Inadequate presentation of tumor antigens by host professional antigen-presenting cells (APCs), including dendritic cells (DCs), is one potential mechanism for the escape of tumors from the host immune system. Here, we show that human cancer cell lines release a soluble factor or factors that dramatically affect DC maturation from precursors without affecting the function of relatively mature DCs. One factor responsible for these effects was identified as vascular endothelial growth factor (VEGF). Thus, VEGF may play a broader role in the pathogenesis of cancer than was previously thought, and therapeutic blockade of VEGF action may improve prospects for immunotherapy as well as inhibit tumor neovasculature.

Tumours express a variety of novel epitopes which represent potential immune targets, and thus clinically evident tumours are thought to have effectively avoided immune recognition and elimination. Transporters associated with antigen presentation (TAP) are thought to be responsible for conveying intracellular peptides into the endoplasmic reticulum for complex formation with class I MHC and subsequent recognition by cytotoxic T lymphocytes. In this study, we evaluated 79 human solid tumours and cell lines for genetic abnormalities in TAP1 that might have led to an acquired loss of antigen presenting ability. A novel sequence (R659Q) was discovered near the ATP binding site in a human small cell lung cancer (SCLC) cell line, H1436. This cell line is heterozygous for this allele, but only the R659Q allele is transcribed into RNA. Even though the R659Q protein is expressed, these cells act as if they were TAP deficient by peptide binding and antigen presentation studies, which are restored after transfection of a functional TAP1 allele. This is the first evidence for a naturally occuring protein structural defect resulting in defective peptide transport in a human solid tumour.

The escape of tumor cells from immune recognition is a central problem in tumor immunology. Here, we examined the functional role of somatic beta 2-microglobulin (beta2m) gene mutations in human lung and breast cancers. Using single-strand conformational polymorphism (SSCP) analysis and DNA sequencing, we found mutations in the beta2m gene in 2 of 110 tested lung, colon and breast tumors and tumor cell lines. No mutations were identified in 63 breast cancer tumors, in B-lymphoblastoid cell lines or normal tissues from these or other patients. In these cell lines, beta2m protein was undetectable by Western blot analysis and there was no MHC class I on their cell surface even after treatment with interferon-gamma. Transfection of these tumor cell lines with the beta2m gene, but not addition of purified beta2m protein restored MHC expression without addition of exogenous pepticles, indicating that endogenous beta2m expression is necessary for proper intracellular MHC assembly and stabilization by endogeneous pepticles. Mutation in beta2m caused cell line H2009 to be resistant to specific lysis by influenza virus-specific CTL from HLA matched donors, and transfection of the beta2m gene restored this killing. A small cell lung cancer cell line with low class I expression and with a normal beta2m genomic sequence nonetheless also demonstrated increased class I expression after transfection of the beta2m expression vector alone, indicating that the availability of beta2m may be rate limiting for MHC assembly in this line. Our results indicate that somatic mutations or selective loss of expression of the beta2m gene in human lung cancer is rare, but can cause defective MHC class I expression and function allowing these cells to escape recognition by cytotoxic T cells.

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