To study the function of Ig-alpha in the selection of autoreactive B cells,
we have analyzed mb-1 cytoplasmic truncation mutant mice (mb-1(Delta c/Del
ta c)), which coexpress transgenes encoding hen egg lysozyme (HEL) and HEL-
specific immunoglobulin. We demonstrate that in the presence of soluble MEL
(sHEL) and dependent on the mb-1(Delta c) mutation, most immature B cells
bearing the MEL-specific Ig transgene undergo rearrangements of endogenous
kappa light chains, resulting in loss of MEL specificity. Moreover, immatur
e B cells from Ig-alpha mutant mice respond to BCR cross-linking with an ex
aggerated and prolonged calcium response and induction of protein tyrosine
phosphorylation. Our data imply a negative signaling role for Ig-alpha in i
mmature B cells.