Macrophages (mσ) are one of the main phagocytic cells involved in innate immunity having the ability to recognize, internalize and destroy diverse pathogens. Although the testes are considered immunologically privileged, with possible immunoinhibiting molecules (Hutson, 1994), testicular macrophages are known to interact with Leydig cells and modulate testosterone production. The pineal hormone melatonin (MLT) has numerous effects on the immune system (Maestroni, 1999) and peritoneal macrophages are known to possess MLT membrane receptors (Garc’a-Perganeda et al. 1999). Recently it has also been shown that diverse immune functions are affected by diabetes (Geerlings & Hoepelman, 1999).

The aim of this study was to examine whether melatonin modulates the phagocytic activity of testicular macrophages taken from control and streptozotocin (STZ)-induced diabetic rats in vitro.

Experiments were conducted on Wistar rats (350Ð400 g). Animals were killed by cervical dislocation. Testicular macrophages were washed out with PBS (30 ml/testes) and cell suspensions of 10⁶ cells ml^-1 prepared. mσ (200 µl) were incubated for 30 min at 37 °C, washed with pre-warmed PBS and 20 µl of 1 % latex beads suspension (1.1 µm diameter) added. The effect of 10^-7 M, 10^-6 M and 10^-5 M MLT was tested on mσ from control (n = 5) and diabetic (n = 5) animals. Phagocytic index (PI) was calculated as the number of particles ingested by 100 mσ. Student’s paired t test was used for statistical analysis and results were expressed as means ± S.E.M.

Melatonin (10^-7 M) significantly increased (P < 0.05) the PI in testicular mσ from control animals (PI = 68.125 ± 5 compared with PI = 46.875 ± 7 in mσ incubated with PBS). No such effect was observed on the cells from diabetic rats. Moreover, the phagocytic activity of mσ isolated from healthy rats was 20 % higher than that of mσ isolated from diabetic animals.

These results indicate that exogenous MLT may stimulate the phagocytic activity of testicular macrophages in vitro. Diabetes, on the other hand, may have immunosuppressive effects on these cells. The mechanism of MLT action on testicular macrophages is to be investigated.

All procedures accord with current UK legislation.