Immunogenicity of H-2 positive and H-2 negative clones of a mouse tumour, GR9

MHC/HLA Class I Loss in Cancer Cells

In this chapter I describe Tumour Immune Escape mechanisms associated with MHC/HLA class I loss in human and experimental tumours. Different altered HLA class-I phenotypes can be observed that are produced by different molecular mechanisms. Experimental and histological evidences are summarized indicating that at the early stages of tumour development there is an enormous variety of tumour clones with different MHC class I expression patterns. This phase is followed by a strong T cell mediated immune-selection of MHC/HLA class-I negative tumour cells in the primary tumour lesion. This transition period results in a formation of a tumour composed only of HLA-class I negative cells. An updated description of this process observed in a large variety of human tumors is included. In the second section I focus on MHC/HLA class I alterations observed in mouse and human metastases, and describe the generation of different tumor cell clones with altered MHC class I phenotypes, which could be similar or different from the original tumor clone. The biological and immunological relevance of these observations is discussed. Finally, the interesting phenomenon of metastatic dormancy is analyzed in association with a particular MHC class I negative tumor phenotype.

Generation of MHC class I diversity in primary tumors and selection of the malignant phenotype

Intratumor heterogeneity among cancer cells is promoted by reversible or irreversible genetic alterations and by different microenvironmental factors. There is considerable experimental evidence of the presence of a variety of malignant cell clones with a wide diversity of major histocompatibility class I (MHC-I) expression during early stages of tumor development. This variety of MHC-I phenotypes may define the evolution of a particular tumor. Loss of MHC-I molecules frequently results in immune escape of MHC-negative or -deficient tumor cells from the host T cell-mediated immune response. We review here the results obtained by our group and other researchers in animal models and humans, showing how MHC-I intratumor heterogeneity may affect local oncogenicity and metastatic progression. In particular, we summarize the data obtained in an experimental mouse cancer model of a methylcholanthrene-induced fibrosarcoma (GR9), in which isolated clones with different MHC-I expression patterns demonstrated distinct local tumor growth rates and metastatic capacities. The observed "explosion of diversity" of MHC-I phenotypes in primary tumor clones and the molecular mechanism ("hard"/irreversible or "soft"/reversible) responsible for a given MHC-I alteration might determine not only the metastatic capacity of the cells but also their response to immunotherapy. We also illustrate the generation of further MHC heterogeneity during metastatic colonization and discuss different strategies to favor tumor rejection by counteracting MHC-I loss. Finally, we highlight the role of MHC-I genes in tumor dormancy and cell cycle control.

Implication of the β2-microglobulin gene in the generation of tumor escape phenotypes

Classical MHC molecules present processed peptides from endogenous protein antigens on the cell surface, which allows CD8(+) cytotoxic T lymphocytes (CTLs) to recognize and respond to the abnormal antigen repertoire of hazardous cells, including tumor cells. The light chain, β2-microglobulin (β2m), is an essential constant component of all trimeric MHC class I molecules. There is convincing evidence that β2m deficiency generates immune escape phenotypes in different tumor entities, with an exceptionally high frequency in colorectal carcinoma (CRC) and melanoma. Damage of a single β2m gene by LOH on chromosome 15 may be sufficient to generate a tumor cell precommitted to escape. In addition, this genetic lesion is followed in some tumors by a mutation of the second gene (point mutation or insertion/deletion), which produces a tumor cell unable to express any HLA class I molecule. The pattern of mutations found in microsatellite unstable colorectal carcinoma (MSI-H CRC) and melanoma showed a striking similarity, namely the predominance of frameshift mutations in repetitive CT elements. This review emphasizes common but also distinct molecular mechanisms of β2m loss in both tumor types. It also summarizes recent studies that point to an acquired β2m deficiency in response to cancer immunotherapy, a barrier to successful vaccination or adoptive cellular therapy.

