The Natural Abundance of ?~2-light Chains in Inbred Mice*

A small fract ion of the l ight (L) 1 chains of mouse immunog lobu l ins (Igs) are of the X-type (1). O f the -~ 22 X-chains from mouse mye loma tumors whose amino acid sequences have been pa r t i a l ly or fully de te rmined , all bu t one have the same constant (C) d o m a i n sequence (2, 3). T h e exception, L 3t5, is the l ight chain of the mye loma pro te in (M315) p roduced by p l a s m a c y t o m a MOPC-315 . L 315, cal led a X2-ehain (4), differs from the o ther mouse h-chains, cal led X1, at 29 of 110 posit ions in the C region and, c o m p a r e d to the p ro to type Xl-chain (L1°4), at 1 1 posit ions in the var iable (V) region (5). The mouse in which M O P C 3 1 5 arose was o f the B A L B / c 2 type; it der ived from the seventh backcross to pu reb red B A L B / c mice beg inn ing with a (BALB/c× C57BI /Ka) F1 hyb r id (6). Because other X2-chains have so far not been descr ibed in B A L B / c myelomas , it has long seemed possible that the L 3a~ chain might be an allelic var ian t (of C 5 7 B I / K a origin) of the Xl-chains p roduced in BALB/c . However , with so m a n y scat tered C d o m a i n amino acid differences be tween X1 and L 3t~ a more a t t rac t ive possibi l i ty is tha t L axg represents a rarely expressed L-chain isotype, X2, ra ther than a X-allotype (7). Accordingly , the object of the present s tudy was to establish whe ther L315-1ike (X2) chains exist in Igs from B A L B / c and other mouse strains and, if so, at wha t level. A d v a n t a g e was taken of the Fv f ragment of M315 (V~ 15 and V ~ 5) to ob ta in separa te a n t i b o d y (Ab) p repa ra t ions to the C and V regions of L 3x5 (anti-C~ 15 and ant i-V~ 16 , respectively). W i t h these Ab prepara t ions and a25I-labeled L 31~, two r ad io immunoas says were deve loped to measure the frequencies, in mice of different ages, strains, and i m m u n i z a t i o n background , o f serum Igs whose L chains have the respective ant igenic