Eighteen cases of hairy cell leukemia were studied with a battery of polyclonal anti-Ig and nine monoclonal antibodies to determine the lineage of the hairy cells (HC) and the stage of their maturation arrest. Hairy cells tend to nonspecifically bind many antisera and precautions had to be taken to avoid nonspecific fluorescence of the cells. All but one case was reactive with one light chain type and one or more heavy chain isotype antisera as reported before. All cases studied were positive for monomorphic HLA-DR determinants, using monoclonal antibody 7.2. Most cases tested (6/7) were positive with the B-lineage related antibody PI 153/3. While most cases (15/18) were nonreactive with the B-lineage related antibody BA-1, they became positive (5/5) following in vitro culture. Seven out of nine cases were reactive with OKM1. Common acute lymphoblastic leukemia antigen (CALLA) was absent in all (15) cases tested and the ALL associated structure p24/BA-2 was absent from 16 of 18 cases. HC from none of the cases were clearly positive with the T-cell antibodies 9.6., or TA-1, whereas in only 1/18 the cells reacted with T101. The results of this study support the B cell lineage of most HC, and show the presence of multiple phenotypes. In combination with the surface Ib present on the cells, a hierarchy of phenotypes is postulated, with SIg-ormu delta, BA- 1+, PI 153/3+, HLA-DR+ being the most immature, and SIg(alpha) gamma, BA-1-, PI 153/3-, HLA-DR+ the most mature.

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