Multiple mechanisms of immune evasion can coexist in melanoma tumor cell lines derived from the same patient

Progressive tumor growth may be associated with suppression of the immune response. Many different mechanisms may contribute to immune evasion. We investigated some of these mechanisms in melanoma cells lines generated from two patients. These cell lines show a complex pattern of altered HLA expression; however, the resulting phenotype did not satisfactorily explain the simultaneous evasion of T and NK cell cytotoxicity. Two additional alterations have now been detected in these melanoma cell lines: (1) resistance to FAS-induced apoptosis caused by defective FAS gene expression, and (2) constitutive expression of immunosuppressive cytokines. Our results show that several of the major mechanisms for immune evasion may coexist in a single tumor. This suggests that tumor progression may give rise to an extremely resistant phenotype, which may be an impediment to some immunotherapeutic strategies. We hypothesize that the simultaneous presence of several mechanisms involved in tumor immune evasion must be the result of progressive selection of characteristics that are advantageous for tumor survival in a competent host. Our findings do not support the possibility that FASL expression is a common mechanism of evasion of immune response in melanoma cells.

Microsatellite instability analysis in tumors with different mechanisms for total loss of HLA expression

Down-regulation of the expression of major histocompatibility complex molecules is a frequent event that is associated with the poor immunogenicity of tumor cells. Acquired resistance to T-cell-based immunotherapy has been associated with loss of functional beta2-microglobulin expression. This anomaly appears to be particularly relevant in tumors exhibiting a defect in DNA-mismatch repair, and induces structural abnormalities in HLA cell-surface expression that are not reversible by cytokine treatment. We examined HLA expression in 118 melanoma, colon or larynx tumors to identify total loss of HLA class I expression with or without somatic beta2-microglobulin gene mutation. Microsatellite instability was investigated in these tumors to determine whether a replication error phenotype (RER+) implied a particular alteration in HLA phenotype. A total of 7.6% of the tumors showed the RER+ phenotype, and 12.7% were HLA-ABC-negative. In the RER+ group, only one tumor was HLA-ABC-negative and no beta2-microglobulin mutation was identified. In contrast, in the HLA-ABC-negative group, only one tumor showed microsatellite instability. None of the three melanomas that contained beta2-microglobulin mutation exhibited the mutator phenotype. These findings suggest that beta2-microglobulin mutation in human melanoma tumors may arise through a mechanism that does not necessarily involve microsatellite instability. Our results also indicate that somatic mutations of the beta2-microglobulin gene are not the main mechanism of total loss of HLA expression.

Chromosome loss is the most frequent mechanism contributing to HLA haplotype loss in human tumors

Loss of heterozygosity (LOH) in the short arm of chromosome 6 (6p) was detected in samples obtained from colon (13.8%), larynx (17.6%) and melanoma (15.3%) tumors. The parallel study of HLA-antigen expression in tumor tissues using locus- and polymorphic-specific antibodies in combination with LOH microsatellite analysis on 6p allowed us to establish that LOH in chromosome 6 is a representative phenomenon in most tumor cells present in a given tumor tissue. In most cases, specific HLA alleles had been lost in a predominant population of tumor cells, indicating that LOH is a non-irrelevant mutation that probably confers a selective advantage for survival of the mutant cell. We also demonstrate that LOH frequently occurred through chromosome loss rather than somatic recombination. LOH at all loci studied on the p and q arms of chromosome 6 was observed in at least 56.2% (9/17) cases. This HLA-associated microsatellite analysis was a useful tool for classifying tumors as LOH-positive or -negative, and therefore to consider a patient as a potential non-responder or responder in a vaccination trial.

Differential effect on U937 cell differentiation by targeting transcriptional factors implicated in tissue- or stage-specific induced integrin expression

The inhibition of transcription factor functions was used to define their role in phorbol ester-induced cellular differentiation of a monocytic cell line, U937. We demonstrate a differential effect on cell adhesion and differentiation: antisense or competitive binding with double-stranded oligonucleotides antagonized the functions of AP-1, NF-kappaB, and PU.1 transcriptional factors. In the presence of phorbol 12-myristate 13-acetate (PMA), U937 cells attached to the plastic surface and cells were characterized by marked expression of beta2-integrin molecules on the cell surface. We show that the in vivo differentiation of U937 cells appears to occur normally in the absence of AP-1 activity. In contrast, the addition to the cell culture of phosphorothioate oligonucleotides that contained the NF-kappaB or PU.1 binding sites significantly inhibited U937 differentiation. The absence of NF-kappaB led to pleiotropic effects with a clear reduction in the expression of integrin and other lineage-specific myeloid antigens on the cell surface. In contrast, the absence of PU.1 had a more restricted effect on integrin expresion on the cell surface, probably as a result of blockage of CD18 gene expression.

Microsatellite instability in cervical intraepithelial neoplasia

We studied the incidence of microsatellite instability (MI) in lesions defined as cervical intraepithelial neoplasia (CIN). The human papillomavirus (HPV) status of the tissues was also determined. DNA from tissue samples and autologous lymphocytes were studied for five loci located within or adjacent to the DNA mismatch repair genes. Replicate errors were detected in 7 out of 47 (14.8%) samples of cervical tissue from 24 women. Our results indicate that the defect in DNA repair-associated genes does not appear to be necessary for the selection of clones which progress from dysplasia to carcinoma. Although HPV DNA of highly oncogenic types (16/18) was detected in most cervical lesions and may be an important factor for MI, we also detected MI in two loci in HPV-negative normal tissue, indicating that further events can also be involved in mismatch repair defects.

In vivo and in vitro generation of a new altered HLA phenotype in melanoma-tumour-cell variants expressing a single HLA-class-I allele

A new HLA-class-I altered phenotype is described in melanoma. This phenotype is the result of a combination of HLA-B-locus down-regulation and HLA-haplotype loss. The alteration was found in 2 melanoma cell lines generated from 2 patients; one was derived from an in vivo lesion (FM37) and the other was obtained after in vitro immunoselection (R22.2). The R22.2 cell line was isolated from FM55P, a cell line derived from a primary melanoma, after in vitro treatment with a heterologous HLA-A2-restricted cytotoxic-T-lymphocyte (CTL) clone. Two additional cell lines from patient 55 were obtained from 2 s.c. metastases (FM55M1 and FM55M2). Iso-electric focusing and flow-cytometric studies showed a significant reduction in the expression of both HLA-B alleles in all cell lines studied. The expression of HLA-B-locus products recovered completely after IFN-gamma treatment of FM55P, M1 and M2. In contrast, FM37 and R22.2 tumour cells showed an additional HLA defect: the absence of one HLA haplotype. Simple tandem-repeat polymorphism markers spanning chromosome 6 showed that DNA from the 2 samples (FM37 and R22.2) showed loss of heterozygosity (LOH). In both cases, homozygosity was observed on 6p, which maps the HLA region, the final consequence being a tumour cell that expressed a single HLA-class-I allele (HLA-A3 and HLA-A1 respectively). FM37 cells may thus reflect the in vivo counterpart of resistance to lysis by HLA-A2-restricted tumour-infiltrating lymphocytes.