Secretion of 10-kDa and 12-kDa Thioredoxin Species from Blood Monocytes and Transformed Leukocytes

Thioredoxins (TRX) are ubiquitous, small redox-active proteins with multiple functions, including antioxidant, cytoprotective, and chemoattractant activities. In addition to a 12-kDa intracellular form, extracellular 10-kDa and 12-kDa TRX have been defined. The biological activities of the 10-kDa TRX were previously measured as eosinophil cytotoxicity enhancing activity or B-cell stimulatory activity. Cytotrophoblastic cell lines also release a 10-kDa TRX form. To study the biological role of 10-kDa TRX, we established two highly sensitive enzyme-linked immuno-spot assays (ELISPOT), which detect secreted truncated 10-kDa and full-length 12-kDa TRX at the single cell level. TRX secretion was investigated in several cell lines including the T-helper cell hybridoma MP6, the Jurkat T-cell leukemia, the U-937 myelomonocytic leukemia, and the 3B6, EBV-transformed, lymphoblastoid B-cell line. The highest number of secreting cells was found in 3B6 cultures, median = 34 (quartiles, 27–39) per well (10⁵ cells). Peripheral blood monocytes isolated from healthy donors secreted significantly more TRX after stimulation with ionomycin, phorbol myristate acetate (PMA), fMLP, and lipopolysaccharide (LPS), compared to unstimulated cells. Oxidative stress induced by thioloxidant diamide also induced the secretion of both truncated and full-length TRX measured in ELISPOT (p = 0.047 and p = 0.031, respectively). The biological activity of the truncated and full-length forms was tested in a cell migration assay. Truncated TRX was devoid of protein disulfide reductase activity, but retained strong chemoattractant activity for human monocytes, in the same range as full-length TRX, as previously reported (Bertini et al., 1999).