An experimental approach to enumerate the genes coding for immunoglobulin variable-regions.

Critical to our understanding of the immune system diversity is the determination of the number of germ line V genes. The total number of V genes is given by the product: number of subgroups x number of germ line genes per subgroup. Studies of kappa chains and of embryonic DNA indicate 5-10 V genes per subgroup. Statistical analysis of the limited sequence data of mouse kappa chains suggest about 50 V kappa subgroups. We report here a general approach for direct estimation of the number of VL and VH subgroups expressed in normal spleen, and present data for V kappa. The kappa mRNA of the spleen is a heterogeneous population where different V kappa are linked to the same C kappa, i.e. C kappa equals total V kappa. The ratio C kappa/distinct V kappa approximates the number of subgroups since V kappa of the same subgroup cross hybridize while V kappa of different subgroups do not. This ratio was determined by molecular hybridization of cloned C kappa and V kappa DNA probes with spleen mRNA. The results indicate the expression of 280 V kappa subgroups in mouse. Assuming an average of 7 genes per subgroup, we estimate about 2000 V kappa germ line genes.