Innate immunity plays an important role in host defense after severe insult. gammadelta T lymphocytes are recognized as the first line of defense against microbial invasion. In this study, we evaluated gammadelta T lymphocytes in the peripheral blood of patients with severe systemic inflammatory response syndrome (SIRS), and examined on role of these cells. Thirty-seven patients with severe SIRS (SIRS criteria and serum C-reactive protein > or = 10 mg/dL) and 27 healthy volunteers were studied. Severe SIRS was caused by trauma in 14 patients (Injury Severity Score of 30.1 +/- 10.8) and by sepsis in 23 patients. The counts of gammadelta and alphabeta T lymphocytes were determined by flow cytometry of cells stained with monoclonal antibodies to gammadelta and alphabeta T lymphocyte receptors. The activation of these cells was evaluated by flow cytometry of cells stained with monoclonal antibodies to CD69 and HLA-DR. Serial counts and activation of gammadelta and alphabeta T lymphocytes were also determined in eight trauma patients (Injury Severity Score of 31.0 +/- 13.5) during a 2-week observation period. The count of gammadelta T lymphocytes in the peripheral blood of SIRS patients (30.1 +/- 6.0/microL) was significantly lower (P < 0.05) than that of the healthy volunteers (104.3 +/- 10.9/microL). The expression of CD69, an index of early activation of T lymphocytes, was significantly greater on gammadelta T lymphocytes from SIRS patients (patients 23.9% +/- 3.4%, healthy controls 4.8% +/- 0.6%, P < 0.05). In trauma patients, the expression of CD69 on gammadelta T lymphocytes increased rapidly within 48 h after injuries. In conclusion, gammadelta T lymphocytes are activated and decreased in the peripheral blood of severe SIRS patients. In trauma patients, the activation of gammadelta T lymphocytes occurs in the fairly acute phase after injuries. These results suggest a significant role for gammadelta T lymphocytes as early responders after severe insult.