Characterization of immature sinus histiocytes (monocytoid cells) in reactive lymph nodes by use of monoclonal antibodies.

The distinctive mononuclear cells that appear in the cortical sinuses of lymph nodes in toxoplasma lymphadenitis and other conditions have been termed "immature sinus histiocytes," although these cells have neither enzyme histochemical nor ultrastructural features of histiocytes. The authors stained these cells in frozen sections of six reactive lymph nodes, using the immunoperoxidase technique, with monoclonal antibodies to T cell, B cell, and monocyte antigens and with heteroantisera to immunoglobulin and lysozyme. The immature sinus histiocytes stained with a monoclonal antibody to B cells (anti-B1) and had immunoglobulin of the IgG class. They did not react with monoclonal antibodies to T cells or monocytes or with antilysozyme. In contrast, medullary sinus histiocytes in three additional lymph nodes were B1-negative and reacted with anti-T4, anti-M1, and anti-lysozyme. These results indicate that immature sinus histiocytes are IgG-bearing B lymphocytes rather than histiocytes. Their role in the immune response remains enigmatic.