Inhibition of HLA class I antigen and mRNA expression induced by Rous sarcoma virus in transformed human fibroblasts

Blocking oncogenic RAS enhances tumour cell surface MHC class I expression but does not alter susceptibility to cytotoxic lymphocytes

Mutations in the RAS family of oncogenes are highly prevalent in human cancer and, amongst its manifold effects, oncogenic RAS impairs the expression of components of the antigen presentation pathway. This allows evasion of cytotoxic T lymphocytes (CTL). CTL and natural killer (NK) cells are reciprocally regulated by MHC class I molecules and any gain in CTL recognition obtained by therapeutic inactivation of oncogenic RAS may be offset by reduced NK cell activation. We have investigated the consequences of targeted inactivation of oncogenic RAS on the recognition by both CTL and NK cells. Inactivation of oncogenic RAS, either by genetic deletion or inactivation with an inducible intracellular domain antibody (iDAb), increased MHC class I expression in human colorectal cell lines. The common RAS mutations, at codons 12, 13 and 61, all inhibited antigen presentation. Although MHC class I modulates the activity of both CTL and NK cells, the enhanced MHC class I expression resulting from inactivation of mutant KRAS did not significantly affect the in vitro recognition of these cell lines by either class of cytotoxic lymphocyte. These results show that oncogenic RAS and its downstream signalling pathways modulate the antigen presentation pathway and that this inhibition is reversible. However, the magnitude of these effects was not sufficient to alter the in vitro recognition of tumour cell lines by either CTL or NK cells.

Role of single-stranded DNA regions and Y-box proteins in transcriptional regulation of viral and cellular genes

Single-stranded regions, known to be important for optimal rates of transcription, have been observed in the promoters of several cellular genes as well as in the promoters of many pathogenic viruses. Several host-encoded, single-stranded DNA binding proteins capable of binding these regions have been purified and their genes isolated. In this review, information available about single-stranded regions present within various promoters and the interaction of a novel class of single-stranded DNA binding transcription factors belonging to the Y-box family of proteins is reviewed. Mechanisms by which these proteins influence transcription of both cellular and viral genes are proposed.

Down-regulation of the MHC class I antigen-processing machinery after oncogenic transformation of murine fibroblasts

Malignant transformation is often associated with genetic alterations providing tumor cells with mechanisms for escape from immune surveillance. Human and murine tumors of various origin as well as in vitro models of viral and oncogenic transformation express reduced levels of major histocompatibility complex (MHC) class I antigens resulting in decreased sensitivity to MHC class I-restricted cytotoxic T lymphocyte (CTL)-mediated lysis. We here investigate whether the suppressed MHC class I surface expression of ras-transformed fibroblasts is due to dysregulation of the genes of the antigen-processing machinery, the peptide transporters TAP-1 and TAP-2 and the proteasome subunits LMP-2 and LMP-7, and whether it can be restored by gene transfer. In comparison to parental NIH3T3 cells, the ras oncogenic transformants revealed reduced TAP and LMP mRNA expression and impaired function of these genes, leading to deficient peptide transport and peptide loading of MHC class I molecules resulting in instable expression of the MHC class I complex on the cell surface. Enhanced H-2 surface expression due to stabilization of the MHC class I complex could be achieved by culturing ras transformants at low, unphysiological temperature (26 degrees C) or by loading these cells with either exogenous human beta2-microglobulin or MHC class I-binding peptide alone or in combination. Furthermore, interferon-gamma treatment was capable to enhance the expression of TAP, LMP and MHC class I molecules in both parental as well as ras-transformed fibroblasts. Stable transfection of the human TAP-1 cDNA into ras transformants caused a partial reconstitution of the peptide transport and an enhancement of the MHC class I surface expression, whereas the level of MHC class I biosynthesis was not affected by TAP-1 overexpression in parental cells. Together these results point to the existence of an association between oncogenic transformation and deficiencies in the MHC class I antigen-restricted immunosurveillance, suggesting intervention strategies involving specific MHC class I-binding peptides or transfection of the LMP and/or TAP genes to overcome the expression of the immune escape phenotype.

