Aquatic Toxicology 81 (2007) 79–89 Long-term exposure to arsenic affects head kidney and impairs humoral immune responses of Clarias batrachus Debabrata Ghosh a , Soma Datta a , Shelley Bhattacharya b , Shibnath Mazumder a,∗ a Immunobiology Laboratory, School of Life Sciences, Visva-Bharati University, Santiniketan 731235, India b Environmental Toxicology Laboratory, School of Life Sciences, Visva-Bharati University, Santiniketan 731235, India Received 29 July 2006; received in revised form 5 November 2006; accepted 7 November 2006 Abstract The present study was aimed at determining the effects of long-term arsenic exposure on the head kidney (HK) and ensuing humoral immune responses in Clarias batrachus L. Long-term exposure (150 days) to non-lethal concentrations of arsenic (42.42 M) resulted in significant time- dependent alterations in HK cell number eventually affecting the HK somatic index. Prolonged exposure to arsenic also suppressed HK-B cell proliferation and led to significant reduction in serum immunoglobulin levels and antigen-specific serum bacterial agglutinin titers. A decline in the number of antigen-specific plaque-forming cells with duration of arsenic exposure was noted in the HK. Enzyme linked immunosorbent assays further revealed that arsenic exposure inhibited the release of “IL-4 like factors” from HK-T cells. Histological studies documented time-dependent changes in the structure and cellular composition of HK characterized by extensive lymphocytopenia, decrease in melano-macrophage population and hemosiderin accumulation. From exposure-challenge studies with Aeromonas hydrophila it was evident that pathogens could efficiently disseminate and colonize distant host tissues in the exposed fish. Moreover, the ability to decrease the pathogen load was also significantly reduced in the arsenic-exposed fish. Thus long-term exposure to non-lethal concentrations of arsenic affects HK and interferes with the humoral immune system of C. batrachus rendering them immunocompromised and susceptible to pathogenic challenge. © 2006 Elsevier B.V. All rights reserved. Keywords: Arsenic; Head kidney; Humoral immune responses; Lymphoproliferation; Clarias batrachus; Aeromonas hydrophila 1. Introduction The semimetal arsenic is an important environmental toxi- cant (Oremland and Stolz, 2003; Ratnaike, 2006). Among the different hazardous chemicals directly introduced to aquatic environment arsenic has been reported to be most alarming (ATSDR, 2002). There appears to be a continuous and alarming influx of this toxicant to aquatic environment worldwide from both naturally occurring and anthropogenic sources (Goering et al., 1999). Consequently, it is taken up by various organisms (Bernstam and Nriagu, 2000). The chemistry of inorganic arsenic in aquatic environment is very complex especially with the changes in pH and avail- able oxygen concentrations (Roy and Saha, 2002). It has been observed that arsenic exists in aquatic environment either in arsenite (As 3+ ) or arsenate (As 5+ ) form, which is inter-converted through redox and methylation reactions (Philips, 1994; Eisler, ∗ Corresponding author. Tel.: +91 3463 261268; fax: +91 3463 261268. E-mail address: shibnath1@yahoo.co.in (S. Mazumder). 1988). Arsenic in As 5+ form is the least toxic while As 3+ has been found to be more toxic both under in vivo and in vitro conditions (Cervantes et al., 1994). Following entry inside the cell these two forms also react differentially with arsenite binding to –SH groups in proteins while arsenate interfering with the phosphory- lation reactions (Andrew et al., 2003; Bears et al., 2006). In order to avoid the toxic effects of arsenic, organisms have evolved dif- ferent mechanisms for biotransformation of arsenic to less toxic forms which are then excreted (Vahter, 2002; Bears et al., 2006). Environmental contaminants selectively target the immune system, often acting on the acquired immune system by sup- pressing antibody production and memory responses as well on innate immune functions by lowering macrophage activity (Fournier et al., 2000) and maturation (Sakurai et al., 2006). There are also reports on the role of arsenic as modulators of co- receptors present in lymphocytes (Galicia et al., 2003). Arsenic is known to inhibit the JAK-STAT pathway in human lympho- cytes and interfere with the release of different cytokines (Cheng et al., 2004). Arsenic exposure reduced delayed-type hypersen- sitivity reaction, inhibited mitogen-activated T cell proliferation, increased free intracellular Ca 2+ production and induced early 0166-445X/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aquatox.2006.11.004