Co-expression of surface immunoglobulin and T3 on hairy cells

Whereas it is generally believed that most cases of hairy cell leukaemia (HCL) represent B cell neoplasms, it is reported here that 4 of 14 cases of HCL not only express monotypic surface immunoglobulin (SIg) but also react with the T cell-specific monoclonal antibody (Mab) UCHT1 by indirect immunofluorescence or immunoperoxidase staining. Although both UCHT1 and OKT3 recognize the T3 molecule, which is expressed in all normal T cells, UCHT1+ hairy cells (HCs) were consistently OKT3-. Spurious reactivity of UCHT1 antibodies with SIg+ HCs was excluded by showing that lymphocytes sensitized with an irrelevant mouse Mab did not stain with second layer antibodies and lymphocytes sensitized with second layer antibodies alone were always completely unreactive. Moreover, in 1 case the determinants demonstrable by both UCHT1 and anti-Ig were strongly re-expressed after capping and shedding, i.e. appeared to be endogenous. It is concluded that HCs with a hybrid T-B surface phenotype are more common than hitherto reported, that HCs may express T3 molecules as well as SIg but that determinants recognized by OKT3 may be cryptic or buried in the HC membrane.