The human immunodeficiency virus type 1 (HIV-1) CD4 receptor and its central role in promotion of HIV-1 infection

Interactions between the viral envelope glycoprotein gp120 and the cell surface receptor CD4 are responsible for the entry of human immunodeficiency virus type 1 (HIV-1) into host cells in the vast majority of cases. HIV-1 replication is commonly followed by the disappearance or receptor downmodulation of cell surface CD4. This potentially renders cells nonsusceptible to subsequent infection by HIV-1, as well as by other viruses that use CD4 as a portal of entry. Disappearance of CD4 from the cell surface is mediated by several different viral proteins that act at various stages through the course of the viral life cycle, and it occurs in T-cell lines, peripheral blood CD4+ lymphocytes, and monocytes of both primary and cell line origin. At the cell surface, gp120 itself and in the form of antigen-antibody complexes can trigger cellular pathways leading to CD4 internalization. Intracellularly, the mechanisms leading to CD4 downmodulation by HIV-1 are multiple and complex; these include degradation of CD4 by Vpu, formation of intracellular complexes between CD4 and the envelope precursor gp160, and internalization by the Nef protein. Each of the above doubtless contributes to the ultimate depletion of cell surface CD4, although the relative contribution of each mechanism and the manner in which they interact remain to be definitively established.

Differential modulation of plasminogen activator gene expression by oncogene-encoded protein tyrosine kinases

Urokinase-type plasminogen activator (uPA) gene transcription is increased > or = 50-fold in chicken embryo fibroblasts (CEF) following transformation by the protein tyrosine kinase pp60v-src. Protein phosphorylation appears to play a critical role in uPA gene expression in these cells; protein kinase C-activating phorbol esters cooperate with pp60v-src to synergistically increase uPA mRNA, whereas cyclic AMP (cAMP)-dependent protein kinase-activating agents (e.g., 8-bromo cAMP) repress uPA mRNA levels. To explore the relationship between transforming oncogenes and uPA gene expression, uPA mRNA levels were measured in CEF infected with selected avian retroviruses. We report that v-ras and the transforming protein tyrosine kinases v-src, v-yes, and v-ros all increase cellular uPA mRNAs. However, transformation with the protein tyrosine kinase encoded by v-erbB, or the nuclear proteins encoded by v-jun, v-ski, or v-myc, did not increase uPA mRNA detectably. Ras and all of the protein tyrosine kinases analyzed, including the v-erbB product, but none of the nuclear oncoproteins sensitized cells to phorbol ester induction of uPA gene expression. Thus, increased uPA gene expression is not simply a secondary consequence of cell transformation but, rather, is regulated or comodulated by only a subset of oncogene products. Analysis of cells expressing site-directed mutants of pp60v-src showed that the induction of the uPA gene is dependent on protein tyrosine kinase catalytic activity, myristylation, and plasma membrane localization. However, these properties together are not sufficient; an additional feature in the src homology 2 domain is also required. The major sites of serine phosphorylation, serines 12 and 17, and the autophosphorylation site, tyrosine 416, are not essential for uPA gene induction. However, the reduction of uPA mRNA in pp60v-src-transformed cells by 8-bromo cAMP is dependent on tyrosine 416.

Differential modulation of plasminogen activator gene expression by oncogene-encoded protein tyrosine kinases

Urokinase-type plasminogen activator (uPA) gene transcription is increased > or = 50-fold in chicken embryo fibroblasts (CEF) following transformation by the protein tyrosine kinase pp60v-src. Protein phosphorylation appears to play a critical role in uPA gene expression in these cells; protein kinase C-activating phorbol esters cooperate with pp60v-src to synergistically increase uPA mRNA, whereas cyclic AMP (cAMP)-dependent protein kinase-activating agents (e.g., 8-bromo cAMP) repress uPA mRNA levels. To explore the relationship between transforming oncogenes and uPA gene expression, uPA mRNA levels were measured in CEF infected with selected avian retroviruses. We report that v-ras and the transforming protein tyrosine kinases v-src, v-yes, and v-ros all increase cellular uPA mRNAs. However, transformation with the protein tyrosine kinase encoded by v-erbB, or the nuclear proteins encoded by v-jun, v-ski, or v-myc, did not increase uPA mRNA detectably. Ras and all of the protein tyrosine kinases analyzed, including the v-erbB product, but none of the nuclear oncoproteins sensitized cells to phorbol ester induction of uPA gene expression. Thus, increased uPA gene expression is not simply a secondary consequence of cell transformation but, rather, is regulated or comodulated by only a subset of oncogene products. Analysis of cells expressing site-directed mutants of pp60v-src showed that the induction of the uPA gene is dependent on protein tyrosine kinase catalytic activity, myristylation, and plasma membrane localization. However, these properties together are not sufficient; an additional feature in the src homology 2 domain is also required. The major sites of serine phosphorylation, serines 12 and 17, and the autophosphorylation site, tyrosine 416, are not essential for uPA gene induction. However, the reduction of uPA mRNA in pp60v-src-transformed cells by 8-bromo cAMP is dependent on tyrosine 416.

