WITHIN GERMINAL-CENTERS, ISOTYPE SWITCHING OF IMMUNOGLOBULIN GENES OCCURS AFTER THE ONSET OF SOMATIC MUTATION

Human tonsillar B cells were separated into naive IgD(+)CD38(-)CD23(-) (Bm1) and IgD(+)CD38(-)CD23(+) (Bm2), germinal center IgD(-)CD38(+)CD 23(-) centroblasts (Bm3) and IgD(-)CD38(+)CD77(-) centrocytes (Bm4), a nd memory IgD(-)CD38(-) (Bm5) subsets. Previous IgV(H) sequence analys is concluded that the triggering of somatic mutations occurs during th e transition from Bm2 subset into the Bm3 subset. To determine the ini tiation of isotype switching, sterile transcript expression was analyz ed by amplification, cloning, and sequencing. A selective sterile I ga mma, I alpha, and I epsilon expression was observed at centrocyte (Bm4 ) stage, suggesting that isotype switch is triggered within germinal c enters, after somatic mutation is initiated within centroblasts (Bm3). Finally, the high level of 5'S gamma-S mu 3' DNA switching circles ob served in germinal center B cells indicates that within human tonsils, germinal center is a major location for isotype switching.