The V4-34 Encoded Anti-i Autoantibodies Recognize a Large Subset of Human and Mouse B-cells

Autoantibodies to the i, I and Pr2 carbohydrate determinants bind red blood cells, preferentially at low temperature in vitro. Using multiparameter flow cytometric analyses, we demonstrate that each of these autoantibodies also react with human and mouse lymphocytes at physiologic temperatures. The anti-Pr2 autoantibody recognizes a glycoprotein determinant(s) expressed by a subset of both T and B lymphocytes. In contrast, the binding of anti-i and anti-I antibodies each is restricted to B-lymphocytes. The anti-i autoantibody binds to over 50% of all B cells, whereas the anti-I antibody reacts with less than 10% of either tonsillar or blood B cells. Prior studies identified that the B cell isoform of CD45 (B220) has the linear poly-N-acetyllactosamine that forms the "i" determinant. Because anti-B220 antibodies recently have been reported to influence T-dependent B-cell isotype switching, we tested each antibody for its ability to influence the production of secondary Ig isotypes by murine splenocytes co-cultured with a stimulator helper T cell clone. We find that addition of anti-i antibody increases the proportion of B cells secreting secondary Ig isotypes. In contrast, the anti-I antibody had no such effect. These findings imply that stimulation of B cells through the highly conserved carbohydrate determinant that forms the "i" antigen may be of physiologic importance in T-dependent B-cell differentiation.