Neuropeptide regulation of immunity. The immunosuppressive activity of alpha-melanocyte-stimulating hormone (alpha-MSH)

The ocular microenvironment is an extreme example of regional immunity. Within its microenvironment, expression of delayed type hypersensitivity (DTH) is suppressed. This immunosuppression is mediated in part by the constitutive expression of alpha-MSH. Previously we have found that alpha-MSH suppresses the production of IFN-gamma by activated effector T cells. Recently we have found that alpha-MSH can mediate induction of TGF-beta-producing T cells that act as regulatory T cells. This has encouraged us to further examine the potential for alpha-MSH to suppress T cell-mediated inflammation (autoimmune disease) and to regulate lymphokine production by effector T cells. When alpha-MSH was injected i.v. into mice at the time of peak retinal inflammation, the severity of experimental autoimmune uveitis (EAU) was significantly suppressed. Effector T cells activated in vitro in the presence of alpha-MSH proliferated and produced IL-4 and enhanced levels of TGF-beta while their IFN-gamma and IL-10 production was suppressed. The alpha-MSH-treated T cells functioned as regulatory T cells by suppressing in vitro IFN-gamma production by other inflammatory T cells. This regulatory activity was the function of alpha-MSH-treated CD4+ CD25+ T cells. Therefore, alpha-MSH mediates immunosuppression by inducing a differential expression of lymphokine production and by inducing activation of regulatory functions in T cells. This implies that alpha-MSH may take part in regional mechanisms of immunosuppression and possibly peripheral tolerance. Thus, alpha-MSH can be used to suppress autoimmune disease and possibly reestablish tolerance to autoantigens.

Suppression of nitric oxide generated by inflammatory macrophages by calcitonin gene-related peptide in aqueous humor

PURPOSE

Ocular immune privilege is mediated in part by the activity of constitutively produced immunosuppressive cytokines and neuropeptides. Aqueous humor was examined for content of calcitonin gene-related peptide (CGRP), and the potential of CGRP to mediate immunosuppressive activity within aqueous humor was determined.

METHODS

The concentration of CGRP in fresh, normal rabbit aqueous humor was assayed by competitive enzyme-linked immunosorbent assay. The ability of CGRP to suppress interferon (IFN)-gamma production by antigen-stimulated, primed lymph node cells was examined by assaying supernatants of stimulated CGRP-treated, primed T-cell cultures for IFN-gamma. The anti-inflammatory activity of aqueous humor and CGRP was assayed by treating IFN-gamma-lipopolysaccharide (LPS)-activated RAW 264.7 cells (macrophages) with aqueous humor, aqueous humor plus anti-CGRP antibody, or CGRP alone. Culture supernatants of the treated macrophages were examined for nitrite by Griess reagent. The production of inducible nitric oxide synthase (NOS2) protein was examined by immunoblotting cell lysates of treated activated macrophages.

RESULTS

The constitutive level of CGRP in fresh, normal rabbit aqueous humor was 5+/-1 x 10(-5) M. At its ocular concentration, CGRP did not inhibit IFN-gamma production by stimulated effector T cells, but it suppressed nitric oxide generation by activated macrophages. Neutralization of CGRP in normal rabbit aqueous humor prevented the aqueous humor from suppressing nitric oxide generation by macrophages. Neither CGRP nor aqueous humor suppressed NOS2 protein synthesis in activated inflammatory macrophages.

CONCLUSIONS

Calcitonin gene-related peptide is a constitutive neuropeptide in aqueous humor. Through CGRP, aqueous humor suppresses nitric oxide production by activated macrophages. This suppression appears to result from inhibiting NOS2 enzymatic activity, rather than from suppressing NOS2 synthesis. The results imply that the ocular microenvironment has diverse immunoregulatory mechanisms that suppress induction, activation, and mediation of immunogenic inflammation.

Aqueous humor induces transforming growth factor-beta (TGF-beta)-producing regulatory T-cells

PURPOSE

The intraocular microenvironment is an immune-privileged site where immunogenic inflammation is suppressed. Suppression of immunogenic inflammation has been associated with immunosuppressive factors found in aqueous humor produced by ocular tissues. To further understand the mechanisms suppressing immunogenic inflammation in the eye, we have examined the production of lymphokines by primed T-cells activated in the presence of aqueous humor.

METHODS

Enriched in vivo primed T-cells were T-cell receptor-stimulated in the presence of fresh aqueous humor. The culture supernatant was assayed for IFN-gamma and IL-4 by sandwich ELISA. TGF-beta production by T-cells stimulated in the presence of aqueous humor was assayed by a TGF-beta bioassay of the culture supernatant and by quantitative RT-PCR for TGF-beta 1 mRNA expression. Aqueous humor-treated T-cells were assayed for their capacity to suppress IFN-gamma production by stimulated, primed T-cells.

RESULTS

Aqueous humor-enhanced proliferation but irreversibly suppressed production of both IFN-gamma and IL-4 by in vitro-activated, in vivo-primed T-cells. Aqueous humor induced in vivo primed T-cells to produce TGF-beta in vitro, and these TGF-beta-producing T-cells suppressed IFN-gamma production by other T-cells activated in co-cultures.

CONCLUSIONS

Aqueous humor alters the functional program of TCR-ligand-activated, primed T-cells, converting the cells to TGF-beta-producing regulatory cells.