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Patients with rheumatoid arthritis have decreased numbers of T mu lymphocytes in their peripheral blood. To find out whether these low number of T mu lymphocytes were associated with the presence of anti-lymphocyte antibodies, the sera of 27 patients with definite or classical rheumatoid arthritis (RA) were investigated for the presence of autoantibodies against subsets of lymphocytes. In addition the numbers of T, T mu, T gamma and B lymphocytes in the peripheral blood of these patients were investigated. Patients with active RA showed lower numbers of T mu lymphocytes in their peripheral blood than patients with inactive RA. However, both groups of RA patients had significantly decreased numbers of T mu lymphocytes in their peripheral blood as compared with 22 age matched healthy donors. Moreover, mainly in patients with active RA cold reactive antibodies were found directed against T mu and B lymphocytes, but never against T gamma lymphocytes of healthy donors. Similar results were found in the indirect immunofluorescence procedure when tested for reactivity against T-cell subsets. This serum reactivity was not caused by rheumatoid factors or antinuclear antibodies. Since RA sera after precipitation with 2.5% polyethyleneglycol, still showed cytotoxicity against T and B lymphocytes, it is suggested that this serum reactivity is not caused by immune complexes but by antibodies.

Cerebriform mononuclear cells (CMC) constitute a morphologically distinct subpopulation of T cells in healthy individuals. They are characterized ultrastructurally by a highly indented nucleus, a high nucleus-to-cytoplasm ratio, condensed chromatin along the nuclear membrane and a scanty cytoplasm. In order to characterize the peripheral blood CMC by enzyme-histochemistry and membrane characteristics, lymphocyte fractions enriched for T cells, T mu cells, T gamma cells and T cells without Fc mu and Fc gamma receptors (T0 cell fraction), or depleted of T cells, were investigated for the presence of alpha-naphthylacetate esterase and acid phosphatase at light and electron microscopic levels. CMC were found exclusively in the T mu-enriched and T0 cell fraction, indicating that these cells have either an Fc mu receptor or no Fc receptor at all. Except for their nuclear indentation, both the CMC in the T mu-enriched fractions and the CMC in the T0 fractions ultrastructurally resembled the characteristic cell in the T mu fraction (TM-type cell), but differed from the characteristic cell in the T gamma fraction (TG-type cell). Moreover, like the TM-type cells all CMC showed paranuclear dots of alpha-naphthylacetate esterase and acid phosphatase activity in their cytoplasm. From these observations it was concluded that CMC without Fc mu receptors were either stimulated T mu cells or precursor T mu cells. Thus CMC in healthy individuals constitute a distinct subpopulation of T cells not only morphologically but also histochemically and immunologically.