Newcastle disease, avian influenza and infectious bursal disease are the three most common infectious diseases in the poultry production, which have brought huge losses to the poultry industry. In this study, the classical vaccines of the three diseases were used to prepare chicken immune models, so the research results not only provide references for in-depth understanding the functions and characteristics of miR-20a-5p in different vaccine immune responses, but also have positive research significance for the immune prevention and treatment of the three diseases. Serum antibody detection results showed that the antibody levels of the IBD group, ND group, and H9 group gradually increased after immunization, reaching a peak on 21 dpi and then slowly decreasing, while no antibody was detected in the control group, indicating that the three vaccine-immunized chicken models were successfully prepared (the results were in attachment 1) (Wang 2020; Liu 2020).
2.1 Expression characteristics analysis of serum circulating miR-20a-5p in different immune responses
Analyzing the changes in the expression levels of serum circulating miR-20a-5p can indicate the functional characteristics of miR-20a-5p in different immune responses. The qRT-PCR results showed that, in IBD group, except for the significantly up-regulation on 21dpi (P<0.01), circulating miR-20a-5p were all down-regulated at the other time points and especially during 1-5 dpi (P<0.05). In ND group, the serum levels of miR-20a-5p were significantly up-regulated on 2dpi and 3dpi during the innate immunity stage (1-4dpi) (Abbas 2012; Murphy 2011) (P<0.05); during the transition stage from innate immunity to acquired immunity, miR-20a-5p was significantly up-regulated on 5dpi (P<0.05); during the acquired immunity period (7-35dpi) (Abbas 2012; Murphy 2011), circulating miR-20a-5p was significantly down-regulated on 7dpi and 28dpi, but significantly up-regulated to the peak (P<0.01) on 21dpi. In H9 group, the circulating miR-20a-5p was down-regulated at all time points and especially on 2dpi, 21dpi and 28dpi (P<0.01) (Figure 1).
By comparison with the control group, circulating miR-20a-5p showed a similar downregulation change pattern in IBD group and H9 group from 1dpi to 5dpi, which was the opposite to ND group. The results suggested that circulating miR-20a-5p actively participated in the innate immune response and the initial response of acquired immunity of the three immune responses, but there were differences in functions. Whether the significant down-regulations of miR-20a-5p in IBD group and H9 group during the innate immunity stage were related to the immunosuppression of IBDV (Gimeno and Schat 2018) and inactivated H9 vaccine remained to study in depth. Interestingly, in the acquired immunity stage, circulating miR-20a-5p showed the most significant differences and changes on 21dpi (significant increasing in IBD and ND groups, but decreasing in H9 group), which indicated that 21dpi was a key time node worth exploring.
2.3 Expression characteristics analysis of tissue miR-20a-5p in different immune responses
Through comprehensively analyzing the changes of serum circulating miR-20a-5p in different immune responses, we selected the three time points of 2dpi (innate immunity stage), 5dpi (transition stage from innate immunity to acquired immunity) and 21dpi (acquired immunity stage) with significant differences to further compare and analyze the expression characteristics of tissues miR-20a-5p in the three immune responses. On 2dpi, compared with the control group, miR-20a-5p was significantly up-regulated in the IBD group (P<0.01); in the ND group, except for the down-regulation of thymus, cecal tonsil and glandular stomach, miR-20a-5p was significantly up-regulated in the other tissues (P<0.01); in the H9 group, except for the thymus and spleen, miR-20a-5p was significantly up-regulated in the other tissues (P<0.01) (Figure 2A). These results suggested that the roles of miR-20a-5p in the innate immunity of these three vaccines were different, but there were certain similarities between IBD group and H9 group according relative expression levels, and different from ND group, which was consistent with the results of serum analysis.
On 5dpi, compared with the control group, miR-20a-5p was significantly down-regulated in the spleen, thymus, and glandular stomach in the IBD group (P<0.05), and up-regulated in the other tissues; in the ND group, miR-20a-5p was significantly down-regulated in all candidate tissues except for the small intestine (P<0.05); in the H9 group, miR-20a-5p was up-regulated in the cecal tonsil and glandular stomach, but significantly down-regulated in the other tissues (except for colon) (P<0.01) (Figure 2B). The results showed that miR-20a-5p actively participated in the initial stage of the acquired immunity stage. On the whole, the expression patterns of the ND group and H9 group were similar, and which had a great difference from that of IBD group.
On 21dpi, compared with the control group, miR-20a-5p was significantly up-regulated in the heart, colon, and cecal tonsils in the IBD group (P<0.05) and significantly down-regulated in the spleen, small intestine, and glandular stomach (P<0.05); in the ND group, miR-20a-5p was significantly down-regulated (P<0.01), but significantly up-regulated in the bursa and no change in the heart; in the H9 group, except for the up-regulation of miR-20a-5p in the bursa, it was down-regulated in the other tissues (Figure 2C). The results showed that miR-20a-5p actively participated in the phase of acquired immunity. Basically, the expression patterns of the ND group and H9 group were also similar, and which were different from the IBD group.
By comparing the changes of expression levels between the three time points, in control group, we interestingly found that miR-20a-5p showed a significant up-regulation trend in the spleen from 2dpi to 21dpi. Studies showed that miR-20a could promote Th cell differentiation (Wu et al. 2018), indicating that miR-20a-5p was positively related to the development of chicken spleen. In the IBD group, the expression levels of miR-20a-5p in the bursa of Fabricius showed a significant downward trend from 2dpi to 21dpi, and miR-20a was involved in regulating B cell responses (Xu et al. 2015; Wu et al. 2018), suggesting that miR-20a-5p was negatively related to the immune response in bursa of Fabricius. In addition, the expression levels of miR-20a-5p had similarities and differences in different immune treatment groups. For example, miR-20a-5p was significantly up-regulated in the heart, bursa of Fabricius, colon and small intestine in the three groups on 2dpi, significantly down-regulated in the spleen and thymus on 5dpi, and significantly down-regulated in the spleen and glandular stomach on 21dpi, suggesting that miR-20a-5p performed similar functions in different stages of the three immune responses. However, the differences of miR-20a-5p in the three immune responses were more significant than its similarities. For instances, in IBD group, miR-20a-5p was up-regulated in the thymus on 2dpi, colon on 5dpi, and cecal tonsil on 21dpi, while it was down-regulated in the same tissues in ND group and H9 group; and in IBD group, miR-20a-5p was significantly up-regulated in the heart on 21dpi, but no significant changes in ND group and H9 group. Basically, the expression distributions of tissue miR-20a-5p of ND group and H9 group at the last two time points were similar, which were different from that of IBD group, indicating that miR-20a-5p expression was related to the type of vaccine and tropism of virus, but the specific mechanism needed to be studied in the future.
In conclusion, miR-20a-5p actively participated in the whole processes of immune responses and played different roles in different immune responses. 2dpi, 5dpi and 21dpi were the possible key time points for the significant changes of serum circulating miR-20a-5p in the three immune responses, and the spleen, thymus and bursa of Fabricius were the differentially expressed tissues worthy of attention. This study can provide a positive theoretical reference for further studying the immune function of miR-20a-5p and its potential application as a molecular marker.