Absence of streptococcal protein G (PG)-specific determinant in the Fab region of human IgG2.

It has been previously reported that CH1 Fab protein G-contact site is responsible for the widespread recognition of mouse and human IgG Fab by PG. Here we present evidence that PG binding to F(ab′)₂ is restricted, as indicated by the lack of reactivity with PG-Sepharose columns of a portion of F(ab′)₂ fragments obtained by pepsin digestion of human IgG from a commercial immunoglobulin preparation for intravenous use or purified from sera of two healthy blood donors and two patients with polyclonal hypergammaglobuinaemia. Isoelectric focusing showed that F(ab′)₂ fragments that did not bind PG focused in a lower pH range compared with those which did. Testing of the Fab fractions with MoAbs to κ and λ light chains or to γ1, γ2 and γ3-Fab subclass determinants showed that γ2-F(ab′)₂ were mainly found in the PG non-reactive F(ab′)₂ fraction, and that this distribution was not influenced by the L chain isotype. These results indicate that the PG-specific binding determinant(s) is not expressed in the F(ab′)₂ region of most human IgG2. [ABSTRACT FROM AUTHOR]