Cytoplasmic fibrils in mixed lymphocyte cultures.

N ULTRASTRUCTURAL COMPARISON of the cells which develop in cultures of mixtures of lymphocytes from allogenic donors and those which result from stimulation by phytohemagglutinin ( PHA ) has demonstrated morphologic differences. One of the most striking of these is the frequent presence of bundles of fibrils in transformed lymphocytes of tile mixed lymphocyte cultures. There have been scattered reports, in recent years, listed by Biava,’ of cytoplasmic fibrils in a number of different cell types. Bessis2 described a crescent shaped fibrillar structure in the cells of acute granulocytic leukemia in 1957, and since then similar collections of fibriis have been reported in other hematologic cell types in both malignant and benign conditions. 3 Reports of the presence of cytoplasmic fibrils in lymphocytes have apparently been limited to a description of bundles of fibrils in thoracic duct lymphocytes,6 occasional inconspicuous fibrils in peripheral lymphocytes,3 and rare individual fibrils in lymphocytes exposed to PHA.7 The latter is in agreement with our own experience. The frequent occurrence of bundles of cytoplasmic fibrils in mixed lymphocyte cultures, therefore, has been most impressive and, in fact, appears to provide an unusual opportunity for ( 1 ) the morphologic characterization and localization of the fibrils and (2) the determination of their chemical composition by cytochemical and autoradiographic technics. The fibrils were observed not only in the blast cells of the mixed lymphocyte cultures, but also in a high percentage of lymphocytes with increased cytoplasm and organdies. These will be referred to as activated lymphocytes to distinguish them from tile small or dormant peripheral 1)100(1 lymphocytes. Tiley appear to be similar to the blood lymphocytes commonly classified as medium or large, or in some instances atypical lymphocytes. Whether the presence of the bundles of fibriis ill the mixed lymphocyte cultures in contrast to their absence in phytohemagglutmm cultures is a morphologic expression of a fundamental difference in the stimuli-i.e., anti-