Evidence for Direct Inhibition of MHC-Restricted Antigen Processing by Dexamethasone.

Dexamethasone (Dex) was shown to inhibit the differentiation, maturation, and antigen-presenting function of dendritic cells (DC) when added during DC generation or maturation stages. Here, we examined the direct effects of Dex on MHC-restricted antigen processing. Macrophages were incubated with microencapsulated ovalbumin (OVA) in the presence of different concentrations of Dex for 2 h, and the efficacy of OVA peptide presentation was evaluated using OVA-specific CD8 and CD4 T cells. Dex inhibited both class I- and class II-restricted presentation of OVA to T cells; this inhibitory effect on antigen presentation was much more potent in immature macrophages than in mature macrophages. The presentation of the exogenously added OVA peptide SIINFEKL was not blocked by Dex. In addition, short-term treatment of macrophages with Dex had no discernible effects on the phagocytic activity, total expression levels of MHC molecules or co-stimulatory molecules. These results demonstrate that Dex inhibits intracellular processing events of phagocytosed antigens in macrophages.

Dendritic cells process antigens encapsulated in a biodegradable polymer, poly(D,L-lactide-co-glycolide), via an alternate class I MHC processing pathway.

Biodegradable nanospheres generated from a biocompatible polymer, poly(D,L-lactide-co-glycolide) (PLGA), have been studied extensively as implantable reservoirs for sustained-release drug delivery. PLGA-nanospheres have also been studied as vehicles to deliver antigens to phagocytes. The intracellular processing pathway of antigens delivered to phagocytes by PLGA particles was studied in the present study. Ovalbumin (OVA) encapsulated with PLGA (OVA-nanosphere) was efficiently captured, processed and presented on class I major histocompatibility complex (MHC-I) by dendritic cells (DCs). The MHC-I processing of OVA-nanospheres was resistant to lactacystin, a proteosome inhibitor, and brefeldin A, which blocks anterograde transport from the endoplasmic reticulum (ER) through the Golgi apparatus. Chloroquine, which inhibits phagolysosomal enzymes by increasing phagolysosomal pH, inhibited MHC-I processing of OVA-nanospheres. In addition, DCs generated from TAP-/- mice were markedly suppressed in MHC-I processing of OVA-nanospheres. These results demonstrate that DCs process phagocytosed OVA-nanospheres via a vacuolar alternate MHC-I pathway for presentation of OVA peptides to T lymphocytes.

Calcineurin inhibitors block MHC-restricted antigen presentation in vivo.

APCs, like T cells, are affected by calcineurin inhibitors. In this study, we show that calcineurin inhibitors efficiently block MHC-restricted exogenous Ag presentation in vivo. Mice were injected with clinical doses of tacrolimus (FK-506) followed by soluble OVA, and dendritic cells (DCs) were isolated from lymph nodes and spleens. The efficacy of OVA peptide presentation by DCs was evaluated using OVA-specific CD8 and CD4 T cells. Tacrolimus inhibited both class I- and class II-restricted DC presentation of OVA to T cells. Tacrolimus also inhibited both class I- and class II-restricted presentation of OVA in peritoneal macrophages isolated from mice injected with tacrolimus followed by soluble OVA. Tacrolimus-treated peritoneal macrophages, however, were able to present synthetic OVA peptide, SIINFEKL. Inclusion of cyclosporine A to biodegradable microspheres containing OVA greatly reduced their capacity to induce OVA-specific CTL response in mice. These findings provide novel insight into the mode of action of calcineurin inhibitors and have important implications for clinical immunosuppression regimens.

Synergistic activation of monocytes by polysaccharides isolated from Salicornia herbacea and interferon-gamma.

The hot water extract of Salicornia herbacea, SHE, has recently been shown to have strong immunomodulatory activity. In the present study, we purified the polysaccharides, termed SHP, from SHE preparation and examined their immunomodulatory activity alone and in combination with interferon (IFN)-gamma. The combination of SHP and IFN-gamma synergistically inhibited the growth of the mouse monocytic cell line, RAW 264.7, inducing further differentiation to strongly adherent macrophages. The differentiation-inducing activity of SHP alone and in combination with IFN-gamma was confirmed by changes in the expression of differentiation antigens such as CD11b, CD18 and CD24. In addition, the combination of SHP and IFN-gamma synergistically activated RAW cells to produce cytokines such as tumor necrosis factor (TNF)-alpha and interleukin (IL)-1 beta, and nitric oxide (NO). The synergistic activity of SHP was more prominent when SHP concentration was low. Increased production of TNF-alpha, IL-1 beta and NO was correlated with an increased level of their respective transcripts. These results confirm that Salicornia herbacea contains immunomodulatory polysaccharides that activate monocytes synergistically with small doses of IFN-gamma.