Mouse hepatitis virus A59 increases steady-state levels of MHC mRNAs in primary glial cell cultures and in the murine central nervous system

Infection of mixed glial cell cultures with mouse hepatitis virus (MHV)-A59 results in an approximately six-fold increase in the level of major histocompatibility complex (MHC) class I mRNA. In situ hybridization of glial cell cultures infected with MHV-A59 again showed enhanced MHC mRNA expression, both in infected and uninfected cells. These results extend our earlier finding that MHC surface antigens are enhanced on astrocytes and oligodendrocytes after MHV-A59 infection and suggest that this enhancement is a result of an increase in the steady-state level of MHC mRNA. We further demonstrate that increases in MHC mRNA occur in the murine central nervous system (CNS) following infection in vivo. Northern blot analysis of RNA from the brains of infected animals showed transient expression of both MHC class I and class II mRNA over the first 14 days of infection. Expression coincided with viral replication and clearance. In situ hybridization of brain sections from infected animals showed that class I and class II expression was widespread throughout all portions of the brain and in uninfected as well as infected cells. Viral RNA, in contrast, was observed in small foci of cells and mostly within the limbic system. Thus enhancement of MHC mRNA was not restricted either to areas of infection or inflammation. The spatial relationship between viral and MHC expression supports our hypothesis that a soluble mediator is involved in the mechanism of the increase in MHC levels. The fact that MHC induction occurs in vivo as well as in vitro suggests MHC may be important in the mechanism of MHV-induced disease.

Modulation of MHC antigen expression by viruses and oncogenes

It is becoming increasingly clear that regulation of MHC antigen expression by viruses and oncogenes, leading to either immune evasion or autoimmunity, is widespread and important in disease. At a recent meeting*, which brought together workers interested in tumour immunology, viral infection and the MHC, a number of mechanisms for the regulation of MHC antigen expression were revealed and the importance of balanced expression of MHC gene products to effective immunity was underlined.

Frequency of abnormal expression of HLA-A,B,C and HLA-DR molecules, invariant chain, and LFA-3 (CD58) in colorectal carcinoma and its impact on tumor recurrence

HLA-A,B,C and HLA-DR molecules are involved in cognate LFA-3 (CD58) in antigen-independent T-cell/target-cell interaction. T-cell-mediated host-versus-tumor response might therefore depend on the presence of both types of molecules on the surface of the target cell. To investigate whether presence or absence of these molecules in colorectal carcinoma influences the recurrence rate, 149 patients who underwent curative surgery were surveyed for a maximum of 65 months (mean, 48 months). As determined by immunohistochemistry, aberrant reduction of HLA-A,B,C determinants was observed in 34.9 and a complete loss in 8.7% of the tumor specimens. An induction of HLA-DR molecules was found in 55.0 and of the HLA-DR-associated invariant chain (Ii) in 81.9%. An abnormal reduction of LFA-3 was detected in 43.6%, while a complete loss of this structure was observed in 6.7%. Reduction/loss of HLA-A,B,C was correlated with reduction/loss of LFA-3 (p = 0.03). In contrast to the prognostic role of tumor stage and grade, the presence vs. absence of all these structures was not correlated with the recurrence rate. We conclude that, although encoded on different chromosomes, an abnormal reduction/loss of HLA-A,B,C and LFA-3 might be the consequence of one transacting down-regulating signal. However, the resulting deviant immunophenotypes do not profoundly influence survival and growth potential of residual tumor cells.

