EXPRESSION OF CLASS-II MHC MOLECULES IN THE RAT PINEAL-GLAND DURING DEVELOPMENT AND EFFECTS OF TREATMENT WITH CARBON-TETRACHLORIDE

Cells expressing major histocompatibility complex (MHC) class II (Ia) antigen have been examined during the development of rat pineals and i n the pineal gland of adult rats treated with carbon tetrachloride. Ce lls positive for MHC class II are first detected in the pineal gland o f the 7-day-old rat. These positive cells increase in number gradually during development, MHC class II immunoreactivity reaching adult leve ls at 4 weeks after birth. The MHC class II antigen is intensely label ed on the cell surface, and labeled cells are distributed throughout t he organ, several positive cells being gathered into groups. The posit ive cells are small (7-12 mu m in diameter), irregular in shape, and f requently exhibit one or more processes. At the electron-microscopic l evel, the cytoplasm of positive cells contains few organelles, various ly sized empty vacuoles, and a few electron-dense lysosome-like struct ures. Pinealocytes with synaptic ribbons have been found adjacent to i mmunoreactive cells. Double-immunoperoxidase staining for MRC OX6, MRC OX42, and ED1 results in OX6(-)/ED1(+)/OX42(+), OX6(-)/ED1(-)/OX42(+) , and OX6(+)/ED1(-)/OX42(-) cells. These findings suggest that OX6-pos itive cells in the pineal can be considered as peripheral dendritic ce lls. The number of cells expressing MHC class II (Ia) antigen signific antly increases in the pineal gland of rats after treatment with carbo n tetrachloride (P<0.005). Our results indicate that at least some of the OX6-positive cells migrate into the gland from the circulation und er these conditions.