MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) CLASS II-POSITIVE DENDRITIC CELLS IN THE RAT IRIS - IN-SITU DEVELOPMENT FROM MHC CLASS II-NEGATIVE PRECURSORS

Purpose. To examine the postnatal development of major histocompatibil ity complex (MHC) class II-positive dendritic cells (DC) in the iris o f the normal rat eye. Methods. Single-and double-color immunomorpholog ic studies were performed on whole mounts prepared from rat iris taken at selected postnatal ages (2 to 3 days to 78 weeks). Immunopositive cells were enumerated, using a quantitative light microscope, and MHC class II expression on individual cells was assessed by microdensitome tric analysis. Results. Major histocompatibility class II-positive DCs in the iris developed in an age-dependent manner and reached adult-eq uivalent density and structure at approximately 10 weeks of age, consi derably later than previously described in other DC populations in the rat. In contrast, the anti-rat DC monoclonal antibody OX62 revealed a population of cells present at adult-equivalent levels as early as 3 weeks after birth. Dual-color immunostaining and microdensitometric an alysis demonstrated that during postnatal growth, development of the n etwork of MHC class II-positive DCs was a consequence of the progressi ve increase in expression of MHC class II antigen by OX62-positive cel ls. Conclusions. During postnatal growth, the DC population of the iri s develops initially as an OX62-positive-MHC class II-negative populat ion, which then develops increasing MHC class II expression in situ an d finally resembles classic DC populations in other tissue sites. Matu ration of the iris DC population is temporally delayed compared with t ime to maturation in other tissue sites in the rat.