CD22 associates with the human surface IgM-B-cell antigen receptor complex.

The B-cell surface molecule CD22, when cross-linked, modulates signaling through the surface IgM (sIgM)-B-cell receptor (BCR) complex. Here we analyzed the basis of this interaction between CD22 and the human sIgM complex. After lysis of B cells or B-cell lines in digitonin, CD22 coimmunoprecipitated a kinase activity that in vitro-phosphorylated two polypeptides of 150 and 130 kDa on tyrosine residues. By immunoblot analysis with a rabbit anti-serum specific for a synthetic peptide of CD22, we found these proteins to be CD22 itself. Furthermore, the phosphorylated 150-kDa CD22 was found in the sIgM-BCR complex maintained by digitonin, along with Ig alpha/mb-1, Ig beta/B29, and a 75-kDa polypeptide precipitated by an antiserum specific to protein-tyrosine kinase PTK72. CD22 is likely to be an important signaling partner in the sIgM-BCR complex since it is very rapidly and strikingly phosphorylated after sIgM is cross-linked and since it contains the antigen recognition homology I (ARHI) motif, present in other antigen receptor molecules.