Immunohistochemical comparative investigations of lymphatic tissue in reactive processes, myasthenic thymuses and Hodgkin's disease.

In this study coexpression of cytokeratin and desmin, and occasionally also Ki-1 antigen, was displayed in extrafollicular reticulum cells of reactive lymph nodes. The absence or expression of trace amounts of these proteins in normal lymphoid tissue suggests that activation of T cell regions is correlated with the increased frequency of cytokeratin, desmin and Ki-1 expressing cells, and therefore may be a transient phenomenon. S-100-positive interdigitating reticular cells were found occasionally in extrafollicular T cell region of normal lymph nodes. They were, however, more numerous in reactive lymphatic tissue. In the myasthenic thymuses cells forming Hassall's corpuscles displayed coexpression of cytokeratin, desmin and Ki-1 antigen. Medullary epithelial cells were also cytokeratin-positive and, additionally, Ki-1 antigen was expressed on some cells dispersed in whole thymic tissue. S-100-positive interdigitating reticular cells were especially numerous in the thymic medulla and some of them found inside the Hassall's corpuscles. In Hodgkin's disease deficiency of cytokeratin and desmin in extrafollicular reticulum cells is a constant phenomenon in spite of a classic inflammatory background. However, Ki-1 antigen displayed Reed-Sternberg cells which, similar as some thymic cell elements, appear to originate from stromal perivascular mesenchyme. This fact suggests that Reed-Sternberg cells in Hodgkin's disease are pathologic counterparts of extrafollicular reticulum cells which represent a cellular differentiation defect to produce desmin and cytokeratin but with a possibility of Ki-1 antigen expression. The consequence of this may be the disregulation of immune system and the observed immunologic abnormalities. Further studies are needed to elucidate the role of Epstein-Barr viruses in this process. S-100-positive interdigitating reticular cells were in close contact with Reed-Sternberg cells and they were especially large and with numerous cells processes in the mixed cellularity (MC) subtype. The occurrence of interdigitating reticulum cells with S-100 protein expression, especially numerous in the T cell region activated of peripheral lymphatic tissue, as well as their close contact with Reed-Sternberg cells and with cells forming Hassall's corpuscles suggest their eventual possible role in the function of the immune system.