IMMUNOBIOLOGY Impaired Ig class switch in mice deficient for the X-linked lymphoproliferative disease gene Sap

X-linked lymphoproliferative disease (XLP) is characterized by abnormal im-mune responses to Epstein-Barr virus attributed to inactivating mutations of the SAP gene. Previous studies showed im-munoglobulin E (IgE) deficiency and low serum IgG levels in Sap-deficient mice before and after viral infections, which are associated with impaired CD4 T-helper function. In the present work, we find that signaling lymphocytic activation molecule (SLAM)–associated protein (SAP) is expressed in B cells and this expression is down-regulated after stimu-lation with lipopolysaccharide (LPS) and interleukin 4 (IL-4). We demonstrate that B cells from Sap-deficient mice exhibit reduced IgG and IgA production in vitro. This impairment correlates with de-creased circular transcript levels of I, I2a, I2b, and I3 after stimulation, which indicate a defective Ig switch recombina-tion in Sap-deficient B cells. While XLP is believed to cause defects in T, natural killer T (NKT), and natural killer (NK) cells, our results indicate that B cells are also affected. (Blood. 2005;106:2069-2075) © 2005 by The American Society of Hematolog