Distribution of immune cells in the epididymis of the aging brown Norway rat is segment-specific and related to the luminal content

Remarkable changes occur during aging in the testis and epididymis of the B rown Norway rat. A dramatic increase in the number of halo cells, which are present in the epididymal epithelium and originate from the immune system, is found in animals of increasing age. Halo cells have been postulated to be either lymphocytes or monocytes, We hypothesized that halo cells are a m ixture of different immune cells and that their relative composition change s with age. To verify this hypothesis, markers for helper T lymphocytes, cy totoxic T lymphocytes, B lymphocytes, and monocytes-macrophages were used t o identify the major categories of immune cells in the epididymides of Brow n Norway rats ranging in age from 3 to 24 mo. The numbers of immunocompeten t cells in the epididymis were determined in relation to age, epididymal se gment, and luminal content. We found that monocytes, helper T lymphocytes, and cytotoxic T lymphocytes belong to the population of halo cells. In addi tion, a segment-specific increase with age in the number of these immune ce lls was noted. Finally, we report a segment-specific recruitment of cytotox ic T lymphocytes and monocytes-macrophages in the epididymal epithelium of aged rats whose epididymal lumen contained few spermatozoa. We postulate th at accumulation of damaged epithelial cells and antigens of germ cell origi n, leaking through a dysfunctional blood-epididymis barrier, may contribute to the active recruitment of immune cells with age.