Epidemiological evidence, in vitro studies and animal models suggest that exposure to the bacterial endotoxin lipopolysaccharide (LPS) can influence the development and severity of asthma. Although it is known that signaling through Toll-like receptors (TLR) is required for adaptive T helper cell type 1 and 2 responses, it is unclear whether the LPS ligand TLR 4 is expressed on CD4(+) and CD8(+) T-lymphocytes and if so, whether LPS could modulate the T(H)1 or T(H)2 response in this context. The present authors have, therefore, examined the expression of TLR 4 on peripheral blood CD4(+) and CD8(+) T-lymphocytes using RT-PCR method and FACS analyses. Furthermore, the authors have studied the IL-12-induced expression of the T(H)1-associated cytokine INF-gamma and the IL-4-induced expression of the T(H)2-specific cytokine IL-5 in the presence of LPS using ELISA and compared nine atopic asthmatic subjects and eleven nonatopic normal volunteers. There was an increased anti-CD3/anti-CD28-induced IL-5 expression in T cells of asthmatics compared with normals (p<0.01). In the presence of IL-4 (10 ng/ml), there was an additional increase in IL-5 expression and this additional increase was greater in T cells of normals compared with asthmatics (p<0.05). There was an expression of INF-gamma in anti-CD3/anti-CD28-induced T-lymphocytes without differences between both groups (NS). In the presence of IL-12 (10 ng/ml), there was an increase in INF-gamma release without differences between normals and asthmatics (NS). In the presence of different concentrations of LPS (10 ng/ml, 1 mug/ml), there was a decrease in IL-4-induced IL-5 expression without differences in both groups, indicating an intact T(H)2 response to bacterial endotoxin LPS in asthma. Interestingly, LPS increased the IL-12-induced INF-gamma release in a concentration-dependent manner in T-lymphocytes of normals but this could not be found in T cells of asthmatics, indicating an impaired T(H)1 response to bacterial endotoxin LPS in asthma. In addition, there was a TLR 4 expression on CD4(+) T-lymphocytes of normals and to a lesser extent in asthmatics but this TLR 4 expression could not be found on CD8(+) T cells of both groups. In conclusion, there may be an impaired concentration-dependent LPS-induced T(H)1 rather than a T(H)2 response in allergic adult asthmatics compared with normal volunteers. One reason for this could be a reduced TLR 4 expression on CD4(+) T-lymphocytes of asthmatic subjects.