Abstract：CD4 is a membrane-bound glycoprotein found on T cells, and also an important surface marker of helper T lymphocyte. This molecule widely involves in recognition of specific antigens in association with major histocompatibility complexⅡ(MHCⅡ) molecules and transmembrane signal transduction. Expression of CD4 is critical for cell-mediated immune response and defense mechanisms. In this study, the yak (Bos grunniens) CD4 open reading frame (ORF) was cloned and sequenced using RT-PCR, and the CD4 mRNA and protein levels of adult yak thymus, spleen, mesenteric lymph nodes and haemal nodes were detected by means of qRT-PCR (quantitative Real-time PCR), Western blots and immunohistochemical streptavidin biotin-peroxidase complex (SABC) method. The results showed that yak CD4 ORF contained 1 188 bp, encoded 396 amino acids. The cDNA sequences of yak CD4 revealed significant homology with Bos taurus. The results of qRT-PCR and Western blots showed that CD4 mRNA and protein widely expressed in lymphoid tissues. The expression levels in the thymus was higher than that in other lymphoid tissues (P＜0.01); additionally the levels of haemal nodes was similar to that of mesenteric lymph nodes, and both of them were higher than those for spleen (P＜0.01). The immunohistochemical results showed that the CD4+ cells were distributed in the cotex and medulla of yak thymus. In the spleen, many CD4+ cells were located in the periarteriolar lymphoid sheaths and red pulp. Moreover CD4+ cells in mesenteric lymph nodes and haemal nodes were mainly presented in the mantle zone and medulla cord. The results indicated the thymus was a primary 1ymphoid tissue responsible for CD4+ T lymphocytes exportation and thymic production may influence the T lymphocytes extent of secondary lymphoid tissues. Then haemal nodes might have similar functions as the mesenteric lymph nodes and spleen. This study would provide fundamental data for immunogenetics and immunobiology, and also bring a new insight into timely immunization, the pathogenesis and disease prevention.
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