DAO 46:15-21 (2001)  -  doi:10.3354/dao046015

Streptococcus iniae inhibition of apoptosis of nonspecific cytotoxic cells: a mechanism of activation of innate immunity in teleosts

Susan L. Taylor1, Liliana Jaso-Friedmann1, Andrew B. Allison1, Avi Eldar2, Donald L. Evans1,*

1Department of Medical Microbiology and Parasitology, College of Veterinary Medicine, University of Georgia, Athens, Georgia 30602, USA
2Fish Disease Lab, Kimron Veterinary Institute, PO Box 12, Bet Dagan, 50250, Israel
*Corresponding author. E-mail:

ABSTRACT: Nonspecific cytotoxic cells (NCC) may provide innate anti-bacterial resistance against Streptococcus iniae infections in tilapia. The mechanism of immunity would be elaboration and release of various cytokines, augmentation of inflammation and amplification of increased antigen processing. To investigate bacterial regulation of NCC function, 2 different processes of cellular pathology were examined: apoptosis and necrosis. Different isolates of S. iniae from diseased teleosts, a dolphin and a human were tested. All isolates were examined for their ability to produce apoptosis and/or necrosis on freshly purified tilapia NCC and on a tilapia continuous cell line (i.e. TMB-8 cells). Two different isolates (9033 and 173) inhibited the outer membrane expression of phosphatidylserine (PS) by NCC, an early sign of apoptosis. This occurred at 4 h post-treatment and lasted throughout the 24 h treatment period. All other isolates either did not differ from control levels or produced a small increase in PS expression by NCC. The early reduction in PS expression occurred concomitantly with increased necrosis associated with nonspecific DNA fragmentation. Two-color flow cytometry (Annexin-V vs propidium iodide staining) demonstrated the specificity of Annexin-V binding. Experiments were also done to determine the effects of S. iniae on TMB-8 cells. Treated TMB-8 cells did not produce appreciable Annexin-V binding. Compared to the ATCC strain, 9033 produced high levels of necrosis-associated DNA fragmentation of TMB-8 cells at 4 and 8 h post-treatment. These data indicated that different isolates of S. iniae may regulate NCC anti-bacterial resistance by causing reduced levels of programmed cell death (PCD), increased necrosis and associated enhancement of inflammatory responses. Understanding the relevance of these bacterial effects on NCC may be an important consideration in the evaluation of isolates used in vaccine/ bacterin production.

KEY WORDS: Nonspecific cytotoxic cells · Tilapia · Streptococcus iniae · Apoptosis · Necrosis · Annexin-V · Innate immunity · DNA · Programmed cell death · Teleosts

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