In old BALB/c mice, bone marrow pre-B cell and surrogate light chain reduction is associated with increased B cell reactivity to phosphorylcholine, but reduced T15 idiotype dominance

In young adult BALB/c mice, antibodies to phosphorylcholine (PC) bearing the T15 (TEPC 15) idiotype confer protection against pneumococcal infections. In old age, even though PC reactive B cells are often increased, the proportion of T15⁺ antibodies declines. We hypothesize that limited surrogate light chain (SLC) and compromise of the pre-B cell receptor checkpoint in old mice contribute to both reduced new B cell generation and changes in the anti-PC antibodies seen in old age. In old mice: 1) early pre-B cell loss is most pronounced at the preBCR checkpoint; however, the reduced pool of early pre-B cells continues to proliferate consistent with preBCR signaling; 2) increased PC reactivity is seen in bone marrow immature B cells; 3) deficient SLC promotes increased B cell PC reactivity and diminished T15 idiotype expression; and 4) as pre-B cell losses and reduced SLC become progressively more severe, increased T15 negative PC reactive B cells occur. These results associate a reduction in pre-B cells, imposed at the preBCR checkpoint, with increased reactivity to PC, but more limited expression of the protective T15 idiotype among PC reactive antibodies in old age.