A rearranged λ2 light gene chain retards but does not excludex and λ1 expression

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Abstract
Mice transgenic for the λ2 light chain of MOPC315 were established. In newborn transgenics (TG), λ2 was the only light chain found on B cells. However, by day 21, λ2highxlow as well as λ2lowxhigh double expresser populations were emerging. λ2 was found on an increased fraction of serum immunoglobulins (Ig), this fraction declined with age. Correspondingly, x and λ1 expression was suppressed in young mice but increased with age. In adult mice x or λ1 were often co‐expressed with λ2 in single serum Ig molecules. Most B cell hybridomas from an adult TG secreted λ2,x mixed molecules and had rearranged their x chain genes. One λ2,λ1 hybridoma and even a λ2,xλ1 hybridoma were also found. In conclusion, isotypic exclusion in λ2 TG is complete in newborns but becomes increasingly leaky with age. Antigen probably expands the λ2lowxhigh B cell population; this population is most likely the major source of serum Ig in adult λ2 TG mice. In contrast, the λ2highxlow population, a major fraction of which is CD5+μlowδlow, appears only infrequently to develop into antibody‐secreting plasma cells.