Pseudomonas aeruginosa is a gram-negative bacterium that is responsibl
e for devastating acute and chronic infections, which include bronchie
ctasis in cystic fibrosis, nosocomial pneumonia, and infection of burn
wounds. Previous studies have demonstrated that these patients have i
mpaired host responses, including cell-mediated immune responses, whic
h are important in anti-Pseudomonas host defense. The P. aeruginosa ex
oproduct, exoenzyme S, has a number of characteristics which suggest t
hat it might be important in cell-mediated immunity. To determine whet
her exoenzyme S activates lymphocytes to proliferate, peripheral blood
mononuclear cells (PBMC) from normal volunteers were stimulated with
purified exoenzyme S, and the lymphocyte response was assessed by meas
uring [H-3]thymidine uptake and by counting the number of cells after
various times in culture. Ninety-five percent of healthy adult donors
had a lymphocyte response to exoenzyme S, The optimal lymphocyte respo
nse occurred on day 7, with 4 x 10(5) PBMC per microtiter well when ce
lls were stimulated with 10 mu g exoenzyme S per mi, [H-3]thymidine up
take correlated with an increase in the number of mononuclear cells, i
ndicating that proliferation occurred, In unseparated PBMC, T cells, a
nd to a lesser extent B cells, proliferated. Purified T cells prolifer
ated, while purified B cells proliferated only after the addition of i
rradiated T cells. Thus, T lymphocytes are necessary and sufficient fo
r the proliferative response to exoenzyme S, We speculate that exoenzy
me S from P. aeruginosa is important in T-lymphocyte-mediated host def
ense to P. aeruginosa, In strategies to enhance impaired cell-mediated
immunity, exoenzyme S should be considered as a potential stimulant.