Expression of HLA-A,B,C, beta 2-microglobulin (beta 2m), HLA-DR, -DP, -DQ and of HLA-D-associated invariant chain (Ii) in soft-tissue tumors

Non-neoplastic mesenchymal cells, along with 33 benign and 87 malignant soft-tissue tumors (STT) were examined for expression of HLA-A,B,C, beta 2-microglobulin (beta 2m), HLA-DR, -DP, and -DQ molecules and the HLA-D associated invariant chain (Ii). Serial frozen sections were immunostained using monoclonal antibodies (MAbs) to monomorphic framework determinants of HLA sublocus products, beta 2m and Ii, and to CD53, a recently defined broadly distributed pan-leucocyte molecule. Compared with the normal state, an induction/neo-expression of HLA-A,B,C/beta 2m was found in a considerable number of tumors of muscle, peripheral nerve, cartilage-forming, adipose, and vascular tissues. Conversely, some tumors of fibrous origin and of autonomic ganglia showed an abnormal abrogation/loss of HLA-A,B,C/beta 2m with respect to their cells of origin. Small, round tumor cells present in various types of STT exhibited a heterogenous pattern of expression of these molecules with a preponderance of HLA-A, B,C/beta 2m-negativity. HLA-D/Ii determinants were rarely detectable in STT. Besides their expression in some fibrohistiocytic tumors, they were only occasionally found in tumors of smooth-muscle, peripheral-nerve and vascular origin as well as in one clear-cell sarcoma. In all tumors but one, there was no microtopographic association between HLA-D/Ii-positive tumor cells and inflammatory cells. CD53 allowed discrimination between dendritic interstitial cells (DIC) and neoplastic cells and additionally revealed that, in contrast to other solid tumors, STT are generally characterized by an extreme scarcity of lymphohistiocytic infiltrates. Our data indicate that, aside from very rare exceptions, aberrant induction or abrogation of MHC molecules in STT occurs in the absence of lymphohistiocytic stromal infiltrates, suggesting that these alterations might not be a consequence of local cytokine effects.

Expression of HLA class I antigens in human tumors and their involvement in tumor growth

A decreased expression of major histocompatibility complex (MHC) class I antigens is a common feature of many experimental and human tumors and can often be correlated with malignancy grade. In fact, reduction of class I antigens is associated in most tumors with an enhanced ability to elude immune surveillance. Loss of HLA-A,B,C antigens ranges from a decrease in the percentage of A,B,C-positive cells to selective loss of particular antigens and total loss of class I molecule expression. In man, this has been documented in melanomas, carcinomas, lymphomas, neuroblastoma and acute leukemias. The reduction in membrane antigens is generally associated with a parallel fall in immunoprecipitable intracellular proteins and the corresponding mRNAs in the absence of structural changes in the coding genes. The literature concerning the above mentioned topics is reviewed and discussed.

Expression of HLA ABC and DR antigens in thyroid neoplasia and correlation with mononuclear leukocyte infiltration

The rejection of tumor cells by the immune system depends on the production of tumor-associated antigens and the expression of HLA antigens on these cells. We therefore studied the expression of HLA ABC and DR antigens in malignant and benign thyroid disorders and correlated it with the types and extent of mononuclear cell infiltration. In the normal thyroid, HLA ABC expression was weak and focal, while it was diffusely present in benign disorders and in most but not all malignancies. HLA DR antigens, while absent or infrequently expressed in normal thyroid, were strongly but often focally expressed in all cases of autoimmune thyroid disease (AITD), as well as in most cases of malignant tumors and benign epithelium surrounding these tumors, and colloid nodule disease. There was a T cell predominance in all disorders, and the CD4+/CD8+ ratio was greater than 1 in most benign and malignant specimens. There was a direct correlation between the high expression of HLA antigens and dense inflammatory infiltration in AITD but not in most tumor specimens. Lack of such correlation suggests that the expression of HLA antigens is an autonomous event in tumors, independent of cellular infiltrate.

