Germinal Centers (GC) of secondary lymphoid tissues are critical to mounting a high-affinity humoral immune response. B cells within the GC undergo rapid clonal expansion and selection while diversifying their antibody genes through class switch recombination and somatic hypermutation. Although it is generally believed that GC B cells employ a unique proliferative program to accommodate these processes, very little is known about how the GC-associated cell cycle is orchestrated. The D-type cyclins are important regulators of the G1 phase of the cell cycle and are the ultimate targets of many mitogenic and oncogenic stimuli. The Cyclin D3 gene is rearranged and over-expressed in certain mature B cell malignancies, and its overexpression has been reported to predict poor clinical outcome in patients with diffuse large B cell lymphoma. It has been observed that during their development, B cells switch from expressing cyclin D2 to cyclin D3 when they are recruited into the GC response. It is unclear, however, whether this switch simply reflects a change in the transcription factors that govern cyclin expression or serves a biological mandate. Here we report that mice deficient in cyclin D3 are profoundly impaired in their ability to form GCs as measured by immunohistochemistry and flow cytometry. Production of antigen-specific antibodies and affinity maturation, as ascertained by ELISA, are concomitantly reduced in these animals. These phenotypes can be at least partially explained by a significant block in the G1-phase of the cell cycle of GC B cells in vivo. Interestingly, this block in the G1-S transition is observed despite an apparent compensatory increase in cyclin D2 expression. In addition, naive B cells activated in vitro by either LPS or LPS and IL-4 display only minor changes in cell-cycle profile, suggesting that a specific requirement for cyclin D3 is unique to GC B cells. We also find moderately reduced Bcl6 mRNA expression in both naìˆve and GC B cells from the cyclin D3 knockout mice. Since Bcl6 is a master regulator of the GC response, decreased activity of this transcriptional repressor may further contribute to the severity of the GC phenotype. This is the first demonstration that cyclin D3 plays a unique role during the GC response in that it is required for its optimal structure and function. In addition to expanding appreciation for the cell type- and tissue-specific functions of the three D-type cyclin molecules, our findings have implications for understanding the role of Cyclin D3 in human B cell lymphomas.

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