Abstract: 【Objective】 To investigate the differences of spleen weight, histological structure and miRNA expression profile in Beijing Fatty Chicken and Leghorn Chicken, we screened the candidate genes related with the immune response capacity difference in two kind of chickens, so as to lay a foundation for poultry breeding for disease resistance. 【Method】 45 one-day-age Beijing Fatty Chickens and 45 female Leghorn pullets hatched in the corresponding period were selected as the experimental materials, fed in the same nutritional and environmental condition. The body weight and spleen weight were measured in 42 days. Three individuals were randomly selected for each species (body weight was close to the average value of the group). Half of the spleen was made into the paraffin sections. The spleen histological structure, number of splenic corpuscles and thickness of the periarterial lymphatic sheath were observed using HE staining. The small RNA was extracted from other half of the spleen tissue and composed into 2 RNA pools through balanced mix. The cDNA library was constructed. The high-throughput sequencing was conducted using a Solexa platform. The expressed genes were obtained through comparing the sequencing result and reference genome database. The new miRNAs gene was predicted using Mireap software. The gene expression was calculated using RPKM (reads per kb per million reads) method. The differentially expressed genes were screened according to the criteria of FDR（false discovery rate）＜0.001 and |log₂ ratio (T/CK)| ≥1. The target genes differentially expressing miRNA were predicted using TargetScan and Pictar softwares, the GO and KEGG database functions were annotated. The enrichment analysis of the target gene protein was conducted using DAVID software. The predicted target genes were intersected with the known QTL region annotation genes related with spleen weight and spleen index. 【Result】The body weight of the Beijing Fatty Chicken and spleen weight were significantly higher than those of Leghorns (P＜0.01). Histological observation showed that the tissue structure of the spleen was integrated in the two chicken breeds. The white pulp, red pulp, splenic corpuscle and periarterial lymphatic sheath could be clearly observed. Compared with the Leghorn chicken, the number of splenic corpuscles was more, periaterial lymphatic sheaths were thicker and intrathecal lymphocytes were more intensive in the Beijing Fatty Chicken. 486 known miRNAs, 130 candidate miRNAs and 216 coexpressed miRNAs were identified in the two groups of samples using high-throughput sequencing. There were 35 kinds of miRNAs differentially expressed between two groups. Among them, 8 kinds were up-regulated and 27 kinds were down-regulated in the Beijing Fatty Chicken. Their fold change was 1.002-10.41. The first five expressive abundances of differentially expressed genes were miR-2954, miR-6606-5p, miR-146b-5p, miR-21-3p and miR-21-5p. The bioinformatics analysis of 2767 target genes showed that they were involved in the biological processes of T, B lymphocyte cell differentiation, immune organ development, protein phosphorylation and cell morphogenesis. They were significantly enriched in ubiquitin mediated proteolysis, apoptosis, phosphatidylinositol signaling system etc. 14 pathways. The miR-6606-5p and miR-146b-5p common target genes- BLM and miR-2954 target gene IRF8 were localized in the QTL region related with spleen weight or spleen index. 【Conclusion】(1) There were some differences in spleen weight, histological structure and miRNA expression profile of the 42-day-old Beijing Fatty Chickens and Leghorn Chickens. (2) The differential high abundance miRNA (miR-2954, miR-6606-5p, miR-146b-5p, miR-21-3p and miR-21-5p) might be involved in the regulation of T, B lymphocytes differentiation, proliferation, activation and other processes through regulating target gene, so as to affect the spleen weight, organizational structure and immune response capacity.

Key words: spleen, microRNA, Solexa sequencing, differentially expressed genes