Cell Surface Expression Of MHC Class Research Articles

Crosstalk Between PKA and Epac Regulates the Phenotypic Maturation and Function of Human Dendritic Cells

The cAMP-dependent signaling pathways that orchestrate dendritic cell (DC) maturation remain to be defined in detail. Although cAMP was previously thought to signal exclusively through protein kinase A (PKA), it is now clear that cAMP also activates exchange protein activated by cAMP (Epac), a second major cAMP effector. Whether cAMP signaling via PKA is sufficient to drive DC maturation or whether Epac plays a role has not been examined. In this study, we used cAMP analogs to selectively activate PKA or Epac in human monocyte-derived DCs and examined the effect of these signaling pathways on several hallmarks of DC maturation. We show that PKA activation induces DC maturation as evidenced by the increased cell-surface expression of MHC class II, costimulatory molecules, and the maturation marker CD83. PKA activation also reduces DC endocytosis and stimulates chemotaxis to the lymph node-associated chemokines CXCL12 and CCL21. Although PKA signaling largely suppresses cytokine production, the net effect of PKA activation translates to enhanced DC activation of allogeneic T cells. In contrast to the stimulatory effects of PKA, Epac signaling has no effect on DC maturation or function. Rather, Epac suppresses the effects of PKA when both pathways are activated simultaneously. These data reveal a previously unrecognized crosstalk between the PKA and Epac signaling pathways in DCs and raise the possibility that therapeutics targeting PKA may generate immunogenic DCs, whereas those that activate Epac may produce tolerogenic DCs capable of attenuating allergic or autoimmune disease.

Cigarette Smoke Exposure Impairs Dendritic Cell Maturation and T Cell Proliferation in Thoracic Lymph Nodes of Mice

Robbins CS, Franco F, Mouded M, Cernadas M, Shapiro SD. J Immunol. 2008;180(10):6623–6628 PURPOSE OF THE STUDY. Airborne antigens are processed and presented by respiratory tract dendritic cells (DCs). The purpose of this study was to determine the consequences of cigarette-smoke exposure on DC function in mice. METHODS. Mice were exposed to cigarette smoke 5 days per week for 1 month. There was also a control group of mice unexposed to smoke. Mice then underwent intratracheal injection of ovalbumin-fluorescein isothiocyanate (FITC) or ovalbumin and were killed at varying time points thereafter depending on the aspect of DC function being studied (migration, maturation, or T-cell activation). Lung cells were isolated, and the numbers of DCs were determined in exposed and unexposed mice by flow cytometry. Migration of lung DCs to thoracic lymph nodes (TLNs) was assessed by measuring CD11c+FITC+ cells. Maturation of TLN DCs after ovalbumin exposure was then determining by assessing expression of cell-surface costimulatory molecules (CD80, CD86, MHCII, CD40). T-cell proliferation was assessed by measuring T-cell–derived cytokine production by TLN cells in vitro. The staining profile was assessed to measure T-cell proliferation in vivo. RESULTS. The number of lung DCs was decreased among mice exposed to cigarette smoke. Exposed mice did not exhibit decreased migration of lung DCs to TLNs, as evidenced by similar numbers of TLN CD11c+FITC+ cells as in unexposed mice. Although no differences were observed in migration, smoking suppressed DC maturation in the lymph nodes as demonstrated by decreased cell-surface expression of MHC class II and the costimulatory molecules CD80 and CD86. T-cell proliferation was impaired in smoke-exposed mice, evidenced by reduced interleukin 2 production from CD4 T cells as compared with controls. T-cell proliferation was also decreased in vivo: smoke-exposed mice had a lower percentage of T cells undergoing ≥6 cycles or cell division compared with controls. CONCLUSIONS. These findings suggest that cigarette smoke is associated with defects in DC maturation and suppressed thoracic lymph node CD4+ T-cell proliferation in mice. REVIEWER COMMENTS. This study suggests that cigarette-smoke exposure in mice leads to impaired DC maturation and thereby diminished T-cell clonal expansion in the TLNs. The findings imply that, although the T-cell effector program remains intact, fewer cells might be available to perform those duties. The authors asserted that the observed effects on DC function might serve as a potential mechanism for cigarette smoke causing impaired tumor surveillance. In addition, it is suggested that impaired CD4+ T-cell function might predispose patients with obstructive lung disease to infections and exacerbations. Additional studies will help to elucidate the specific mechanisms by which cigarette-smoke exposure exerts its harmful immunologic effects.

