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

Nonspecific cytotoxic cells (NCC) may provide innate anti-bacterial resista nce against Streptococcus iniae infections in tilapia. The mechanism of imm unity would be elaboration and release of various cytokines, augmentation o f inflammation and amplification of increased antigen processing, To invest igate bacterial regulation of NCC function, 2 different processes of cellul ar pathology were examined: apoptosis and necrosis. Different isolates of S . iniae from diseased teleosts, a dolphin and a human were tested. All isol ates were examined for their ability to produce apoptosis and/or necrosis o n 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 mem brane expression of phosphatidylserine (PS) by NCC, an early sign of apopto sis. This occurred at 4 h post-treatment and lasted throughout the 24 h tre atment 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 bindin g, 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-associat ed DNA fragmentation of TMB-8 cells at 4 and 8 h posttreatment. These data indicated that different isolates of S. iniae may regulate NCC anti-bacteri al resistance by causing reduced levels of programmed cell death (PCD), inc reased necrosis and associated enhancement of inflammatory responses, Under standing the relevance of these bacterial effects on NCC may be an importan t consideration in the evaluation of isolates used in vaccine/bacterin prod uction.