Neonatal surgical castration of male pigs reduces thymic growth but has moderate consequences on thymocytes

Thymus integrates numerous signals from the neuroendocrine-immune system, including sex steroids, glucocorticoids and catecholamines. Neonatal surgical castration, commonly practiced in pig husbandry modifies thymic hormonal environment, e.g. sex steroids and probably glucocorticoids and catecholamines, which are important modulators of thymic function. This study aimed at investigating, in pubescent male pigs, the consequences of neonatal suppression of testicular hormones on thymic T-cell differentiation and hormonal control of thymocyte proliferation. A total of 34 male pigs were allocated to 2 experimental groups: control intact males (CT) and males surgically castrated at 5 or 6 d of age (SC). At slaughter, thymus was weighed and thymic samples were collected to determine fat content, distribution of thymocyte subsets by identification of CD1, CD4, CD8, γδ T cell receptor (TCR) cell surface markers and to measure thymocyte proliferation in presence of cortisol, norepinephrine and sex steroids. Results showed that absolute and relative thymus weights were greater (P < 0.01, P < 0.01 respectively) whereas thymic fat content was less (P < 0.01) in CT than in SC pigs. Surgical castration did not change the frequency of CD1+ immature thymocytes. The proportion of γδ T cells tended to be greater in CT than in SC pigs (P < 0.1) but the proportions of CD4+, CD8+ and CD4+CD8+ thymocytes were similar in both groups (P > 0.1) indicating that the Tαβ lineage was not influenced by early castration. Proliferation of thymocytes in response to concanavalin A (ConA) was greater in CT than in SC pigs (P < 0.05). Cortisol and norepinephrine decreased the ConA-induced proliferation in CT and SC pigs (P < 0.05). In addition, proliferation of thymocytes was less inhibited by norepinephrine in SC than in CT males (P < 0.05). The greatest concentration of testosterone (25 ng mL-1) increased (SC males, P < 0.05) or tended to increase (CT males, P < 0.1) the proliferative responsiveness to Con A but the lowest dose (2.5 ng mL-1) and the greatest dose of testosterone combined with estradiol had no significant effect (P > 0.1). Overall, our data show little effect of neonatal castration on thymocyte differentiation as well as of sex hormones on thymocyte proliferation. However, thymic cells seem to be more sensitive to the inhibitory influence of norepinephrine in CT than in CS pigs. The significance of such difference for animal health remains to be explored.