Gene regulation by tyrosine kinases: src protein activates various promoters, including c-fos

A promoter of the nuclear proto-oncogene fos was activated by cotransfection with the viral src gene. Ability to transactivate the c-fos promoter was dependent on tyrosine kinase activity, because (i) src mutants which have reduced tyrosine kinase activity due to mutation of Tyr-416 to Phe showed lower promoter activation, (ii) pp60c-src mutants which have increased tyrosine kinase activity due to mutation of Tyr-527 to Phe also augmented c-fos promoter induction, and (iii) mutation in the ATP-binding site of pp60v-src strongly suppressed c-fos promoter activation. Tyrosine kinase activity alone, however, was not sufficient for promoter activation, because of pp60v-src mutant which lacked its myristylation site and consequently membrane association showed no increased c-fos promoter activation. Both the tyrosine kinase- and membrane-association-defective mutants were also unable to induce transformation. Therefore, phosphorylation of membrane-associated substrates appears to be required for both gene expression and cellular transformation by the src protein. Two regions of the c-fos promoter located between positions -362 and -324 and positions -323 and -294 were responsive to src stimulation. We believe that protein tyrosine phosphorylation represents an important step of signal transduction from the membrane to the nucleus.

Autocrine induction of major histocompatibility complex class I antigen expression results from induction of beta interferon in oncogene-transformed BALB/c-3T3 cells

By varying growth conditions, we identified a novel mechanism of autocrine regulation of major histocompatibility complex (MHC) class I gene expression by induction of beta interferon gene expression in transformed BALB/c-3T3 cells. Low-serum conditions enhanced MHC class I antigen expression in v-rasKi- and v-mos-transformed BALB/c-3T3 cells but not in untransformed BALB/c-3T3 cells. Transformed and untransformed cells grown under standard serum conditions (10% bovine calf serum) expressed similar cell surface levels of MHC class I antigens. However, low-serum conditions (0.5% bovine calf serum) induced four- to ninefold increases in cell surface levels of MHC class I antigens in both v-rasKi- and v-mos-transformed cells but not in untransformed cells. These increases in MHC class I gene expression were seen at both the mRNA and cell surface protein levels and involved not only the heavy-chain component of the class I antigens but also beta 2 microglobulin. Beta 1 interferon mRNA and beta interferon-inducible 2',5'-oligoadenylate synthetase mRNA were induced by growth under low-serum conditions in transformed BALB/c-3T3 cells, and antibodies to beta interferon blocked the induction of MHC class I antigen expression by serum deprivation in these cells. These results demonstrate that growth under low-serum conditions leads to induction of beta interferon expression in oncogene-transformed cells which then directly mediates autocrine enhancement of MHC class I gene expression.

Expression of HLA-A, -B, -C, -DR, -DP, -DQ, and of HLA-D-associated invariant chain (Ii) in non-neoplastic mammary epithelium, fibroadenoma, adenoma, and carcinoma of the breast

Non-neoplastic mammary gland, 20 benign tumors and 206 carcinomas of the breast were immunohistochemically examined for expression of HLA-A, -B, -C, HLA-DR, -DP, and -DQ molecules and the HLA-D associated invariant chain (Ii). In contrast to cells from benign lesions, tumor cells of 51.2% of carcinomas had an abnormally low content of HLA-A, -B, and -C determinants ranging from reduction of antigenic density per cell (28.8%) over an incomplete (15.6%) to complete loss of antigens (6.8%). Associated with lymphohistiocytic stromal infiltrates, HLA-D/Ii determinants were found to be induced in benign duct and acinar epithelium after the order Ii greater than or equal to HLA-DR greater than or equal to HLA-DP greater than or equal to HLA-DQ. These antigens were also expressed, mostly noncoordinately, in 55.5% of carcinomas, and in 98 cases according to the above order. In 28.6%, Ii expression clearly exceeded HLA-D antigen expression; conversely, 6.2% contained HLA-DR+/Ii- tumor cell subsets. In breast carcinoma, the association of reduced HLA-A, -B, and -C expression and a noninduction of HLA-DR was highly significant (P less than 0.0009), suggesting an abnormal signal acting down-regulating on the expression of both classes of antigens. Because the modality of HLA-A, -B, and -C and HLA-D/Ii expression correlated with neither tumor type nor grade, it might be an independent parameter.

