Introduction: Despite the activation of lymphocytes by tumor antigens, the immune system is often inefficient in eliminating tumor cells and preventing progressive tumor growth. It has been reported that naturally occurring antibodies against over-expressed or mutant proteins in tumor cells occur at high frequency in the serum of breast cancer patients. However, spontaneous regression of breast cancer is rare and in contrast, the presence of the antibody in patient serum may correlate with poor prognosis. The purpose of this research was to study the role of naturally occurring human antibody in controlling human breast cancer cell survival. Methods: Total RNA isolated from the PBMC of a patient with breast cancer was used for RT-PCR and the positive PCR products were ligated into a pComb3 display phage for constructing a recombinant cDNA expression library of human IgG Fab. Primary breast cancer cell lysate was used to screen the antibody library. The tumor antigen binding ability of antibodies was tested by ELISA analysis. Breast cancer cell lines were treated with antibodies at various concentrations. The cancer cell growth was measured by 3H-Thymidine uptake assay and the cell apoptosis measured by FACS with FITC annexin V. Results: A cDNA expression library containing 1.2 x 105 primary human IgG Fab clones has been successfully constructed. Twenty clones were screened from the library; which represented 14 different transcripts by sequence homology search from the GenBank data bases. ELISA was performed on three clones; all showed stronger binding to antigens from primary breast cancer and breast cancer cell lines than to one from normal breast tissue. Functional studies showed that antibodies exhibited different biological effects on breast cancer cell survival in vitro when used at various concentrations. At low level (2 μg/ml) some clones stimulated growth of MDA-MB-231 breast cancer cell line but had no effect on MCF-7 and SK-BR-3 cell growth in vitro, indicating that these antibodies can promote breast cancer cell growth by binding to some tumor antigen that expressed in various levels among different cell lines. Treatment with high level of these clones (20 μg/ml) had no effect to promote cancer cell growth of above three cell lines. However, two tested antibody clones showed strong effect to block etoposide-induced cell apoptosis in all three above cell lines, which occurred with a Fab concentration as low as 2 μg/ml and reached to a complete blocking with a high level at 20 μg/ml. Conclusions: Our studies provide evidence to explain why naturally occurring immune response is often inefficient in eliminating tumor cells and preventing progressive growth. These findings suggest that serum auto-antibodies to tumor-associated antigens may initially contribute to cancer promotion when immune response developed at a low level and finally involve in tumor immune escaping with a high level of immune response.

[Proc Amer Assoc Cancer Res, Volume 45, 2004]