KU70 IS REQUIRED FOR LATE B-CELL DEVELOPMENT AND IMMUNOGLOBULIN HEAVY-CHAIN CLASS SWITCHING

Immunoglobulin (Ig) heavy chain (HC) class switch recombination (CSR) is a late B cell process that involves intrachromosomal DNA rearrangem ent. Ku70 and Ku80 form a DNA end-binding complex required for DNA dou ble strand break repair and V(D)J recombination. Ku70(-/-) (K70T) mice , like recombination activating gene (RAG)-1- or RAG-2-deficient (R1T or R2T) mice, have impaired B and T cell development at an early proge nitor stage, which is thought to result at least in part from defectiv e V(D)J recombination (Gu, Y., IC.J. Seidl, G.A. Rathbun, C. Zhu, J.P. Manis, N. van der Steep, L. Davidson, H.L. Cheng, J.M. Sekiguchi, It. Frank, et al. 1997. Immunity. 7:653-665; Ouyang, H., A. Nussenzweig, A. Kurimasa, V.C. Soares, X. Li, C. Cordon-Cardo, W. Li, N. Cheong, M. Nussenzweig, G. Iliakis, et al. 1997.J. Exp.. Mcd. 186:921-929). Ther efore, to examine the potential role of Ku70 in CSR, we generated K70T mice that carry a germline Ig HC locus in which the JH region was rep laced with a functionally rearranged VH(D)JH and Ig lambda light chain transgene (referred to as K70T/HL mice). Previously, we have shown th at B cells from R1T or R2T mice carrying these rearranged Ig genes (R1 T/HL or R2T/HL mice) can undergo CSR to IgG isotypes (Lansford, R., J. Manis, E. Sonoda, K. Rajewsky, and F. Alt. 1998. Int. Immunol. 10.325 -332). K70T/HL mice had significant numbers of peripheral surface IgM( +) B cells, which generated serum IgM levels similar to those of R2T/H L mice. However, in contrast to R2T/HL mice, K70T/HL mice had no detec table serum IgG isotypes. In vitro culture of K70T/HL B cells with age nts that induce CSR in normal or R2T/HL B cells did lead to the induct ion of germline CH transcripts, indicating that initial signaling path ways for CSR were intact in K70T/HL cells. However, treatment with suc h agents did not lead to detectable CSII by K70T/HL B cells, and inste ad, led to cell death within 72 h. We conclude that Ku70 is required f or the generation of B cells that have undergone Ig HC class switching . Potential roles for Ku70 in the CSR process are discussed.