Research Paper In vitro Effects of Lipopolysaccharide and Stress Hormones on Phagocytosis and Nitric Oxide Production by Enriched Head Kidney Macrophage Cultures in the Catfish Heteropneustes fossilis R KUMAR and K P JOY 1,* Department of Zoology, Centre of Advanced Study, Banaras Hindu University, Varanasi 221 005, India 1 Present address: Department of Biotechnology, Cochin University of Science and Technology, Kochi 682 022, India (Received on 04 September 2017; Revised on 22 November 2017; Accepted on 25 November 2017) Head kidney (HK) is the embryonic pronephric kidney retained in adult teleosts and contains the adrenal homologues (interrenal and chromaffin cells) and hemopoietic tissue. The adrenal homologues secrete the stress hormones glucorticoids (cortisol) and catecholamines, and the hemopoietic tissue is a major source of monocytes/macrophages and neutrophils that serve as the first line of defence against invading pathogens. In the present study, head kidney macrophage-enriched preparations of Heteropneustes fossilis were used to demonstrate the dynamics of phagocytosis and nitric oxide (NO) production in the presence of lipopolysaccharide (LPS; bacterial toxin), the synthetic glucocorticoid dexamethasone and/or catecholamines. The incubation of enriched macrophage cultures with LPS for 6 h at 20 o C stimulated phagocytosis of yeast cells and fluorescent latex beads. On the other hand, the cortisol agonist dexamethasone (10 nM) inhibited phagocytosis of yeast cells and latex beads under similar conditions. LPS stimulated inducible NO synthase (iNOS)-like expression in the macrophage cultures, which was inhibited by dexamethasone in co-incubations. Complementary to the iNOS expression, LPS stimulated NO production (nitrite level), which was inhibited by the NO synthase inhibitor L-NMMA. Dexamethasone inhibited basal as well as the LPS-induced stimulation of NO. The catecholamines epinephrine and norepinephrine did not alter the basal NO level but inhibited the LPS-induced stimulation of NO. Dopamine stimulated NO production only at a higher concentration. The results provide evidence for the existence of an endocrine-immune interaction at the level of the head kidney to modulate macrophage activity and immune functions in the catfish. Keywords: Head Kidney Macrophages; Catecholamines; Dexamethasone; Endocrine-Immune Interaction *Author for Correspondence: E-mail: keerikkattilpailyjoy@yahoo.com Proc Indian Natn Sci Acad 84 No. 3 September 2018 pp. 707-721  Printed in India. DOI: 10.16943/ptinsa/2018/49312 Introduction In fish, the innate immune response is the first line of defence against invading pathogens, executed by phagocytes comprising of neutrophils, monocytes and macrophages (Secombes and Fletcher, 1992; Magor and Magor, 2001). Neuroendocrine and autonomic nervous systems modulate immune functions (Castillo et al., 2009; Verburg-van Kemenade et al., 2011; Nardocci et al., 2014). The head kidney (HK) is the embryonic pronephric kidney containing endocrine (interrenal and chromaffin cells) and immune (macrophages and granulocytes) tissues and serves as an anatomical site for endocrine-immune interactions in fish (Verburg-van Kemenade et al., 1994, 2011; Kumar et al., 2016). Macrophages and neutrophils release extracellular traps that kill pathogens (Pijanowski et al., 2015). Phagocytosis is stimulated in the presence of bacterial toxins like lipopolysaccharide (LPS) through the mediation of nitric oxide (NO), reactive oxygen species (ROS), proinflammatory cytokines and chemokines (Pressley et al., 2005; Jensch-Junior et al ., 2006; Kalgraff et al., 2011; Verburg-van Kemenade et al., 2011). The head kidney hemopoietic tissue, which surrounds the endocrine tissues, contain not only free moving macrophages, but also macrophage-aggregates called melano-macrophage centres (MMCs) involved in various steps in Published Online on 29 January 2018