The kappa gene repertoire of human neonatal B cells

The kappa chain repertoire of individual IgD(+) human neonatal B cells was analyzed using a single cell PCR technique. A total of 104 productive and 90 non-productive VkappaJkappa rearrangements from three cord blood B cell samples were sequenced and compared to the adult IgM(+) peripheral B cell VkappaJkappa repertoire. All six Vkappa families were present in neonatal B cells, but the distribution was not random. In the non-productive repertoire Vkappa2 and Vkappa6 families were less frequent, Vkappa1 and Vkappa3 families were as frequent, and Vkappa4 and Vkappa5 families were more frequent than expected from random chance. Notably, the Vkappa2 family was negatively selected into the productive repertoire. In contrast, the Vkappa1 family was positively selected because of positive selection of three specific genes, O12/O2, L12a and L9. B3 (Vkappa4) and B2 (Vkappa5) were over-represented in the non-productive repertoire and then were expressed less frequently in the productive repertoire. In contrast, the Vkappa3 family gene, A27, was also over-represented in the non-productive repertoire but not further selected into the productive repertoire. Compared to the adult repertoire, junctional diversity was less marked because of a diminished influence of TdT activity, whereas the mean CDR3 length was comparable to that of normal adult B cells. Comparison of the distribution of Vkappa and Jkappa genes with those found in normal adult subjects suggested that there was less receptor editing in neonatal B cells. When neonatal CD5(+) B cells were compared with CD5(-) IgD(+) B cells, it was noted that the Vkappa gene A30 was used only in CD5(+) B cells in both the productive and non-productive repertoires. The results indicate that the usage of Vkappa genes by neonatal B cells is biased by both intrinsic molecular processes and selection. The evidence of selection indicates that the Vkappa repertoire is shaped by self antigens, since exposure to exogenous antigens is limited at the time of birth.