Antibody Repertoire Development in Fetal and Neonatal Piglets. XI. The Relationship of Variable Heavy Chain Gene Usage and the Genomic Organization of the Variable Heavy Chain Locus

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Abstract
In this study, we have mapped the 3′ H chain V region (VH) genes and those in the H chain diversity, H chain joining, and 5′ portion of the H chain constant locus. We show that swine possess only two functional H chain diversity segments and only one functional H chain joining segment. These data help to explain more than a decade of observations on the preimmune repertoire of this species and reveal the vulnerability of swine to natural or designed mutational events. The results are consistent with earlier studies on the region containing Enh, Cμ, and Cδ while revealing that the ancestral IgG3 is the most 5′ Cγ gene. We also observed a recent duplication (∼1.6 million years ago) in the VH locus that contains six of the seven VH genes that comprise 75% of the preimmune repertoire. Because there are no known transfers of immune regulators or Ags that cross the placenta as in mice and humans, fetal VH usage must be intrinsically regulated. Therefore, we quantified VH usage in fetal piglets and demonstrated that usage is independent of the position of VH genes in the genome; the most 3′ functional VH gene (IGHV2) is rarely used, whereas certain upstream genes (IGHV14 and IGHV15) are predominately used early in fetal liver but seldom thereafter. Similar to previous studies, three VH genes account for 40% of the repertoire and six for ∼70%. This limited combinatorial diversity of the porcine VH repertoire further emphasizes the dependence on CDR3 diversity for generating the preimmune Ab repertoire of this species.