Striped bass Morone saxatilis were studied in order to characterize their immune responses over the short term following challenge with Mycobacterium marinum. The expression of immunity-related genes quickly increased following infection with M. marinum, but these genes were subsequently down-regulated despite the fact that bacterial counts remained high. The number of monocytes and neutrophils also initially increased at 1 d postinfection. This confirms the importance of these types of cells in initial inflammation and mycobacterial infection in striped bass. The phagocytic index of splenic leukocytes over these same time frames did not change significantly following infection. The discrete window in which inflammatory mechanisms were stimulated in striped bass may be related to the intracellular nature of this pathogen.KEY WORDS: Mycobacteriosis · Striped bass · Gene expression · Interleukin-1β · IL-1β · Tumor necrosis factor-α · TNF-α · Natural resistance-associated macrophage protein · Nramp · Transforming growth factor-β · TGF-β Resale or republication not permitted without written consent of the publisherDis Aquat Org 94: [117][118][119][120][121][122][123][124] 2011 culosis in humans due to its relatively fast growth (~4 h generation time) and its genetic similarity to M. tuberculosis (Tonjum et al. 1998, Cosma et al. 2003. Several aquatic animal models have been used in the past to study immune responses to infection with M. marinum, including goldfish Carassius auratus, zebrafish Danio rerio, leopard frog Rana pipiens and medaka Oryzias latipes (Ramakrishnan et al. 1997, Talaat et al. 1998, Decostere et al. 2004, Broussard & Ennis 2007.Due to the chronic nature of disease caused by Mycobacterium marinum, previous studies have concentrated on host responses over the long term, looking at the formation and persistence of granulomas in host organs. Only the work of Burge et al. (2004) and Harms et al. (2003) have addressed the early-phase response, and in those cases only in vitro. Here, we investigate the initiation of immune responses in the striped bass Morone saxatilis to an in vivo challenge from M. marinum using markers for inflammation , tumor necrosis factor-α [INF-α], transforming growth factor-β [IGF-β] expression) and bacterial killing mechanisms (natural resistance-associated macrophage protein [Nramp] expression, phagocytic index). MATERIALS AND METHODSExperimental fish and maintenance. One-year-old striped bass Morone saxatilis, Choptank River, Maryland strain, were obtained from Delmarva Aquatics. These fish were transported to the Cooperative Oxford Laboratory (COL) to allow for fish and system acclimation. A total of 100 fish (mean weight = 50 ± 5 g) were randomly stocked in sixteen 568 l circular tanks at 12 to 13 fish per tank and allowed to acclimatize for 1 mo. Experimental conditions were set as: photoperiod 12h light: 12 h dark, pH 8.2, salinity 10 ppt, temperature 21°C. Water quality (dO, NH 3 , NH 4 , NO 2 ) remained in a healthy range for the species (Harrell et al...