Developmental Dynamics of Immunoglobulin Gene Diversity in the Horse

The development of B lymphocytes and immunoglobulin gene diversity was investigated in foals to better understand immunity against pathogens in early life, and improve neonatal vaccination strategies.  Ig diversity is generated through several mechanisms, including gene recombination with selective gene usage and somatic hypermutation.  Our previous investigation of B cell markers and Ig heavy chain isotype expression revealed that equine fetal B cells express mature and co-stimulatory B cell molecules and most Ig heavy chain isotype transcripts.  Rapid amplification of cDNA ends (RACE) libraries were generated from equine fetal, pre-suckle neonatal, 1 to 3 month-old foal, and adult horse lymphoid tissues, and used to amplify heavy and lambda light chain transcripts from each age.  Thirty unique clones per gene were sequenced from each age, and germline genes were annotated by comparison with the equine genome sequence (EquCab2.0).  For the Ig heavy chain IGHV, IGHD, and IGHJ genes, similar gene usage profiles were found throughout life.  In contrast, the usage of Ig lambda light chain genes differed significantly with time (p = 0.03).  Unexpectedly, Ig heavy and light chain gene sequence diversity increased between mid-gestation and birth (p < 0.0001 and p = 0.0005, respectively).  Increases in Ig gene sequence diversity were also observed between the foal and adult ages (p < 0.0001), as expected.  This work reveals that both the heavy and light chains contribute to Ig diversity during equine fetal life, before antigen exposure.