Immunoglobulin heavy chain gene expression in peripheral blood B lymphocytes

cDNA libraries for IgM heavy chain variable regions were prepared from unmanipulatedperipheral blood lymphocytes of two healthy people. Partial sequencing of 103 clones revealed V_H gene family use and complete CDR3 and J_H sequences. The libraries differed in the two subjects. In one person's cDNA the V_H5 family was overexpressed and the V_H3 family underexpressed relative to genomic complexity. In the second person's cDNA, V_H3 was most frequently expressed. In both libraries, J_H4 was most frequent. V_H segments of several clones were closely related to those in fetal repertoires. However, there was also evidence of mutation in many cDNAs. Three clones differed from the single nonpolymorphic V_H6 germline gene by 7-13 bases. Clones with several differences from V_H5 germline gene V_H251 were identified. CDR3 segments were highly diverse. J_H portions of several CDR3's differed from germline J_H sequences. 44% of the clones had D_H genes related to the D_LR and D_XP families, most with differences from germline sequences. In 11 D_LR2-related sequences, several base substitutions could not be accounted for by polymorphism. Thus, circulating IgM-producing B cell populations include selected clones, some of which are encoded by variable region gene segments that have mutated from the germline form.