Enhancement of antigen expression on acute lymphoblastic leukemia (ALL) cells by cell incubation.

Leukemic cells from blood and/or marrow specimens of 64 acute lymphoblastic leukemia (ALL) patients were examined for the expression of the Ia-like antigen (Ia) or ALL cell-associated antigen (ALLA) employing indirect immunofluorescent techniques before and after overnight incubation (18 hours) at room temperature in Roswell Park Memorial Institute medium 1640. In 20 cases, the post-incubation cells showed an augmented intensity of the fluorescent stain and an increased (greater than 15%) percentage of the ALLA and/or Ia-bearing cells. Evaluation of cell viability after incubation and of non-specific staining in control preparations using normal rabbit serum in lieu of specific rabbit antiserum to ALLA or Ia excluded selective cell death and increased non-specific stain as causes of the observed increase in fluorescent cell staining. The enhanced antigen expression was observed in those samples in which antigens were detectable on only a small fraction of the leukemic cells. The results demonstrate the necessity of re-examination of leukemic cells for the presence of ALLA and Ia following overnight incubation especially in antigen negative cases.