Background and objectives: The redox-regulatory protein thioredoxin has several functions including transcriptional regulation, and antioxidant, cytokine, and chemokine activities. We have previously shown that extracellular thioredoxin protects B-cell chronic lymphocytic leukemia (CLL) cells from apoptosis in vitro. In this study we were interested to determine whether thioredoxin is produced by cells surrounding the CLL cells in the in vivo microenvironment and whether this cell-derived thioredoxin has any leukemia growth-promoting effect in vitro.
Design and methods: Lymph nodes from CLL patients (n=25) were analyzed for thioredoxin expression by immunohistology. Stromal cells purified from the lymph nodes were analyzed for thioredoxin secretion at the single cell level using an ELIspot assay. The survival effect of the stromal-derived thioredoxin was tested by co-culturing stromal- and CLL cells with and without Fab-fragments of an anti-thioredoxin antibody.
Results: The results indicated that the thioredoxin production correlated with the amount of proliferating cells and was mainly localized to the proliferation centers (pseudofollicles) in the CLL lymph nodes. The leukemia cells per se showed minimal thioredoxin levels; in contrast, stromal cells strongly expressed thioredoxin. Purified primary stromal cells, which secreted extracellular thioredoxin, significantly protected the CLL cells from undergoing apoptosis in 72 h co-cultures. Interestingly, this anti-apoptotic effect could be abrogated by addition of Fab-fragments of an anti- thioredoxin antibody.
Interpretation and conclusions: In conclusion, we have shown that stromal cells in the lymph node microenvironment produce thioredoxin and that the thioredoxin production is localized to the proliferation centers of the CLL lymph nodes. In addition, thioredoxin produced by purified stromal cells rescued CLL cells from apoptosis in vitro.