The tumour suppressor Fhit positively regulates MHC class I expression on cancer cells

MHC class I (MHC-I) molecules are ubiquitously expressed on the cells of an organism. Study of the regulation of these molecules in normal and disease conditions is important. In tumour cells, the expression of MHC-I molecules is very frequently lost, allowing these cells to evade the immune response. Cancers of different histology have shown total loss of MHC-I molecule expression, due to a coordinated transcriptional down-regulation of various antigen-processing machinery (APM) components and/or MHC-I heavy chains. The mechanisms responsible for these alterations remain unclear. We determined the possible genes involved by comparing MHC-I-positive with MHC-I-negative murine metastases derived from the same fibrosarcoma tumour clone. MHC-I-negative metastases showed transcriptional down-regulation of APM and MHC-I heavy chains. The use of microarrays and subtraction cDNA libraries revealed four candidate genes responsible for this alteration, but two of them were ruled out by real-time RT-PCR analyses. The other two genes, AP-2α and Fhit tumour suppressors, were studied by using siRNA to silence their expression in a MHC-I-positive metastatic cell line. AP-2α inhibition did not modify transcriptional expression of APM components or MHC-I heavy chains or surface expression of MHC-I. In contrast, silencing of the Fhit gene produced the transcriptional down-regulation of APM components and MHC-I heavy chains and decreased MHC-I surface expression. Moreover, transfection of Fhit in MHC-I-negative tumour cell lines restored MHC-I cell surface expression. These data indicate that defects in Fhit expression may promote MHC-I down-regulation in cancer cells and allow escape from immunosurveillance(#).

Gene transfer preferentially selects MHC class I positive tumour cells and enhances tumour immunogenicity

The modulated expression of MHC class I on tumour tissue is well documented. Although the effect of MHC class I expression on the tumorigenicity and immunogenicity of MHC class I negative tumour cell lines has been rigorously studied, less is known about the validity of gene transfer and selection in cell lines with a mixed MHC class I phenotype. To address this issue we identified a C26 cell subline that consists of distinct populations of MHC class I (H-2D/K) positive and negative cells. Transient transfection experiments using liposome-based transfer showed a lower transgene expression in MHC class I negative cells. In addition, MHC class I negative cells were more sensitive to antibiotic selection. This led to the generation of fully MHC class I positive cell lines. In contrast to C26 cells, all transfectants were rejected in vivo and induced protection against the parental tumour cells in rechallenge experiments. Tumour cell specificity of the immune response was demonstrated in in vitro cytokine secretion and cytotoxicity assays. Transfectants expressing CD40 ligand and hygromycin phosphotransferase were not more immunogenic than cells expressing hygromycin resistance alone. We suggest that the MHC class I positive phenotype of the C26 transfectants had a bearing on their immunogenicity, because selected MHC class I positive cells were more immunogenic than parental C26 cells and could induce specific anti-tumour immune responses. These data demonstrate that the generation of tumour cell transfectants can lead to the selection of subpopulations that show an altered phenotype compared to the parental cell line and display altered immunogenicity independent of selection marker genes or other immune modulatory genes. Our results show the importance of monitoring gene transfer in the whole tumour cell population, especially for the evaluation of in vivo therapies targeted to heterogeneous tumour cell populations.

Expression of MHC class I antigen in Morris rat hepatoma and normal liver

Expression of major histocompatibility complex (MHC) class I antigen in Morris rat hepatoma 7777 and normal rat liver tissues was studied by immunohistochemical techniques, Western blot and radiobinding assay. It was found that hepatoma tissue displayed a pattern of enhanced MHC class I antigen expression compared with normal liver. This phenomenon may represent an uncommon pattern of MHC class I antigen expression of liver tumor. The influence of enhanced MHC class I antigen expression on the development and spread of tumors as well as the possible mechanisms of this enhancement are discussed.

