The Functions Of Transcription Elongation Factor ELL3in Murine B Cells

ELL3as an elongation factor belongs to ELL (eleven-nineteen lysine-rich leukemia gene) family which includes3members:ELL1, ELL2and ELL3. In contrast to ELL1and ELL2which are expressed ubiquitously, ELL3is highly and specifically expressed in B cells. Although ELL3could promote RNA synthesis through increasing RNA polymerase catalytic rate in vitro, the biological function of ELL3in vivo is not clear. Transcription is known to be indispensable for somatic hypermutation (SHM), and transcription pausing and stalling causes multiple clustered mutations by AID. As ELL3is specifically expressed in B cells, it is interesting to investigate whether ELL3plays a special role in B cells. The two major questions we set out to ask are:Is the ELL3involved in the SHM process? Does ELL3deficiency affect SHM (somatic hypermutation) pattern or distribution?To address these questions, we generated ELL3knockout (KO) C57/B6mice. The development of B and T cells was normal in ELL3knock out mice. The KO mice showed a deficiency in responding to immunization of different doses of T-dependent antigen, whereas T-independent antigen immunization induced a normal immunoglobulin production. Consistent with T-dependent immune deficiency, the germinal center formation was impaired in ELL3KO mice. As ELL3is not expressed in T cells, it may affect T-dependent immune response through altering B cell functions.In vitro, live cell percentage decreased in ELL3KO B cells under high doses of anti-IgM stimulation, which may indicate that ELL3plays a role in B cell survival and cause germinal center death by changing expression of some specific genes. To find out the target genes of ELL3, we sequenced the transcriptome of germinal center B cells using RNA-seq, which revealed an up-regulation offos gene in ELL3KO mice. This is in accordance with a previous finding that fos over-expression caused B cell death in germinal center of fos transgenic mice. Furthermore, we analyzed the SHM after T-dependent immunization, showing that mutaion frequency of JH4intronic region was significantly decreased in KO mice, and the VH186.2mutation frequency was normal, which may be due to selection of high affinity antibody secreting cells in germinal center. The decrease of the JH4intronic mutations frequency was consistent with the germinal center death in KO mice. All these results together suggest that ELL3plays pivotal roles in T-dependent immune response and germinal center formation.The findings reported in this thesis are helpful to understand how the transcription elongation factor is involved in the immune response, and may also help to improve the clinical diagnosis of specific category of immune deficiency.