B cell development and VDJ rearrangement in the fetal pig

Hematopoietic activity of the swine has been documented in three phases during fetal ontogeny. The hematopoietic system develops first in the yolk sac, then in fetal liver and finally in the bone marrow. Using flow cytometry (FCM) and molecular biological techniques we show that B-cell lymphogenesis and the appearance of B cells follows a pattern. First, VDJ rearrangement occurs at the 20th day of gestation (DG20) in the yolk sac at a time when light chain transcription is absent. Next, B-cell lymphogenesis is detected at DG30 in the fetal liver. Thereafter, bone marrow becomes the major B lymphopoietic organ (DG45). In yolk sac and fetal liver, more than 90% of the VDJ rearrangements were in-frame but expression of micro heavy chain could not be clearly detected by FCM. However, cells with a putative phenotype of B-cell precursors are present. These cells express high levels of MHC class II (SLA-DR) and low levels of CD2 and CD25. CDR3 length analysis (spectratyping) indicates that the heavy chain repertoire is oligoclonal at this time with large inter-animal variations. Consistent with our earlier reports, fetal VDJ rearrangements are not mutated and there is no evidence for an age-dependent increase in TdT activity or a change in V(H) and D(H) usage from those used by B-cells formed in the yolk sac or fetal liver. However, our findings indicate major differences in the regulatory environment and/or selective pressures in yolk sac and fetal liver versus bone marrow. In contrast with the yolk sac and fetal liver, the proportion of in-frame VDJ rearrangements in the bone marrow correspond to a value indicative of random recombination.