Immunohistochemical analysis of the thymus in newborn and adult yaks (Bos grunniens)

The thymus is the site of development and maturation of functional T lymphocytes and is critically important to the immune system. The purpose of this study was to examine the expression of markers of T lymphocytes, macrophages, dendritic cells, B lymphocytes and plasmocytes in the yak thymus. Twenty healthy male yaks were divided into newborn (2-4 weeks old, n = 10) and adult (3-4 years old, n = 10) group. qRT-PCR was used to evaluate the mRNA expression level of the main markers of the studied cell types. Immunohistochemistry was used to detect the distribution of CD3+ T lymphocytes, CD68+ macrophages, SIRPα+ dendritic cells, CD79α+ B lymphocytes, IgA and IgG+ plasmocytes. Within the same age group, the mRNA expression of CD3ε was highest (P < 0.05), followed by that of CD68, SIRPα, CD79α, IgG and IgA. Furthermore, CD3ε, CD68, and SIRPα mRNA expression levels were higher in newborn yaks than in the adult ones (P < 0.05), whereas those of CD79α, IgA, and IgG were higher in adults (P < 0.05). Immunohistochemical results showed localization of CD3+ T lymphocytes in the thymic cortex and medulla. CD68+ macrophages, SIRPα+ dendritic cells, CD79α+ B lymphocytes, IgA+ and IgG+ plasmocytes were mainly observed in the cortico-medullary region and medulla. In the same age group, the frequency of CD3+ T lymphocytes was higher than that of CD68+ macrophages and SIRPα+ dendritic cells (P < 0.05), followed by those of CD79α+ B lymphocytes and IgA+ and IgG+ plasmocytes. No significant difference was observed between B lymphocyte and plasmocyte frequencies in the yak thymus in both age groups (P > 0.05). The frequency of CD3+, CD68+ and SIRPα+ cells decreased from newborns to adults (P < 0.05). However, the frequencies of CD79α+, IgA+ and IgG+ cells increased from newborn to adult yaks (P < 0.05). The thymus of newborn yaks is well-developed, with higher numbers of T lymphocytes, macrophages, and dendritic cells than those in the adult thymus. However, higher frequencies of plasmocytes and B lymphocytes were detected in the adult thymus, suggesting that adults may better resist infections through humoralimmunity as this organ undergoes involution. Furthermore, there was no significant difference in the number of IgA and IgG plasmocytes, which differs from what is observed in rodents and humans. This difference might be related to the fact that yaks live in low-oxygen plateaus.