Metastasis of mouse T lymphoma cells is controlled by the level of major histocompatibility complex class I H-2Dk antigens

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.

The metastatic T-cell hybridoma antigen/P-selectin glycoprotein ligand 1 is required for hematogenous metastasis of lymphomas

Using variants of the murine BW5147 lymphoma cell-line, we have previously identified 3 monoclonal antibodies (MAbs) that discriminate between metastatic and nonmetastatic BW5147-derived T-cell hybridomas and lymphomas, as well as BW5147-unrelated T-lymphomas. These MAbs were reported to recognize an identical membrane-associated sialoglycoprotein, termed "metastatic T-cell hybridoma antigen" (MTH-Ag). Here, we document that the expression pattern of the MTH-Ag on metastatic and nonmetastatic BW5147 variants correlates with that of the P-selectin glycoprotein ligand 1 (PSGL-1), a sialomucin involved in leukocyte recruitment to sites of inflammation. Moreover, the MAbs against the MTH-Ag recognize PSGL-1 when it is transfected in MTH-Ag-negative BW5147 variants, suggesting that the MTH-Ag is PSGL-1. Overexpression of MTH-Ag/PSGL-1 in MTH-Ag-negative BW5147 variants did not affect their in vivo malignancy. Yet, down-regulation of MTH-Ag/PSGL-1 expression on metastatic, MTH-Ag-positive BW5147 variants, using an RNA interference (RNAi) approach, resulted, in a dose-dependent manner, in a significant reduction of liver and spleen colonization and a delay in mortality of the recipient mice upon intravenous inoculation. Collectively, these results demonstrate that, although MTH-Ag/PSGL-1 overexpression alone may not be sufficient for successful dissemination and organ colonization, MTH-Ag/PSGL-1 plays a critical role in hematogenous metastasis of lymphoid cancer cells.

Multiple effects of transfection with interleukin 2 and/or interferon gamma on the behavior of mouse T lymphoma cells

We have previously reported that transfection of mouse interferon gamma (IFN-gamma) in H-2Kk-positive BW variants (BW-Sp3) abolishes tumorigenicity and reduces metastatic capacity. To further increase the immunogenicity of BW-Sp3 cells, the gene for human interleukin 2 (huIL-2) was transfected in these cancer cells. Single BW-Sp3(huIL-2) and double BW-Sp3(huIL-2+IFN-gamma) transfectants were generated and their behavior was investigated as compared to parental and IFN-gamma-transfected BW-Sp3. Although expression of huIL-2 was equally effective as IFN-gamma in preventing tumor formation and reducing experimental metastasis, it did not confer protection to spontaneous metastases and even reversed the anti-metastatic activity of IFN-gamma. Inoculation of the BW variants in immunocompromised mice revealed that expression of IL-2 activates both T cells and aspecific immune effectors. However, in immunocompromised mice a clear pro-metastatic effect of IL-2 was recorded. Analysis of membrane antigens on the different variants showed a selective effect of huIL-2 on the expression of two surface antigens, i.e. L-selectin and metastatic T cell hybridoma antigen (MTH), which may contribute to metastasis. Hence upon expression of huIL-2 in T lymphoma variants, tumor outcome will depend on the balanced effects of the transfected cytokines on the immune response and the redirected effect on tumor progression.

Cancer vaccines: challenges and potential solutions

Almost a century has passed since immunotherapy of cancer was first attempted using cancer immunogens (vaccines); however, its clinical impact remains modest. Although initial concerns about a lack of human tumor antigens have decreased, prevailing issues include inefficient procedures for immunization and downregulated expression of major histocompatibility complex (MHC) class I molecules in tumor cells. While immunization can be improved, deficient MHC class I expression remains a problem, because it hampers the ability of tumor cells to present antigens for killing by CD8+ T cells. These are the major mediators of tumor destruction, and they have little or no activity against antigen-negative bystander cells. However, there are reasons to be optimistic that therapeutic vaccination against cancer antigens might become a reality at last.