Immunohistochemical Evaluation of J-Chain Expression by Intra- and Extra-follicular Immunoglobulin-producing Human Tonsillar Cells

Abstract

Cytoplasmic J‐chain expression by Ig‐producing cells was characterized immunohistochemically in normal specimens of palatine and nasopharyngeal tonsils (adenoids). Altogether follicular immunocytes, which were mainly of the IgG and IgM isotypes, showed a much higher percentage of J‐chain positivity than the extrafollicular ones, in agreement with the idea that this polypeptide is principally a marker of differentiating or newly differentiated Ig‐producing cells. Thus, in concurrence with the predominating IgG isotype. J‐chain expression was almost 50% in the germinal centres of lymphoid follicles but only about 2% in the extrafollicular compartment. By contrast, a substantial number of J‐chain‐positive IgA immunocytes were found in the Jailer compartment. Since it is believed that extrafollicular Ig‐producing cells are mainly derived from follicular centre cells, this result indicated that significant isotype switching is involved in tonsillar B‐cell differentiation. Most tonsillar IgD immunocytes were J‐chain‐positive, in accordance with the notion that IgD expression is a feature of relatively early clonal development. It is postulated that J‐chain‐positive B‐cell blasts may leave the tonsils and, through isotype switching, contribute to the dimer‐producing IgA‐cell populations normally found in the exocrine glands of the upper aero‐digestive tract.