Lymphocyte development in fetal piglets: Facts and surprises

The developing porcine fetus offers an excellent opportunity for the study of lymphocyte development. Studies on B cell, αβ T cells and γδ T cells in the last decade have expanded our knowledge of lymphocyte development in pigs. These studies have revealed several interesting differences between swine, mice and humans. For example, porcine peripheral lymphocytes include CD4 +CD8 + αβ T cells and an abundance of γδ T cells that may even prevail over the αβ population. There are numerous CD2 − γδ T cells in the blood and a large number of CD8αα-bearing cells that include NK cells, conventional γδ and αβ T cells. All porcine B lymphocytes are CD25 lo and sIgM + B cells may differ in the expression of CD2 antigen. Unlike mice, porcine B cells appear ∼2 weeks before T cells and progenitors undergo VDJ H rearrangement at 20th day of gestation (DG20) in the yolk sac and DG30 in the fetal liver before consummating high level lymphogenesis in the bone marrow after DG45. Early B cells show an unexpectedly high proportion of in-frame rearrangements, undergo switch recombination in thymus on DG60 and use N-region insertion from the time of the earliest VDJ rearrangement. The genomic repertoire of V H, D H and J H genes is small compared to mice and humans and swine appear to depend on junctional diversity for the majority of their repertoire. The limited V H repertoire of swine contrasts sharply with the porcine TCRβ repertoire, which is extensive, extraordinarily conserved and nearly identical to that in humans. Therefore, swine present an example of two highly related receptor systems that have diverged in the same species.