Location and sequence of rearranged immunoglobulin genes in human thymus

Thymic B cells are a proliferating B cell population concentrated in normal thymic medulla. They are large cells, some with dendritic morphology, and are not associated with any organized follicular structure. Previous work in this laboratory has shown that most of these B cells are surrounded by tightly adherent thymocytes. The literature on human thymic B cells contains many inconsistencies. There is no consensus on whether they express CD5. Even the existence of thymic B cells has been questioned. In this study we have undertaken the first analysis of rearranged immunoglobulin (Ig) genes, looking in particular for evidence of affinity maturation. The Ig VH genes of human thymic B cells in this study are those of the fetal repertoire, though the resemblence to fetal Ig genes is limited in other respects. They are mostly unmutated, but the presence of mutated sequences suggests that this is not a uniform population, as has been previously indicated by phenotypic studies.