Two subpopulation of circulating human T cells forming rosettes with neuraminidase-treated sheep erythrocytes were purified on the basis of the presence of receptors for IgG (TG cells) or for IgM (TM cells), and were shown to have distinguishing morphological and histochemical characteristics. TM cells had the general features of typical small- or medium-sized lymphocytes; most were easily identifiable by distinctive cytoplasmic accumulations, usually one and sometimes two large spots, of nonspecific acid esterase activity. The release of the vesicular contents on short-term culture of TG cells was inhibited by cytochalasin B. Definition of these distinguishing characteristics of TM and TG cells provides a basis for practical enumeration of these functionally distinct subpopulations of human T cells. Some of the TG cells were capable of endocytosis of IgG antibody-coated erythrocytes.

We show here that murine erythroleukemia (MEL) cells, following induction with hexamethylene bisacetamide, accumulate high mobility group (HMG)t protein onto the external surface of the cell in a membrane-associated form detectable by immunostaining with a specific anti-HMGt protein antibody. This association is maximal at a time corresponding to cell commitment. At longer times, immunostainable cells are progressively reduced and become almost completely undetectable along with the appearance of hemoglobin molecules. Binding to MEL cells does not affect the native molecular structure of "MGt protein. The type of functional correlation between "MGt protein and MEL cell differentiation is suggested by the observation that if an anti-HMGt protein antibody is added at the same time of the inducer almost complete inhibition of cell differentiation is observed, whereas if the antibody is added within the time period in which cells undergo through irreversible commitment, inhibition progressively disappears. A correlation between MEL cell commitment and the biological effect of HMGt protein can thus be consistently suggested.

No abstract

The ultrastructural and cytochemical features of human peripheral blood TG cells (T cells with receptors IgG) and of the cells of the so-called third population (non-T, non-B cells with high avidity receptors for IgG) have been investigated and compared. Both TG and third-population cells (TPC) contained acid hydrolases with a paranuclear localization of alpha-naphthyl acid esterase, beta-glucuronidase or acid phosphatase. At the electron microscopy level, TG and TPC were indistinguishable and displayed rough cell surface, indented nuclei, abundant cytoplasm with predominance of the smooth over the rough membranes and peroxidase-negative granules. A large proportion of cells of the TPC could form rosettes with sheep erythrocytes after treatment with neuraminidase. The observed close similarities between TG and TPC may suggest that both cell types belong to a special subset of T cells. However, the alternative hypothesis that both TG and TPC are part of a subset unrelated to T cells, such as a new non-T, non-B cell population, or even of the monocytic-macrophage lineage, is also discussed.

No abstract