The Effect Of UHRF2 On The Maturation Of Germinal Center B Cells

Germinal centres (GCs) are transient structures that formgenerate within peripheral lymphoid organs in response to T cell-dependent antigen. Within GCs, B cells expressing high-affinity antibodies develop and differentiate into antibody-secreting plasma cells and memory B cells that mediate and sustain protection against invading pathogens. Besides, GC B cells proliferate at a rate that is unparalleled in mammalian tissues. The same genetic mechanisms that enable the development of high-affinity immunoglobulin receptors of different isotype classes are involved in the malignant transformation of B cells. The regulation mechanism of this process is important both for basic research and clinical study.UHRF2 is an "intermodular hub" that occupies a central position in the cell cycle network, and facilitates coordination among the cell cycle machinery, the ubiquitin-proteasome system, and the epigenetic system. Previous studies suggest that UHRF2 can not only interacts with cyclins, CDKs, p53, pRB, PCNA, HDAC1, DNMTs, G9a, methylated histone H3 lysine 9 and methylated DNA, but also ubiquitinates cyclins Dl and E1, and induces G1 arrest. By integrating different signals, UHRF2 can play a key role in coordinating the cell cycle regulation network and epigenetic signaling pathways, and regulates the differentiation of cells. It is reported that UHRF2 gene, a candidate tumor suppressor, is frequently lost in tumors.To determine the function of UHRF2 in B cells, we isolated human naive B cells, pre-germinal center B cells, GC B cells and memory B cells from human tonsil by flow cytometry (FACS). The comparative expression level of UHRF2 was determined by both RT-PCR and western blot. Human germinal center B cells express the highest level of UHRF2 among all the B cell subsets analyzed. Besides, the expression level of UHRF2 in the memory B cells is similar to naive B cells. These data indicate that UHRF2 may play an important role in germinal center.To detect the function of UHRF2 in vivo, we got UHRF2 total knockout mice developed by cas9 technology, and detected the impact on B cells after knocking out UHRF2. The result shows that the percentage of pro-B, pre-B, immature B, transitional B and mature B cells were similar between UHRF2 knockout mice and wildtype littermates. However, the percentage of GC B cells in peyer’s patch were significantly increased in UHRF2 knockout mice compared to wildtype littermates.In addition, following immunization with NP25-CGG, the percentage of GC B cells in spleen was significantly upregulated in UHRF2-deficient mice as that under unchallenged condition. In contrast, the antibody production of first immune response and secondary immune response did not show any significant difference.Combined with the reports that overexpressing UHRF2 may induce G1 arrest, our results suggest that the regulation of B cell proliferation will be out of control after knocking out UHRF2, which maybe the cause of the abnormally increased proportion of GC B cells. Therefore, UHRF2 maybe a potential tumor suppressor in B cell lymphomas.