YY1 Is Required for Germinal Center B Cell Development.

YY1 has been implicated as a master regulator of germinal center B cell development as YY1 binding sites are frequently present in promoters of germinal center-expressed genes. YY1 is known to be important for other stages of B cell development including the pro-B and pre-B cells stages. To determine if YY1 plays a critical role in germinal center development, we evaluated YY1 expression during B cell development, and used a YY1 conditional knock-out approach for deletion of YY1 in germinal center B cells (CRE driven by the immunoglobulin heavy chain γ1 switch region promoter; γ1-CRE). We found that YY1 is most highly expressed in germinal center B cells and is increased 3 fold in splenic B cells activated by treatment with anti-IgM and anti-CD40. In addition, deletion of the yy1 gene by action of γ1-CRE recombinase resulted in significant loss of GC cells in both un-immunized and immunized contexts with corresponding loss of serum IgG1. Our results show a crucial role for YY1 in the germinal center reaction

Interleukin-7-dependent B lymphocytes are required for the anti-pneumococcal polysaccharide response and protective immunity to Streptococcus pneumoniae

Unlike human adults or adult mice, young children or young mice respond poorly to pneumococcal polysaccharides (PPS). In mice, B1b lymphocytes are the major responders to a variety of bacterial polysaccharides including PPS. Despite having B1b cells, young mice are severely impaired in responding to PPS, suggesting that B cells in the young are distinct from those in adults. Since B lymphopoeisis early in life is largely Interleukin-7 (IL-7)-independent, while in adults it is IL-7-dependent, we hypothesize that B cells developed in the presence of IL-7 are required for generating anti-PPS antibody responses. In support of this, we found that despite having B1b cells, young wildtype and adult mice deficient either in IL-7 or IL-7Rα are severely impaired in responding to Pneumovax®23 vaccine, and do not survive pneumococcal challenge. Furthermore, we found that transgenic expression of IL-7 promotes the anti-PPS response in young and confers protective immunity to young mice. To translate these findings to human infants we have utilized neonatal NOD/SCID/gcnull mice engrafted with human umbilical cord blood CD34+ hematopoietic stem cells to create a Human Immune System mouse (HISmouse) model. We have found that these HISmice generate several B cell subsets including B1 (CD19+CD20+CD27+CD43+CD70-CD69-) and the majority of them exhibit an immature phenotype. Moreover, just as young children, HISmice responded poorly to PPS. IL-7 is produced mainly by non-hematopoietic stromal cells, and unlike the human IL-7, the murine IL-7 is poor stimulator of human B lymphocyte development. Although our data indicate that IL-7-dependent B cells are crucial for generating anti-polysaccharide response, we also found that enforced expression of a polysaccharide (a1,3, dextran)-specific B cell antigen receptor heavy chain (VH J558) in mice can overcome the lack of anti-polysaccharide antibody responses in young mice even in the absence of an IL-7-dependent B lymphopoiesis

YY1 is required for germinal center B cell development and immunoglobulin class switching.

<p><b>(A)</b> Spleen cells from non-immunized <i>YY1</i>^<i>f/</i>, <i>γ1CRE</i> and <i>YY1</i>^<i>f/</i>f <i>γ1CRE</i> mice were stained with various antibodies to identify total B cells (CD19⁺AA4.1⁺, upper panel) and germinal center B cells (GC-B, DUMP^-IgD^-GL7^hiCD95^hi, lower panel). Percentages and number of <b>(B)</b> total B cells, and <b>(C)</b> GC-B cells per spleen of <i>YY1</i>^<i>f/</i>, <i>γ1CRE</i> and <i>YY1</i>^<i>f/</i>f <i>γ1CRE</i> mice. Fig A-C are from three independent experiments (<i>n</i> = 3 mice for each genotype). <b>(D)</b> We used ELISA to detect various isotypes of serum immunoglobulins from <i>YY1</i>^<i>f/</i>, <i>γ1CRE</i> and <i>YY1</i>^<i>f/</i>f <i>γ1CRE</i> mice. The concentration of IgM, IgA, total IgG, as well as IgG subclasses, IgG1, IgG2 and IgG3 were measured from sera samples that were obtained from four experiments (<i>n</i> ≥ 4 mice for each genotype). Asterisks indicate p<0.001.</p