Terminal deoxynucleotidyl transferase-positive B cell precursors in fetal lymph nodes and extrahemopoietic tissues

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Abstract
Terminal deoxynucleotidyl transferase (TdT)-positive B cell precursors populate fetal lymph nodes (LN) of mid-term (16th–22th week) fetuses. These cells have a B cell phenotype (HLA-DR, CD45R, CD22, μ) and lack mature differentiation antigens (CD1c, CD20, L26). TdT cells are also present in the extranodal mesenteric mesenchyme, pancreas and LN primordia. Extranodal TdT cells are found among proliferating (i.e. Ki-67) B cells and lack leukocyte function-associated antigen (LFA-1) and HLA-DQ; these latter molecules are acquired by nodal TdT elements. Adult LN, adult and fetal spleen contain less TdT cells than fetal LN. The fetal LN contains the full range of the B cell differentiation compartment, from the TdT precursor cell to the mature B lymphocyte.