Detection of the common acute lymphoblastic leukaemia antigen (CALLA) on B cells from human fetal tissues. A multiple phenotypic characterization

Fetal bone marrow liver and spleen of gestational age 15-20 weeks contain CALLA⁺, HLA-DR⁺ lymphoid cells. We show that a proportion of them express surface membrane immunoglobulins (SIg) as well as B cell differentiation antigens. A multiple phenotypic analysis reveals that CALLA⁺ fetal B cells are: HLA-DR⁺, SIg⁺, FMC8⁺, BA1⁺, Y29.55⁺ or Y29.55^-, B2⁺, TdT^-. Tissue specific phenotypic differences concern the expression of B7 and HLA-DC on spleen but not on marrow B cells. This study indicates that the distribution of the CALL antigen across the B cell committed lineage is much wider in fetal than neonatal life since CALLA⁺ B cells have not yet been detected in bone marrow and peripheral blood of normal infants and adults. In addition, the interpretation of our phenotypic data suggests that fetal bone marrow B cells are more immature than those present in the spleen, thus, further supporting the evidence that the bone marrow is the organ of B cell lymphopoiesis.