The mitogenic activity of type III bacterial Ig binding proteins (protein G) for human peripheral blood lymphocytes is not related to their ability to react with human serum albumin or IgG.

The mitogenic potential of bacterial IgG Fc binding proteins for human PBL is controversial. Wild type and recombinant type III IgG Fc binding proteins induce a wide spectrum of proliferative responses ranging from non-mitogenic to potent responses. To understand the reason for these differences, three recombinant forms of a type III IgG Fc binding protein derived from a single human group C streptococcal strain, 26RP66, were generated. Form I bound human IgG and human serum albumin, form II bound IgG alone and form III bound human serum albumin alone. These functionally distinct forms were compared with the corresponding wild type preparation from the same strain for mitogenic potential. A mitogenic response was induced only with the form I recombinant or the native wild type protein. These proteins shared the functional characteristics of binding human serum albumin and IgG. Mixtures of the IgG binding (form II) and human serum albumin binding fragments (form III) failed to reconstitute the mitogenic potential of the full length proteins. These results demonstrate that the type III IgG Fc binding protein has mitogenic potential for human PBL that is not related to its ability to react with human serum albumin or IgG.