Low affinity binding of mouse immunoglobulin to human CD5+ B cells

Abstract

Polyreactive immunoglobulin (Ig) secreted by CD5-bearing B cells has the capacity to bind a broad range of self and foreign antigens. Flow cytometric analysis was used to detect low-affinity binding of mouse Ig molecules to the surface of CD5-bearing B cells from patients with chronic lymphocytic leukaemia (CLL). Mouse Ig isotypes G, A, and M, and IgG subclasses G1, G2a, and G3 bound to the B cell surface via a mechanism not involving the antigen-binding site of the mouse Ig molecule. Fab and F(ab')2 fragments of mouse Ig associated with the CLL cells in a similar manner to intact Ig, indicating that Fc receptor interactions were not involved. Dissociation of the mouse Ig from the B cell surface by three washing steps distinguished this lower-affinity binding from high-affinity binding which occurs through the antigen-binding site of a mouse monoclonal antibody (MoAb) when it recognizes a specific cell surface epitope. Blocking studies suggest that the low-affinity binding occurred via surface IgM and surface IgD (sIgM/sIgD) on the CD5-bearing B cells. The results are consistent with the expression of polyreactive Ig on the surface of CD5-bearing B cells in CLL.