Effect of MHC class-I transfection on local tumor growth and metastasis in an H-2-negative clone derived from a chemically induced fibrosarcoma

GR9 is a chemically induced fibrosarcoma composed of clones with different H-2 class-I expression. These clones differ with respect to local growth and spontaneous metastasis. The B9 clone (H-2 negative) is highly tumorigenic (local growth) but of low metastatic potential (spontaneous metastasis assay). We have analyzed the effect that transfection of H-2Dd and H-2Kd genes on this clone have upon local growth, lung colonization after i.v. injection and ability to form spontaneous metastases. The results showed that the effect on local growth of transfection of the Kd-gene was stronger than that of the Dd gene. In addition, B9 co-transfected with H-2Kd and Dd genes showed the highest immunogenic properties in syngeneic BALB/c mice. Interestingly, the pSV2-neo transfected clone gave almost the same result as that obtained with Dd transfection. Lung colonization after i.v. injection of the different clones (experimental metastasis), paralleled the results obtained for local growth: the number of lung nodules followed the cadence KdDd less than Kd less than Dd less than pSV2. Survival of mice was always inversely correlated with local growth, e.g., all mice injected with 5 x 10(5) B9 H-2KdDd transfected cells survived. In contrast, no mice injected with the B9 control did. These differences were abrogated in irradiated and nude BALB/c mice. Finally, all transfected clones remained non-metastatic in a spontaneous metastasis assay, behaving as the control, non-transfected B9 cells.

Damage in B2m genes and DNA methylation of H-2 genes are involved in loss of expression of class I MHC products on the membrane of LR.4, a cell line derivative of the T-cell lymphoma L5178Y

We have isolated an H-2 deficient cell line (LR.4) from the T-cell lymphoma L5178Y which grew without restrictions in the peritoneal cavity of different inbred strains of mice. The use of polyclonal anti-H-2 antiserum and complement indicated that LR.4 cells did not express class I determinants on the cell membrane. Southern blots of genomic DNA of LR.4 cells showed that B2m genes were severely damaged and that class I H-2 genes were extensively methylated. Consequently, LR.4 cells failed to transcribe mRNAs for both B2m and class I H-2 genes. On the other hand, specific immunity to LR.4 was demonstrated in C57BL/6J mice since, in subsequent challenges with either LR.4 or EL4.4, LR.4 did not grow, whereas EL4.4 grew and killed the mice. In C57BL/6J mice, rejection of LR.4 was accompanied by the production of cytotoxic antibodies. The immune response induced in C57BL/6J mice was determined by non-H-2 antigenic determinants in LR.4 cells.

Differential mRNA levels of c-myc, c-fos and MHC class I in several clones of a murine fibrosarcoma

We have evaluated the relationship between MHC class-I, c-myc and c-fos proto-oncogene expression in several clones of a methylcholanthrene-induced fibrosarcoma (GR9) which originated in a BALB/c mouse. These clones represent a heterogeneous population and differ markedly with regard to H-2 class-I cell-surface expression, local tumor growth, NK sensitivity and metastatic potential. In the present study we show that cell-surface expression of MHC class-I antigens correlates inversely with levels of c-myc mRNA transcripts. On the other hand, mRNA levels of c-fos are correlated directly with the amount of mRNA of MHC class I. Treatment of the B9 clone with gamma interferon increased mRNA transcription and surface expression of H-2 class-I antigens, while c-myc transcription was simultaneously down-regulated. In contrast, c-fos mRNA levels remained unaltered.

MHC antigens on human tumors

MHC class I antigens on tumor cells are expected to play an important role because they regulate the sensitivity to antitumoral immunological mechanisms. Overall or selective qualitative or quantitative changes in MHC molecules may modify the recognition of tumor cells by components of the immune system. It seems clear that MHC antigen expression on tumor cells is important in triggering the immune response by autologous lymphocytes. A deficiency in or lack of MHC class I antigens may have profound effects on T and NK cell activity. In experimental models, variation in the expression of MHC class I antigens has been shown to exert a decisive influence on local tumor growth and metastasis. However, there is little information about the influence of selective loss of individual locus products on the behavior of human tumor cells. Total and selective HLA losses have been found in a large variety of tumors, and different mechanisms have been shown to be responsible for these changes. In some examples, HLA losses are associated with a poor degree of tissue differentiation and poor prognosis. In other tumors, however, no such association has been found. We do not know whether HLA class II expression in neoplastic cells plays an immunological role, although, with the exception of melanoma, HLA class II expression is more frequently observed in tumors with a more favorable prognosis. Finally, there is no doubt that we need to learn more about how to manipulate the expression of MHC class I and II antigens in human tumors, in order to stimulate immune response.(ABSTRACT TRUNCATED AT 250 WORDS)

