| CD40 is a B cell receptor whose binding gives rise to regulatory signals including growth stimulation, differentiation, isotype switching, and upregulation of Fas. CD40 also is commonly expressed in epithelial cancers such as breast cancer but its physiologic role is still being explored. We hypothesize that CD40 has a growth regulatory function in human breast cancer cells. The function of CD40 on breast carcinoma cells was examined with soluble recombinant CD40L molecules (gp39 and CD40LT). The biologic activity of affinity-purified gp39 was verified by a B cell proliferative assay. Gp39 and CD40LT significantly reduced 3H-thymidine uptake in CD40-positive BT-20 and T47D cells of up to 40%, but did not affect the growth of CD40-negative MCF-7 or ZR-75-1 cells. Similarly, growth inhibition was observed in T47D cells following coincubation with CD40L-transfected L cells (55.0 +/- 8.9%) or with CD40L + peripheral blood lymphocytes (39.7 +/- 3.7%). Untransfected L cells and non-CD40L expressing lymphocytes did not produce significant growth inhibition. The in vivo antitumorigenic effects of CD40L were examined using a subcutaneous xenograft model. Pretreatment with soluble CD40L produced xenograft growth inhibitory effects of up to 67% which were reversed by cotreatment with a CD40L-neutralizing antibody. In vitro analysis indicated that up to 31% of CD40+ breast carcinoma cells underwent apoptosis after CD40L treatment. An upregulation of pro-apoptotic elements, Bax and Bak, was observed, indicating that the Bcl-2 family of proteins may be involved in the observed growth inhibition. To explore the clinical relevance of CD40-CD40L interaction, immunohistochemical analysis was carried out to characterize CD40 and CD40L expression in breast cancer patient biopsies. All of the infiltrating ductal and lobular breast carcinomas, and carcinomas in situ tested expressed CD40. Tumor cells also expressed CD40L in the majority of infiltrating ductal and lobular carcinomas, and carcinomas in situ. Tumor infiltrating mononuclear cells from primary tumor tissues expressed CD40 but less commonly expressed CD40L. These observation suggest that the CD40 signaling pathway is active in human breast carcinoma cells. However, tumor infiltrating lymphocytes from primary tumor tissues may be limited in their capacity to directly modulate tumor growth through the CD40-CD40L loop. |
