Occult leukocyte and tumor‐associated antigens assessed by flow cytometry: A Review

We have developed a method for assessing the reactivity of monoclonal antibodies with intracellular antigens. This procedure has been utilized to evaluate the reliability of leukocyte surface antigens as markers of lymphoid cell lineages and for leukemic cell classification, and to further examine the specificity of tumor-associated antigens. Occult expression of certain leukocyte antigens was found in human large granular lymphocytes (LGL), the cells responsible for natural killer (NK) activity. These included the T65 antigen, a T-cell-associated antigen recognized by monoclonal antibodies T101 and Leu-1; the 140Kd 6–cell-associated glycoprotein recognized by monoclonal antibody 82; and the molecules recognized by the monocytel macrophage antibody MO-2. Thus, T-cell, B-cell, and monocyte markers were detected intracellularly in human LGL but not on the surface. Similarly, normal T-cells expressed the 6-2 and MO-2 markers intracellularly. The pan-T-cell antigen Leu-4 (OKT-3) was found to be expressed intracellularly in normal monocytes and in B-cell chronic lymphocytic leukemia cells. In addition, a secreted 100Kd human melanoma-associated antigen as well as vH-ras-p21 oncogene product could be demonstrated intracellularly in high amounts in human tumor and viral-transformed cells. These results indicate that leukocyte differentiation and tumor-associated antigens are expressed in a wider variety of cells than previously detected on the cell surface; and therefore cell lineage and specificity studies require a reassessment.