High Levels of MeCP2 Depress MHC Class I Expression in Neuronal Cells

BackgroundThe expression of MHC class I genes is repressed in mature neurons. The molecular basis of this regulation is poorly understood, but the genes are particularly rich in CpG islands. MeCP2 is a transcriptional repressor that binds to methylated CpG dinucleotides; mutations in this protein also cause the neurodevelopmental disease called Rett syndrome. Because MHC class I molecules play a role in neuronal connectivity, we hypothesised that MeCP2 might repress MHC class I expression in the CNS and that this might play a role in the pathology of Rett syndrome.MethodologyWe show here that transiently transfected cells expressing high levels of MeCP2 specifically downregulate cell-surface expression of MHC class I molecules in the neuronal cell line N2A and they prevent the induction of MHC class I expression in response to interferon in these cells, supporting our first hypothesis. Surprisingly, however, overexpression of the mutated forms of MeCP2 that cause Rett syndrome had a similar effect on MHC class I expression as the wild-type protein. Immunohistological analyses of brain slices from MECP2 knockout mice (the MeCP2tm1.1Bird strain) demonstrated a small but reproducible increase in MHC class I when compared to their wild type littermates, but we found no difference in MHC class I expression in primary cultures of mixed glial cells (mainly neurons and astrocytes) from the knockout and wild-type mice.ConclusionThese data suggest that high levels of MeCP2, such as those found in mature neurons, may contribute to the repression of MHC expression, but we find no evidence that MeCP2 regulation of MHC class I is important for the pathogenesis of Rett syndrome.

HFE cross-talks with the MHC class I antigen presentation pathway

AbstractHFE is a protein known to be involved in iron metabolism; yet, other than its homology with major histocompatibility complex (MH95) class I molecules, it has not been described as having an immunologic function. Here we report that peripheral blood mononuclear cells (PBMCs) from patients with hereditary hemochromatosis (HH) carrying the C282Y mutation in HFE have reduced cell-surface expression of MHC class I due to an enhanced endocytosis rate of MHC class I molecules caused by premature peptide and β2-microglobulin dissociation. This faster turnover also leads to increased expression levels of cell-surface free class I heavy chains in mutant PBMCs. Biochemical analysis indicates an earlier peptide loading and endoplasmic reticulum maturation of MHC class I molecules in C282Y mutant cells. Thermostability assays further showed that in HFE mutants the MHC class I peptide loading gives rise to low-stability heterotrimers that dissociate prematurely during its intracellular traffic. The present results suggest the existence of an intriguing cross-talk between a particular HFE mutation and the classical MHC class I route. These findings constitute the first description of peptide presentation pathway abnormalities linked to HFE and provide additional evidence for the occurrence of immunologic defects in patients with HH.

Human retinoblastoma: in vitro differentiation and immunoglobulin superfamily antigen modulation by retinoic acid.

Suspension and attachment cultures of Y79 human retinoblastoma cells were treated with all-trans retinoic acid (RA) for up to 10 days to assess its effect on growth and cell-surface expression of immunoglobulin superfamily antigens MHC class I and class II, ICAM-1, NCAM and Thy1. RA up to 10 microM induced growth inhibition, and marked morphological differentiation with extension of prominent processes resembling neurites was seen in attachment cultures. However, above 10 microM RA produced extensive cell death. We also observed increased cell-surface expression of MHC class I, ICAM-1, NCAM and Thy1 on Y79 cells treated with 10 microM over 10 days; constitutive MHC class II expression was not apparent, nor did RA treatment appear to induce Y79 cells to express MHC class immunoreactivity. The up-modulation of cell-adhesion molecules (NCAM, ICAM-1 and Thy1) and immune recognition molecules (NCAM, ICAM-1 and MHC class I), associated with reduced growth and tumour cell differentiation, suggests that RA may have a potential role in regulating the growth and development of retinoblastoma tumours.

Flavivirus-induced up-regulation of MHC class I antigens; implications for the induction of CD8+ T-cell-mediated autoimmunity.

Infection of a wide variety of cells of human, mouse and other species' origin by flaviviruses such as WNV, YF, Den, MVE, KUN and JE, increases the cell-surface expression of MHC class I. This MHC class I up-regulation is not due to increased MHC class I synthesis per se, but the result of increased peptide availability in the ER for MHC class I assembly. This is most likely due to the interaction of the viral polyprotein with the ER membrane during viral replication. Flavivirus infection can overcome peptide deficiency in TAP-deficient or non-permissive cell lines such as RMA-S and Syrian hamster cells, BHK and NIL-2. The consequence of this increased MHC class I expression manifests itself in reduced susceptibility to NK cells and augmented lysis by Tc cells. In mice, long-term flavivirus-immune Tc cell memory formation is impaired, following the appearance of strong anti-self Tc cell reactivity observed in in vitro cultures from splenocytes of flavivirus-primed animals. We hypothesize that flavivirus-induced MHC class I up-regulation leads to transient T-cell autoimmunity, followed by down-regulation of both autoimmunity and virus-specific Tc cell memory. Furthermore, we speculate that flavivirus infections of humans in the tropics may be responsible for the observed lower incidence of overt autoimmunity in these geographic regions than in temperate climates where flaviviruses are not endemic.