabeta(-)) cells and increased those of all TCRalphabeta(high) subsets, suggesting an increased rate of early thymocyte differentiation, more efficient positive selection and further maturation of the selected cells.

CONCLUSIONS

The effect of GnRH-A administration is affected by the presence of ovarian steroids.

Changes in thymopoiesis in myasthenia gravis

This study was undertaken to investigate T-cell maturation in hyperplastic thymi of patients suffering from myasthenia gravis (MG). For this purpose, the expression of the major differentiational molecules (CD4, CD8, and CD3/TCRalphabeta) and that of the regulatory and activation molecules on thymocytes from MG patients and control subjects were estimated by flow cytometric analysis. In the MG patients the increase in relative proportion of immature (CD4-8- TCRalphabeta-) and the most mature (CD4+8- TCRalphabetahigh and CD4-8- TCRhigh encompassing immunoregulatory NKT) thymocytes followed by a decrease in that of CD4+8+CD3-/TCRalphabeta- cells was found. Furthermore, in these patients the relative proportion of CD4+HLA-DR+ and CD4+71+ cells was increased, whereas that of CD4+25+ cells was slightly, but significantly, decreased (reflecting, most likely, decreased contribution of T reg cells bearing this phenotype). Moreover, in MG thymi the percentage of CD45RA+ cells was reduced indicating changes in the selection processes. In keeping with this finding the reduced thymocyte apoptotic index and percentage of cells bearing apoptosing (CD4-8- TCRalphabetalow) phenotype were detected. In conclusion, the study demonstrates substantial changes in intrathymic differentiation of T cells in hyperplastic MG thymi and suggests alterations in selection events providing an increased escape of potentially autoreactive T-cell clones, on one side, and an altered maturation and/or selection of immunoregulatory cells (NKT and CD4+8-25+ T reg cells) keeping these cell clones under control, on the other side.

Neonatal androgenization affects the intrathymic T-cell maturation in rats

The thymus structure, expression of CD4/CD8/TCRalphabeta on thymocytes and thymocyte proliferative and apoptotic indexes were analyzed in sexually immature 30-day-old and in sexually mature 60-day-old female rats neonatally androgenized (NA) by subcutaneous injection of 500 microg testosterone propionate/day on days 1-3 and in their vehicle-administered counterparts. The treatment affected normal thymus development. Thus, at 30 days of age, there was a reduction in the thymus weight, reflecting a decrease in the main thymic compartments. However, at 60 days of age, thymus weight did not significantly differ from that in age-matched controls, since the cortical volume enlargement was followed by a proportional decrease in the medullary volume. In rats of both ages, the changes in thymic compartments most likely reflected alterations in the size of both lymphoid and nonlymphoid components. Furthermore, in NA rats, substantial changes in thymocyte phenotypic characteristics were registered, in spite of their age. In both groups of NA rats, a decrease in the relative proportion of the least mature CD4-8-TCRalphabeta- cells and in that of CD4+8- TCRalphabeta-/TCRalphabeta(low) cells followed by an increase in the percentage of their successor CD4+8+TCRalphabeta-/TCRalphabeta(low) cells was detected. In addition, in 30-day-old NA rats, the relative proportions of CD4+8+TCRalphabeta(high) cells (just positively selected) and that of mature single positive (CD4+8- and CD4-8+) and CD4-8- double negative TCRalphabeta(high) cells, were reduced, while in 60-day-old NA rats only the percentage of CD4+8+TCRalphabeta(high) thymocytes was decreased. Thus, the study showed that the changes in the development of the hypothalamo-pituitary-gonadal axis induced by neonatal androgenization may affect the thymus development and intrathymic T-cell maturation.

