MHC class I expression on human tumour cells and their susceptibility to NK lysis

Natural mechanisms protecting against cancer

Carcinogenesis is a multistage process. At each step of this process, there are natural mechanisms protecting against development of cancer. The majority of cancers in humans is induced by carcinogenic factors present in our environment including our food. However, some natural substances present in our diet or synthesized in our cells are able to block, trap or decompose reactive oxygen species (ROS) participating in carcinogenesis. Carcinogens can also be removed from our cells. If DNA damage occurs, it is repaired in most of the cases. Unrepaired DNA alterations can be fixed as mutations in proliferating cells only and mutations of very few strategic genes can induce tumor formation, the most relevant are those activating proto-oncogenes and inactivating tumor suppressor genes. A series of mutations and/or epigenetic changes is required to drive transformation of a normal cell into malignant tumor. The apparently unrestricted growth has to be accompanied by a mechanism preserving telomeres which otherwise shorten with succeeding cell divisions leading to growth arrest. Tumor can not develop beyond the size of 1-2mm in diameter without the induction of angiogenesis which is regulated by natural inhibitors. To invade the surrounding tissues epithelial tumor cells have to lose some adhesion molecules keeping them attached to each other and to produce enzymes able to dissolve the elements of the basement membrane. On the other hand, acquisition of other adhesion molecules enables interaction of circulating tumor cells with endothelial cells facilitating extravasation and metastasis. One of the last barriers protecting against cancer is the activity of the immune system. Both innate and adaptive immunity participates in anti-tumor effects including the activity of natural killer (NK) cells, natural killer T cells, macrophages, neutrophils and eosinophils, complement, various cytokines, specific antibodies, and specific T cytotoxic cells. Upon activation neutrophils and macrophages are able to kill tumor cells but they can also release ROS, angiogenic and immunosuppressive substances. Many cytokines belonging to different families display anti-tumor activity but their role in natural anti-tumor defense remains largely to be established.

Intensity of class I antigen expression on human tumour cell lines and its relevance to the efficiency of non-MHC-restricted killing

A modified tetrazolium reduction assay (MTT) was used to assess the relation between HLA class I antigen expression on tumour cells and their susceptibility as a target for non-MHC restricted LAK/NK cytotoxicity using interleukin-2 activated peripheral blood mononuclear cells (MNC) from normal individuals. At 20/1 effector/target ratio this ranged from no killing to 77%. The efficiency of killing was dependent on duration of effector cell culture with IL-2, peaking at day 10 and declining thereafter. This killing could be enhanced by addition of other cytokines including interferons alpha, beta and gamma. Study of a panel of 15 tumour cell lines using a single effector showed that there was no statistically significant inverse correlation (using Spearman rank test) between the degree of tumour class I expression and LAK/NK killing at 20/1 (r = 0.23 P = 0.39) and 10/1 (r = 0.30, P = 0.27) and at 5/1 E/T ratio r = 0.47, P = 0.08) respectively. Lack of inverse correlation between these two parameters came from study of one bladder tumour line (FEN), whose absent class I antigens had been corrected by transfection with beta 2 microglobulin gene. At high E/T ratio (20/1) there was an increase in the susceptibility of target cells to lysis (36% parent cell, 45% transfected cell), whilst at lower E/T ratios (1/1) there was significantly more killing of the non-transfected cells (10% vs 31%). The addition of anti-class I antibody W6/32 increased killing by 18% but this was non-specific as the same increase occurred with a class II antibody. These data suggest that overall there was not an inverse correlation between class I expression and LAK/NK killing at high E/T ratios, whilst at low (5/1 or lower) E/T ratios this correlation nearly reached statistical significance suggesting that the conflicting literature reports may be due to a threshold levels of effector cells above which the masking effects of MHC antigens disappears.

Histocompatibility antigens and natural killer susceptibility

The central question of the nature of the structure(s) involved in the recognition of targets by natural killer (NK) cells remains unresolved. Although NK-mediated cytotoxicity is not MHC-restricted, it has been suggested that these cells could recognize the targets more effectively in the absence of MHC class I antigens. In this paper we review the contradictory results obtained when studying the NK susceptibility of cell lines which constitutively express different levels of MHC antigens, or which have been induced to express MHC antigens by gene transfection or gamma-interferon treatment. Taken together, the results indicate that MHC antigens play a differential role in NK lysis depending on the nature of the target cells used; MHC class I antigens play a role in the NK resistance of cells from a hematopoietic lineage, but this does not extend to cells from other origins. The data reviewed also support the hypothesis that MHC class I antigens induced NK resistance by interfering with target structures, and that multiple NK molecules are involved in NK-mediated lysis as part of a possible advanced recognition system.