A weakly tumorigenic phenotype with high MHC class-I expression is associated with high metastatic potential after surgical removal of the primary murine fibrosarcoma

We have examined the metastatic capacity of different clones of a chemically induced fibrosarcoma GR9. These clones have previously been characterized for their H-2 class-I and class-II phenotype, NK sensitivity and local tumor growth. Our present data show that clones which express low amounts of H-2 class-I antigens are poorly metastatic in a post-surgical spontaneous metastasis assay, while those expressing high levels of class-I antigens possess a high metastatic capacity. These results correlate inversely with local growth patterns of the clones. High metastatic capacity was associated with resistance to NK cells. In an experimental metastasis assay, based on intravenous administration of in vitro carried GR9 clones to syngeneic BALB/c mice, an opposite result to the post-surgical assay was obtained. Gamma-IFN treatment of B9 clones (H-2-deficient) enhanced H-2 class-I expression and diminished experimentally induced metastases. Metastatic colonies, from the spontaneous metastasis assay, obtained from different organs, showed changes in the ratio H-2K/H-2D. There was a tendency for down-regulation of the expression of H-2K molecules in H-2-positive clones and for up-regulation of H-2D expression in H-2-negative clones.

NK sensitivity and lung clearance of MHC-class-I-deficient cells within a heterogeneous fibrosarcoma

The present study examines clonal variations in NK sensitivity in a methylcholanthrene-induced fibrosarcoma. Previous studies of clones from this tumor have shown considerable heterogeneity in H-2 expression, and an association between deleted or low levels of class-I products and increased tumorigenicity after subcutaneous implantation in immunocompetent syngeneic mice. Here, fibrosarcoma clones with no or low expression of MHC-class-I products were found to be sensitive to NK-mediated lysis, while clones with high levels of MHC-class-I expression were relatively resistant. One H-2+ (G2) and one H-2- (B9) clone were chosen for more detailed studies. Cold-target competition assays and conjugate cytotoxicity assays in agarose showed that splenic effector cells bound equally well to the H-2+ and H-2- tumor clone, although only the latter was sensitive to NK cell lysis. Treatment with 50 U/ml of rIFN-gamma for 48 hr increased the levels of H-2 expression and made both clones more resistant to NK-mediated lysis. In vivo studies with radiolabelled tumor cells showed that cells from the H-2+ clone survived better than cells from the H-2- clone in the pulmonary capillary bed after i.v. inoculation. This difference disappeared in mice treated with anti-asialo GM1 serum, known to deplete NK cell activity.

Generation of syngeneic anti-tumour double negative cells (CD4- CD8-), with cytotoxic activity against clones of different MHC class I expression

In vivo immunization and secondary culture techniques were used to generate cytotoxic T lymphocytes against a tumour cell clone obtained from a methylcholantrene-induced fibrosarcoma. Our CTLs differed from classical CTLs (Ly2+ and MHC restricted) in that our lymphocytes exhibited a non-MHC restricted cytotoxic activity directed to the original tumour and to other MCA-induced tumours, but have failed to demonstrate any killing activity against a wide range of tumour cell lines of diverse origin (including NK sensitive targets). Depletion and phenotypic studies demonstrated that these cells bear the Thyl.2 antigen but are negative for both Ly2 and L3T4 antigens. These CTLs may belong to a double negative subset (CD4-, CD8-) involved in the anti-tumour response.