Thymopoiesis following chronic blockade of beta-adrenoceptors

The present study was undertaken in order to further clarify putative role of the adrenergic innervation in the regulation of the intrathymic T-cell maturation. For this purpose adult male DA rats were subjected to either 4-day- or 16-day-long propranolol treatment (0.40 mg propranolol/100 g/day, s.c.) and the expression of CD4/8/TCRalphabeta on thymocytes, as well as thymocyte proliferative and apoptotic index, was assessed in these animals by flow cytometric analysis. Propranolol treatment, in spite of duration, increased both the thymocyte proliferative and apoptotic index (vs. respective vehicle-treated controls). In 4-day-treated animals the thymus cellularity and thymus weight remained unaltered, while in 16-day-treated rats the values of both of these parameters were reduced (since increase in the thymocyte apoptotic index overcame that in the proliferative index). The treatments of both durations affected the thymocyte phenotypic profile in a similar pattern, but the changes were more pronounced in rats exposed to the treatment of longer duration. The relative proportion of the least mature CD4-8- double negative (DN) TCRalphabeta- cells was increased, those of thymocytes at distinct differentiational stages on the transitional route to the CD4+8+ double positive (DP) TCRalphabetalow stage decreased (all subsets of TCRalphabeta- in both groups of rats, and those with low expression of TCRalphabeta in rats subjected to 16-day-long treatment) or unaltered (all subsets of TCRalphabetalow cells in 4-day-treated rats). Furthermore, the percentage of CD4+8+ DP TCRalphabetalow cells was significantly elevated, as well as those of the most mature CD4+8- TCRalphabetahigh and CD4-8+ TCRalphabetahigh cells (the increase in the percentage of former was much more conspicuous than that of the latter), while the relative proportion of their direct detectable precursors (CD4+8+ DP TCRalphabetahigh) was reduced. Thus, the present study: i) further supports notion of pharmacological manipulation of adrenergic action as an efficient means in modulation of the T-cell development, and hence T-cell-dependent immune response, and ii) provides more specific insight into T-cell maturation sequence point/s particularly sensitive to beta-adrenoceptor ligand action.

[Binding of natural IgM autoantibodies DJ to T-lymphocytes and T-cell hybridomas and its possible role in apoptosis]

INTRODUCTION

Natural autoantibodies can recognize self-antigens expressed on damaged cell membranes and participate in their elimination from the body and maintainance of homeostasis. It has been shown earlier that human monoclonal immunoglobulin M DJ isolated from sera of patients with Waldenstrom macroglobulinemia, possesses characteristics of natural antibodies/autoantibodies and binds to phosphoryl choline. In this paper we analyzed binding of IgM DJ to the membrane of thymocytes, T-lymphocytes derived from lymph node and to T-cell hybridomas.

MATERIAL AND METHODS

Human monoclonal IgM antibodies were isolated by means of euglobulin precipitation and used for immunization of BALB/c mice. T-lymphocytes were isolated from lymph nodes and stimulated by antigen or Concanavalin A, used as a mitogen. For obtaining T-T hybridoma, fusion was done with BW 5147 cell line. Binding of IgM DJ to T-lymphocytes and T-hybridomas was detected by flow cytometry.

RESULTS

We presented IgM DJ binding to the membrane of thymocytes and lymphocytes of the lymph nodes from nonimmunized mice. Binding with similar characteristics was demonstrated for T-lymphocytes isolated from lymph nodes from immunized mice before in vitro stimulation, which significantly increased after stimulation either by mitogen or by antigen. The highest level of binding of IgM DJ was obtained in the population of in vitro cultivated lymphocytes, not stimulated in vitro. By analyzing cells which bound IgM DJ, it has been established that the examined cell population presented with characteristics of cells undergoing apoptosis.

CONCLUSION

These results indicate that IgM DJ can be involved in maintenance of homeostasis by elimination of cells expressing phosphoryl choline during apoptosis.

Alterations in thymopoiesis in intact and peripubertally orchidectomized adult rats of different age

In adult rats 3, 6 and 9 months post-orchidectomy performed at the age of 30 days the thymus weight, thymocyte yield and relative proportions of thymocyte subsets (delineated by expression of CD4/CD8 molecules and TCRalphabeta) were analyzed in order to elucidate a putative role of male gonadal hormones in the shaping of thymus size and intrathymic T cell maturation. In 4-month-old control rats the thymus size and cellularity returned to the corresponding levels in 1-month-old rats. These levels were sustained during the following 6 months. In spite of that, the distribution of the main thymocyte subsets in these rats was subjected to significant changes, probably due to an age-associated diminishing thymus ability to provide efficient T cell differentiation. The results added further weight to a potential feedback regulatory role of CD4+8- cells in thymopoiesis. Furthermore, they revealed that the orchidectomy-induced (i) enlargement of the thymus size and enrichment of the thymic lymphoid cell content are of a limited duration; and (ii) alterations in the relative proportion of thymocytes become quantitatively more pronounced with duration of the gonadal deprivation. Thus, the study also indicates that the age-associated changes in gonadal hormones may be, at least partly, responsible for the age-related reshaping of the T cell maturation sequence, and hence for remodeling T cell dependent immune functions.

