The ELL-associated Factor2Negatively Regulates The Survival Of Germinal Center B Cell

The germinal center (GC) B cells represent a unique cell population that arise in the secondary lymphoid organs during an immune reponse against T-dependent (T-D) antigens. These rapidly dividing cells undergo somatic hypermutation (SHM) and class switch recombination (CSR) of the immunoglobulin genes and those with increased affinity for antigen are selected to survive and differentiate into antibody-producing plasma cells or memory B cells. It is not well understood, however, how the survival of the GC B cells is regulated during an immune response.We found that the ELL-associated factor2(Eaf2) is selectively and highly expressed in GC B cells. Overexpression of EAF2in normal B cell can induce significantly increased cell death. To further explore the function of Eaf2in B cells, we generated C57BL/6background knockout mice. We found that B cell development in bone marrow and maturation in spleen was normal. B cell responses to various stimuli and serum Ig levels were also normal.However, GC B cells from Peyer’s patches of Eaf2-deficient mice exhibited enhanced survival compared with WT GC B cells under different in vitro culture conditions. By immunohistochemical staining of spleen sections at2weeks after immunization with the T-D antigen NP-CGG (4-hydroxy-3-nitrophenyl-acetyl coupled to chicken gamma-globulin), we confirmed that Eaf2-deficient GCs had fewer TUNEL+apoptotic cells than WT mice. In addition, the size of PNA+GC in Eaf2-deficient mice was enlarged by2-fold compared with WT mice.Consistently, mice deficient in Eaf2contained a higher frequency of GC B and memory B cells than did WT mice at2weeks after immunization. However, the numbers of GC and memory B cells decreased to normal levels at6and9weeks after immunization. The titers of the total and high affinity NP-specific antibodies were both increased in Eaf2-deficient mice. While the SHM in the JH4intronic region occurred normally, both the mutation frequency and ratio of the amino acid replacement mutations in the NP-specific VH186.2gene were moderately reduced, suggesting that affinity maturation against NP was compromised. Finally, by adoptive transfer experiment, we found that the absence of Eaf2did not impair the memory response.These results provide direct evidence that Eaf2mediates GC B cell apoptosis and is essential for preventing excessive GC expansion and for the affinity maturation of GC B cells. To our knowledge, this is the only molecule that directly affects the sizes of GCs in response to a T-D antigen. Our findings provide new insights into the mechanisms that regulate the dynamic GC reaction and memory and plasma cell differentiation during humoral immune responses.