Sex Steroids Modulate Fish Immune Response

For some time behavioural and ecological studies have suggested that sex steroid hormones regulate several immune processes in fish. For example, the immunocompetence handicap hypothesis relates the heritability of parasite resistance with secondary sexual ornaments, which are determined and maintained by androgens. Such ornaments are probably a good indicator to potential mates of genetic resistance to infections (Dijkstra et al., 2007; Roberts et al., 2004). Among vertebrates, the prevalence and intensity of parasitic infections is higher in males than females (Klein, 2004). Some fish species show altered sex steroid hormones levels upon parasite infection. The main alterations recorded upon infection are decreases in androgen, estrogen and vitelogenin serum levels (Hecker & Karbe, 2005). For example, during an infective period of vibriosis, silver seabream showed gradually increasing testosterone serum levels, whereas serum estradiol levels significantly decreased at an early stage of infection and remained low until death. This process coincided with increasing macrophages phagocytic activity (Deane et al., 2001). Such field studies prompted immunologists to try to establish how sex steroid hormones are able to alter the functions of the circulating leukocytes. In fish, most existing information on reproductive-immune interactions deals with the modulation of immune responses by circulating hormones, including cortisol, growth hormone, prolactin and reproductive hormones and some proopiomelanocortin-derived peptides (Engelsma et al., 2002; Harris & Bird, 2000). Although the exact effect of these endocrine mediators depends on the species, in general, they are known to modulate immune responses by integrating the activities of all the systems. In this way they help to adapt the organism to its environment (Lutton & Callard, 2006). From a reproductive biology point of view, the leukocytes located in mammalian gonads orchestrate important reproductive physiology processes, including gametogenesis and steroidogenesis. A long time has passed since leukocytes were first described in the gonad of teleosts. Since them, several types of leukocytes have been described in the testis of different teleost species using light and electron microscopy. Moreover, differences in the number and localization of leukocytes within the testis have also been observed during the different stages of the reproductive cycle (Besseau & Faliex, 1994; Billard, 1983; Bruslé-Sicard & Fourcault, 1997; Lo Nostro, 2004; Scott & Sumpter, 1989). Thus, in the gametogenic activity and spawning stages some macrophages have been described in the interstitial tissue of the rainbow trout testis (Loir et al., 1995), whereas in the post-spawning stage a