Gene regulation by tyrosine kinases: src protein activates various promoters, including c-fos

A promoter of the nuclear proto-oncogene fos was activated by cotransfection with the viral src gene. Ability to transactivate the c-fos promoter was dependent on tyrosine kinase activity, because (i) src mutants which have reduced tyrosine kinase activity due to mutation of Tyr-416 to Phe showed lower promoter activation, (ii) pp60c-src mutants which have increased tyrosine kinase activity due to mutation of Tyr-527 to Phe also augmented c-fos promoter induction, and (iii) mutation in the ATP-binding site of pp60v-src strongly suppressed c-fos promoter activation. Tyrosine kinase activity alone, however, was not sufficient for promoter activation, because of pp60v-src mutant which lacked its myristylation site and consequently membrane association showed no increased c-fos promoter activation. Both the tyrosine kinase- and membrane-association-defective mutants were also unable to induce transformation. Therefore, phosphorylation of membrane-associated substrates appears to be required for both gene expression and cellular transformation by the src protein. Two regions of the c-fos promoter located between positions -362 and -324 and positions -323 and -294 were responsive to src stimulation. We believe that protein tyrosine phosphorylation represents an important step of signal transduction from the membrane to the nucleus.

Regulation of IFN-gamma-induced host cell MHC antigen expression by Kirsten MSV and MLV. I. Effects on class I antigen expression

We have reported previously that the Kirsten murine sarcoma virus (Ki-MSV) that carries the v-Ki-ras oncogene prevents C3H10T 1/2 fibroblasts from being able to respond to interferon-gamma (IFN-gamma) with the expression of the class II major histocompatibility complex (MHC) antigen, H-2A. In this report we investigate further as to whether MSV or its parent virus Kirsten murine leukaemia virus (Ki-MLV) is able to reduce host class I MHC antigen expression. The results demonstrate that class I expression is diminished in MSV-infected cells over a time-course of 7 days after exposure to IFN-gamma and over a range of IFN-gamma concentrations. The optimal concentration of IFN-gamma for maximal class I expression remained unchanged. Cells infected with Ki-MLV, which failed to abolish the induction by IFN-gamma of class II antigens, also expressed lower levels of class I antigens, similar to those for cells infected with Ki-MSV, after exposure to IFN-gamma. It is likely therefore that the inhibition of class I induction is due to genetic material shared between the viruses, principally in the long terminal repeats (LTR), and hence that the mechanism of action is distinct from that responsible for the abolition of class II induction by Ki-MSV alone. Since class I antigens are required for CD8+ T cells (mainly cytotoxic T cells) to recognize (foreign) antigen this reduction in class I expression might lead to reduced visibility of infected cells to T cells and thus might contribute to the tumorigenicity of Ki-MSV-infected cells.

Autocrine induction of major histocompatibility complex class I antigen expression results from induction of beta interferon in oncogene-transformed BALB/c-3T3 cells

By varying growth conditions, we identified a novel mechanism of autocrine regulation of major histocompatibility complex (MHC) class I gene expression by induction of beta interferon gene expression in transformed BALB/c-3T3 cells. Low-serum conditions enhanced MHC class I antigen expression in v-rasKi- and v-mos-transformed BALB/c-3T3 cells but not in untransformed BALB/c-3T3 cells. Transformed and untransformed cells grown under standard serum conditions (10% bovine calf serum) expressed similar cell surface levels of MHC class I antigens. However, low-serum conditions (0.5% bovine calf serum) induced four- to ninefold increases in cell surface levels of MHC class I antigens in both v-rasKi- and v-mos-transformed cells but not in untransformed cells. These increases in MHC class I gene expression were seen at both the mRNA and cell surface protein levels and involved not only the heavy-chain component of the class I antigens but also beta 2 microglobulin. Beta 1 interferon mRNA and beta interferon-inducible 2',5'-oligoadenylate synthetase mRNA were induced by growth under low-serum conditions in transformed BALB/c-3T3 cells, and antibodies to beta interferon blocked the induction of MHC class I antigen expression by serum deprivation in these cells. These results demonstrate that growth under low-serum conditions leads to induction of beta interferon expression in oncogene-transformed cells which then directly mediates autocrine enhancement of MHC class I gene expression.