Abstract:

This study investigated the effects of feeding weaned yak calves with milk replacer at a range of levels on the immune function of the spleen and thymus， using a transcriptomics approach to analyze gene transcript profiles. Fourteen healthy male yak calves were selected for this experiment. The calves were 42 days old at the start of the experiment， had a consistent genetic background， and an average weight of （36.30±1.09） kg. The calves were randomly assigned to either a low milk replacer group （dC， 0.8% of average body weight） or a high milk replacer group （dH， 1.2% of average body weight）. The calves had ad libitum access to calf starter and alfalfa （Medicago sativa） hay. After a 30-day preliminary period， when calves reached a body weight of more than 60 kg and a daily solid feed intake exceeding 1 kg， five calves from each group were randomly selected for slaughter and sample collection. Spleen and thymus tissues were collected for analysis. The main results were as follows： 1） Transcriptomics analysis of spleen tissue revealed significant upregulation of immune-related genes （e.g.， CXCL14， CXCL11， CXCL9） in the spleen of the dH group. Differentially expressed genes （DEGs） between the dH and dC groups were significantly enriched in pathways such as viral protein interaction with cytokine and cytokine receptor， cellular DNA-sensing pathway， chemokine signaling pathway， and RIG-Ⅰ-like receptor signaling pathway， suggesting enhanced antiviral capacity of the spleen. 2） In thymus tissue， the dH group exhibited increased transcript levels of antigen presentation-related genes （HSPA6， HSPA1A） and Toll/Imd pathway genes （MAPK10）. Furthermore， the DEGs were significantly enriched in pathways including antigen processing and presentation， cytochrome P450 xenobiotic metabolism， and the Toll and Imd signaling pathway. 3） Quantitative real-time PCR （qRT-PCR） analyses of thymus and spleen tissues corroborated the transcriptomic sequencing results， confirming the accuracy of the sequencing data. In conclusion， compared with the dC group， the dH group displayed changes in gene expression in the spleen indicative of enhanced immune responses to viral proteins， including the activation of virus defense-related pathways such as the interferon signaling pathway. Similarly， antigen-presenting cell processing and presentation functions were improved in the thymus of calves in the dH group， indicative of increased immune competence. These findings provide a theoretical basis for nutritional interventions aimed at promoting early immune development in yak calves.

Key words: yak calves, milk replacer feeding level, spleen, thymus, transcriptomics