Recombinant Protein A Activity in SCID Mouse Models of Human B Cell Tumors.

Protein A from Staphylococcus Aureus acts as a potent B cell superantigen through specific interaction with B cell receptors displaying variable heavy chain sequences from the VH3 clan. In humans, 30–50% of all B cells are VH3-positive, so protein A activity may serve to interfere with host immune responses to S. Aureus through deletion of normal B cell subsets. Indeed, protein A-mediated B cell deletion has been demonstrated in transgenic mice with nearly all B cells expressing VH3-positive surface Ig [Goodyear, J. Immunol176(4), 2262, 2006]. However, it is conceivable that this activity may be usurped to target clonal B cell diseases with a VH3-positive cell origin. In the current study, the superantigen function of a recombinant therapeutic form of protein A (TpA) was exploited to investigate potential anti-tumor activity of protein A in in vitro and in vivo models of B cell neoplastic disease. TpA initiated apoptotic cell death of VH3-positive B cell tumor lines in culture within 24–48 of exposure as evidenced by alteration in mitochondrial membrane potential, activation of caspases, and external exposure of phosphatidylserine in the cell membrane. By 72–96 hours, cell viability was reduced by >50%. Addition of human IgG was essential for cell killing and optimal activity was observed at a 2:1 molar ratio of IgG to TpA. In SCID mice engrafted with the non-Hodgkin’s lymphoma cell line 2F7, TpA administered along with human IgG resulted in modest but statistically significant extension of median survival. In two independent studies employing a SCID mouse KMS-12-BM tumor model, TpA along with 40mg/kg human IgG produced a dose-dependent, reproducible therapeutic effect. At the highest dose studied (30mg/kg), median survival was 45 days compared to median survival of 27 days for control animals. Increased frequency of TpA administration also positively impacted median survival. While human IgG was required for optimal response, a five-fold increase in IgG diminished TpA anti-tumor activity, further underscoring the importance of the ratio of TpA to IgG in cell killing. These results demonstrate that the capacity of protein A to act as a B cell superantigen is not limited to normal cells but can be manifest on transformed cells as well. Since the VH3 clan is highly represented in the human B cell repertoire and is over-represented in some B cell neoplastic diseases, protein A may have utility as a therapeutic agent in cases of B cell malignancy where a VH3 origin is identified.