Thymic medullary cells expressing B lymphocyte antigens.

A small subpopulation of human thymic medullary cells was found to express B cell-restricted and -associated antigens in various combinations. The cells were detected in fetal, juvenile, and adult thymi using indirect immunoenzymatic methods and monoclonal antibodies (MoAbs). Morphologically, they could be subdivided into small, round lymphoid cells, accounting for less than 1% of medullary lymphoid cells, and into a larger variant, even more infrequent in number and asteroid in shape because of short cytoplasmic processes. Immunophenotype (CD19+, CD20+, CD22+, CD37+, IgM+, IgD+) and morphology of the first cell type led to the conclusion that the lymphoid cells were B lymphocytes. The second, asteroid cell type constantly expressed CD20 and inconstantly expressed IgM, CD19, CD22, and CD37; they were often found to form rosettes with non-B lymphocytes. It can be concluded that a small number of B cells and asteroid cells of still uncertain origin, but expressing B cell-restricted antigens, are constitutive elements of the fetal and adult thymic medulla. It can be hypothesized that the asteroid cell might represent a novel type of thymic accessory cell and that the rosetting of non-B lymphocytes around this asteroid cell might simulate or in fact be the earliest B cell interaction of maturing T cells.