Identification of a transcription factor that binds to the S box of the I-A beta gene of the major histocompatibility complex

Immunosenescence of macrophages: reduced MHC class II gene expression

In order to determine the effect of aging on macrophages, we produced bone marrow-derived macrophages in vitro from young and aged mice. We analyzed the effect of aging on the genomic expression of macrophages in these conditions, without the influence of other cell types that may be affected by aging. Macrophages from young and aged mice were present in similar numbers and showed an identical degree of differentiation, cell size, DNA content and cell surface markers. After incubation with interferon-gamma (IFN-gamma), the expression at the cell surface of the MHC class II gene IA complex product and the levels of intracellular IAbeta protein and mRNA were lower in aged macrophages. Moreover, the transcription of IAbeta gene was impaired in aged macrophages. The amount of transcription factors that bound to the W and X boxes, but not to the Y box of the IAbeta promoter gene were lower in aged macrophages. Similar levels of CIITA mRNA were found after IFN-gamma treatment of both young and aged macrophages. This shows that neither the initial cascade that starts after the interaction of IFN-gamma with the receptor, nor the second signals involved in the expression of CIITA, are impaired in aged macrophages. These data could explain, at least in part, the impaired immune response associated to senescence.

IFN-gamma-dependent transcription of MHC class II IA is impaired in macrophages from aged mice

To determine the effect of aging on IFN-gamma-induced MHC class II antigen expression, we produced bone marrow-derived macrophages in vitro. In these conditions, we analyzed the effect of aging on the genomic expression of macrophages without the influence of other cell types that may be affected by aging. Although macrophages from young and aged mice showed an identical degree of differentiation, after incubation with IFN-gamma, the expression at the cell surface of the IA complex and the levels of IAbeta protein and mRNA were lower in aged macrophages. Moreover, the transcription of the IAbeta gene was impaired in aged macrophages. The amount of transcription factors that bound to the W and X, but not to the Y, boxes of the IAbeta promoter gene was lower in aged macrophages. Similar levels of CIITA mRNA were found after IFN-gamma treatment of both young and aged macrophages. This shows that neither the initial cascade that starts after the interaction of IFN-gamma with the receptor nor the second signals involved in the expression of CIITA are impaired in aged macrophages. These data indicate that aging is associated with low levels of MHC class II gene induction by IFN-gamma because of impaired transcription.

pRB is required for interferon-gamma-induction of the MHC class II abeta gene

pRB is required for IFN-gamma-induction of MHC class II in human tumor cell lines, providing a potential link between tumor suppressors and the immune system. However, other genes, such as cyclin D1, show pRB-dependency only in tumor cells, so by analogy, pRB may not be necessary for cII-regulation in normal cells. Here, we demonstrate that induction of the mouse MHC class II I-A heterodimer is normal in RB+/+ mouse embryonic fibroblasts (MEFs), but deficient in RB-/- MEFs. Inducibility is restored in RB-/- MEFs stably transfected with wild type RB cDNA or infected with an adenovirus expressing pRB. Thus, involvement of pRB in MHC class II expression is conserved in the mouse and is not an aberrant feature of tumorigenic, aneuploid, human tumor cells. Although cII genes are generally induced in a coordinate fashion, suggesting a common mechanism, we found that pRB was specifically required for induction of the Abeta, but not Aalpha or other MHC cII genes including Ebeta, Ii and H2-Malpha. Finally, IFN-gamma-induction of class II transactivator (CIITA), was pRB-independent, suggesting that pRB works downstream of this master-regulator of MHC class II expression.