In vivo modulation of the splenocyte yield and composition by female sex steroid hormones

The study was designed to shed more light on the controversial role of the two main ovarian steroid hormones (i.e. estradiol and progesterone) in shaping the size and phenotypic characteristics of the splenic lymphocyte pool. For this purpose ovariectomized adult rats (OVX) were treated for 14 subsequent days with either estradiol or progesterone (to attain physiological concentrations of the hormones). Afterwards, the splenocyte yield, and overall number of splenocytes bearing TCR alpha beta receptor, CD4 and CD8 coreceptor were evaluated. Fourteen-day-long ovarian hormone deprivation produced an increase in the splenic weight and splenocyte yield (on the account of a rise in the number of TCR alpha beta- cells), although the number of TCR alpha beta+ cells was reduced as a result of a decrease in the size of the CD4+ cell subpopulation. Replacement of either estradiol or progesterone prevented the increase in splenic weight and reduced the splenocyte yield to values significantly lower than that in sham-OVX rats. Both the treatments completely abolished the effect of ovariectomy on the size of TCR alpha beta- cell population, but had differential effects on that of TCR alpha beta+ cell population; estradiol did not affect its size, while progesterone caused a reduction on the account of a decrease in the numbers of both CD4+ and CD8+ cells. The results suggest that: a) estradiol and progesterone have similar effects on the size of the splenic B cell population and that replacement of either estradiol or progesterone can prevent the effects of ovariectomy on the size of this population and b) estradiol does not affect while progesterone reduces the size of splenic T cell population. Thus, replacement of none of them is able to compensate the removal of gonads.

In vivo modulation of the distribution of thymocyte subsets by female sex steroid hormones

This study examined the effects of the principal ovarian steroids, 17 beta-estradiol (E) and progesterone (P), on the thymic structure and on the intrathymic development of T-cells. Adult female rats were ovariectomized (OVX) and treated for 14 days with physiological doses of either E or P; controls received an equivalent volume of vehicle. Ovariectomy produced a marked increase (vs. sham-operated controls) in thymus weight, which was associated with an increase in the volume and cellularity of both the medulla and cortex. Treatment of OVX rats with E reduced the thymic weight to value, which was significantly lower than that of sham-operated controls decreasing the volume of cortex below level in sham-OVX rats, and reversing the effect of ovariectomy on the volume of medulla. P only prevented the increases in thymus weight and cortical volume induced by OVX. However, unlike E, it had no discernable effect on the medullary volume. E treatment reduced the cellularity of the cortex and medulla to values, which were lower than those of sham-OVX rats, while P only reversed the effects of OVX on the cellularity of both the compartments. Ovariectomy also had a profound effect on the thymocyte profile, increasing the proportion of CD4+8+TCR alpha beta- cells and producing a corresponding decrease in the relative proportions of all TCR alpha beta high cell subsets. The decrease in the latter was opposed by treatment with E or P. However, the sensitivity of the less mature cells (except CD4-8-TCR alpha beta-, the percentage of which was reduced by both hormones) to the two hormones differed. E reduced the relative proportion of CD4-8+TCR alpha beta-, CD4-8+TCR alpha beta low and CD4+8+TCR alpha beta- cells, while P increased the percentage of CD4-8+TCR alpha beta low cells. The results suggest that E and P affect both the lymphoid and nonlymphoid compartments of the thymus, and that while P increases the volume of the nonlymphoid component of the medulla, E has the opposite effect. The finding that ovariectomy decreased while E and P increased the relative proportion of the most mature thymocytes, which include CD4-8-TCR alpha beta high cells that are believed to harbour potentially autoreactive cell clones, is particularly interesting and may relate to the high propensity of autoimmune diseases in females.

Differential effects of chronic propranolol treatment on the phenotypic profile of thymocytes from immature and adult rats

To elucidate a putative role of beta-adrenoceptors in the modulation of intrathymic T-cell maturation, the expression of major differentiational antigens (CD4/CD8 and TCR alphabeta) on the thymocytes from both immature (aged 21 day at the beginning of the treatment) and adult (aged 75 days at the beginning of treatment) male rats subjected to a 15-day-long propranolol treatment (0.40 mg/100 g/day, s.c.) was analyzed by two- and one-color flow cytometry, respectively. Rats of matched age injected with saline served as controls. The propranolol treatment in immature but not adult rats caused a significant reduction in both the relative thymus weight and total thymocyte yield. In addition, a significant increase in the percentage of CD4+ 8+ double-positive cells, with a proportional decrease in the relative proportion of CD4+ 8- single positive cells, was found in immature rats. In contrast, a slight but significant decrease in the percentage of CD4+ 8+ cells with a parallel increase in the relative proportion of CD4+ 8- cells was found in adult rats. In both groups of rats, the percentage of TCR alphabeta(total) thymocytes was increased: in immature rats this was due to an increase in the percentage of TCR alphabeta(low) thymocytes, while in the adult rats it reflected a rise in the relative proportion of TCR alphabeta(high) cells. In conclusion, the study revealed that propranolol treatment in both immature and adult rats alters the relative proportion of CD4+ 8+ and CD4+ 8- thymocytes, but in opposite fashion, and the data suggest that this treatment affects distinct fractions within the population of CD4+ 8+ thymocytes with respect to expression of TCR alphabeta. The results also indicate that, regardless of rat sexual maturity, the development of thymocytes towards CD4- 8+ T-cells is relatively insensitive to long-lasting beta-adrenoceptor blockade.

