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

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 expresses 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 bone 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 suggest 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.