Human Ia alloantigens as cell-differentiation markers of normal and pathologic leukocyte surfaces.

Comparative membrane phenotypes of normal T and B lymphocytes, of normal myeloid cells, and of various leukemic cell types were established by the use of specific Ia‐type alloantisera and ‘classical’ immunological surface markers, including complement receptors. 4%–8% of lymphocytes taken from mixed leukocyte cultures on day 4 showed Ia alloantigens of responder‐type specificity, in addition to two known T‐cell markers on the same cell surface. Together with Ia antiserum inhibition studies, this subpopulation would appear to present a human mixed leukocyte culture (MLC)‐related suppressor T‐cell population. Whilst such cells could not be detected in normal peripheral blood by double fluorescence, a small percentage could readily be demonstrated among cell populations from patients with systemic lupus erythematosus and rheumatoid arthritis (joint fluid). Specific Ia alloantigens were further detected on the immature myeloid cells from bone marrow but not on the more mature myeloid cell types. This establishes the Ia alloantigen system as a valuable cell surface differentiation marker in man. The appearance of complement receptors on myeloid cells was Later than that of Ia alloantigens. Ia alloantigens are expressed on various numbers of leukemic myelo‐ and lympho‐blasts of the so‐called ‘non‐T, non‐B’ cell types. After discontinuous Ficoll gradients to enrich specific density cell groups, mainly the Ia alloantigens and the complement receptors allow a discrimination between acute (AML) and chronic myelocytic leukemia (CML) blasts, inclucling blasts crisis cells. AML cells appear to present a less differentiated cell type than CML blasts crisis cells. For the Latter disease, a more complex picture is obtained since at least three different membrane phenotypes of blasts cells can be identified: an Ia‐positive and an Ia‐negative myeloid blasts and an Ia‐positive lymphoid blasts. The relative quantities of these three elements can vary considerably within even a small group of patients.