Naltrexone prevents ethanol-induced changes in rat thymus

Ethanol is known to suppress the immune response, but the underlying mechanism accounting for the immunosuppression is not clearly elucidated yet. The purpose of this study was to investigate the effect of a single dose of ethanol on relative proportion of the four major rat thymocyte subsets and possible mechanism of its action. To this end, adult female AO rats were treated with: a) ethanol (2 or 4 g/kg, i.p.), b) naltrexone (5 mg/kg, i.p.) followed 45 min later by ethanol (2 or 4 g/kg, i.p.), c) naltrexone (5 mg/kg, i.p.), or d) only saline. Twenty hours later the rats were sacrificed and the proportion of the four major thymocyte populations defined by expression of CD4 and CD8 molecules was analyzed. Flow cytometric analysis revealed that ethanol evoked a decrease in the percentage of double-positive CD4+CD8+ thymocytes followed by a proportional increase in the percentage of single-positive CD4+CD8- cells. Naltrexone pretreatment prevented the ethanol-induced alterations in thymocyte subsets. The results clearly indicate that ethanol affects the process of intrathymic T-cell maturation. It seems that this effect might be mediated by an opioid-dependent mechanism.

Differential effects of gonadectomy on the thymocyte phenotypic profile in male and female rats

As an organ responsible for generation of T-cell repertoire the thymus occupies a central position in establishment of mature immune response. To assess the potential role of the gonadal steroids in development and maintenance of immunological sexual dimorphism, the effects of gonadectomy pre- and postpuberty on the thymocyte profile of male and female rats were examined. Rats aged 30 days or 75 days were gonadectomized; 30 days later the thymic cellularity was estimated and the expression of the cell surface antigens (CD4 and CD8) and the T-cell receptor (TCR) alpha beta was analyzed by flow cytometry. Regardless of age at surgery, the thymus weight and total thymocyte yield were greater in sham-operated males than females; this sexual dimorphism in thymic cellularity persisted after gonadectomy. Sexual dimorphism in the composition of thymocyte subsets was also evident in sham-operated rats, with males expressing a higher percentage of CD4-8- cells, and remained after gonadectomy of adult rats. In male rats, gonadectomy at day 75 increased the percentage of CD4+8- single-positive and TCR alpha beta + cells. In contrast, in females, ovariectomy decreased the percentages of CD4+8- single-positive, CD4-CD8- double-negative, and TCR alpha beta + cells and increased the percentage of CD4+CD8+ double-positive cells. In the immature rats gonadectomy increased the percentages of CD4+CD8- single-positive and TCR alpha beta + thymocytes and decreased the percentages of double-positive and double-negative cells in males, while in the female it increased the percentage of CD4+8- single-positive thymocytes. Gonadectomy at that age abolished the sexual dimorphism in the expression of accessory molecules (i.e., CD4/CD8), but facilitated gender-specific expression of TCR alpha beta. In conclusion, the results suggest that the gonadal steroids are more important for the development than for the maintenance of the sexual dimorphism in the thymocyte composition.

Leflunomide prevents the development of experimentally induced myasthenia gravis

Myasthenia gravis is an autoimmune disease in which autoantibodies specific to the acetylcholine receptor (AChR) are formed, leading to a gradual destruction of the receptors in muscles that are responsible for picking up nerve impulses, and results in weakness and eventual loss of muscle function. The novel immunomodulating drug leflunomide (HWA 486) has been shown to be very effective in preventing and halting ongoing disease in an array of experimental autoimmune disorders and reactions leading to organ graft rejection. Further, recent data from phase II clinical trials indicate that this drug is efficacious and is safe in humans with rheumatoid arthritis. In the studies reported here, we found that rats immunized with AChR-protein and not receiving leflunomide developed experimental myasthenia gravis (EMG) between day 7 and 11 post-immunization, and about 79% of these animals expressed clinical signs of disease. Treatment of AChR-protein immunized rats with leflunomide, from the day of disease induction, totally suppressed the development of EMG. Thus, the results we have obtained using leflunomide in EMG indicate that this drug could be beneficial in combating myasthenia gravis in humans.

