J Med Assoc Thai Vol. 94 Suppl. 7 2011 S131 Correspondence to: Kalaya A, Division of Microbiology and Immunology, Department of Preclinical Science, Faculty of Medicine, Thammasat University, Rangsit Campus, Klong Luang, Pathumthani 12120, Thailand. Phone: 0-2926-9710-1, Fax: 0-2926-9710 E-mail: kalaya@tu.ac.th Crocodylus siamensis Serum and Macrophage Phagocytic Activity Kalaya Aree PhD*, Jindawan Siruntawineti PhD**, Win Chaeychomsri MSc** * Department of Preclinical Science, Faculty of Medicine, Thammasat University, Rangsit Campus, Pathumthani, Thailand ** Department of Zoology, Faculty of Science, Kasetsart University, Bangkok, Thailand Antimicrobial activity of sera from many crocodilian species has been recognized. This activity was proposed to be mediated, at least in part, by complement. Due to the fact that complement proteins have different functions in the immune system, they may be involved in phagocytic process of phagocytes. In the present study, the effects of Siamese crocodile serum on phagocytic activity of macrophages as well as the possible involvement of complement in this process were examined. The results showed increases in the phagocytosis of both Escherichia coli and to a lesser extent, Staphylococcus aureus upon incubation of murine macrophage cell line with fresh crocodile serum (FS). Similar to FS, other crocodile blood products, including freeze dried serum (DS) and freeze dried whole blood (DWB) exhibited phagocytosis-enhancing property. However, the ability of DWB to enhance phagocytosis was less efficient than that of FS and DS, suggesting that serum factors were involved in this process. Treatment of FS with heat at 56 ο C for 30 min deteriorated the effect of FS on bacterial uptake of macrophages, suggesting that complement proteins play a role in the modulation of the phagocytic process. Collectively, the results of the present study suggested that crocodile serum enhances the macrophage phagocytic activity through complement activity and, therefore, may be taken as an alternative medicine for supporting the human immune responses. Keywords: Crocodylus siamensis, Crocodile serum, Phagocytosis, Complement J Med Assoc Thai 2011; 94 (Suppl. 7): S131-S138 Full text. e-Journal: http://www.jmat.mat.or.th Macrophages play an important role in the first line immune defense. Their primary function is phagocytosis, which depends largely on receptors on the cell membrane. These receptors include Toll-like receptors (TLR), mannose receptor (MR) and scavenger receptor (SR), which bind directly to pathogen surface structures, so-called pathogen-associated molecular pattern (PAMP). In addition, phagocytosis is facilitated through opsonization, a process by which coated pathogens are recognized mostly through Fc- (FcR) and complement receptors (CR). The multiple ligand- receptor interactions initiate pathogen internalization, and ultimately killing of pathogens (1,2) . Sera from several crocodilian species have been reported to have antimicrobial activity (3) . American alligator (Alligator mississippiensis) serum was shown to exhibit potent antimicrobial activity against a wide variety of bacteria (4) , viruses (5) and amoeba (6) . From these studies, complement proteins were suggested to have significant antimicrobial functions in alligators. This activity was proposed to be related to the complement protein C3 since antimicrobial activity was inhibited by polyclonal anti-human C3 antibodies (7) . In Siamese crocodiles (Crocodylus siamensis) , antibacterial and antifungal activities were also detected (8,9) . In addition, the antibacterial activity of crocodile complement was observed and appeared to be more effective than that of human (10) . Although the antimicrobial peptide, named crocosin, was recently identified from Siamese crocodile plasma (11) , complement proteins are expected to take part in antimicrobial activity in C. Siamensis. It is well documented that activation of C3 results in the formation of the anaphylatoxin, C3a and the opsonin, C3b. C3a recruits inflammatory cells whereas C3b can either bind to microbial membrane or initiate the formation of membrane attack complex (MAC) (12,13) .