Role of ovarian hormones in age-associated thymic involution revisited
Introduction
The most dramatic age-related change within the immune system is thymic involution. At the cellular level this is exemplified by a profound reduction in thymic size, overall thymocyte content, and number of mature CD4+CD8− and CD4−CD8+ single positive (SP) T cell receptor (TCR)αβhigh cells that exit into the periphery and migrate to secondary lymphoid organs as phenotypically distinguishable recent thymic emigrants (RTEs) (Ye and Kirschner 2002; Luettig et al. 2001; Sempowski et al. 2002). The decrease in thymic output, in combination with gradual exhaustion of the naïve T-cell pool with progressive antigen contact and peripheral T-cell clonal expansion, leads to an age-associated bias towards memory cells (reviewed in Berzins et al. 2002). Thymic changes, apart from narrowing T-cell repertoire diversity, probably also contribute to disruption of T-cell subset balance within the peripheral lymphoid compartment. The age-associated, mainly thymus-dependent, decline in CD4+ lymphocyte abundance in combination with oligoclonal expansions in the CD8 T-cell subset (Clambey et al. 2007; Czesnikiewicz-Guzik et al. 2008) leads to a decrease in the CD4+/CD8+ splenocyte ratio (De Paoli et al. 1988; Gilman-Sachs et al. 1991; Pahlavani and Richardson 1994; Clambey et al. 2007; Czesnikiewicz-Guzik et al. 2008). Clinical consequences of immunosenescence comprise increases in the frequency and severity of infectious diseases and raised incidences of chronic inflammatory disorders, cancer and autoimmunity (Hakim and Gress 2007; Goldberg et al. 2007). The age-related decline in thymic function becomes critical following cytoablative treatments, such as chemo- and radiation-therapy, and acquired immune deficiency syndrome (AIDS), i.e. when it is necessary to resurrect the dramatically impaired T-cell pool.
Given that thymic involution is most evident from puberty onwards, so the start of this process coincides with an increase in gonadal steroid production (Grossman 1985; Bodey et al. 1997), a causal alliance between these events has been proposed. Indeed, in rodents of both sexes, elevation of gonadal hormone levels in young animals induces thymic atrophy similar to that observed in aged rats (Kuhl et al. 1983; Luster et al. 1984; Windmill et al. 1993; Dulos and Bagchus 2001; Oner and Ozan 2002; Yellayi et al. 2002), whereas surgical castration before puberty and in early adulthood prevents thymic involution and reverses the early involutive changes, respectively (Kendall et al. 1990; Windmill et al. 1993; Leposavić et al. 1996; Windmill and Lee 1998; Pejcić-Karapetrović et al. 2001; Heng et al. 2005).
In young adult rodents, estrogen-induced thymic atrophy is marked by a decrease in the ratio of cortex to medulla (Yao and Hou 2004) and changes in thymocyte subset composition, suggesting alterations in the T-cell differentiation/maturation pattern (Screpanti et al. 1991; Leposavić et al. 2001). Multiple intracellular (α and β) and membrane GPR30 estrogen receptor (ER)-specific mechanisms have been implicated in estrogen-induced thymic atrophy (reviewed in Leposavić and Perišić 2008), but the composite picture of cellular and molecular events is not yet clear.
Recent findings that the effects of peripubertal gonadectomy on thymic cellularity are long-lasting but transitory (i.e. 20 weeks post-gonadectomy it is indistinguishable from that in age-matched controls) have led to the hypothesis that peripubertal gonadal steroid changes and initiation of thymic involution are two causally dissociated events (Min et al. 2006). However, there are at least two other plausible explanations for this phenomenon (Pešić et al. 2007; Leposavić and Perišić 2008). Firstly, gonadal steroids may initiate or, more likely, substantially contribute to the initiation of thymic involution, whereas age-associated intrinsic hematopoietic and/or extrinsic stromal defects ensure progression of thymic involution. Secondly, the limited duration of gonadectomy-induced “rejuvenating” effects on the thymus may be due to compensatory production of androgens by thymic cells and estrogens by adrenal and fat tissue (Leposavić and Perišić 2008). Support for a role of the peripubertal increase in gonadal hormone production in the initiation of thymic involution in rats of both sexes comes from data that neonatal orchidectomy (Radojević et al. 2007) and neonatal androgenization (Leposavić et al. 2009) (i.e. experimental manipulations that decelerate sexual maturation) postpone the onset of thymic involution in male and female rats, respectively. On the other hand, the observations that orchidectomy in aged rats leads to complete reversal of thymic involution (Adcock et al. 1986; Greenstein et al. 1986) corroborate the notion that gonadal hormones contribute to the maintenance/progression of thymic involution in male rodents. However, to our best knowledge, there are no data on the effects of ovariectomy on the thymus with advanced chronobiological changes. This seems particularly important in the light of recent findings suggesting that, differently from male mice, the efficiency of gonadectomy at producing thymic changes in female mice is very much age dependent (Hince et al. 2008).
The present study was undertaken to: (i) address whether ovarian hormone ablation influences the progression of already advanced thymic involution, and the causally related decline in thymic naïve cell output, and (ii) if so, to elucidate the cellular and molecular mechanisms underlying this decline in detail. Therefore, T-cell differentiation/maturation was studied in rats ovariectomized (Ovx) when thymic involutive changes were already prominent and RTE levels were quantified in both peripheral blood and spleen as an example of a secondary lymphoid organ. Given that age-associated changes within the immune system, besides thymic-dependent narrowing of the T-cell peripheral repertoire, encompass partly the thymus-dependent decline in CD4/CD8 cell ratio in spleen (De Paoli et al. 1988; Gilman-Sachs et al. 1991; Pahlavani and Richardson 1994), T-cell subset composition following ovarian hormone ablation was also analyzed in this peripheral lymphoid compartment.
Section snippets
Animals and experimental protocol
This study included female Albino Oxford (AO) rats bred in the animal housing facility at the “Branislav Janković” Immunology Research Centre in Belgrade. Animals were kept in polyethylene cages containing sterilized wood shavings in an isolated room under controlled temperature (22±1 °C) and lighting conditions (12 h light:12 h darkness, lights on at 0700 h) and with free access to rat chow and tap water. Ten-month-old rats were bilaterally ovariectomized under pentobarbitone sodium (60 mg/kg;
Ovarian hormone ablation increased thymic weight and cellularity
In 11-month-old rats bilaterally Ovx at the age of 10 months both thymic weight and cellularity were greater (p<0.01) than in age-matched control rats (Table 1). Since there were no significant differences in the mean values of either of these two parameters between sham-Ovx and age-matched intact rats, the data from these two groups of animals were combined in all analyses of results and graphs. Furthermore, relative thymic weight (thymic weight per 100 g body weight) and thymocyte number
Discussion
The present study demonstrated that gonadal ablation in rats may reverse even well advanced age-associated thymic changes. This conclusion emerges from our findings that both thymic weight and cellularity were greater in 11-month-old rats deprived of ovarian hormones for a month than in both 10- and 11-month-old non-Ovx controls. Thus, it seems that physiological levels of ovarian hormones are implicated not only in the initiation (Leposavić and Perišić 2008; Leposavić et al. 2009), but also in
Acknowledgement
This work was supported by Grant no. 145049 from the Ministry of Science and Technological Development of the Republic of Serbia.
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