Reactivity of antihuman thymocyte serum with acute leukemic blasts

The presence of receptors for sheep erythrocytes (E) on some leukemic lymphoblasts is suggestive but not conclusive evidence for a thymic origin of these lymphoid cells. Normal and leukemic lymphoid populations were therefore examined to determine if the formation of spontaneous E rosettes correlated with the presence of thymus-associated cell surface antigens. Lymphoblasts from four of 12 children with acute lymphoblastic leukemia (ALL) formed spontaneous E rosettes at 4 degrees C and 37 degrees C (E+) and bound rabbit antihuman thymocyte serum, as determined by indirect immunofluorescence and by an indirect radiolabeled antibody assay. The blasts from the other eight children did not form E rosettes at 4 or 37 degrees C (E-) and did not bind antithymocyte serum. Absorption of the antiserum with peripheral blood leukocytes removed all detectable reactivity with normal peripheral blood lymphocytes, normal bone marrow cells, and E- bone marrow cells, but did not remove all reactivity with thymus cells or E+ leukemic blasts. Absorption of the antiserum with thymus from one donor removed all reactivity against thymus cells from another donor. After absorption with normal peripheral blood leukocytes, the antithymocyte serum reacted with E+ leukemic blasts but not with remission lymphocytes from the same patients. Thus, there are at least two distinct thymus-associated antigens-one that is present on T lymphocytes from peripheral blood as well as thymus and another that is present only on normal thymus and on ALL blasts that are identified by their ability to form E rosettes at 37 degrees C.