The human (Psi L+mu(-)) proB complex: Cell surface expression and biochemical structure of a putative transducing receptor

The surrogate light chain (Psi L) associates with mu and Ig alpha-Ig beta c hains to form the preB-cell receptor that plays a critical role in early B- cell differentiation. Discrepancies exist in human concerning the existence of Psi L(+)mu(-) proB cells and the biochemical structure of such a proB-c ell complex remains elusive. Among new antihuman VpreB monoclonal antibodie s (MoAbs), 5 of the gamma kappa isotype bound to recombinant and native Vpr eB protein with high affinity. They recognized 4 discrete epitopes, upon wh ich 2 were in the extra-loop fragment. Such MoAbs detected the Psi L at the cell surface of either preB or on both proB and preB cells. The previously reported SLC1/SLC2 MoAbs recognize a conformational epitope specific for t he mu/Psi L association in accordance with their preB-cell reactivity. Usin g the proB/preB 4G7 MoAb, Psi L cell surface expression was detected on nor mal bone marrow, not only on CD34(-)CD19(+) preB but also on CD34(+)CD19(+) proB cells. Futhermore, this MoAb identified Psi L(+)mu(-) fresh proB leuk emic cells of the TEL/AML1 type. Biochemical studies showed that, at the pr oB stage, the Psi L is associated noncovalently with two proteins of 105 an d 130 kD. Triggering of this complex induces intracellular Ca2+ flux, sugge sting that the Psi L may be involved in a new receptor at this early step o f the B-cell differentiation. (C) 1999 by The American Society of Hematolog y.