Exoenzyme S from Pseudomonas aeruginosa induces apoptosis in T lymphocytes

Exoenzyme S from Pseudomonas aeruginosa is a unique T cell mitogen; it is a powerful immunostimulus that activates a large proportion of T cells, but results in delayed and reduced lymphocyte proliferation. This study was performed to explain the discrepancy between early T cell activation and subsequent proliferation. Studies revealed that exoenzyme S induced rapid and unsustained surface expression of CD69, but could not induce interleukin-2 receptor alpha (IL-2R alpha) up-regulation on T cells. IL-2 was undetectable in supernatants and addition of rIL-2 could not reverse the unresponsiveness, indicating that anergy was not involved. Exoenzyme S induced membrane phosphatidylserine translocation, DNA hypodiploidy, and DNA fragmentation, implicating apoptosis as the mechanism for the unresponsiveness. Exoenzyme S-induced apoptosis shows features of both propriocidal and death by neglect, suggesting shared characteristics of an intermediate pathway. Thus, a Pseudomonas exoproduct induces T cell apoptosis, which may contribute to the pathogenesis of Pseudomonas infections in diseases such as cystic fibrosis.