Prepubertal castration alters the phenotypic profile of adult rat thymocytes

To assess the gonadal influence on the maturation of the thymus, rats were orchidectomized at different periods critical for programming of both gonadal and immune functions, and the composition of the intrathymic thymocyte population was determined in adults by flow cytometric analysis of the surface phenotype. The relative proportion of thymocytes expressing CD4, CD8 and T cell receptor (TCR)alpha beta was measured in the adult rats castrated at 1, 7 and 30 days. Castration performed at postnatal day 1 did not significantly affect expression of these molecules. However, in rats subjected to the surgery at day 7, a significant decrease in the proportion of CD4+CD8+ double-positive cells was found, in parallel with a proportional increase in the percentage of CD4+CD8- single-positive (SP) cells. In rats castrated at age 30 days, in addition to these changes, a small but significant increase in the percentage of CD4-CD8+ SP thymocytes was measured. Castration performed at age 7 and 30 days also caused an enrichment in the thymocyte population expressing TCR alpha beta, probably related to the increase of CD4+CD8- SP cells (7 days) and both CD4+CD8- and CD4-CD8+ SP cells (30 days). The total yield of thymocytes was increased in all experimental groups. We conclude that T cell maturation sequences in the adult thymus are altered if gonadal influence is removed during certain stages of thymus development.

Isolation, cultivation and phenotypic characterization of rat thymic dendritic cells

Rat thymic dendritic cells (TDC) were isolated from thymic cell suspension by Nycodenz gradients of different densities and osmolarities. After cultivation of these cells for 3 days in the conditioned medium (TE-R 2.5 + HT supernatant) prepared by cocultivation of a medullary thymic epithelial cell line (TE-R 2.5) and hydrocortisone resistant thymocytes, the purity (75-85%) and survival of TDC significantly increased. The supernatant contained moderate activities of IL-1 and IL-6, low levels of TNF-alpha and IL-2 and factor(s) that strongly stimulated the proliferation of a mouse macrophage cell line RAW 264.7. TDC survival in culture was significantly increased by GM-CSF and decreased by IL-6 and IFN-gamma. The phenotype of TDC was studied by flow cytometry using a panel of monoclonal antibodies (mAb) to rat cell surface markers. It was found that almost all freshly isolated TDC expressed major histocompatibility complex (MHC) class I and class II molecules as well as CD45. Most TDC were LFA-1 (CD11a)+, CD18+, ICAM-1 (CD54)+ and CD53 (OX- 44)+, but only certain subsets expressed CD11b and thymocyte markers Thy1, CD2, CD4 and CD8. Upregulation in the expression of almost all the markers was observed after cultivation of TDC. In addition, cultivated, but not freshly isolated TDC expressed CD25 (IL-2R alpha) and CD45RC (OX-22) molecules. Cultivated TDC had strong accessory function in autologous thymocyte proliferation.

Thymus changes in experimentally induced myasthenia gravis

Experimental myasthenia gravis (EMG) was elicited in female AO rats, 8-12 weeks of age, by injection of 100 micrograms/rat Torpedo marmorata acetylcholine receptor (AChR)-protein incorporated in CFA. Bordetella pertussis, 24 x 10(9) microorganisms, rat, was injected simultaneously as additional adjuvant. Rats were sacrificed on the day of appearance of the clinical signs of EMG, and thymuses were used for histological analysis using stereologic method, and thymocyte subsets were estimated by flow cytometry. Two and three colour fluorescence was applied to determine DN (CD4-CD8-), DP (CD4+CD8+), SP-CD4+ (CD4+CD8-) and SP-CD8+ (CD4-CD8+) subsets, as well as thymocytes expressing TCR alpha/beta. Rats immunized with BSA and rats injected with saline were used as controls. From 56 rats immunized with AChR-protein, 44 rats developed the disease, between day 7 and 11 after immunization. Severity of disease varied from + to + + +. Stereologic analysis of tissue sections revealed a highly significant reduction of thymic cortex and hypertrophy of medulla in EMG thymuses. Similar, but very slight changes were observed in thymuses of rats immunized with BSA. Percentages of DN, SP-CD4+, and SP-CD8+ subpopulations were significantly increased, while the percentage of DP population showed a marked decrease. These preliminary data suggest an alteration of thymocyte maturational events. Whether these changes could be responsible for the initiation of autoimmunity, or are occurring as a secondary phenomenon, after EMG was already established following the injection of cross reactive antigen, is a matter for discussion.