Human Ig heavy chain CDR3 regions in adult bone marrow pre-B cells display an adult phenotype of diversity: evidence for structural selection of DH amino acid sequences

Ig repertoires generated at various developmental stages differ markedly indiversity. It is well documented that Ig H chain genes in human fetal liverare limited with regard to N-regional diversity and use of diversityelements. It is unclear whether these characteristics persist in pre-B cellH chain genes of adult bone marrow. Using Ig H chain CDR3 fingerprintingand sequence analysis, we analyzed the diversity of Ig H chain thirdcomplementarity determining regions (HCDR3) in adult bone marrow pre-B andmature B lymphocytes. Pre-B cell HCDR3 sequences exhibited adultcharacteristics with respect to HCDR3 size, distribution of N regions andusage of diversity elements. This suggested that pre-B cells in adults aredistinct from fetal B cell precursors with regard to Ig H chaindiversification mechanisms. At the DNA sequence level, HCDR3 diversity inmature B cells was similar to that in pre-B cells. Pre-B HCDR3s, however,frequently contained a consecutive stretch of hydrophobic amino acids,which were rare in mature B cells. We propose that highly hydrophobic pre-BHCDR3s may be negatively selected on the basis of structural limitationsimposed by the antigen binding site. At the same time, usage of hydrophilicHCDR3 sequences (thought to support HCDR3 loop formation) may be promotedby positive selection.