Mechanisms involved in NK resistance induced by interferon-gamma

Human tumor cell lines were treated with interferon-gamma (IFN-gamma) and then used as target cells in NK assays to measure their ability to form conjugates and stimulate the production of NK cytotoxic factors (NKCF) and to determine their susceptibility to NKCF lysis. K562 and cell lines RS1, RS3, RS7, CAC, and CAP2, obtained from solid brain tumors, were used as targets, and peripheral blood lymphocytes (PBL) from normal donors were used as effector cells. IFN-gamma-treated cell lines had a decreased susceptibility to NKCF lysis and a decreased ability to induce the release of these factors without affecting target-effector cell binding. These results were not due to changes in HLA class I antigen expression, given that the level of HLA class I antigens on the tumor cell lines was not affected, the only exception being K562. In an attempt to further clarify the possible influence of HLA class I expression on K562, IFN-gamma-pretreated K562 cells were separated into HLA class I positive and HLA class I negative subsets for the NK assays. The results showed that both populations behaved similarly upon target-effector conjugate formation, whereas the HLA class I positive population showed a reduced susceptibility to lysis by NK cells and NKCF. Thus, these results establish that NK resistance induced by IFN-gamma is mediated by blocking the target cell's ability to activate NK cell triggering and release of NKCF and by blocking its susceptibility to lysis by these factors. This analysis helps to clarify not only the NK process but also the controversial regulatory effect of IFN in NK lysis.

Sera from patients with colon, breast and lung cancer induce resistance to lysis mediated by NK cytotoxic factors (NKCF)

Natural killer (NK) cells are involved in the antitumoral immunologic mechanism. These cells act through the release of cytotoxic molecules defined as NK cytotoxic factors (NKCF). Inhibitory factors of NK and NKCF mediated lysis have been described in in vitro assays. This study evaluates the induction of resistance to NKCF cytotoxicity by sera from 27 patients with colon, breast and lung cancer. Addition of these sera to the cytolytic assay where K562 cells and concentrated NKCF were used, induced resistance to NKCF mediated cytotoxicity in 21 cases (77%). The sera from the group with metastasis blocked NKCF lysis more markedly than the group with local tumours. However, no differences were observed when the groups with colon, breast and lung cancers were compared. This blocking effect was not found to be related to gamma interferon (IFN) levels. In a previous study, we described a tumour factor (NK-RIF) produced by human cell lines derived from metastatic adenocarcinomas. This factor blocked lysis of tumour target cells by NK cells. Consequently, it is proposed that the release of similar tumour factors with a capacity to induce resistance to NKCF may be involved in tumour growth and metastatic spreading in in vivo.

Analysis of the mechanisms involved in NK resistance induced by a new tumor factor NK-RIF

The mechanisms involved in susceptibility or resistance of neoplasic cells to lysis by NK cells are not well known. We have recently described a 12-kDa factor (NK-RIF), produced and released by different tumor cell lines, making K562 resistant to NK lysis without affecting the cytotoxic function of NK effector cells. In this paper we further study the mechanism involved in NK resistance of K562 mediated by NK-RIF and its biological implications. The results show that NK-RIF does not affect the binding capacity of target and effector cells nor the levels of HLA class I antigen expression on the target cells, as a proof that resistance to NK-mediated lysis is not always associated with a defect in target effector binding or with an increased MHC class I antigen expression. However NK-RIF-treated K562 loses its capacity to induce NK cell activation and the subsequent capacity to release NKCF and makes K562 resistant to lysis by NKCF. Therefore our results show that induction of resistance to NK cytotoxicity can be the result of the modulation of target structures responsible for inducing effector cell activation without affecting target/effector binding molecules. This indicates that the structures involved in adherence and activation of NK cells have a different nature and that molecules other than HLA participate in NK resistance.