The presence of biologically significant concentrations of glucocorticoids but little or no cortisol binding globulin within aqueous humor: relevance to immune privilege in the anterior chamber of the eye.

PURPOSE: Immunosuppressive factors in aqueous humor (AH) contribute to the immune-privileged status of the anterior chamber of the eye. One such factor is transforming growth factor-beta (TGF-beta); other relevant inhibitors have not been fully identified. The authors examined AH to search for other putative inhibitors and to determine their effect on TGF-beta inhibitory activity.
METHODS: Radioimmunoassays (RIA) were used to detect the presence of hydrocortisone, corticosterone, cortisol binding globulin (CBG), and alpha-melanocyte stimulating hormone (alpha-MSH) in AH. The ability of these factors to inhibit murine thymocyte proliferation stimulated by phytohemagglutinin-interleukin 1 (PHA/IL-1) and proliferation of a TGF-beta-sensitive cell line (CCL64) in vitro was examined. The ability of hydrocortisone to inhibit a one-way mixed lymphocyte reaction (MLR) and the ability of epidermal cells to present soluble tumor-associated antigens (TAA) for elicitation of immunity in mice in the concentration range present in AH was also examined.
RESULTS: Hydrocortisone was detected in mouse, rat, and human AH (10.8 +/- 1.1 ng/ml, 9.3 +/- 2.1 ng/ml, and 18.0 +/- 1.0 ng/ml, respectively; mean +/- SEM), as was corticosterone (2.7 +/- 0.9 ng/ml, 2.2 +/- 0.3 ng/ml, and 0.7 +/- 0.1 ng/ml, respectively). Whereas normal plasma contains a binding protein for corticosteroids (i.e., CBG), the concentration in mouse, rat, and human AH was less than the level detectable by an RIA. Hydrocortisone inhibited PHA/IL-1-stimulated murine thymocyte proliferation and CCL64 cell proliferation in the concentration range present in AH. When hydrocortisone was combined with TGF-beta 2 (125 pg/ml), the degree of inhibition observed was greater than with either alone. Corticosterone inhibited thymocyte costimulation only slightly at concentrations present in AH but was inhibitory for CCL64 cells. alpha-MSH was also detected in AH. The concentration present had only slight inhibitory effects for CCL64 cell proliferation and did not enhance TGF-beta 2-mediated (62 pg/ml to 250 pg/ml) inhibition of CCL64 or thymocyte proliferation. Hydrocortisone inhibited the one-way MLR in the concentration range present in AH and, at 10 ng/ml, inhibited the ability of epidermal cells to present TAA for elicitation of delayed-type hypersensitivity in tumor-immune mice.
CONCLUSIONS: These results show that AH contains biologically relevant concentrations of glucocorticoids and that CBG is relatively absent so that glucocorticoids present are largely free, and they suggest that regional sites take advantage of the activities of multiple factors to maintain an immune-privileged status.