Restricted κ chain expression in early ontogeny: Biased utilization of Vκ exons and preferential Vκ-Jκ recombinations

Abstract

To determine the extent of κ chain diversity in the preimmune repertoire early in development, κ cDNA libraries were analyzed from 15-d old fetal omentum, 18-d-old fetal liver, and 3-wk old bone marrow. An anchored polymerase chain reaction approach was used to avoid bias for particular Vκ families. From the sequence analysis of 27 bone marrow clones, 10 different families and 20 unique Vκ genes were identified. In contrast, the Vκ expression in the fetus is highly restricted and clearly differs from the broader distribution seen in 3-wk-old bone marrow. Although several Vκ families were represented in the fetal library including Vκ9, Vκ10, Vκ4,5, Vκ8, and Vκ1, one or two members of individual families were observed repeatedly. The fetal liver and omentum libraries were found to be largely overlapping. Given the Vκ families/exons identified in the fetal sequences, the mechanism of κ rearrangements in the early repertoire appears to occur predominantly by inversion. Importantly, the fetal repertoire was further restricted by dominant Vκ-Jκ combinations such as Vκ4,5-Jκ5, Vκ9-Jκ4, and Vκ10-Jκ1. Since in some cases independent rearrangements could be established, the results indicate a bias for particular Vκ-Jκ joins. The results also suggest that clonal expansion/selection in the fetal repertoire takes place after light chain rearrangement as opposed to at the pre-B cell level in the bone marrow. The restriction observed in κ light chain expression together with known restrictions in gene usage and junctional diversity at the heavy chain level indicate a remarkably conserved fetal repertoire.