Comparison of [3H]thymidine incorporation with MTT- and MTS-based bioassays for human and murine IL-2 and IL-4 analysis. Tetrazolium assays provide markedly enhanced sensitivity.

The sensitivity of [3H]thymidine incorporation and MTT/MTS colorimetric bioassays for detection and quantitation of murine and human IL-4 and IL-2 were compared in CT.4S, CT.h4S and HT-2 bioassays respectively. We reasoned that low levels of cytokine, insufficient to induce cell proliferation (thus, DNA synthesis and [3H]thymidine incorporation), may be sufficient to maintain the viability of the bioassay cells in culture. Because colorimetric assays such as those employing MTT (3-(4,5 dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) or MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-( 4-sulfonyl)-2H- tetrazolium) measure conversion of these salts to intensely colored formazan products by mitochondrial enzymes independent of whether proliferation is induced, we reasoned that such assays could be superior for detection of low levels of cytokine protein. Direct comparison of these approaches demonstrated that the MTT- and MTS-based assays were consistently able to detect 2-16-fold lower cytokine levels than methods based on [3H]thymidine incorporation. Moreover, the MTT and MTS assays exhibited higher precision with standard deviations of < 1-4% vs. 5-15% for thymidine incorporation. This finding is of particular importance in approaches such as limiting dilution analysis, or primary bulk culture of antigen-stimulated human lymphocytes, where levels of cytokine production may be extremely low.