CD5 is a potential selecting ligand for B-cell surface immunoglobulin: A possible role in maintenance and selective expansion of normal and malignantB cells

Although the function of CD5 on B cells is unknown, previous studies sugges ted that CD5 interaction with V-H framework regions of surface immunoglobul ins (Igs) may contribute to survival and expansion of B cells, Here we used B-chronic lymphocytic leukemia (B-CLL) cells and transformed B-cell Lines from normal and B-CLL patients to study CD5-Ig interactions. Immobilized Ig binds and permits isolation of CD5 from lysates of CD5-expressing cell lin es. Immunoglobulins or Fab fragments of different V-H families varied in th eir effective ness as inhibitors of anti-CD5 staining of CLL cells, appendi x and tonsil tissue sections. Human Ig also binds to purified recombinant C D5. We show here for the first time that the unconventional Ig-CD5 interact ion maps to the extracellular CD5-D2 domain whereas conventional epitopes r ecognized by anti-CD5 antibodies are localized in the D1 domain of CD5. We propose that interactions of V-H framework regions with CD5 as a ligand may maintain, select or expand normal, autoimmune or transformed B cells and a lso contribute to skewing of the normal V-H repertoire.