Phenotypic and Functional Characterization

Although the CD5 (T1) antigen was initially described as a pan-T cell membrane glycoprotein, we report that 14 of 40 normal individuals were found to have 5% or greater of their blood mononuclear cells characterized as CD3 (T3)+ but CDSby dual immunofluorescence flow cytometry. These cells expressed normal quantities of surface CD3 and CD2 but low levels of CD7, were CD8' and CD4-, and CD16-. In order to determine whether cells of this phenotype were functional, six CD5cytolytic T lymphocyte (CTL) clones isolated from normal individuals were studied. The CDSCTL clones all demonstrated normal cytolytic activity against appropriate target cells. Monoclonal antibodies (MAbs) directed against CD3, CD8, CD2, and lymphocyte function-associated antigen 3, but not against CD5, inhibited cytolytic activity. Changes in intracellular calcium (ICa2+I,) in response to anti-CD5 and antiCD3 MAbs were measured. Stimulation by anti-CD5 MAb alone did not give rise to a change in ICa2+I. However, under conditions of limiting concentrations of anti-CD3 MAb, preincubation of normal CD5', but not CD5-, clones with anti-CD5 MAb led to a dramatic enhancement in the ability of anti-CD3 MAb to elicit a rise in ICa211. We conclude that CD5T lymphocytes represent a normal lymphoid phenotype. Although CD5 may be involved in T cell activation when present, these CD5CTL clones appear to express normal cytolytic activity.